Temperature Gauge Fix and Calibration

The symptoms suggested that water was not circulating because the temperature gauge was reading hot, but the radiator was cool.

The system was flushed, the thermostat replaced, the radiator re-cored with an extra row of cooling fins, and the temperature sender renewed, but the problem persisted.

So an after-market temperature gauge with matching sensor was fitted to check exactly what temperature the car was running at.

This confirmed that the engine was, in fact, not over-heating but that the Saab gauge was faulty.

The Saab gauge was disassembled for investigation.
It appeared that the Saab gauge used a wire wound coil for both the moving and fixed parts of the gauge. This would mean that if the voltage of the car varied, both the needle and fixed magnet would change by the same amount, giving a stable reading.

On the back of the gauge there was a fixed resistor (82 ohms) and a wire wound resistor. The 82 ohm resistor was connected to one of the coils and then the wire wound one to the other. The gauge would be calibrated (during manufacture) by adding or subtracting windings from the wire wound resistor.

It was suspected there were some shorts in the wire wound resistor giving a higher than normal reading. The meant the gauge still worked - but over-read.
So the wire wound resistor was disconnected and replaced with a 100 ohm variable resistor.
The car was warmed up to working temperature (checked using the after market gauge) and then the variable resistor was adjusted to get the needle on the 'N'

Several test runs were performed to check the gauge operation.
Then the resistance of the variable resistor was checked and found to be 38 Ohms.
The original wire wound coil measured 33 Ohms making the gauge read hot.

To finish, the variable resistor was replaced with a 30 Ohm and an 8.2 Ohm resistor wired in series.