voltage drop at idle

[Lee250]

when I drive at night with the headlights on and hit the brake pedal and come to a stop my voltage drops down to almost 12, I installed a voltmeter in the car. Its like the alternator isn't putting out enough power. I already have a brand new alternator installed and it used to do the same with the old one. I was thinking maybe I have a ground problem somewhere. Anybody else have this problem? It will do this during the day without the lights on but dosen't drop as much.

[Lee250]

ok I checked the numbers and it drops to about 13 with headlights off and with headlights on and car stopped at red light it actually goes below 12 volts.but if I put it in neutral it goes up a little.

[scarney1988]

Has your battery been load tested recently?

Is your battery the right size for your car?

Are your battery connections clean and secure?

What is your idle speed when it drops down?

How is the tune of your engine, is it running good or does it need a tune-up?

Do you have any aftermarket accessories installed i.e. amps, subs, underglow, etc... ?

Was the alternator tested after it was installed?

Just trying to help you start to narrow the problem down.

Good luck

[Lee250]

batterys good, its actually way overated for the application. connections are good, engine speed when in gear is about 750-800, in neutral its about 1200-1300. engine runs great. have aftermarket stereo with sub and 2 amps. but it still does this even when they are turned off. I think its a ground problem cause the voltage will drop a little even when in neutral and you rev the engine a bit.

[Kingdom934]

I've seen something similar with the interior light w/ door open/closed. The dim interior light usually is cased by a bad contact, such as a ground. Since you live in hawaii...like me, you know exactly how much corrosion likes our cars.

[Lee250]

anybody have a diagram for ground locations for a gen1

[Eugee]

Just about every light bulb and electronic device has it's own ground to the chaise. I had a similar problem once and it turned out to be a bad connection between a tail light and the body. I would start checking continuity with a meter. Check Ohms, should be zero, between ground on bulb socket to the body. While your their check continuity between hot and body, their should be none. Hope this helps. You can just follow the negative connection on the battery for the chaise ground. Good Hunting, Eugee- 86 Liftback

[scarney1988]

Just making a few suggestions for things to check.

1)With car warmed up and running. Set DMM or Analog meter to DCVolts and put the positive lead on the 10mm output nut on the alternator and the negative on the positive terminal of the battery. What is your reading? Voltage drop of output cable

2) Turn car off, with engine bay hot. Disconnect 10mm nut holding down output of alternator. Disconnect negative and positive battery cables. Put positive lead on eyelet (wire harness side) that would go over stud on alternator and the negative lead on the positive battery cable clamp (that is disconnected and not touching battery). Whats the value? Impedance of output cable

3)If you email me, I will attempt to respond with factory wiring diagram.

4)What brand alternator did your replace it with, and was it new?

5)Main ground should go to transmission and to driver front fender. Can do the same thing as step one with car running and placing positive lead on ground cable eyelet and negative lead negative battery terminal. What is value? Voltage drop of ground cables.

Is there a 4 pin connector going into alternator? Are there 2 white wires, 1 yellow, and 1 black with a yellow tracer?

Deffinetly possible your there are some faulty ground connections. However, these simple electrical check can narrow your problem down and require little to no investigation.