Wrong & unstable

QI have a positiveearth Mini 850. I’ve fitted a periodcorrect temperature gauge using a preexisting sender hole in the cylinderhead. The gauge has two terminals and a light. I fitted a supply wire to the gauge and ran a wire from the gauge to the sender unit. The gauge just rises gradually to the red zone as the engine warms up. I’ve checked the earths and changed the sender. It’s not running hot. What have I missed? Allan Robinson, via email

AWe think you’ve missed the instrument voltage stabiliser. This was standard on most cars with Lucas electrics.

Your gauge is a thermal-type: so the current running through to the sender warms up a bimetallic strip, which bends and moves the needle up the scale. In the days of dynamos, battery voltage could vary between 11V and 15V, depending on engine speed, headlight use, etc. This would make the gauges very inconsistent. So behind the instrument panel is a regulator that feeds a constant 10.5V to the instruments, regardless of the battery voltage. Without the regulator, your gauge is being over-driven, which we think may be causing your false reading. You’ll need to find a stabiliser suitable for positive-earth electrics.

Bear in mind that senders and gauges are matched pairs. Unless you have a pair of the correct part number to suit each other, you’re unlikely to get an correct reading.