OK Guys this one has me (and the mechanic) stumped on our 1999 Ford Explorer Eddie Bauer. After having the battery light turn on, we changed the alternator and battery. However, this did not solve the problem and we’ve gone through several alternators. The battery light still comes on when we idle in traffic for extended periods of time (such as at the Holland Tunnel for an hour!) or drive during a hot day. Recently, it was bad enough to fully drain the battery as well.
Any ideas?
You need to carefully check all of the wiring to the alternator. The factory service manual gives the correct steps for this. There’s no reason to guess at it and keep changing parts out. Do the proper tests.
If it’s not in the wiring, then it could be the PCM. It has some control over the alternator. However, do the tests to confirm this before replacing this expensive part.
I assume that the belt and belt tensioner were checked and found to be good or replaced.
Like Tardis stated, a good logical troubleshooting procedure will find the trouble. Going from what you stated, as a hunch I suggest you check the voltage difference between the alternator output and the positive battery post while there is a good load on the alternator. You should see very little difference in voltage if the wire connection between them is ok. Also check the grounding from the battery to the chassis for any problem. If those things are ok then there is a problem with the wiring to the back of the alternator or the PCM.
Don’t forget the ground strap from the engine to the body.
Hi Tardis. Thank you very much for the suggestions. I feel very much as you suggested – like we’re swapping alternators that are more than likely fine and not fixing the underlying problem. This is reinforced in my mind by the alternators that were removed passed bench tests. However, it seems that replacing the alternator is what they continue to lean towards (even the Ford Service groups – two of them). It’s a single belt in the system so I assume it’s at the proper tension or system failure would happen.
Would it make sense that only when the car is idling for a long time or driving on a very hot day may be pointing to something else? I’m wondering if the alternator is supplying the rated current/voltage but that some part somewhere is taking too much and thus tapping into the battery supply. Or perhaps the internal regulator is choking off the current with the excess heat? The mechanic did mention the PCM as a possibility during the last round of changes.
I believe that mechanics did check the voltage at the terminals and alternator and found them to be the same while the car was running. However, it takes some time for the system to exhibit the problem so I suspect a slow load imbalance – meaning the car is drawing more than the alternator can supply or is supplying which eventually drains the battery. The odd behavior is that the battery indicator can drop slowly or suddenly jump back to a normal charge. Which suggested to me that perhaps something is cycling in the car.
Would the PCM do this? Is there a good test to determine if the PCM is failing?
Yep. He suggested putting a heavier duty grounding strap to the body just to be sure. We did that as well. It’s a real puzzler!
The PCM is most likely controlling the field circuit of the alternator which controls how much current the alternator is supposed to put out to keep up with the load on the battery. There could be something going on inside the PCM that is heating up and causes a problem. You should be able to monitor the voltage on the wire going to the rear of the alternator to see what is going on when the trouble is occuring. At this point I would not be surprised if the PCM has a problem. The trouble is acting like a failure within a electronic circuit.