I have read and reread previous posts on this subject and still can't manage to fix this thing.

Speedo intermitantly went wild for a while then became almost totally inoperable. I Checked at the speed sensor in the diff cover and found that the insulation had been worn away from both wires at the plug, and since this is a twisted-pair type harness, the two wires were in contact. I fixed the insulation problem and the speedo worked for a minute and quit. I pulled over, crawled under the car and knocked on the sensor w/ my pocket knife. The speedo worked for another minute and quit. I removed, cleaned, and reinstalled the sensor. Speedo worked for another minute and quit. I replaced the sensor w/ a used unit, but no dice.

OK, here's where it gets really fun. I checked both terminals in the sensor harness for shorts to ground with an ohm meter. The green wire had no continuity to ground, but the brown wire showed just over 0.1 ohm to ground. I unplugged the harness from the speedo head in the dash. The harness was now unplugged at both ends. The 0.1 ohm reading went away (no continuity). What I'm getting at here is that the speedo head in the dash causes the brown wire to short to ground @ 0.1 ohm or there about.

Questions: Is the speedo head supposed to do that? Is it symptomatic of a defective speedo head?

If the speedo head is supposed to cause a short to ground, what're the chances my used speedo sensor is no-good?

Please help...Thanks.

Al

 

I should have added that I am working on a 91 740 2.3 A/T.