I have a '99 Jeep Wrangler, 6cyl engine. I replaced the alternator as preventive maintenance (brilliant!) about 1 week ago. All was well for daily use and a 3-hour (each way) trip over the weekend. Yesterday, shortly after starting out, the voltage dropped off the bottom of the gauge. If I turned off the engine and restarted, it would pop back up, but after 1-2 minutes, drop off the scale again. Each time I did this, the initial voltage would be a bit less than the last. Finally, it would not pop back up after a restart and I made it to my destination on battery power.
Today, another new alternator showed low voltage - on to alternator #3. The third one showed adequate voltage but gave the same symptoms both before and after the battery replacement.
What would happen after one or two minutes of running that could make the alternator decide to cut out?
Many thanks from Dallas, TX!
Check your battery temp sensor. It’s on the battery tray under the battery. It will alter the pulsing of the fields by the PCM which is the voltage regulator …so to speak.
If the temp sensor is convincing the PCM that the battery is overheated, would this cause the alternator to cut out completely or just keep it from trying to charge the battery? Or is it even able to do one and not the other?
I think the battery charging circuit runs through the PCM module so the trouble may be with it. It could also be that there is a problem in the wiring causing the trouble. It does seem to act like a wire connection is heating up and causing a problem. It may be a good idea to measure the voltage between the alternator output and the positive battery post. If things are ok there should be less than .25 volts across the lead.
If the temp sensor is convincing the PCM that the battery is overheated, would this cause the alternator to cut out completely or just keep it from trying to charge the battery? Or is it even able to do one and not the other?
It would not cut it out, per se~. It would limit the frequency of enabling of the field to produce current. That is, it would limit the output of the alt. If you’ve got demand in excess of that, you’re going down.
There is no voltage regulator in your alt. As has been said, the PCM merely turns it ON or OFF. The bat temp sensor can and will alter this ON:OFF situation as it reads battery temp. Correctly or otherwise.
Pull the sensor. See if the condition goes away.