I'm working on a friends van right now that is suffering from several problems. We had a Check Engine light, a CMP sensor intermittent fault, no coolant temp showing on the dash, and a very happy speedo (moved around even when the vehicle was not moving).
I started cleaning electrical connectors under the hood and checking sensors. The coolant sensor was OK so I moved on to the next set of electrical connections. I found when I grabbed the main umbilical cable in the harness at the rear of the engine the dash readings would change. I took the connector apart and cleaned them with contact cleaner then sprayed them down with a silicone grease then put the connector back together. This solved the CEL, the CMP fault and the coolant reading on the dash.
It did not solve the crazy speedo. I disconnected the speed sensor connector and the speedo still acted the same. When the car is moving the speedo works fine. The tranny shifts fine. It only goes crazy when the car is sitting still. Revving the engine makes the speedo go up and down. I'm wondering if the problem is a chaffed wiring harness or a bad ground somewhere. I'm going to put a scope on the leads and see if there is noise or a ground fault that I can detect at the speed sensor end of the harness.
Any other things I should be checking? | 
