P1682, PCM issue?

XJ is throwing me a curveball. Left work for lunch the other day and noticed my Check Gauges light is on. Voltmeter is sitting on 9 (bottomed out). Grab my DVOM and check the battery, 13.6-13.7V. We have a discrepancy. Key on, voltmeter shows correct source voltage (a little over 12V). Start vehicle, voltmeter shows ~12V and then bottoms out. Get home and pull a P1682. Run the gauge self-test, all gauges sweep normal.

Clear code and everything works as normal. 3 days later, I start up the XJ this morning and once again the gauge bottoms out. Clear the code, start it up and gauge continues to be bottomed out currently.

I haven't pinned out the PCM yet, but I'm guessing that might be what I'm looking at?

Why do you think the problem is not what the trouble code describes ?

When the dashboard gauge shows 9 volts randomly, the alternator usually is faulty. Most often the brushes are worn out. The primary symptom is randomly experiencing 9 volts on the gauge that often returns to 12+ volts when you hit a bump in the road, or stop and re-start the engine. I would also inspect/test the battery temp sensor, its wire plug, and wire harness.

P1682 CHRYSLER - Charging System Voltage Too Low

Possible causes

•Faulty generator (alternator)
•Defective or Loose generator belt
•Generator harness is open or shorted
•Generator circuit poor electrical connection


P1682 CHRYSLER Description

The Powertrain Control Module (PCM) tires to maintain a system voltage between 12.9 and 15.0 volts. The voltage determined by the PCM as the final goal for the charging system is called "control" voltage. This control voltage is determined from the battery temperature sensor (ambient sensor). The control voltage is compared to the sensed voltage continuously during running.

Tim_MN said:

Why would you think the problem is not what the trouble code describes ?

When the dashboard gauge shows 9 volts randomly, the alternator usually is faulty. Most often the brushes are worn out. I would also inspect the battery temp sensor, its wire plug, and wire harness.

P1682 CHRYSLER - Charging System Voltage Too Low

Possible causes

•Faulty generator (alternator)
•Defective or Loose generator belt
•Generator harness is open or shorted
•Generator circuit poor electrical connection


P1682 CHRYSLER Description

The Powertrain Control Module (PCM) tires to maintain a system voltage between 12.9 and 15.0 volts. The voltage determined by the PCM as the final goal for the charging system is called "control" voltage. This control voltage is determined from the battery temperature sensor (ambient sensor). The control voltage is compared to the sensed voltage continuously during running.

Click to expand...

Why would the PCM send a signal to the gauge that voltage is 9V when I'm reading over 13.5V at the alternator with a DVOM?

The PCM signal to the gauges is deliberately altered to alert the driver of an issue.

Tim_MN said:

The PCM signal to the gauges is deliberately altered to alert the driver of an issue.

Click to expand...

Well that's dumb. :dunce:

Bad alternator or battery going south

Could it be an issue with just the battery temp sensor? Alternator is still putting out like it has for years.

Whelp, the other day the alternator quit charging on me. A buddy of mine said he had also had a fairly similar experience with his Ram awhile back and replaced the alternator brushes.

Replaced the brushes and the alternator is charging again! Of course, the battery was fairly old and running without charging killed it off, so it got a new battery as well.