Just recently this started happening:

While running, when I press the brake pedal my headlights /interior O/H lights dim slightly and then return immediately to normal brightness.

This is also accompanied by a slight drop in RPM at idle. The lights dim even if I hold the RPM's up 2,3,4k doesn't matter. Still a dimming and RPM drop for about 1/2 a sec.

The voltage -while running- on the Scanguage shows 13.3V at idle (headlamps on), and then drops to 12.9V while depressing the brake pedal. Returns immediately to 13.3V. I verified these readings with the voltmeter too.

When the truck is not running- headlights on/ brake depressed- no dimming at all.

I checked all my battery connections and the alternator main wire- spotless and tight.

Think I have a (going)bad alternator, bad diodes,regulator? I'm 99% sure I have the smaller alt. (100a) and I do demand a lot of it- dual batteries, thermo electric fridge, etc.


I found this thread from a while ago which seems to be the exact same problem (ironically I responded to it, lol).

Lights Dim when Applying Brakes



Anybody else had this issue- resolution?

yep... sounds like your alternator is on its way out

Remember what supplies power during running conditions.

I figured as much- I just don't understand why they dim and them return to normal brightness while the brake lights are still on. I figured they would stay dim???



BTW- The dealer wants "about $900" to install the alternator- LMAO

If it's indeed bad, I will be upgrading to the factory 130a. I have been looking into it at length and barring the rear bolt to the block it shouldn't be too horrible. Found a reman. 130a for less than $200 too.

I will also post a complete how-to like the oil-change thread.

Just out of curiosity, can you do the same voltmeter test with the truck off? Meaning, you'll have a nominal voltage at 11.9v-12.2v and when you press the brake pedal, it should theoretically drop, right?

And, while the truck is running, can you check to see what the voltage is when you first turn the steering wheel? Does the voltage drop similarly to when you depress the brake pedal?

What I'm trying to determine is if the voltage drop is consistently -.5v regardless of starting point, why does it only affect when the brake pedal is depressed?

i imagine they dim because because it is under load when you press the brake pedal... additional power consumption. $900?! did that come with a ass wax and happy ending?

I should clarify that- They will also dim if I try to close a window that is already closed- you hear the window motor strain against the stop for a second. In other words, a "significant" amperage draw, I guess.

Gimme a minute to check what you asked about and I'll post the results. Thanks Bobby!

OK-

Truck off (lights on):
12.3v, Brakes on: 12.2v No heavy voltage drop/ no lights dim

Truck Running(lights on):
13.74v, Brakes on: immediate drop to 13.05v for a split second (lights dim) than climbs back to 13.68v and holds.

I put a huge load on the alternator- Hibeams/foglights/all my accessory lights/rear defroster/fan on high, etc and the alternator holds the voltage at 13.4v at idle. I could hear it whining under the strain.

Also- No change in voltage at all when the steering wheel is turned.

Interesting. You have a .1v drop with the truck off. Theres a .7v drop with it on. Even though the voltage is higher with the truck on. It's obviously in the alternator circuit, but I couldn't explain why it only affects it if it's running. Even if it drops to 13.05v running, it's still higher than the nominal 12.2v when it's off.

I'm sure you're on the right track with the alternator, but I'm still surprised with the testing.

I think it has something to do with the brake booster/ABS circuit maybe...I dunno but that's the main difference between running/off/brakes.


If you get a chance- want to test yours the same way and see what you get?

PLEASE?

I get very similar numbers. I've never noticed the lights dimming and can't really tell right now because it's broad daylight. But yeah, I only get a .1v drop with the truck off and a .7v drop with it running. I did notice that the rpms increase as I turn the steering wheel back and forth thus not causing any drop in voltage.

The other weird anomaly is the voltage drop when first hitting the brakes is .7v or so. But, if you immediately hit the brakes again, there is no drop in voltage. Almost as if the brake circuit needs an extra second to reinitialize before it causes a big enough load to drop .7v.

One more thing to add... I have HIDs so there's a ballast that's probably equalizing the load so I won't notice the headlights dimming. My testing is done with an Odyssey battery and Kodiak dual system. I tested it without anything else on and batteries isolated.

Just out of curiosity antman, (couple things to check):

With the key turned 'off', you've already documented your electrical status. But what happens when you do the exact same test with the engine 'off' (lights on...etc ), but w/ the key in the 'on position'; main electrical relay closed and all circuits are live? Is there still a substantial voltage drop? I'm trying to establish that the engine/alternator has or has not anything to do with this.

Also... do that test again with the engine running and you stepping on the brake pedal, but this time, disconnect the brake light switch; just trying to isolate the problem to one system or the other. So by doing this, you'll know if the trouble is related to the brake light system or if you can give that system a 'clean bill of health'!

I've got a nagging suspicion that your alternator has nothing to do with this but rather a short/shunted circuit. Brake lights don't draw that much current (enough to dim the other lights) and if yours did, you would certainly blow a fuse somewhere. "Just my 1.5 cents" Cheers!

Thanks for checking- I did notice that it didn't dim ALL the times that I pressed the brake pedal. Just like you observed, there is only a big drop the first time you hit the brakes, not immediately afterwards. That's what got me thinking it may be a booster/ABS thing...


Thanks for taking the time to check in Greg. I'll give your suggestions a try tomorrow if I can. I could probably just remove the brake bulbs completely and see what happens... Damn- Now I'm actually hoping it's the alternator, lol!

Hey jeff, I am sitting in the 4Runner right now and guess what... Mine does the same thing. Turned on the map light, tried to close an already closed window and *boom* map light dims for a split second. Hit the brakes and *pow!* and map light...wait for it...only dims the first time you hit the brake. So, can you come down and fix mine too?

Without checking in person, I'd guess you are witnessing a momentary voltage drop when the brake booster activates. Basically the same situation some of you may see when the home AC or clothes dryer kicks on. I've never noticed on either of mine if they do it or not. Annoying, yes. An actual problem, I don't believe so. If anything, I would suspect a weak, but not really bad, battery. I would think that, to totally kill this condition, you could use a large capacitor inline with the supply to the brake booster. Dual batteries may help dampen the voltage drop as well.