quirky electrical fault

[dogstar]

well, it was discovered a couple days ago, and last night it was acting up, then today on the way home, i noticed it again.

basically whenever i put the brakes on at mid range rpm (3000-5000ish) the voltage drops drastically.
the stereo turns off and you can hear it in the engines running.

im guessing its the alternator dying, but im really not too sure, could it be a short?

has anyone run into a similar problem?

thanks in advance

[lechner]

A short in the brake light circuit is most likely. At higher rpms the coil uses more juice than at idle, so you may not notice it at idle. To test if it is this or the alternator, try turning on the headlights at mid rpm with the radio going. If it's the alternator, they should cause more of a problem than the brake lights.

[Aktunka]

Well, I have never owned a Supra, but I have owned many a Toyota before, mostly 1st Gen Celicas, and what you are describing is almost identical to what would happen to me whenever an alternator was in the process of dying. Although it makes more sense that headlights would effect it more, that was never the case for me.

Since the stereo I always ran had two amps, one for subwoofers and one for mid/high range, I knew I was having an alternator go bad when I would be driving around and then use the brakes, and the mid-high amp would cut off until I let my foot off the brakes.

My guess is if it follows the same sort of pattern like that that I had, it would be a possible upcoming alternator failure. If it is a short in the brakes circuit somewhere, I would expect that you would see the same problem whether you were moving at high rpm or sitting still at idle. Good luck whatever the cause though, but I would probably run the alt in for testing.

[dogstar]

thanks for the replies gents, but Aktunka hit the nail on the head.
i didnt really think much of it the other night, but i noticed the subwoofer amp kicking off when the brakes were applied or when other accessories demanded power.

hmmm, well, i suppose its as good an excuse as any to get a mkIII alternator, just glad im parking the car for winter soon.
and if that doesnt solve the problem, i start looking elsewhere, so if anyone else has any suggestions, feel free to add them.

also, how is the best way to go about tracking down that short? if its intermittent and condition dependant that seems like itd be a bitch to find.

[Aktunka]

Well, IF a new alternator does not take care of the problem (but my money would say that it will) and you think you might have a short, I think a good strategy for trying to track it down would be to start by first just making sure the fuse area is ok (never know when a leak can cause some corrosion you didn't even know about) then to start at the areas at the ends of the circuit. Like starting from the brake pedal or the brake lights themselves. It has been my experience that chances are a problem would most likely occur at one of these points, mainly because they are most open to outside problems like water, movement, air buffeting them, etc. If there is a short and it is somewhere in the wiring as it goes throughout the car, that would be a lot tougher to trace...

Anyhow, alternator probably will fix, if not, then good luck to ya!

[Dave A.]

The stock MKII alternator is going to have a hard time keeping the battery charged if you have a couple of heavy duty accessories (ie: high power stereo) that draw a lot of current. The added current draw can actually damage your alternator and battery over a period of time. A lot of current demand gets placed on both the battery and the alternator when you crank up your stereo and other high powered goodies. The MKIII alternator is a good, reliable upgrade. Pair that up with a good high capacity battery and you shouldn't have anymore problems.

I had this same problem the other night when I drove home from Tampa in my Corolla. The headlights and dash lighting started to dim more and more as I drove. Luckily for me, I was still able to make it home before the engine died. The alternator was keeping the electrical system alive with a toasted battery, but just barely. I had to make darn sure that the RPM didn't drop at a stoplight in order to keep the alternator alive and prevent the engine from stalling. I replaced the battery and all is well, but I'm going to rebuild my overtaxed alternator as well just to play it safe.

A short in the wiring should blow a fuse! Has this been the case? Try upgrading your alt. and battery.

[Dave A.]

You might also want to consider investing in a flux... :roll: umm...Monster Capacitor to help reduce some of the load on the battery and charging system when your subwoofers fire. :lol:

Sometimes stereo systems can be a real blow to the wallet. If you want to play, you gotta pay. 8)

[Dave A.]

Try stepping on the brake at idle with the headlights On. If the voltmeter drops, try stepping on the gas to raise the RPM and see if the voltmeter reading picks back up.

[dogstar]

hmmm, alternator it is i guess...just hope i can get one soon.

my stereo isnt huge, but i guess its a little louder than average, and yeah, i agree, the mkII alt is woefully underpowered.

the battery is a good high capacity, and i really dont think its the problem... i hope not too, as its nearly new, but after i get the alternator i guess ill find out.

[SupraDOHC]

basically whenever i put the brakes on at mid range rpm (3000-5000ish) the voltage drops drastically.
the stereo turns off and you can hear it in the engines running.

This just started happening to me tonight, but mine also does it at idle. Havent been able to test anything but now I know where to start after finding this thread.

[oper8or]

The best way I know to test and see if its the Alt is to disconnect the battery when at idle. If the car dies. Guess what. Bad alt. That is the way I have always done it. Then again I'm not bright when it comes to testing things the way you guys are. I'm the dumb one in the bunch. But that is why I do so much reading in here. Hoping like hell to catch a tid bit of info that can help me learn. :oops:

[SupraDOHC]

This morning I went out and cleaned the terminals on the battery, checked fuses, and then went and started the car. Instead of it jumping to 1500-2000rpm and warming up it idled down right away and sat at 500rpm. I can rev it and it runs just fine and now the voltage doesnt drop even if I put the headlights on and turn the heater on high. I was actually hoping it was the alternator since I still have a warranty on the one in the car, but now im stumped....
I've got an '84 TSRM and im gonna run through and test the alternator anyways.