Hello all,
Helping a neighbor with an 84 Delta (307). The Amp idiot light came on, when I drove it I noticed the idle went high and stayed that way when it was in drive, though not so high I couldn't brake at stop signs. Also noticed the turn signals started blinking slower than normal while I was test driving it. I was thinking alternator or maybe a loose belt, but the weird thing with the idle got me wondering if I was missing something.

Anybody has any ideas, let me know. Thanks!

 

The first thing to check is the basic condition of the alternator with a simple digital multimeter (DMM). While the car is running at idle speed (in park) with no accessories running (turn off heater, AC, stereo, lights, etc.) measure the voltage at the battery. The voltage should be ~14.3V @ idle. Increase the RPM to ~2500 RPM while continuously reading the battery voltage. A good working alternator should be able to produce ~14.3V recharging the battery.

Is the voltage at the battery ~14.3V at idle? Does the voltage at the battery increase when the RPM is increased to ~2500RPM?

Perform the same operation (above) while turning on the headlights, AC, heater, stereo, etc. and monitor the voltage at the battery noting any changes in voltage as accessories are turned on. Note those voltages as accessories are turned on. It's possible the voltage regulator might be suspect (or possibly another issue); yet, a quick determination of battery voltage while the car is running is a basic starting point in diagnosis.

 

Your computer-controlled 307 uses a vacuum-operated Idle Load Compensator to switch to high idle under certain conditions (like when the A/C is on). A solenoid valve controls vacuum to this ILC. A low voltage condition can cause the solenoid valve to become inoperative, and the default condition for the ILC is high idle.

 

Thanks for the advice. I tried the old trick where you start the car, let it run a bit and then disconnect the negative battery cable to see if the alternator is keeping the car running (car died), but then I realized that may not work on every car, especially when they start having computers. Yeah, I'm an amateur. That's ok, this is how I learn!

 

That "old trick" was developed during a time when they used generators instead of alternators. There are folks who will attempt that old trick today on modern alternators but it's not the safest method - especially on newer (rather than older) vehicles which depend on various electrical & electronic circuitry. Best to employ a DMM. I think Joe's input is valuable RE: ILC default condition during low voltage = high idle.

 

Thanks, Vintage Chief, I'll retire that old trick and get a DMM.

 

OK, so I got a DMM! Battery showed 12.4 while the car wasn't running, 12.11 when started, and 11.84 with headlights on. Engine at about 2500 RPM put it back up to 12.07.

So if I understand this correctly, it looks like the alternator isn't charging properly any more.

Thanks and let me know if you have any more ideas or comments. I appreciate the advice. I'm an amateur mechanic at best and electrical was never my strong point.

 

If like to add this was a pointless trick back when Edsels were new. It takes at most a couple amps to keep the ignition system going, I wouldn’t consider a charging system capable of a couple amps adequate for anything less than a riding mower. Not to mention the potential damage from voltage spikes to sensitive electronics, or battery explosions from sparks, the list goes on.

 

Correct. If you'd care to validate, you can drive to a close/nearby automotive supply store. They can (free of charge) place a load on your ALT & perform a test to validate the ALT is not charging.

 

An alternators that is not charging is usually easy and inexpensive to fix. Try reading up on it and do it yourself if you have time.

Good luck!!!

 

Well, with your help and a youtube video, I changed out the alternator and swapped the belt while I was at it. Now the DMM is showing 14.1 when the car is running. I appreciate all the good advice. This site has the best people on it!

Cheers.
Chris

 

Good deal. Happy motoring.