Speedometer Issues......Help???

[Black Beast]

Ok so I’ve had my K-5 for about 6 years now, I love the truck, but I’ve got a problem and for once I’m absolutely stuck on how to fix it.

I’ve got a 1991 Blazer 350/700R4/NP241 combo with 10 bolt axles (3.73 gears) and 32 inch tires. Last year I blew out the rear end (silly me entering it in a truck pull), and swapped the 3.08 10 bolters for a set with 3.73 gears. Just before I swapped the axles, my speedometer went nuts, it started bouncing between 0 mph and whatever speed I was actually going. It did this for about 10 miles and went away. I swapped out the axles and drove the truck for a while, no problems. Several weeks later, the problem came back for a few days and then went away. I knew that my speedometer was off, cause of the new gears, so I performed the DRAC module modification, where you remove the jumpers and install a 8 DIP switch block in place to recalibrate the DRAC module (found on tbichips.com/drac). This fixed the speedometer calibration, but the problem came back a few days later except this time the speedometer went dead, the needle lifts off the resting pin when you turn the power on, but never moves from there. It went away a few days later, I figured it was a bad Vehicle Speed Sensor, so I coughed up the $70 and replaced it. I drove the truck for a while, but a few days later the problem came back. So now I’m out 70 bucks and I still don’t have a working speedometer…….HELP!!!

I don’t think that my modification made it any worse, I’m guessing it’s a bad wire or something……anyone have a wiring schematic?? Or could it be the DRAC module, or worse the ECU?? I don’t want to keep shotgunning this. Thoughts??

 

[4X4HIGH]

It could be a bad speedometer head. I would either take your speedo head to a speedo shop and have them look at it (if you have one near you) or look for a used one from someone on here selling one.

 

[Fordum]

Well, that was a classic speedometer cable going dry symptom, until I realized it was electronic<G>.
If the speedo was dead, for a while, but came back to life as soon as you replaced the VSS, do a real careful wiring check before you do anything expensive. I would concentrate on the end close to the VSS, because you may have moved something when you replaced it and cleared a short or restored a connection.
Be sure to unplug the wire going to it and examine the plug really good. May have a pin or connector loose inside.

J.

 

[Russell]

Wiring is simple on those electric speedometers.

The DRAC receives a 12V ignition controlled feed (pink/black wire, should be hot with the key in the ON and START position), and has a ground wire (black, or black / white) which should be a solid connection to chassis ground. The DRAC module also has two wires that run down to the sensor on the transfercase, and then several single wire outputs. One will go to your speedometer head, one will go to your ECM, one to your cruise control module (if so equipped), and one last one that will connect to your ABS module (if so equipped).

Your speedometer head should have a 12V power supply, and a solid ground (should be fine if the rest of your gauges work, power feed and ground is shared), and the single signal wire from the DRAC.