While driving home Friday the speedo would go from 60-80 miles an hour as I was driving 70 at the same time the brake light on the dash come on, no other lights or problems with the other gages. I checked the battery cables for tightness (all cables tight and clean). My truck is stock and has recently had the lift pump replaced with an OEM style pump. I have not had any other problems with this truck; it’s my daily driver and just need a few ideas here. Up to 73,000 miles. Drove to work this morning and onlt the light came back on, would you think the spedo may be caused by the extreme heat we are having in Southwest Oklahoma (PCM, ECM) getting hot? Where are the othe grounds talked about that need to be cleaned?

 

The first thing to check is the speed sensor in the rear-end. Pull it out and clean the end, sometimes they will have some metal shavings stuck to them from normal wear in the gears. If this doesn't solve the problem I would replace the sensor.

 

Update, I believe that my problem is with some electric part (sensor or motor) being heat sensitive as here in Oklahoma we are dealing with weeks of an oppressive heat wave, 80 degrees in the morning and 106+ in the afternoon. I state this because this morning it was 70 degrees and the brake light on the dash did not come on. Over the couple weeks it has been above 80 for the morning low and about half way to work that light come on and the speedo will act erratic at the same time the light comes on. I have not cleaned the speed sensor in the rear yet as time has not presented itself. Any other ideas?

 

Good advice about having a look at the sensor. While you are back there, have a good look at its connector for corrosion, and follow its wiring forward to where it disappears alongside the fuel tank. There are a few feet where the sensor wiring is exposed, along the axle and forward by the leaf spring, and if your truck is used on gravel a lot, those wires may be bare and damaged.