speedometer calibration?

[Andres]

I put this on the conversion thread and got no responses. I'm installing the drivetrain from a 2003 RAM 2500CTD in a '93 Hummer H1. I'm almost done.



I won't have ABS on my Hummer. Will my speedometer work (at all) and can it be adjusted? I've read various threads that say it's part of the PCM programming, some say part of the ABS. Any ideas or suggestions?



For what it's worth, the Hummer has gear reduction hubs and 42" tires. The rest of the drivetrain is all Dodge, including the dash and the transmission shifter on the column.



Thanks in advance.

 

[rbattelle]

Good question. My manual indicates the speedometer recieves signals from the PCM, but it doesn't indicate where the PCM gets its signals. Based on my time in the winter doing donuts, I believe the signal comes from the rear wheel speed sensor, which is mounted atop the rear differential. So, to get a speedo signal you'd want to pull that sensor and make up a "tone ring" (really a magnetic ring) of an appropriate diameter mounted to either the driveshaft or one of your wheels. Figuring out the "appropriate diameter" will be the trick. You'd have to find out what the rear axle ratio on the donor vehicle was, then make a ring whose rotation corresponds to the driveshaft(?) rpm vs. speed function that was on the donor.



If you can't get the right size ring, a Dodge dealer can reprogram the PCM for the rev/mile you need.



-Ryan

 

[klenger]

I thought the PCM was only used on trucks with an auto trans. My guess is that it's controlled by the ECM.

 

[sag2]

Most times it comes from the wheel speed (or ring gear) sensor to the CAB (controller antilock brake). From there the CAB busses it to the PCM (or ECM on manual or newer trucks) and to the cluster.

So if you have all the factory Dodge pieces and wiring, it should work assuming you do what Ryan suggests on the proper tone ring location and spacing. If you attach the tone wheel to the axle and suppose there is a 2-1 reduction, the slots would need to be twice as far apart as the factory tone wheel as the axle is spinning at twice the speed of the dodge axle at the same road speed.

 

[Jeremiah]

Programming the speedo

My Dodge dealer reprogrammed my speedo when I put 19. 5" wheels on my truck. I had to tell him how revs per mile they did (622) and I got the truck back 30 minutes later. 'Course my wallet was $65 lighter. Kinda a high price to pay but I hate having a speedo that's not right on. Plus, the mpgs aren't accurate unless the speedo is right.