Symptom: One or more of the following conditions: Transmission stuck in 3rd gear, Instrument Cluster inoperative, DTC’s P0740, P0753, P0758, P0785, P1860.

Theory: This may be old news to some of you, but many techs haven’t dealt with this issue before, yet it is a widespread problem. The vehicle may have the instrument cluster go dead and/or the transmission stop shifting. A common cause for this is loss of power to the transmission and instrument cluster due to a faulty ignition-switch or wiring harness problem.

The Test: The first thing to know is that this problem may be hard to duplicate in the shop, as everything may check fine most of the time. With an uncooperative vehicle, you may have to use a voltmeter to monitor things until the problem occurs, which might include a road test. You are trying to determine if the ignition-switch is at fault or if there is a problem in the wiring harness. More often than not, it will be the ignition-switch. Find the appropriate fuse to monitor for your particular vehicle:

1997 S-T Series 1998-2001 S-T 1997-2000 C-K Series 1999-2001 Sierra-Silverado

Trans-24 fuse Cluster-11 fuse Trans-20 fuse Ignition-0 fuse

Watch the voltage at the appropriate fuse when the problem occurs. If the voltage goes away at the fuse, the ignition-switch is the problem. If the voltage stays at the fuse during the problem, further testing of the wiring harness between the fuse panel and transmission/instrument-cluster will be needed.

The Fix: Replace ignition-switch or repair harness according to the results of above testing.