Ok, my speedo is still acting like a tach. I have checked the back of the cluster and all of the wires and connections seem to be fine the battery is grounded correctly, I even added another chassis to engine ground. I took the bulkhead connection for the wiring harness off and cleaned it and put some di electric grease on it and everything worked fine for a couple of days, but now it is back to it's old tricks again. By the way our crew cab work truck is doing the same thing. It's driving me nuts.

 

So I just tried coasting with the engine off and the ignition on and the speedo works fine. Any more input on this would be appreciated.

 

That's a toughie. Still sounds like a grounding issue, try unplugging the PCM and cleaning the connections in the plug with some contact cleaner. The VSS gets its ground through the PCM so that may be a problem. Also, the sensor grounds are all spliced together near the black plug down under the intake manifold. There may be issues with the splice or the connection in that plug. Leaky fuel heater gaskets get that area all slimy so who knows what the connections look like.

 

Thanks Dave, I'm in the process of checking that out right now. I unplugged the vss and plugged it back in yesterday and it worked great for about an hour. On my way to work this morning the speedo read 100 except between shifts.

 

any chance of the VSS or ratio adapter rubbing against the transmission tunnel when the engine is powered up? That would explain it working properly idling and coasting. This is easily fixed by loosening the big nuts and repositioning the units.

 

Oh, you're talking about your speedometer, I thought it was your swimwear...

Seriously it sounds like a broken/bad ground.