Alternator cuts out at idle and back in when taking off

My alternator stops working when the car is at idle in drive. works when at 1000 rpm or over. When in park it kicks in and out. The voltage fluctuates from 11.5 volts at idle to 14 when it is not idling. Mechanic replaced alternator to no avail. The battery has been on trickle charge and is showing 13.5 volts. When I start the car and put in drive, the voltage again dropped to 11.5. When this happens I can hear the fuel pump slow down and the lights dim and the car runs rough. runs great when I am taking off. I know the alternator must be cutting out at idle since the fully charged battery is showing 13.5 with engine off and 11.5 when idling. Mechanic suggests replacing pcm. What else should I do? also found water under carpet in the passenger side floor. where would this be from sunroof or air con box?

Hi Roy. First: do not replace the PCM unless you have a result from a test that specifically identifies this as the cause. The PCM is not likely to fail without reason, and if it did I feel you would be seeing bigger problems than you see now. Also this is an expensive part to be using trial & error method fixes.

Unlike the PCM, which is extremely reliable, aftermarket alternators are not. I once replaced an alternator on my Riviera only to find it was putting out high voltage. I took the car to a different shop the same day - they tested a fault in the unit. Swapped in a 2nd "new" alternator and issue solved. Not saying this is definitely your problem, but it is worth having the unit checked by a different party, imo. I am told by the local parts store in my town that based on their frequency of returned items, about 1 in 4 alternators are DOA at the time of purchase.

Another possible cause: the battery is monitored by a thermistor, which sends a signal to the alternator. This signal tells the alternator to regulate the voltage higher or lower, so depending on the temperature of the battery the alternator will charge at between 11.5-14.5 volts. The thermistor wire is a small light blue wire loomed with a couple others that plug into the back of the unit. You can test this wire by reading its voltage with a meter while the car is running. Changing RPM you can watch to see if the thermistor wire is reporting a corresponding change.

If the thermistor wire is at fault, it could be the thermistor itself is bad, or the battery is somehow heating and cooling rapidly. In this case it should be replaced.

Best of luck. Let us know what you find.

I found that the sunroof seal was shot as well as a leaking tube, just happens to be the one right beside the computer under the dash. I saw another guy who open his up and noticed corrosion on the computer. Could have been the same issue in his case. Carpet insulation soaked as well and I am drying it out. Doing a new sunroof seal, one of two left in the country I assume, lucky to have found it at the dealer. I am curious to see what it looks like under the dash as the water pours out right beside the glove box. I agree that PCM does not usually fail. Might be fine once I look at it. Grounds in my case look great.