Attention: TDR Forum Junkies 
For anyone else who may encounter this on an archaic truck. Not long ago, my speedo started wandering. Never over the current speed, sometimes at the speed, often well under teh speed, and sometimes dropping to zero. Seemed to be more stable on smooth road and less stable on rough road.
I checked under the hood; all seemed OK at the CAB. I check the TruSpeed box connections; they were OK.
I checked the rear diff connections. Connector outer shell is broken, but the sealed connection is OK. Looked where the harness goes up beside the fuel tank. The tank seemed to be pinching the harness. I moved it a bit and all was OK for a couple days. So I figgered the problem was there. Loosened the tank and shoved it over some; the harness and split-loom are undamaged, so the problem is still elsewhere. Zip-tied the harness connector over the diff together and tied it to the brake cable sheath. And nothing. No speedo at all. Aha! I'm close!
So I look under there some more. Get a light. Add my +2.5 cheaters on top of my +2 clipons (so I can see up real close). The split loom's loose on the pigtail. And one wire looks a little worse for the wear. So I bring the pigtail in where there's more light. And find the red wire to the sensor is broken through and the insulation half gone right there. When it was bent, it was making intermittent connections, and getting worse on rough roads. Pulled the pin out of the connector (pull the white piece out, pull the lock away from the pin and slide the pin out). Splice and solder a piece of wire in, wrap it in tape, wrap both wires in tape, wrap the split-loom with electrical tape onto the connector. Re-install, zip-tie the connector together and tie it to the brake cable.
And all is well again.
The moral: when you have an intermittent electrical problem that gets wors on rough roads, look for and feel for small faults in wiring and insulation and evidence of shorts and maybe sparking. Same thing when I figgered out what was wrong with the driver's seat controls. Looked from one end of the harness to the other. And happened to notice a little damage and deposit where the +12V line shorted against the seat frame. Re-insulated and the seat was fine again.
I checked under the hood; all seemed OK at the CAB. I check the TruSpeed box connections; they were OK.
I checked the rear diff connections. Connector outer shell is broken, but the sealed connection is OK. Looked where the harness goes up beside the fuel tank. The tank seemed to be pinching the harness. I moved it a bit and all was OK for a couple days. So I figgered the problem was there. Loosened the tank and shoved it over some; the harness and split-loom are undamaged, so the problem is still elsewhere. Zip-tied the harness connector over the diff together and tied it to the brake cable sheath. And nothing. No speedo at all. Aha! I'm close!
So I look under there some more. Get a light. Add my +2.5 cheaters on top of my +2 clipons (so I can see up real close). The split loom's loose on the pigtail. And one wire looks a little worse for the wear. So I bring the pigtail in where there's more light. And find the red wire to the sensor is broken through and the insulation half gone right there. When it was bent, it was making intermittent connections, and getting worse on rough roads. Pulled the pin out of the connector (pull the white piece out, pull the lock away from the pin and slide the pin out). Splice and solder a piece of wire in, wrap it in tape, wrap both wires in tape, wrap the split-loom with electrical tape onto the connector. Re-install, zip-tie the connector together and tie it to the brake cable.
And all is well again.
The moral: when you have an intermittent electrical problem that gets wors on rough roads, look for and feel for small faults in wiring and insulation and evidence of shorts and maybe sparking. Same thing when I figgered out what was wrong with the driver's seat controls. Looked from one end of the harness to the other. And happened to notice a little damage and deposit where the +12V line shorted against the seat frame. Re-insulated and the seat was fine again.
