Dontcha hate those electrical issues? I got one.

[tinmanSC]

I've been having a strange electrical issue with my 1995 Lexus SC300. It has ~200K on it now and has served me well, except for this one issue. After a long drive (say, an hour or more) when I put additional electrical load on the car, the voltage drops substantially. Normally, when driving around, you may hit your brake and the brake lights will come on. In this case, when hitting the brake, everything cuts out for just a second. The radio, the head lights, dash lights, then everything comes back on as if nothing had been wrong.

It's not always that bad, sometimes it's just enough to cause a blip in the radio, or just enough to notice if I'm looking at a light on the dash, but the more i drive the worse it gets. Like I said, it starts after about an hour, and if I keep driving it can get so bad that the engine will cut out for a second if i hit the brake, or turn on the headlights, any additional electrical load. It has been happening for quite awhile now. In my efforts to correct the issue I have replaced the battery and the alternator. I have also replaced the serpentine belt. The problem persists. Any ideas?

 

[EricF]

my guess would've been worn brushes in the alternator.. but since you've replaced it.. I'm going to say check the wires for anything brittle. resistance may have increased due to age.

 

[sunfire]

Does it happen when the AC kicks in?

 

[tinmanSC]

Originally Posted By: sunfire
Does it happen when the AC kicks in?

No, it seems unrelated to the A/C.

 

[sunfire]

Originally Posted By: tinmanSC
Originally Posted By: sunfire
Does it happen when the AC kicks in?

No, it seems unrelated to the A/C.


I would think the AC would put more load than your headlights. The problem may not be entirely load related. I'm just throwing this out there.

So with your headlights off cruising on the highway not hitting the brakes. And the AC blasting you never noticed a blip?

 

[OVERKILL]

Check your grounds!

 

[FL_Rob]

You've got a short somewhere ,but not a dead short.It could be a bad ground..or Something is getting heat soaked and expanding then grounding out.Since you mentioned brakes(Switch)is that the only time it occurs?..does this happen using ANY electrical, or just certain combinations?

 

[Quest]

Check your grounds and also load test your battery.

The only culprit remains would be alternator

Q.

 

[FL_Rob]

He already replaced the alt,but I've seen bad parts right outta the box,too.Is there any chance there's an external regulator? I can't imagine there would be .No,I have to go back to grounds or arc-ing.

 

[mechjames]

Change your battery clamps. If it still persists, try throwing another battery in there and see what happens. I've seen relatively new batteries do weird things.

 

[JHZR2]

If this car is like our 94 previa, there is a fuse/circuit breaker device on the positive terminal of the battery. I have to wonder if it is getting flaky.

 

[punisher]

Originally Posted By: OVERK1LL
Check your grounds!


Do this first.

 

[CarbonSteel]

It would seem like you have a loose connection and the shift in motion of the car when hitting the brake is causing the connection to break.

As others have stated check your ground connections.

 

[eljefino]

I like the idea of the connection, but am biased toward something slightly unusual.

Could easily be the battery, but could also be that thinner wire coming off the battery going to the underhood fusebox, or something in that fusebox. It would have to be "mainline" enough to affect engine running and lights; a problem at say the ignition switch would get one or the other, and downstream of that are individual circuits with small fuses that would blow saving the rest of the system.

Sometimes, owing to assembly process, you find quick-connects in the main battery terminal wires. (Dodge dakotas have obvious ones.) These are miserable with corrosion and loose contact after a long while.

 

[Pop_Rivit]

I had a similar situation once. When I was checking the grounds I also checked the battery cables and discovered that the positive cable was green and corroded inside. The outer cover of the cable looked perfect.

Replacing the battery cables corrected the issue.

 

[sleddriver]

Originally Posted By: punisher
Originally Posted By: OVERK1LL
Check your grounds!


Do this first.


I agree. If it only happens when you apply your brakes, then by doing so, an iffy ground is finding a better ground through your brake circuit. Check the major grounds first (engine ground strap(s), neg battery to the chassis, etc), then look in your fuse box/panel for any signs of overheated connections, melted insulation, dark color on terminals, wire corrosion, etc.

 

[mechtech2]

Go back and start with the cables. Clean ALL ends and where they set. Power and grounds.
Check the resistance.
Battery OK? Even if new, is the alt putting out OK?

 

[tinmanSC]

Thanks to everyone who replied. Looks like I'm headed to the garage with multimeter and a battery terminal brush.

 

[meep]

your best angle of attack is to set to volts, turn on a bunch of accessories (A/C, lights, defrost grid) and measure voltage drops on both + and - sides. batt (-) to body. batt (-) to alt frame. alt frame to body.

same for +. check for diffs from alt, to batt, to fuse block, etc.

once you find anything greater than 0.3-4 volts or so then start cleaning.

or just build a supplemental harness, especially if it is ground.

some loss is normal.

ground conductivity is compromised by the "dirty welds" in body metal that is galvanized.

happy hunting!

 

[tinmanSC]

Thanks to everyone who responded. I've made a home-brew grounding harness for the car from 4 gauge wire, a bunch of lugs, some solder, and a few hours. It works great. I made 7 or 8 cables (i don't remember how many) and connected them to many points on the engine and body. I got my hands on a manual for the SC300 that details all the grounding points and used that as a starting off point, connecting to a few other points and then to the battery. It works great and the issue seems to have been resolved! woot!