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Diagnosing bad alternator - Rectifier?

'95 Mercury Cougar, ~170k miles, V8


Recently noticed that my odometer had stopped working. I found helpful website showing how to change a broken gear and get the odometer running again. Successfully got the instrument cluster out, found the broken gear, and ordered a replacement. Part of the process to remove the instrument cluster is removing the ignition lock cylinder. I had already removed the cylinder when I decided to slow down and follow the advice in my shop manual to disconnect the battery (airbag safety). (See pain with cylinder removal here http://act…

When I pulled the negative terminal off, I got more sparking than normal (the car was off but ignition was in ‘run’ as it must be to remove the key cylinder). I now suspect that this step may have damaged something.

Problem description:

Worked on car sunday night when above mentioned sparking occured. Drove to work Monday morning, and partially home when the car stalled. Total miles traveled after working on car until car died was ~18-20. The car didn’t immediatley die, but became very week. If I gave it any gas it missed and sputtered, but idling seemed OK. Managed to pull to side of road and wait for help. Battery was very week. Coworker came by and we jump started the car, it died again while we were packing up jumper cables. Jumped again and let it charge for a bit (from good car). Drove ~2-3 miles and had to repeat. Did this 3 more times to get home. (Definitely owe coworker lunch). When the car started to get weak prior to dying, it would run significantly better if I had the accelerator barely depressed. Trying to accelerate caused stuttering. I think these symptons point to week spark, hopefully just because of the low battery.

Is it likely that this is just a week battery caused by a non functioning alternator? What are the chances that I only need to replace the rectifier/diodes on the alternator? Any troubleshooting tips in that regard?

I think the trouble you are having is due to a weak charge on the battery. If you haven’t checked the fuses for a problem yet then do that. You may need to replace the alternator possibly. If the alternator is bad I think you would be better off replacing it with a remanufactured one. Then all the components will be new.

It sounds to me like your alternator is not charging. Once started, a car should continue to run even with a really bad battery if the alternator is charging.

It may be a diode, or it may be a number of other things. Maybe the brushes have worn too short to reach the slip rings. Bad regulator. Open field. Open or shorted stator winding.

One more thing, you didn’t mention anything about seeing a alternator idiot light. The current that goes through the idiot light serves as the initial “pump priming” field current to slightly magnetize the field so the alternator can generate enough voltage to power its own field. It is the alternator’s field rectifier matching the battery voltage that causes this light to go out, same voltage on both sides of the filiment=no current flow.
If this light is burned out or you failed to reconnect it while you had the speedometer out, it can make the alternator not charge.

Running the vehicle with a dead battery probably damaged the alternator. Alternators are not battery chargers. They’re designed to only maintain the charge on a battery.

Hmm, I think you stumbled on the main problem. Since I don’t have the replacement odometer gear yet, I haven’t reassembled the speedometer. I left the entire instrument cluster out of the car. I’ll put the cluster back in place, jump it again, and see how it does.

Some charging systems require the current through the idiot light to start the alternator i.e. the initial current for the field comes through the idiot light. Without that light in the circuit the alternator would not produce charging current. If you measured only 12 volts with the engine running, the alternator is not charging the system. With the alternator charging the voltage across the battery should be 13.5 to 14.5 volts DC.

Hope that helps.