lights/power/voltage drops

Ughh

I'm getting pronounced dimming of all lights (headlight, dash, you name it) when anything else draws power-brakes, signals, fan. The van actually shut off the other night when I turned the blower on, so I can't ignore this any longer. (That's a lie, I certainly can, and might.)

The headlights stay dimmer with foot on the brake when the van is off, but return to normal in a second when its running. I have LED headlights (and that's the only place I have LED, to my knowledge) so the first thing I did was remove them and replace with the halogens. No change, so I put the LED back in. I checked it with the van off and key on, no change, so its apparently not the alternator (right?). I was due for a new battery anyway so I bought one, no change. I've cleaned and replaced the battery terminals and cleaned the ground on the alternator and the one inside the right front fender, no change. I ran a new battery cable from the battery to ground, no change.

I know there are a dozen or two more scattered ground locations, but this is a rust free van so they should be good. I think this must be wiring somewhere. But I have little patience for endlessly chasing ghost after ghost. If it wasn't threatening to turn off the engine or cause a police pullover I wouldn't waste another minute on it.

Being a recalcitrant and unapologetic electric incompetent, I would need some ideas on what to check next, and how to check it. I have not been able to find another Previa that had this problem. It seems to be a problem with some later 4runners, but no solutions were found. They have accepted it as "normal" supposedly caused by brake booster.

It may have gotten worse since I replaced my blower, but I drive the van so little except when traveling I'm not sure. It is wired the exact same wonky way its always been wired.

I would think the next suspect is the alternator itself. Go somewhere (Autozone, etc.) where they can hook up a charging circuit analyzer. The battery isn't particularly good at dealing with any sustained current draw by itself - so if you had the ignition on but the car wasn't running, and saw the headlights dim when you pressed the brakes or turned on the fan, that's par for the course.

I did have the (old) battery and alternator tested at 2 different parts stores, though I can't say exactly what tests their device did. The battery showed a loss of cranking power but it was charging properly. I don't know if this covered the specific testing you are thinking of but I would guess so?

No idea then. I don't what they test at the parts stores - if it's just voltage at the battery, that's not great. The gold standard is some kind of load test where a drain is deliberately placed on your system to see if it can provide the spec'd amps.

I think they just check the voltage output, but we did put it under load with all electricals turned on

Added resistance from anything along the path from battery + through loads then to chassis ground is a problem. Anything that has a higher resistance than normal can assume a larger share of the total voltage than normal, leaving a lower-than-normal voltage on the thing you want powered.

Let's say your lights are dim when just powered from the battery at 12V because other parts of the current path have a high resistance and have too much of the total voltage across them . Turn the van on, and the alternator increases the battery "+" to around 14V (in order to charge it). That additional voltage divided across that current path now give the lights enough voltage "share" to brighten up. Turning on the fan, lighting the brakes, etc. might pull the battery "+" voltage down somewhat, lowering the lights' voltage share and dimming them again.

Maybe also turn your attention to the wiring out of the battery + terminal into "fusible links" and fuses. There could be corrosion/broken/fraying wires & connectors there, upping the resistance and reducing the voltage and current available to most everything. I've seen discussions here about the Vans but can't recall anything specific about the Previa. It's likely to be a PITA to deal with - I haven't had the pleasure yet. It might explain your engine conk-out as well.

Measurements of the charging system aren't going to address that "downstream" electricity flow to devices past the battery unless they also measured current draw rather than just voltage at the battery.

The fusible link connections all look clean, but a couple fusible link "filaments" (if that's what they are called) including the MAIN are severely corroded. Not sure if that could cause the problem but they definitely need to be replaced.

Nope. No change with different links. I also disconnected the radio and blower to see if they were involved, nope and nope. :pissed:

Something easy to try - the power for the headlights runs from a fusible link, through a Head Light Relay, through (right & left headlight) fuses, to the headlights, then to ground through the combo light switch on the left stalk. That relay could potentially have developed a high resistance, thus "robbing" the headlights of some voltage. That relay - the third across the top row under the dash cover - is the same as the heater relay to it's left. Swap 'em and see if that indicates the headlight relay is going bad.

Not sure why that would affect the instrument panel lights if that's the problem, but it's a quick experiment. Might as well jiggle that stalk switch a bit if the headlights still dim on the off-chance that's the culprit.

Will check those out, just saw this curious comment on a Craigslist Previa "dent in the tailgate was causing the battery to drain, so we put a cut-off switch on the battery, and that works well."

Attachment 12437

Drat. Tried the fusible link switch, no go. I found some sketchy connections on one headlight wire, but rewired those and no change. Well maybe it helped reduce resistance a little, only time and a night drive will tell. It still dims, I know that.

I'm down to about my last straw. I'm going to open up the tailgate and inspect all of my wires, sockets and bulbs in there. Not because of the dent guy above, but because I remember having a weird electrical issue once where something in the cab wasn't working and Fred on the Yahoo group steered me to checking all of the lights and sure enough a faulty taillight socket or bulb was causing it. I see little chance of that being the case here, but maybe I'll get lucky and find something.

Wayne, I know you said ground points must be good because it's a no-rust van, but there are only 7 ground points (A..G) in three groups: Engine (ABC), Interior (DE), Back (FG). Maybe we could look for functional tests to exclude them as possible failure points.

First test would be the headlights, because it's where you first noticed the problem. The left and right headlights have different ground points. Can you shine the headlights against a wall (or have someone look at them) while you step on the brake to operate the brake light switch? It would be interesting to know if they dim the same amount, or differently.

By the eyeball test I would have to say the headlights dim equally when applying the brakes.

If it was me, I'd want to make absolutely sure the alternator is good, especially since your original post said pretty much any significant load on the electrical system was causing the lights to dim, not just the brakes. Internally the spinning part of the alternator creates a varying voltage that's then altered into a fairly steady voltage by some circuitry. IF there's something wrong with that circuitry, the voltage read by a meter on "DC" volts (more or less an average of what's coming out of the alternator) might read OK, but wouldn't reveal that the alternator's output was actually varying too much. That would be seen by setting the meter to "AC" volts and seeing if the variation in voltage was over around 0.25-0.5 volts. Current is another practical test as the alternator is supposed to adjust its current output depending on the load (the manual specifically calls out measuring current draw out of the alternator of around 30A with the heater fan on high and high beams (obviously less with LEDs)).

I would imagine a dedicated battery/alternator tester would detect any of this, but I've never seen one in use outside of Youtube.

Unless the lights didn't dim when they were testing the charging system and you turned on a bunch of stuff?

Quote:

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Originally Posted by Previologist

By the eyeball test I would have to say the headlights dim equally when applying the brakes.

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Hmm... looking again at Dan's comments prompted another thought: If you remove the STOP fuse (2nd row, 4th from left, 20A), does pressing the brake pedal still dim the headlights?

No. It does not dim with that fuse removed.

OK, with the brake circuit still disconnected from power, does anything else that draws power dim the lights, like the fan on high (turn up the one in the middle AC part too) + interior lights + radio? With the van running at least a bit above idle, so the alternator's on and running at a decent rate. You had mentioned in the beginning it wasn't just brakes that caused the dimming.

Also, since you're kind of being drawn into the mysteries of electrical theory despite your reticence, consider one of these:
"Southwire 16030A Autoselecting CAT IV Multimeter"
I bought one for most of my family members as they're supposedly very easy to use with fewer manual settings required. (Sure, they're still in the blister packs years later except for one used when I was visiting to help fix a lamp, but they don't deliberately monkey with the Dark Arts as you do). Plus, you get to be seen looking at the display, slapping your forehead and exclaiming "1.21gigawatts ?!?"

Quote:

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Originally Posted by man_btc

OK, with the brake circuit still disconnected from power, does anything else that draws power dim the lights, like the fan on high (turn up the one in the middle AC part too) + interior lights + radio? With the van running at least a bit above idle, so the alternator's on and running at a decent rate. You had mentioned in the beginning it wasn't just brakes that caused the dimming.

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Yes, those still cause dimming with the STOP fuse pulled, whether off or with engine running above idle.

I want to clarify my comment about other things causing dimming. The brakes always had the most dimming effect on the headlights, followed (or perhaps equaled) by the front blower. The mid-blower has a less pronounced dimming effect.

The other effects are less, like when I turn on the blinkers the headlights will sort of pulse brighter and dimmer in time, but its not as pronounced as the dimming effect from braking or the front blower.

Also other things will dim certain dash lights, but its hard to compare the magnitude of that to the headlights. I just noticed that it can happen. If I have the ignition on, the turn signals will pulse the dash dummy lights ever so slightly, for example. And that still happens

I have a couple of cheap multimeters, one digital one analog.

Well that all makes sense: larger loads (like stoplights and blower high) causing larger voltage drops.

Quote:

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I have a couple of cheap multimeters, one digital one analog.

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Good - let's use the analog meter. (I hope it's not auto-ranging.) Do you see the connection in the battery fuse block between the 100A ALT link and 50A AM1 link? Can you get the common (low) side of the meter on that point, and the high side of the meter on the battery + (the terminal that the fuse block is mounted on)? What is your meter's lowest DC volts scale?

My analog low range is 10v, then 50. My digital low range is 20v.

I am clear on the fuse block and the 2 fuses but I am not clear what you mean by the connection between the ALT and AM1

Do you mean the metal band (conductor?) on the side that looks sort of like this? (this pic is from an earlier model and is not identical to mine) (EDIT:I will assume that's not what you mean, because on mine there is a lead directly from the + terminal to that metal band)

http://i8.photobucket.com/albums/a23...v/IMG_0215.jpg

But this is probably a good time to mention that (courtesy of previous owner) the fuse block is not mounted normally, it was just sort of tied with wire to the battery hold down and another point on body. Not sure that would make a difference but thought I better mention it

Also I realize now I should really disconnect the plugs and inspect all the lower terminals closely, which I did not do before when I inspected the fuse block.

The 10V range should do fine.

With respect to your picture, I'm at something of a disadvantage, because I no longer have my Previa. But I do have the EWD, and this picture Attachment 12453 shows that fuse block schematically on the first page. You see how, at the top of p.40, the battery directly feeds four of the five links -- all except the 50A AM1, the link that feeds the loads (most important, IGN and ST) that are affected by the higher loads fed by the 100A ALT link?

As Dan was saying, you have classic symptoms of a weak supply, but tests have indicated that battery and alternator are OK. That just leaves the links ALT and AM1. You mentioned some corrosion in that fuse block, so we want to test the voltage between that ALT-AM1 connection (on the meter's low side) and the battery (on the meter's high side) when you apply one of those high loads: stoplight (brake pedal) or blower on high, or headlights (or all three). Whatever voltage reading you see on your meter is subtracted from the battery (or alternator) supply.

Then the next step: put your meter's (-) lead on the AM1 output to the IGN switch. If there is a difference between those two tests, your major drop is through link AM1. If there is an reading of several volts, but no significant difference between those two stops, then the major drop is through the ALT link.

If you don't get a reading of at least 4 volts with one or both of those tests, then switch to the next higher meter range, put your meter across the battery terminals and do the same test - but this time you are looking for a voltage drop, not increase. Please let us know the results.

Sadly, I don't quite understand much of your very helpful post but maybe a light bulb will go in my brain on as I clean up other things I've discovered.

I took the fuse block out, so I'll post some pictures of it that might help narrow down some of the details in your post.

I discovered that a previous knucklehead has cut and spliced all of the wires to the connector plugs. Fortunately they seem well connected and I don't see any corrosion on the ones where I can actually see into the butt connectors. I presume they did that because the connectors are a PIA to remove, so they just cut them off. But why? :cnfsd: :no: NVM, I probably don't want to know.

I pulled the alt fuse and there is some corrosion on it's "filament." It doesn't look terrible but I will replace it anyway. There is a dusty coating on the wire ends at one of the ring terminals that bolt the alt fuse into the block, but it doesn't look terrible either and the ring was fine. I'd like to avoid messing with those wires unless it proves necessary.

The block is missing it's lower cover, but all of the spades on the bottom look pretty good and free of major corrosion.

That dusty connector on the alt1 fuse ring connector (would that be "C" -->>> EA1 in the top schematic?) runs to ground somewhere, I presume, because the other alt1 fuse ring connector goes straight to the + battery terminal. Does it run through anything that should cause increased resistance?

I can follow the wire down under the battery and it then enters a flex conduit and that's where I lose it. Something comes out of that conduit and connects at the alternator with a plastic connector plug, but that is a much smaller wire (might be 2 small wires).

But anyway I am getting variable (and sometimes puzzlingly increasing) resistance between that alt1 fuse connector and ground. I get a minimum of 2000 ohms when i ground it to the negative battery cable (which in turn shows no measurable resistance to ground). And sometimes that ohm reading increases slowly but not always. But its never less than 2000 ohms (2 on the x1k scale).

I traced that conduit a bit more. Another wire the same size as the alt1 wire emerges and bolts to the top of the alternator (worth noting the alt1 end is white wire and the alternator end is black) . Thinking this may be the same wire (the alternator end also has a dusty coating), I disconnected it from the alternator and found about 2000 ohms between its end and the alt1 end. Granted, I do not know if these are the same wire but they must be connected somehow. But when I test between the alternator end of the wire and ground, I get 0 resistance.

I'm not entirely sure how to interpret this, except that if it is the same wire perhaps the alternator end has another path to ground whose connection is hidden somewhere in the conduit, "beyond" the point where the 2000 ohms of resistance is. But I don't know if this resistance is enough to cause my problem or not, so I'm not sure how much effort I should spend tracking it down.

To see what's happening, don't measure resistance (ohms), measure DC volts. Measuring ohms on anything other than an unpowered wire to ground or across a single unconnected component won't be meaningful. Measuring ohms on a circuit with power on it will definitely give you meaningless, wacky numbers.

Besides, significant vehicle voltage variations occur with tiny resistance variations, so even when measuring resistances to ground with no power on, it's in a low, inaccurate range where the meter can see a low-resistance path to ground, but not whether it's low enough. If under power, that point shows zero(ish) volts, then you know it's a good ground.

My issue and continued focus on the power source is because you're not seeing dimming with only one cause (like brake lights and nothing else). I would also think high fusible link resistance wouldn't let your lights be bright enough in the first place, even before anything else started drawing power.

If I were you, I'd want to be confident that the alternator is producing stable DC volts to the battery and vehicle with different loads. If you measure AC volts across the battery (van running*) what do you see? Should not be much, less than 0.5 volts and probably a lot less. What is the DC value you see? Can you look at that DC value with and without the lights and lights+brake lights on? I guess I don't trust the tests the car parts folks did.

*...Van running at a fast idle with the lights and brakes on (&/or fan on high maybe - something to apply a significant load) that is...

Lots of great advice here.

I would only add that the only correct place to have a crimp style butt connector, is in your emergency kit.
They are notorious for causing issues.
Regardless of appearance, they should be cut out and wiring repaired properly (solder and shrink tube).

Inspect the wiring while you're in there. When you strip the end, make sure it's shiny and bright.
If its dull, it is degraded and needs to be replaced.

Clean that "dusty" end and make sure the insulation is the same colour as further away.
Overheated wiring can take on a dusty look as the insulation starts to break down from excessive heat.

I'm going to disagree about soldered wires being better than crimped in the sense that either can be bad if not done properly. You'll find crimped connections all over mission-critical things that move and vibrate, but ones made with proper (read expensive) tooling used by trained technicians on approved connectors and wiring - in other words, not some low-quality connectors mangled by some unskilled rando who bought them and the crimping tool on the cheap, then didn't protect it from the elements properly. That same mook could just as well have soldered together a poorly-fused, brittle connection that wouldn't automatically be an improvement because it wasn't crimped.

Again, voltage measurement is going to tell you if enough resistance has appeared between two points in a circuit to produce a significant voltage drop. That voltage-drop measurement will show up when you put your multimeter's probes* to either side of your suspected item (assuming you can find a place to touch the wiring at both sides of it). If you see almost zero volts, it's not "stealing" enough voltage from the other items in the circuit to be a problem.

* Meter set to measure DC volts and the range that just includes 12V (20 volts?) so you get the most resolution on the display - in the same sense that you wouldn't want a speedometer that goes to 200 MPH because you could barely see the difference between speeds you normally drive at.

Quote:

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Originally Posted by man_btc

To see what's happening, don't measure resistance (ohms), measure DC volts. Measuring ohms on anything other than an unpowered wire to ground or across a single unconnected component won't be meaningful. Measuring ohms on a circuit with power on it will definitely give you meaningless, wacky numbers.

Besides, significant vehicle voltage variations occur with tiny resistance variations, so even when measuring resistances to ground with no power on, it's in a low, inaccurate range where the meter can see a low-resistance path to ground, but not whether it's low enough. If under power, that point shows zero(ish) volts, then you know it's a good ground.

My issue and continued focus on the power source is because you're not seeing dimming with only one cause (like brake lights and nothing else). I would also think high fusible link resistance wouldn't let your lights be bright enough in the first place, even before anything else started drawing power.

If I were you, I'd want to be confident that the alternator is producing stable DC volts to the battery and vehicle with different loads. If you measure AC volts across the battery (van running*) what do you see? Should not be much, less than 0.5 volts and probably a lot less. What is the DC value you see? Can you look at that DC value with and without the lights and lights+brake lights on? I guess I don't trust the tests the car parts folks did.

*...Van running at a fast idle with the lights and brakes on (&/or fan on high maybe - something to apply a significant load) that is...

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I tried these tests except for using fast idle, although I could probably do that if I cut a piece of wood the right length.

For some reason my analog meter wouldn't read ac or dc volts, so I had to use my digital which I'm less familiar with. I got nothing measurable on any AC setting, testing across the battery terminals when running. DC came in at about 14.48 with nothing electric running or with lights on, blower on and brake on (with the brake fuse back in it's place). When adding the hazard lights on top of the other loads it caused pulsing between roughly 14.3x to 14.48. Turn signal alone caused weaker pulsing, only about plus/minus 0.02-0.05 v

Since I applied the brakes using a stick jammed against the seat, any temporary voltage reduction was unknown. I did the test again, setting up a camera under the hood and shooting video of the meter while I press the brakes on and off a few times. Video showed it only dropped to about 14.1 or 14.2 when I hit the brakes.

That doesn't seem like a big drop to me.

Going back to John's post, here are pictures of my fuse block that may help me understand what he was suggesting I do.


Attachment 12456
This is the inboard side, the cable runs from the alt fuse to the flex conduit and (presumably) to the alternator

Attachment 12458

This is the outboard side, a cable runs to the alt fuse from the + terminal.



Attachment 12457



Bottom of the block, no cover but all looks good.


The abs fuse is intentionally absent.

Looks like you are getting a solid voltage from the battery & alternator. No need for a fast-idle IMO, that was more if the voltage at idle was marginal. i.e. close to 12v and not 14-ish where is (and should be).

Quote:

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Originally Posted by John Kaufmann

Well that all makes sense: larger loads (like stoplights and blower high) causing larger voltage drops.


Good - let's use the analog meter. (I hope it's not auto-ranging.) Do you see the connection in the battery fuse block between the 100A ALT link and 50A AM1 link? Can you get the common (low) side of the meter on that point, and the high side of the meter on the battery + (the terminal that the fuse block is mounted on)? What is your meter's lowest DC volts scale?

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Still trying to understand what you are asking here. Language like "common" and "low side" are Greek to me, but I'm guessing you mean the black probe, and the "high side" would be the red? Once we get that cleared up I just need to figure out what you mean by the "connection"

I got curious and disassembled my donor's fusible link box, as in totally extracted all the metal brackets and connectors. I'm putting together an exploded-view diagram, but now that I understand its construction, I saw something odd in your photos. That blue 100A link sits on tabs that extend into the body of the connector. One goes to the Battery +, the other to the alternator (so neither can blow the other up). That link's tabs have holes that line up with the threaded holes on both sides of the housing - and there should be screws tightened into them! (You can see one in your "earlier model" photo.) As in you can't budge or pull that link out without removing those screws. Without that, it's not a tight connection at all, and that's THE main power connection, so just smallish fuse-ey-type spring-loaded connections aren't good enough (the connectors underneath are beefier). Unless you removed the screws for the photo, then never mind and ignore my rant.

As far as the probing goes, dinna worry about which probe goes to which side - if they're "backwards" you'll see a negative voltage, but it's just the absolute value you care about. And you want the smallest meter range that will still cover up to 12 volts, say 20. On larger ranges it'd be like trying to gauge your speed through a school zone with a speedometer from a formula one car.

Also. not sure where you probe stuff if that housing is all buttoned up. At any rate, if those screws were never there, put 'em in and retest. If they were really installed when the block was in use, you could measure the voltage from one to the other (if you can get to them) as that's the drop across the 100A link.

Yeah the screws are there, they just weren't shown in the picture. But I can easily measure across them because the block isn't bolted down. I just tried it quickly and got extremely low voltage (<1.0 if I was reading it right), but being cold and night I don't want to open the garage door. So I only ran it about a minute because I also don't want to die. :rol: I will check it again in the morning.

Same result today. I am only picking up 0.03v or so probing across that Alt fuse. I will follow through on my threat to replace it today, if the snow lets me get to a parts store.

Kind of surprising because that one looked better than the other fuses, but there is a large part of it that is hidden within the plastic and can't be seen.

So that's not the problem I guess.
When you were measuring, were the lights and brakes on? You want a decent amount of current flowing while you measure the voltage because of the possibility that -

• poor connections
• narrowing of the fusible link by corrosion or internal degradation from previous high-current episodes
• whatever

- might reduce its current-handling capacity and thus increasing it's resistance to higher currents.


As an FYI, if you were the type to radically disassemble (i.e. bust up the housing) a long-lived (in Arizona) 100A fusible link to see the "innards," it'd look like this. No charge for the ambience.

Attachment 12470

Attachment 12471

This mornings readings were without anything running. I tested again tonight with everything running-brakes on, lights on, blowers on, radio on flashers and both wipers on, but with wipers or flashers on it just jumped wildly from 10 to 18v. When I turned the wipers and flashers off the most stable reading I got was about 15v.

I already bought a new fuse so I'm going to perform surgery on this one anyway. I'll run the test again with the new one but I expect similar results.

With that white stuff on the ground, I can tell yours isn't in AZ anymore!

Don't use anything that doesn't draw a steady current, like those flashers and wipers - they will make things jumpy and unreadable as you saw. The lights brakes and high blower is fine.
So with that current being drawn, measure the voltage across that 100A link with a small range (couple o' volts) if you have it, 20V if that's the smallest available. Shouldn't be more than say 1/4 - 1/2 volt tops, and I would hope a lot less. Then, to double-check - and use a different meter range - set the meter to 20V, then with the black on ground, touch the red to one, then the other side of the 100A link (14ish volts presumably). Then, take the difference between those two values to get the voltage across the link. Should be around the same as across the link, but the meter may not be all that accurate at very low values. If you can have the idle up a bit, that's more realistic, but not entirely necessary.

And yeah, that donor Previa has never seen the white stuff (well, snow at least) and the rust-free carcass sits cozily in my garage aggravating my wife no end. I did the outside link shot to get accurate lighting and show how even a link in a dry environment gets corroded somewhat. I understand the normal wire issue - you can only get so much current through a fixed cross-section until it starts melting, making that cross-section even more inadequate until it ultimately breaks. Now, fusible links are meant to endure short-term bursts of current and stay intact where a regular fuse would just blow, so maybe there's some internal degradation that increases resistance over time? I don't know enough metallurgy to speculate beyond that.

Now for the latest visual aid, as I know you're not a fan of schematics. This is an attempt at showing the fuse block "blown apart" so you can see how all those metal bits and bobs inside route the electrical paths (I tried my old X-Ray Specs, but all I could see were the bones of my hand wherever I looked). The graphic wound up being a little more "dense" than I hoped - my attempt to make it a little easier to follow was to tint the positive battery connections red and the positive alternator connections blue.

Attachment 12472

I guess I'm about to put my electrical ignorance on full display again, but I'm over it.

Why should I expect to get such tiny voltage (1/4-1/2v) across that fuse even with things running? I would expect closer to the 15 that I am getting. That 15 is DC volts. My lowest range is 20 on the digital meter.

That's a cool diagram, useful but slightly different than mine. You must have bought your x-ray glasses from the Johnson Smith catalog, they were famous for bogus items like that to prey on 12 year old boys. I had their secret radio pen, which actually sort of worked.

Hah! I've still got my radio pen! It's in the garage in the misc-stuff-in-drawers section.

Anyway, if it were easy to do, you could leave your black lead on ground and measure the voltage from where it's highest (the battery/alternator connection) all the way through the lights to ground, measuring lower and lower voltages until you got to (near) "0" at chassis ground. The difference between any two points is how much of the total voltage is "used up" by what's between. Let's say the van is running. The power is coming from the alternator putting out 14 volts. Current flows across the 100A fusible link to charge the battery and separately to power whatever's turned on. Some small amount of the total voltage will be lost along the way through each wire, connector and component to ground, with most of it appearing across the Thing You Want Powered, like the lights. Each non-major component's "absorption" of the total voltage could ideally be measured across its terminals, but that'd be a fraction of a volt that might not be measured accurately on a meter that can't go down to a small scale, so the alternative is to measure the voltage from each side of a thing to ground and subtract them (as they'll be more in the "middle range" of what the meter can display).

So things work fine with the lights on, but press the brakes or turn on the fan and they dim. That additional load is pulling current from two places shared with the light circuit - the alternator output and then at least one fusible link before the wiring splits off from the fusible-link box to the things being powered. The alternator is supposed to sense the need for additional current and then put out more, which is why the first thing done was to check that, while running, the voltage at the battery stayed steady with increased electricity use. One of the other potential problems is that maybe (?) one or more of the fusible links can't carry enough of that increased current. That might be because a fusible link isn't as "thick" as it used to be (the fatter the conductor, the more current it can carry) or something about its internal structure prevents it from passing through the increased current, especially when it gets hotter.

If there's a problem carrying more current through an intact fusible link, its going to manifest itself by taking up a lot more of the circuit's voltage across it's terminals. Let's say when just the lights are on, one side of the main 100A link shows 14v and the other side shows 13.9v, but with the brake lights and fan on, the alternator side still shows 14v, but the other (load) side now shows 12.9. After further drops along the way from wiring. etc. (call it 1 volt) the lights might only see 11.9v, vs the 12.9 they'd normally get.

On the other hand, you didn't see a drop in the battery voltage during the charging tests, so my example above isn't the greatest. If it was me I'd see if I could get a probe into a connector at the bottom of the block to measure voltage - pin 1 on the shorter white connector - white/blue wire - as that would tell you whether something in the block was causing the voltage coming out to the headlights* to be significantly lower with other loads turned on.

*Well, actually the headlight relay, then to the headlight fuses, the lights themselves, then the light switches, then ground :dizzy:

If something in that fusible link block isn't found to be "stealing" more voltage out of the path from the alternator to ground under an increased load, it's back to head-scratching.