91 Blazer Speedometer error.

[dads83k10]

Ok, I have a 83 K10 that I have performed an LS 5.3 swap on. I am running a (NP241) transfer case and instrument cluster from a 91 Blazer.  The VSS runs through the standalone harness, through the ECM and out to the instrument panel. The ecm was reprogramed by Current Performance out of FL, and everything is working fine. The only issue that I am having is that the speedometer and the computer seem to be running 30+ mph faster than what the car is actually running. Normally I would just use my GPS for a speedometer, but at expressway speeds the ECM thinks the car is going over the max speed and is cutting off the fuel pump.

I know that the VSS on the 91 blazer was a 40 pulse system and I passed this information along to CP prior to them programming the ECM, so if anyone has ideas of where I should start looking or how I can trouble shoot this, I would greatly appreciate your help.

[ehjorten]

It is true that most GM VSS's pulse 40 times per revolution, but the output is an AC signal.  You need something to convert the AC signal to a calibrated DC signal.  I am guessing that happens inside the LS ECM.  On the stock '91 that happened through a DRAC or Digial Ratio Adapter Controller.  The DRAC sends 2,000 digital pulses per mile to the ECM (Electronic Control Module), 4,000 digital pulses per mile to the cruise control module and 128,000 digital pulses per mile to the ABS (Anti-Lock Brake) computer.

My guess is that since your speedo is working and only off by some factor, that it is simply a matter of tweaking the programming in the ECM to send the correct number of pulses to the electronic speedo.