Munch18

When I press the brake pedal my headlights dim. Other electrical systems seem affected when ever the brake pedal is depressed. Would this be the voltage regualtor going bad?

wwest

Much more likely you simply need to clean and burnish your battery connection posts and retighten the connections. Should do it every spring.

mooretorque

Munch, I agree with Willard. Even if it's NOT the battery posts (the corrosion doesn't have to be visible to the eye though), that's the first thing the dealer's mechanic should do. But the difference for you is that the dealer will charge $75/hr to do it. Plus they'll likely "check all fluids" and do some other fun, profit-adding services while they're at it, all in the name of that "extra quality care that Lexus owners demand".

DevilDog

Believe it or not, we actually do try to find a problem when an owner brings their car to us. Having fun is not why we are there. There are several things to check first, battery output, total amp draw, alternater output, amp draw across the battery, ground circuit in the headlights, and of course corosion at the battery posts.

It's $88 hr in my part of the u.s.

DD

mooretorque

DD, not maligning skilled mechanics there. Just pointing out something many owners neglect (based on several yrs long ago working as pump jockey back when they did things other than take money behind bulletproof glass) and what seems to be a fairly well documented propensity for dealers to maximize charges from the service dept. For instance: are ALL vehicles brought into your dealership serviced on the "rough duty" schedule??

When I worked at a service station, I "checked all fluids" everytime someone filled up, for the princely sum of $4/hr. Granted that was 30 yrs ago, but $4/hr then doesn't work out to $75/hr (or $88, in your neighborhood) now. And it somehow takes the dealership 0.25 hrs to do that!!!

DevilDog

Point taken, I understand what your trying to say. Sometimes we techs get a little defensive. Some many times we find ourselves defending our actions. I can't speak for every tech, but I love what I do.

Have a great day!

strongsail

Do your brake lights work?

If a ground fault in the brake lights has blown the fuse, current could be leaking to ground and causing voltage drop when you press the brake pedal. Check the fuse - if blown, replace - if it blows again, get the circuit diagnosed - look for physical damage to wires at taillight fixtures and at the brake light switch - - a wiring diagram would be a big help for the diagnosis - - one of these days I'm gonna spring the $$ for the repair manuals and wiring diagram!

strongsail

If you need diagnosis, and don't have the wiring diagrams, or the skills or desire to do it yourself, really consider taking it to a dealer - these cars are too electrically complex to give to somebody who doesn't know the car well - see if you can find somebody like Devildog, who sounds like a straight-ahead honest guy, at your local Lexus store - -

Munch18

I cleaned and checked the battery posts...all looked fine. The fuse is good and all brake lights are fully functional.
The electical drain only occurs when the brake pedal is depressed...no other electical system will cause a drain.

wwest

Does the engine RPM drop at the same time?

Vacuum leak, maybe...

Power brakes use engine vacuum and if the brake vacuum chamber has sprung a leak that could result in loss of engine power...

Mostly just rambling...

Munch18

Nope...no noted drop in power.

I'm going to borrow a volt meter tomorrow and check the flow across the battery off the alternator.

I don't think I'll see anything notable.

wwest

If you raise the engine RPM to about 2k and then step on the brakes does it still happen?

When the engine is at or near idle some alternators cannot supply enough power for all accessories and headlamps, so at idle the battery must make up the difference.

If the lights still dim at high engine RPM then check both ends of both battery cables.

Additionally most cars have the negative end of the battery connected directly to the engine block and then a GOOD ground strap connected from the engine block to the car's body, firewall to the rear of the engine is typical.

Also check the positive cable connections. It often goes directly to the starter solenoid and then there is another cable from there to the fuse block/panel. A poor connection at any of these locations would give your symtoms.