Clearing ECM Trouble codes

Asked by Tireguy71 Dec 17, 2016 at 06:17 PM about the 1991 GMC R/V 3500 Series V3500 4WD

Question type: Maintenance & Repair

I've got a '91 GMC 1 ton dually 4 wheel drive. 454 tbi. Having some drivability
issues, so wanted to check and clear trouble codes. There were 3 stored
codes, but when I try to clear them, one code remains. Code 34. Map sensor.
The other codes cleared, just not the 34. Even after leaving battery
disconnected fo 10+ minutes, the check engine light comes on imediately at
start up. Even when I checked the code bank BEFORE starting the engine,
code 34 is present. Is this an ECM issue?

Another strange thing... With the engine running, I disconnected the Idle air
circuit, the throttle positioning sensor, the map sensor, and even the EGR all
at the same time, and the engine ran the same. Gotta be a computer issue?

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KenF answered 7 years ago

If the ecm is in 'open loop,' it doesn't rely on input from sensors to run engine.

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Tireguy71 answered 7 years ago

So this is normal? I guess I heard about "open loop" and "closed loop" before,, but to be honest I really don't know what it means. Do you think my ECM is functioning properly, based on the little information I've provided? My drivability issue may have nothing at all to do with the computer. I'm just running out of ideas. I was a mechanic years ago, back in the days of carbeurators, points and feeler gauges. Even something as old as my '91 GMC truck seems high tech to me. Any help you or others on this forum would be willing to offer would be greatly appreciated.

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