1993 Van VSS questions

[ThePIGSmith]

I bought my 1993 GMC 3500 series extended length van for its good running 6.2L Diesel engine.
I got it for cheap because its 4L80E was dead.
Later, I bought a used later model 4L80E that was GM Goodwrench reman unit from gasser truck.
I had a local tranny shop install a Diesel torque converter and install it.
They struggled to get this later model 4L80E to work in my early model rig.
The modified my wiring hardness to get my '93 model TCM to talk to this newer 4L80E.

I never drove my van before, but after the tranny swap, the van's speedometer was way off.
I keep wanting to get my GPS out to see how far off, but so far, I have not.

I called the tranny shop to ask if they could reset the speedo and they told me that I would need to go to a dealer.
I stopped by a GMC dealer today and they told me that they did not have a clue what was wrong and they would need it for a 1/2 day for diagnosis (at $65 per hour)

I am creating this posting to ask for help on how to correct my VVS and get my speedo to read accurately.

[HH]

You could try this converter box from Dakota Digital (http://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=126/category_id=287/home_id=59/mode=prod/prd126.htm). It may be a matter of changing the sensor or ring in the trans. You need to match to the gearing and tire size.

[ThePIGSmith]

You could try this converter box from Dakota Digital (http://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=126/category_id=287/home_id=59/mode=prod/prd126.htm). It may be a matter of changing the sensor or ring in the trans. You need to match to the gearing and tire size.

My speedometer is electronic.
Nothing mechanical about it as far as I know

[DmaxMaverick]

My speedometer is electronic.
Nothing mechanical about it as far as I know

True, somewhat. All of them have a mechanical beginning. The Dakota Digital reference is what you need. It is used to alter the signal from the tranny (or transfer case), to a usable/accurate signal. There are other options, but IMO, the price of the DD module offsets the time/effort (and a degree of uncertainty) of hacking your VSSB. Convenience comes with a price, and I think this one is worth it.

[john8662]

You'll need some more info on the van before trying to calibrate the speedometer. You will need to collect the gear ratio off the rear end, and the tire size, from there you can figure out what ratio you need for the VSSB. The VSSB is what calibrates the speedometer. It's possible it's been swapped out during the troubleshootting of the 4L80E in the past, which could explain your readings now. How far off is it?

You can find the info on the VSSB calibration in the 6.5 diesel book, or on various websites online.