Some 2019-2022 Silverado 1500, Sierra 1500; and 2020-2022 Silverado 2500HD/3500HD and Sierra 2500HD/3500HD models may have an illuminated Check Engine MIL at startup along with DTC U0100 (Lost Communication with ECM) set in the Engine Control Module (ECM). There also may be some transmission shifting concerns and DTCs P0700 (Transmission Control Module Requested Malfunction Indicator Lamp Illumination) and/or U18D5 (Serial Data Gateway Module Lost Communication with Engine Control Module) may be set.
These conditions may be due to the ECM missing the accessory wake-up input on circuit 5985. Use a voltmeter to check for power on circuit 5985 at the ECM.
There should be approximately 12 volts present with the Body Control Module (BCM) awake. With the ignition turned off, there may still be voltage present if the BCM is still awake. This is a normal condition.
If no or low voltage is found, use the appropriate wiring diagram for the vehicle and perform normal circuit diagnosis. (Fig. 9)
Fig. 9
The use of a voltmeter and small bulb (for example, 194 bulb) is required to test circuit 5985. The circuit is a low amperage signal circuit and it may not be able to power certain test lights or bulbs.
With a battery charger/maintainer connected, attach one side of a 194 bulb to circuit 5985 and the other side to a good ground (battery negative). Next, wake up the BCM by turning the headlights on or turning the ignition on, and check that the bulb lights. If the bulb does not light, inspect for high resistance/open/shorts in circuit 5985. If the bulb lights, use a voltmeter and measure the voltage across the 194 bulb to make sure there is at least 11 volts. If there is less than 11 volts, inspect for high resistance/open/shorts in circuit 5985.
TIP: A 194 bulb draws approximately 250 ma. Attaching too much of a load to circuit 5985 will pull the voltage down below 11 volts and may lead to misdiagnosis.
Possible areas of concern may be a damaged circuit within the first 12 inches of the wiring harness from the ECM connectors or contact with the ECM/TCM bracket, upper control arm or shock tower. (Fig. 10) Refer to Bulletin #21-NA-149 for other known areas for high resistance/open/shorts in circuit 5985.
Fig. 10
Refer to #PIT6198 for more details.
Thanks to Jim Will
