Car runs normal, while running down the road.

ANONYMOUS

MEMBER
2006 CHEVROLET TRAILBLAZER
180,000 MILES

Car runs normal, while running down the road the voltage gauge will stay at about 14.5. While idling the car will run at 1000 rpm for a few seconds then the voltage gauge will drop to 12 volts, lights will dim, heater blower will slow down, and the rpm will drop to 500.
Any ideas?

2 Replies

JACOBANDNICKOLAS

MECHANIC
109,709 POSTS

Have the alternator tested.

Was this

answer

helpful?

Yes

No

Tuesday, November 13th, 2012 AT 7:46 PM

CARADIODOC

MECHANIC
33,871 POSTS

Failing generator. They develop huge voltage spikes due to their design, and those spikes can destroy the internal diodes and voltage regulator, and they can interfere with computer sensor signals. That can result in a variety of running symptoms. Diodes never short intermittently so to have an intermittent problem, either the regulator is quitting or there is one shorted diode out of the six. That will reduce the generator's maximum output current to exactly one third of its rated value, and that's not enough at idle. System voltage, (14.5 volts, in your case), can be normal with one bad diode, at higher rpm.

Have the generator professionally load-tested. If you find the most current it will deliver is around 30 - 40 amps, you will also find that "ripple" is very high. That will compound the voltage spike issue. Repeat failure of these generators is very common. To reduce that number, replace the battery at the same time unless it is less than about two years old. As they age, they lose their ability to dampen and absorb those spikes.