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Do the RPM gauge supose to move up on dash when cranking?

2004 GMC Envoy 4.2L i am trying to check to see is the crank sensor working by watching the RPM gauge on the dash the gauge don't move up when cranking but it go down below 0 when cranking but when i turn the key loose it go back up to 0 i use my scan tool and watch the RPM on it it range from 120 to 140 is that the correct numbers?

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The tach readout is controlled by software in the Instrument cluster (IC) based on the values sent from the VCM/PCM. The readout on your scanner should show the correct rpm values (120 to 240 is wrong). Therefore, if the engine is running normally, the computer is probably not sending the correct values to the IC, but the crank sensor is OK.

Posted on Jan 02, 2017

  • 8 more comments 
  • Joe Russell Jan 02, 2017

    the engine want start there is no spark what is causing no spark? how do i check the injectors for a signal or do this vehicle have injectors?

  • EdCarCal
    EdCarCal Jan 02, 2017

    Could be a lot of things. First thing is to check for error codes (DTC's) with a scanner.

  • Joe Russell Jan 03, 2017

    codes are P2101,P1516,P2119

  • EdCarCal
    EdCarCal Jan 03, 2017

    All of these point to a problem with the Throttle Actuator Control Motor. However, the problem may also be back at your engine control module (ECM) since you are also getting faulty tach readings. In any case, diagnosis of the problem will probably have to be done at a GM dealer - I expect as a start either the motor or ECM will have to be replaced, and the throttle body internals cleaned.

  • Joe Russell Jan 03, 2017

    if the ECM is bad would there be a code for it? the vehicle was running ok when drove to the store when turn the engine off and tried to start up again the engine would turn over but it wouldn't start i thought it needed a fuel pump so i replace the fuel pump and fuel filter and it still wouldn't start, after that i check for spark wasn't getting no spark at the coils. what all that can cause no spark other than the ECM?

  • Joe Russell Jan 03, 2017

    if the ECM is bad would there be a code for it? the vehicle was running ok when drove to the store when turn the engine off and tried to start up again the engine would turn over but it wouldn't start i thought it needed a fuel pump so i replace the fuel pump and fuel filter and it still wouldn't start, after that i check for spark wasn't getting no spark at the coils. what all that can cause no spark other than the ECM?

  • EdCarCal
    EdCarCal Jan 03, 2017

    There can be a DTC for a faulty ECM but not always, depending on the failure mode. The ECM controls spark timing through the spark control (ESC) module. The ESC may be faulty, but at this point the only practical way to find the problem will be to start by running a full diagnostic on the ECM. Troubleshooting by just replacing parts is not advised - see a dealer.

  • Joe Russell Jan 16, 2017

    i replace a new crank sensor but the engine still want start there is no spark at the coils and spark plugs, will a igniton switch cause no spark?

  • EdCarCal
    EdCarCal Jan 16, 2017

    As I said before, the crank sensor is probably not the problem. Given the DTCs you are seeing, the problem is probably with the ECM or ESC.

  • Joe Russell Jan 21, 2017

    what is the RPM number should be?

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