I took the 7 out for an extended drive in very hot weather on Saturday with the NorCal drivers group. The Speedo behaved perfectly.
So, in conclusion, the speed sensor should have its own independent ground to chassis near the back of the car. And, cut the existing ground connection.
Took the 7 out during the hot part of the day to the Truckee air show & left it parked in the hot sun for a couple of hours, then drove home. Speedo working perfectly. I think the problem is solved. I just need to do an extended drive for final confirmation.
I got an email from James at Caterham with an official “how to fix” pdf. I almost had it right (see my last post), however I had not cut off the old ground point. So I did that and went for a test drive. So far so good – its working correctly. I have to try again in the hot part of the day to give a final confirmation.
I was alerted to a conversation on Blatchat about speedo issues.
One recommendation was to install a chassis ground at the car rear and connect to the ground side of the sensor plug.
I did that and went for a test drive, but no success. As usual, on a hot day when the car got warmed up (~20 miles), the speedo went erratic and putting on the blinkers made it worse.
So I’m still suspecting its a grounding issue, but where?
New ground wire connected to the black (it caught the flash so looks white – its really green).
Grounded to the rear brake pipe T on the de-dion tube.