Power Drain, lose Electrical, Ruff Idlings, car stops

PMASON1

MEMBER
2001 MERCURY MARQUIS
93,000 MILES

I have put in two alternators and I still have a problem. Charging system seems to be functioning until you start driving and then it sucks all the juice right out of the battery until the car stalls. At night I experienced dimming of instrument panel, headlights went out and then the engine started missing and then stalled. I have had the battery tested and it is good and was fully charged when new alternators were installed. I am very perplexed. Help! Do I need a new ECU? Seems like the electrical system is just burning up my alternators. Alternators upon installation were putting out 12.8 volts after I had installed which is the correct voltage. As I check voltage of alternator when car is running shows 12.8 and then gradually starts dropping with time, if you turn on accessories such as a/c and headlights it speeds up or increases the speed at which the voltage continues to drop until battery iis dead and car stalls. No check engine light is on after start up, it goes out after car starts

2 Replies

CARADIODOC

MECHANIC
33,864 POSTS

12.8 volts is absolutely not sufficient. The system must develop between 13.75 and 14.75 volts. A fully charged battery with the engine off will read 12.6 volts so at 12.8 there is no reason for any current to go back into the battery to keep it charged.

Measure the voltages on the small wires on the back of the generator. One must have full battery voltage all the time. If it is missing, look for a blown 15 amp fuse. If it's there, check that the battery warning light is turning on before you start the engine. If it is not, that circuit must be checked for a broken wire or stretched / corroded connector terminal. That is the turn-on circuit to tell the voltage regulator when to wake up and do its thing.