1999 Pontiac Bonneville Voltage Drop

DNJ1216

MEMBER
1999 PONTIAC BONNEVILLE
6 CYL
2WD
AUTOMATIC
207,000 MILES

When Im driving my car after it heats up if I stop at a light and idle my voltage will drop to about 10 and my air condtioner will not push out cold air. I have shut the air off when idleing but it only goes up to about 10.2. When I start driving it goes back to 12.5-13 and it starts fine headlights dont dim so what is the problem. It only does it in Drive if in Nuetral or Park it stays around 12.

I have replaced the alternator and battery and the problem still exsists

1 Reply

CARADIODOC

MECHANIC
33,742 POSTS

Hi dnj1216. Welcome to the forum. Don't rely on the dash gauge for diagnostics. The place to start is by using a cheap digital voltmeter to measure battery voltage while the problem is occurring. A fully charged battery should read near 12.6 volts when the engine is not running. If it is around 11.0 volts or less, there is a shorted cell and the battery must be replaced.

Next, measure battery voltage again while the engine is running. It must be between 13.75 and 14.75 volts. If it is low, suspect a generator problem. Since you already replaced it, measure the voltage right at the large terminal bolted to the back of it. If it is higher there than at the battery, that wire has high resistance or a break in it. We can discuss that further if necessary.

If you installed a used generator, be aware that GM went from the second best generator in the world to the worst pile starting in 1987. It is common to go through four to six of them in the life of the vehicle. This is one time you don't want to install a used part. Due to their design, they have a strong tendency to develop voltage spikes that wreak havoc with the other computers on the car, and those spikes can be interpreted by the voltage regulator as too high voltage so it tries to cut back on the generator's output. The more current is requested, the larger the voltage spikes that are produced, and the lower the voltage the regulator drives the generator to.

Caradiodoc