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1998 GMC K3500 6.5 Turbo Diesel speedometer and RPM issues

Daniel Cassany

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Location
North Port FL
Hi, i have a 1998 GMC K3500 6.5 Turbo Diesel

When I go in the interstate the speedometer goes to the top, the GPS tells me 70 MPH and the truck tells me 100 MPH, also the RPM goes to the limit and the engine is not heard at full capacity but the RPM marks 6000. the truck is shi no I know where to start

Thanks
 
Automatic vehicles use speed sensors to determine when the gear should be shifted up or down. In the event that a speed sensor doesn't work correctly, your car might not shift gears when it needs to. This can cause high RPM as the engine has to work harder.
 
I would suspect a cluster issue. The tach is ran straight from the alternator, the speedo and radio are on one vssb output(4k ppm) the ECM is on 2 others(2k ppm & 40 pulse per output shaft revolution), and the abs is on another(128k ppm). Either the calibration resistor in your cluster is bad, or you have power backfeeding into the ground circuit.

The VSSB could have an issue on the 4k ppm output circuit, but with the tach doing the same thing I would lean towards a cluster issue. Either in the cluster, or in the power or ground to it. There are ways to test it, but most don't have a signal generator to do it.
 
I would suspect a cluster issue. The tach is ran straight from the alternator, the speedo and radio are on one vssb output(4k ppm) the ECM is on 2 others(2k ppm & 40 pulse per output shaft revolution), and the abs is on another(128k ppm). Either the calibration resistor in your cluster is bad, or you have power backfeeding into the ground circuit.

The VSSB could have an issue on the 4k ppm output circuit, but with the tach doing the same thing I would lean towards a cluster issue. Either in the cluster, or in the power or ground to it. There are ways to test it, but most don't have a signal generator to do it.

i checked an cleaned everithing, i should replace?
 
i checked an cleaned everithing, i should replace?
No, diagnose it. Just replacing parts almost always ends up causing more problems than you started with. The grounds on the back of the intake are grounds for the cluster iirc. If you have a DVOM with a good tachometer in it, you can use that to check the outputs. GM used a signal generator that sent out a calibrated signal to check them.
 
UPDATE...
i replaced all the
Instrument Cluster Dash
and now the speed are perfect until 55MPH then stop there, the truck can go on 66 MPH but the speedometer still in 55 MPH and the trans start to shift wear.

before this the VSS was replaced, the DRAC was replaced, i checked the wires and look good, all rest gauge is working good, The RPMs now show good

remember all star because the truck show speed and RPMs double all the time
 
UPDATE!!!!!!

remember the original problem was double speed and RPMs in the Cluster, i replaced the VSS and the Drac and still the same, when i replaced the cluster start to be 100% accuracy until 55 MPH and stay there no matter if the real speed is 70 (by GPS) (the trans is going up and down 1 shift over and over in that moment)

But now with the new Cluster no matter the VSS is new or old, if the Drac is new or old still doing the same, the OBD2 show me error P1643 (i am not sure if this is related)

i checked the wires and grounds and look good, the trans fluid level is in the full site (taked in hot tempeture in park position after shift each D, 3, 2, 1, for 5 seconds)

Thanks for all help
 
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