Code P1781

[Ed McFarland]

A couple of days ago I had a SES light come on
took the truck to the dealer. He said it had
stored a P1781 Code. They "ran a computer update" and all should be well. They took my $70
and I left, got almost home and the light was back on. The truck seems to be alright except for the SES light. Anybody have any ideas on this problem.

Thanks,
Ed

[OC_DMAX]

From the Helms manual, code 1781 is related to a diagnostic routine that the ECM runs on a circuit between the ECM/TCM control modules (ECM = engine control module, TCM = transmission control module).

The TCM uses this control line to tell the ECM to illuminate the SES light on the dash board display (the TCM has detected a fault). During normal driving, the ECM runs a diagnostic that checks the integrity of this control line, and when an improper voltage is detected on the line the ECM sets code 1781. So the ECM thinks there is something wrong with the circuit between the two control modules.

The trouble shooting sequence centers around a wiring harness check, connector checks at both TCM and ECM, and eventual replacement of either TCM or ECM.

Sounds like you need to take it back to the dealer. Anything to do with the ECM should be covered under the engine 5 yr / 100k warranty. The TCM may be covered under the emissions warranty. Read your warranty book that came with the truck carefully.

[Ed McFarland]

OC_DMAX,
Thanks, I appreciate the info.
Ed