98 Ram, Falling Alternator Output

Vehicle: 98 Ram, 5.2L, 2WD, 140 amp output alternator.

When the engine is cold and the battery is fully charged (battery is new, too), my alternator outputs 13.7 VDC. After the engine reaches operating temperature, the truck begins to run rough. The dash voltmeter indicates a gradually dropping output (verified by a direct check of alternator output with VOM) as the engine continues to run. Within 15 minutes, the engine will not idle and the engine must be revved at every stop. Continued operation drains the battery as the alternator fails to charge the battery and power the car’s electrical systems. A short duration bench check of the alternator yields good output. My OBD analyzer says nothing more than the obvious: low alternator output.

This engine’s alternator regulator is integrated in the PCM. Before I replace that expensive unit or the alternator, I want to eliminate a bad sensor (or good sensor reporting a legitimate problem) as the problem.

Anybody out there have any ideas?


Bob in Birmingham, AL

Try comparing the voltages on the wires going to the alternator while it is running cool and working and then after it quits working. See if they change at all.

You may have a charging system concern.But I can’t connect the drivability concern with it.The time frame is to short.A fully charged battery is more than capable of supporting basic electrical demands for 15min.

I think that the PCM uses an open collector output transistor to ground one end of the alternator field. If you can monitor that field wire at the alternator while connected, see what the voltage is reading. It will be bouncing around so either use a analog meter or average the readings of a DVM. If you see the voltage is high there may be a problem with the PCM. If the voltage is close to zero, there is a problem in the alternator because the field is fully on but you are not getting output.

I don’t know on you engine if there is a way to drive the alternator to full output like some GM alternators with the grounding tab. Alternately, you could take the alternator off and have it tested at an auto parts store that has a test unit. An auto electrical shop should probably be able to troubleshoot the problem in short order. The PCM does need to have consistant voltage to operate the injectors correctly.

Hope that helps.