combustables

Hi guys,

I was in stop and go traffic for about 35 minutes yesterday and suddenly the battery voltage display switched by itself from coolant readout and said 11.4V in my 1986.

I turned the radio off and put it in neutral and revved it and the battery voltage went back up to 12.5.

I kept looking at it after that and noticed that whenever I held the brake pedal down, the voltage would start to go down fairly quickly from an average of 13.2 down to 12 or 11.9.

This is new behavior I think. It didn't happen before. I have a new gold top battery and a new 140 amp powermaster alternator and the electrical has been solid for the past 2 months after all of this was installed.

Do you guys think I have a short somewhere ? Do light bulbs go bad and start to "consume" more current? How would I troubleshoot this? I don't think there's anything about it in the FSM but I'll check again.

Any thoughts would be appreciated!

Lee

0Paul Ruggeri

I know you said the alternator is new, but it sounds like its not putting out enough.

MK 82

I would say your new alternator has gone south.

pcolt94

But check the battery voltage with a DVM just to make sure. Go thru some RPM changes, it should go to about 14 depending on the charge the battery has. The voltage readout in the cars are not really accurate.

jfb

Your voltages are too low. Normal alternator output voltage as read by the dash voltmeter is 14.3 volts engine cold and this voltage drops to 13.3 volts as the alternator gets to its operating temperature. You should have seen the red battery symbol light up on the dash telling you of inadequite alternator output. Your brake lights put about a 10 amp load on the electrical system but this is provided for by the alternator. You need to have your alternator checked, your symptoms show it has very low inadequite output, new or not.

JrRifleCoach

Have the battery checked first. You may have a dead cell.

Just cause its new, doesn't mean its perfect.

jon0108

thats how i found out i had a bad battery.While on the rack at the alignment shop the brake pedal was depressed for an extended period of time, in order to center the steering wheel with their tool. The battery went dead. turned out i had a bad cell.

coupeguy2001

I just went out and checked my 86.
The dash volt meter is .5 lower than the actual alternator battery terminal, and checked the battery, and same as the alternator.
And, I have the same problem as you do.
I am going to check the grounds, and maybe swap batteries between my 2 cars.

desertmike1

Check your primary connections i.e Battery, Alternator, Ground. If these look good, then I would suspect an alternator problem. Brake lamps draw the same power new or used, until of course they burn out!

samsonb

When you stay Gold top, are you referring to Optima. Or the Duralast Gold?

If you have a yellow Optima, then that is the wrong battery. That is a deep cell cycle battery that is intended for boats, not cars.

coupeguy2001

I switched the battery out with another one, and the problem went away.

combustables

Hi Guys,

Thanks for all the input. I ordered a new alternator (again) and installed it yesterday and the problem STILL persists.

I do have 2 spal fans that draw about 20-30 amps and 2 stereo amps but they don't seem to drain the battery at all. Only the brake lights do ....

Voltage is at 13.2 -13.4 on the readout until I hold down the brake lights. it quickly goes down to 12.5 and maybe lower if I don't let go the brakes.

I'm beginning to suspect the battery which is a gold top duralast and only 2 months old.

I'll be changing that next.

jfb

Measure the voltage across the battery terminals and see if it follows your dash voltmeter. It should but the battery voltage will be about 0.3 volts higher. This is due to the voltage drop across the ignition switch contacts and the dash voltmeter reads the voltage after the ignition switch. Also, just changing parts with no measurements is not an effective way to fix anything and it can get expensive without the problem being fixed.

samsonb

That is neat that the dash automatically switches if there is a problem. I usually switch between them while driving to make sure everything is in range.

JrRifleCoach

I wonder why the feeling "he isn't reading our post" just came over me?

combustables

Awww c'mon Steve, I read every word you guys wrote. What gave you that impression ???

Lee

JrRifleCoach

Well Lee,
Whenever someone posts a problem, they are usually trying to fix the problem not the symptom. And maybe, just maybe the OP is trying to avoid throwing money at the problem and not simply replace parts until the problem "goes away".

So most of us will take the time to assist and post a reply to guide you in the direction that we each think is the best. Some get it right, and some don't. The OP can usually figure this out by reviewing the posts and asking a few troubleshooting questions that can help him sift out the suggestions that seem the most likely.

So its easy to read that your willing to swap parts or throw money at the problem until the problem goes away. (Can I have all the parts you replaced?)

Cheers Bud!

combustables

Hi Steve,

I promise you I'm not just throwing parts at it. The first few replies implied that the alternator was going south (which actually made sense) so I ordered a replacement knowing that if nothing was wrong with my 2 month old alternator, I would send the new one back.

The battery is from autozone and I'm going to take it back and have them check it. If it's bad, I'll ask for a replacement under warranty.

By the time I read yourr first post about having the battery checked firrst, the new alternator had already shipped so I figure since I'm paying shipping both ways, why not try it.

I'm a troubleshooter by profession and I like the process so throwing parts at the car isn't normally my preferred method although I agree that it LOOKS that way here.

If autozone checks the battery and it turns out to be bad then the problem will be solved. If not, then I will have to chase a probable electrical short in the brake light circuit which I'm prreparing for. I have the fsm and will physically chase the wiring in "hops" to try to figure out what's going on.

I actually hope it's the battery at this point. That's a whole lot easier to fix.

Thanks again to everybody that chimed in. Fixing a vette without the forum is like driving with one eye closed :-)

Lee

JrRifleCoach

Lee,
Thanks for helping explain and setting me straight.
I hope the battery will resolve your problem.

May I also recommend you use a Battery Tender when the car is parked?
This device has made my life much easier for both the Vette and my camper.

combustables

Hi Steve,All,

I just came back from autozone and the battery checked out OK believe it or not. I explained the problem to two guys there who were really helpful and they did recommend I change the brake light switch since myt problem rears it's head when the rear brakes are applied.

It's interesting that they mentioned the brake light switch because I actually replaced the brake booster a month or so ago after it started hissing at me and the pedal got harder.

I had misadjusted the L shape bracket when I put everything back and I drove to the grocery store with the brake lights on at all times. I only noticed what I had done when I came back out. I re-adjusted that metal L bracket and the brake lights came on only when I proessed the pedal.

I'm gonna change the brake light switch anbd see what happens. It was $9 bucks so no big deal. Boy it sure sounds like I'm throwing parts at it again except I DID NOT buy another battery so there :-)

If that doesn't do it, I'll take out the ammeter and start measuring current draw. What a pain that will be.

The rear brake light circuit looks simple enough in the FSM but actually chasing it won't be easy I'm sure.

I'll keep you guys posted.
Lee