Question1970 MGB GT. When engine not running but with the ignition on switch overdrive on, side lamps on or press brake pedal but not all at the same time, the rev counter will go round to 7000 RPM, switch ignition off the rev counter goes back to zero. Have given it a good earth. With the engine running the panel light switched off there is a faint glow from the light when held against the back of the rev counter. Secondly the rev counter is a replacement from Moss but still waves about when in operation. Other than the above the car runs very well.
AnswerHi Robert. Strange electrical faults such as this are usually due to a bad earth.
Turn the sidelights on and check that the gauge illuminates. If all the other gauges glow in the dark but the rev counter does not, then the gauge is not earthed properly.
Next, turn the dashboard illumination rheostat to the off position, switch the ignition on, then try the side lamps. My guess is it will be OK until you turn the instrument illumination back on. The tiny bulb inside the instrument will provide a little power the casing of the instrument unless the instrument is properly earthed.
My third thought is related to the ignition warning lamp. This fits inside the casing of the tachometer, but it is vital that it is fully insulated from the unit. If the insulation has failed or if it is being bypassed, that would be enough to cause your problems.
Is there any possibility that you have got the wiring to these two lamps mixed up?
Finally, it is possible that your new gauge is faulty. Why did you need to replace it anyway?
