Voltage drop low when idling
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Voltage drop low when idling
'92 trans am.
Had new alternator installed a few months ago but I've always had this problem. When I'm driving the car the voltage needle indicates approx 13V. When I come to a stop and the car is at idle, the voltage need drops to approx 9V. The headlights and interior lights get dim and the turn signal blinks at a snails pace. Any idea what causes this or how to fix? As soon as I hit the gas the volt needle jumps back up to 13V.
Had new alternator installed a few months ago but I've always had this problem. When I'm driving the car the voltage needle indicates approx 13V. When I come to a stop and the car is at idle, the voltage need drops to approx 9V. The headlights and interior lights get dim and the turn signal blinks at a snails pace. Any idea what causes this or how to fix? As soon as I hit the gas the volt needle jumps back up to 13V.
Joined: Feb 2001
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From: Sophia, NC
Car: 2016 Camaro SS + 1986 Z28
Re: Voltage drop low when idling
I had this problem, and while there could be actual "problems" somewhere, often with an older car you can perform a "big 3" upgrade and fix your voltage drop. The BIG 3 refers to electrical cables, and are battery ground to chassis, chassis to engine block, and battery positive to Alternator. Go to a car stereo shop and grab some 1 or 0 gauge wire and some ring terminals. Keep in mind your car already has these 3 cables, and you do NOT need to remove them. You can install your upgraded cables IN ADDITION to what you already have. You can always go back later and clean/tidy up any work, remove old worn cables, tuck away your new ones etc. etc.
On a newer car I'd probably be more suspect of something else, but I've had two thirdgens and an old s10 that all showed SOME sort of voltage drop with lights on, and the Big 3 fixed all 3 vehicles.
One caveat to mention though, I had a few subs and amps in my old 86, and I still got dimming with the stereo cranked. A good buddy who did pro stereo installs told me the big 3 helps everything work as it should, in lieu of old wiring, bad grounds, and small gauge wire. It will NOT fix a system that is working properly, but just plain overworked. (big amps, etc.)...you need to actually upgrade the charging system, etc.
On a newer car I'd probably be more suspect of something else, but I've had two thirdgens and an old s10 that all showed SOME sort of voltage drop with lights on, and the Big 3 fixed all 3 vehicles.
One caveat to mention though, I had a few subs and amps in my old 86, and I still got dimming with the stereo cranked. A good buddy who did pro stereo installs told me the big 3 helps everything work as it should, in lieu of old wiring, bad grounds, and small gauge wire. It will NOT fix a system that is working properly, but just plain overworked. (big amps, etc.)...you need to actually upgrade the charging system, etc.
Joined: Mar 2001
Posts: 10,405
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Car: '89 Firebird
Engine: 7.0L
Transmission: T56
Re: Voltage drop low when idling
Under-sized wire would cause voltage drop during all operating conditions, not just idle.
I think the electrical load is exceeding alternator charging capacity at engine idle speed. Alternator charging capacity (available charging current) changes with engine speed. The lower the engine speed the less charging current alternator can provide. Turn the alternator slow enough and it actually shuts off.
There is an immediate voltage drop when electrical load exceeds alternator capacity. At that moment the alternator is giving everything it can and the difference is being provided by the batteries (batteries discharging). The system voltage will rise again when alternator can service all electrical loads.
My guess is somebody has added a lot of electrical stuff to the car and now alternator is overloaded at idle. One option is to remove that electrical equipment. Another option is to get a different pulley that turns alternator at higher speed.
Getting an alternator with more output may make the problem worse. I'm not going to explain why because it will just fill the page with information that doesn't matter to you.
I think the electrical load is exceeding alternator charging capacity at engine idle speed. Alternator charging capacity (available charging current) changes with engine speed. The lower the engine speed the less charging current alternator can provide. Turn the alternator slow enough and it actually shuts off.
There is an immediate voltage drop when electrical load exceeds alternator capacity. At that moment the alternator is giving everything it can and the difference is being provided by the batteries (batteries discharging). The system voltage will rise again when alternator can service all electrical loads.
My guess is somebody has added a lot of electrical stuff to the car and now alternator is overloaded at idle. One option is to remove that electrical equipment. Another option is to get a different pulley that turns alternator at higher speed.
Getting an alternator with more output may make the problem worse. I'm not going to explain why because it will just fill the page with information that doesn't matter to you.
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