Ram gauges going nuts again

Once it turned colder, the dash gauges started acting up again, going dead, lights flashing, etc. I was getting codes P0480 and P0483.

I made a weather front out of a cardboard box and put it between the radiator and the condenser, and disconnected the fan clutch.

Disconnecting it made no difference. Everything is still going crazy. So, at least I can be reasonably confident there’s nothing wrong with the fan clutch; that’s $350 I don’t need to spend.

And this morning I got the code P1652, indicating the J1850 bus is shorting to ground somewhere.

pwong on the Cummins Forum linked a diagnostic procedure for this code that should isolate it from a bad ECM. Either the problem is caused by a bad ECM, or by a short elsewhere (or possibly in another module that is connected to the ECM via the J1850 bus).

I hope it’s not the ECM; they’re not a cheap replacement.

He also linked to three documents which serve as a schematic of the J1850 bus on my truck: 1, 2, 3. I’ve got these same schematics in my 03 Dodge Ram Service Manual, by the way.

The steps are:

  1. Eliminate the ECM as a source of the problem.
  2. Eliminate wiring shorts as a source of the problem.
  3. Eliminate every module connected to the bus as a source of the problem (by unplugging all modules and then plugging each one back in, one at a time, until the problem recurs).

The upshot is that I have to go through the entire engine compartment, checking all the wiring and modules, one-by-one. Hopefully I can find something obviously wrong.