alternator quietly buzzing, key off

[eljefino]

97 ford contour 2.0 does this. Unknown but possibly sordid history, just got the thing. Harness looks good insulation wise, I have heard about the dreaded "harness rot" on these things.

Alt was initially not working, I replaced a fuse that activated the dash idiot lights including the charge light, now a couple days later the alt buzzes (like a tiny rectifier you might find in a "wall wart" AC/DC adaptor.

I have to go in for inspetion tomorrow or I'd start playing with battery cable connections etc. My OBD-II I/M monitors just completed and I don't want to restart this process until after the inspection.

Just wondering what would cause an alternator to quietly buzz inside... and how quickly this could drain my battery.

(Was thinking "shorted diode" or whatever; it hardly matters as any alt fix would be another alt, for me.) If it's an external wiring fault, that would be of interest to me.

 

[punisher]

I think your diag is right on. WIth the engine running I have heard a whine noise proportional to RPM, and with the engine off I have heard everything from a high pitched whine to a buzzing noise. With a cold engine, feel the back of the alternator. If it is warm, you nailed the problem. I really feel for you if you have harness insulation problems. Those are a PITA to replace.

 

[rustlerdude]

Do some voltage drop tests. Start the car and put lots of loads on. Headlights, rear defogger, a/c on max, etc. Put your voltage meter positive lead on the + battery and the negative lead on the + alternator connection. You should have less than a volt of drop. Meaning your voltmeter should read 1.0 volts or less. Anymore than that and you have a wiring problem between your battery and alternator.

 

[eljefino]

Ok I think I figured it out. Knock on wood.

This car uses a relay (#11, underhood fusebox) on the ignition key outputs to run all the PCM controlled whozits. This relay was stuck closed, and that hum was actually coming from the ignition module. (Hard to pinpoint noises under the hood)

There is some other ignition key output that's also vital for running, perhaps the fuel pump, so the car still shut off when I turned the key off. When I restarted though it would idle low and sputter for a second.

As I lifted the relay it came apart on me showing its pretty copper coil innards and oozed a couple drops of oil. Stuck the horn relay in its place and all is well.

Oddly, I put this relay in the horn position (and the horn works, so perhaps I smacked some new life into the relay), so if my car starts making noise, darn it, I'm going to hear about it.

 

[brianl703]

Incidentally, the wiring harness should be pretty easy to replace on a 4-cylinder contour, if the replacement parts are still available. (If not check Green Sales Co they buy all the obsolete Ford parts). The 6-cylinder models are a pain to do.



The degradation on the 4-cylinder model seems to start at the engine coolant temperature sensor.