Car: 1989 Mercury Grand Marquis
OEM Alternator with External Regulator
Observed Condition Car is dying while driving or idling. (Lights Dimming, Clock Dimming)
Apparent Cause: Insufficient electrical energy in the battery to sustain engine operation.
Background: Condition arose during test drive after recent replacement of heater core and brake line. (Car sat for approximately 5 weeks and is not my daily driver) Battery has been replaced (judged defective by parts store and replaced under warranty) and alternator is new OEM unit with approximately 6,000 miles on it. The engine has been replaced with a rebuilt unit (by me) and around 6k miles were put on the car before the heater core and brake line failed.
My Analysis: I've had the alternator bench tested and it bench tests ok. It puts out ~14.5 volts at ~800rpm. The battery also bench tests as OK. I've put a multimeter to the battery with the car off and it measures 11.7 volts. (It's not completely charged) The voltage at the alternator with the ignition off is the same. This is grounding to the chassis. Once the car is started, the voltage at the battery drops slightly and continues to slowly drop as the car runs. The voltage at the alternator continues to be the same as what is at the battery. As a result, I have a feeling that the alternator is not taking over and charging the battery. The measured voltage drop as measured directly from positive terminal on the alternator to the positive terminal on the battery is almost zero. I suspected that the alternator may not be ground correctly but I manually ground the small (secondary?) ground on it to the chassis and saw no change in performance.
I'm now thinking that the external voltage regulator may be causing the problem. I've replaced it with the old OEM unit (It has been replaced before as part of trouble shooting a short circuit after a previous engine swap.) and also saw no change. The only two questionable things that I see are that the plug for the voltage regulator shows some exposed wires. I will check the connectivity today. The only other thing is that there is a coupler between the alternator and voltage regulator carrying two ground wires for which the pins on both sides do not seem to be oriented correctly to ground the alternator. This has, however, not been changed since the engine swap.
My requests, for the group here are for answers to the following questions:
Is there some change point from the heater core change (Whole dash came apart) that could affect this condition?
Is there any more analysis that I can perform before resorting to a professional mechanic? ;-;
Thanks in advance. -MJ