VSS read correctly by PCM but not by instrument cluster?

[weinerschizel]

I have a 2001 Z71 Tahoe. Changed out cluster with one from HD truck so I could have transmission temperature gauge. This is a very common swap.

After changing out the cluster I noticed my speedometer was 85% of actual speed. I used a scanner to see the PCM was registering speed correctly.

I'm looking at calibrating the speed in the PCM however, the speed signal is used by many parts of the truck. I'm not sure if I adjust in the PCM if it will affect cruise control, ABS, Airbags, or have any effect on the instrument cluster? I have my programmer hooked up but don't want to tinker too much till I know more about it.

Additionally, I wonder if there's a configuration in the cluster itself. It has a daughter board with processor and EPROM to store odometer (possibly more?) off which the signal is sent to the gauge motor.

Anybody have experience with calibrating the speedometer on this era of GM product?

Thanks,

PS. I have some notes here on work I've already done to diagnose this problem: https://www.tahoeyukonforum.com/thre.../#post-1465157

[weinerschizel]

Any help? I'm still struggling with this. Have since disassembled my dash wiring harness and under hood harness tracing as many wires to validate are correct as possible.

I need to know what an appropriate PCM VSS to cluster signal is (not VSS to PCM).

It appears my PCM is putting out a 12V square wave with a 1Hrz : 1mph ratio. Is this correct?

I get no love from the folks who sold me the cluster and they took my old one in return. I'm not sure if the cluster is at issue or the truck.

[kevin87turbot]

The PCM should output a 4000PPM signal to the speedometer. I've seen a few clusters that were inaccurate, but usually only by a couple mph. It's very odd that yours is off by 15%. You can decrease the vehicle PPM under the speedometer tab by 15% and that should get you close. Just don't change your trans rev/mile calibration, as that will affect your shift tables.

[BigEd]

The PCM should output a 4000PPM signal to the speedometer. I've seen a few clusters that were inaccurate, but usually only by a couple mph. It's very odd that yours is off by 15%. You can decrease the vehicle PPM under the speedometer tab by 15% and that should get you close. Just don't change your trans rev/mile calibration, as that will affect your shift tables.

Well crap, I just adjusted the wrong thing, but fortunately havent loaded it to the PCM yet..
I was thinking to adjust that setting, but had not run across anything to get better info.
Now that I have, I will try that.
My speedo is off my 4-5mph at 55 mph by gps app on my phone (speedo shows 59-60).
Weird part is that I have the ORIGINAL sized tires on the truck and it was not off until this latest set of tires.
Exact same as the last set, also.

But I will be making the adjustment to the tune and trying it tomorrow.