Driving '91 GM electronic speedometer from a US Shift Controller

Hi, I recently installed a US Shift transmission controller on my 1991 GMC C3500 with 4L80E. It works great, and solved a problem 3 shops couldn't diagnose or fix. In any case, I still cannot get my stock electronic speedometer to drive. The Quick 2 controller has a speedo output wire to connect to the speedo, but their tech svs dept has not been able to supply any worthwhile information on which wire on the PCM harness to connect it to. Has anyone ran into this problem and how did you solve it? I've tried piggybacking (in turn, starting with B5 wire) onto the GM harness wires B5 & B11, which are described as "Transmission output speed signal (4WD)" and "Vehicle speed and transmission output speed signal (2WD)". Neither made the speedo display. There is some question as to which output setting the controller should be set to: 5V AC, 12V pulse, or replicated. I haven't tried "replicated" yet.

Anyone have any thoughts or experience on the connection? Thanks, Gary

your speedometer needs a 2000 ppm signal. use the DRAC that your truck already has. you will need to hook up the output speed sensor back to the DRAC.

Thanks Tayto. To be specific, which wire on the printout you attached am I to connect the Quick 2 speedo output wire to? I'm not real clear which one goes to the drac. Thanks!

I would not use the quick2 to run your speedometer. i am not familiar with the usshift controller so i can't say if it would run the speedometer or not, but I would retain the DRAC for a few reasons: it converts the signal for speedometer (2000ppm), cruise control (4000ppm) and to the PCM. While you are not using transmission control in the PCM anymore it still needs a speed signal for other operations.

If you want to run the DRAC you need to hook the factory wires up to the output speed sensor, you can splice into the us shift harness to run this signal in parallel. This should be C7 & C15 on the DRAC. Hopefully you haven't totally cut out your factory harness...

PS: your DRAC should be located behind the glovebox, near the PCM.

Did those trucks also use a buffer?

the DRAC is also known as a VSSB, vehicle speed sensor buffer

tayto said:

the DRAC is also known as a VSSB, vehicle speed sensor buffer

Click to expand...

Buffer is near the ECM, isn't the DRAC on the cluster? I guess they serve the same function either way.

i have heard the earlier years it was integrated, but i know in 91 w/ a 4L80E it was behind the glove box.

My Drac is integrated into the cluster for sure. I didn't cut out any of the original wiring, I just unplugged it from the transmission and zipped tied it out of the way, then used the US Shift wiring harness from the transmission up to it's mounting spot which in my case is in the glove box. I'll search down the wires you named Tayto and see if I can make the splice. Thanks All!

Gary Gable said:

My Drac is integrated into the cluster for sure. I didn't cut out any of the original wiring, I just unplugged it from the transmission and zipped tied it out of the way, then used the US Shift wiring harness from the transmission up to it's mounting spot which in my case is in the glove box. I'll search down the wires you named Tayto and see if I can make the splice. Thanks All!

Click to expand...

Your original output speed sensor wires should be Light Green w/ black stripe & Purple w/ white stripe. You'll want to splice those in (run in parallel) with the new controllers output speed sensor wires. All your other speedometer wiring will need to go back to "stock".

FWIW, the output speed sensor is a 40 tooth wheel (40 pulses per rotation). This sensor generates it's own AC signal and sends to the DRAC . The DRAC then converts this AC signal into a DC "square wave" signal (2000 ppm/4000 ppm). This is where you can calculate and change for tire size,gear ratio, etc.