Little Bear

2000 C2500 crew cab OBS, 2 wd, 5.7l engine, 4L80e trans. I noticed when going into town that the radar traps they have set up displaying your speed were always 5 MPH lower than what my speedometer was reading. Since there are several of these in town and they all act the same I started wondering if it was my speedometer. Brought my handheld GPS with me the other day and it too shows my speed being 5 MPH slower than what the speedometer was reading.The tires match the size listed on the door and are properly inflated. Any ideas why this is happening? Am I also correct in thinking the only way to correct this is to have the ECM reprogrammed to correct for the difference?

kevinkpk

If you are refrering to the speedometer checks the cops set up, these are calibrated with a tuning fork, so they could be out of adjustment.

Little Bear

OK, but my handheld GPS has proven accurate with other vehicles and I've noticed too when in traffic I'm always the slow one, always thought everyone else was just in a hurry as I was doing the speed limit.

Gumby22

Could be the stepper motor in the cluster starting to go bad. Turn the key on but don’t start it - the cluster should automatically perform a self test and sweep the gauges - all gauges should sweep smoothly from side to side - watch to see if the speedo movement is abnormal.

Vehicle speed is monitored directly by the PCM and data fed to the cluster, cruise module, BCM, suspension control module (if equipped) and radio. Has the cruise quit working or has there been any hard shifting? That would indicate a problem with the VSS and related wiring.

The easiest way to tell if it’s the VSS/related circuits or just the cluster would be to check live data. Compare the speed signal the PCM is receiving to the signal the cluster is receiving - if a discrepancy, troubleshoot the circuit between the PCM and cluster. If not, the cluster/stepper motor is probably the issue.

Little Bear

This is an Old Body Style dash, 96-98, no self test, gauges just jump to what ever the current reading is I don't believe they used the same type of stepper motors as became a problem 03+. Cruise works fine, shifting is smooth. With that said, I do have another gauge cluster from a 96 parts truck that I could try and see how it reads and perhaps at least eliminate the gauge cluster.

Gumby22

I can never remember the year when they started the self test at key on. However, the exact year and style is irrelevant - stepper motors fail for varying reasons no matter if they were manufactured in 1996 or 2020.

In lieu of a sweep test, you can remove the cluster, take off the lens, and manually sweep the speedo. If it feels like there’s any dragging or catching, the motor is bad.

Little Bear

OK I'll take it apart this weekend.

tech2

not sure with the old body style (pre 1999) but the new bodystyle (1999 to 2006) with an ebcm...you can correct the speedo my setting a different tire size in the ebcm.

mountainmanjoe

do you have oversize tires? check your door sticker.

Little Bear

No correct tires. Oversize tires would make it read slow, found that out the hard way with the wife's new to us Jimmy.