08-28-2016, 05:48 AM
For what it's worth, I had the same problem on my 89. Didn't know which pickup did what. Unhooked one and drove about 1/4 mile. that told me which pickup was speedometer. After determining which was for DDEC, I ran a two conductor cable from the MPU to the drivers station. Luckily I was able to run the wire inside up through the floor near the engine compartment by following another cable's protrusion.
The cable was connected to the MPU (without the DDEC) and I measured the voltage and frequency on a multimeter while driving. The voltage needs to be about 2 to 5 volts (it will be an AC signal) and you should see frequency vary in direct proportion to the speed. If that reading is correct, look for a bad connection at the DDEC input. That is where I found my problem, corrosion in the weatherpack DDEC connector.
Hope this helps a bit.
The cable was connected to the MPU (without the DDEC) and I measured the voltage and frequency on a multimeter while driving. The voltage needs to be about 2 to 5 volts (it will be an AC signal) and you should see frequency vary in direct proportion to the speed. If that reading is correct, look for a bad connection at the DDEC input. That is where I found my problem, corrosion in the weatherpack DDEC connector.
Hope this helps a bit.
Gordon Jones
2000-45'-2slide-#567