Hello,
I have a 1991 G30 1 ton, 350 TBI, 4L80E trans with a US Gear overdrive unit. The van started shifting hard. So I ran some diagnostics. The speed sensor in the OD unit was bad. I spliced the factory trans speed sensor back into the wiring harness to bypass the OD unit as I plan on taking it out. Now the speedo is way off. Its saying im going at least 10mph faster than I really am.
Does anyone know how to calibrate a speedo on a 1991 gvan?