Saab 96 - Fixing the Temperature gauge


The Saab showed signs of increasing overheating, especially in hot weather in slow traffic. The problem steadily got worse over time.

Finally the gauge ran into the red and so further investigation was needed to prevent damage to the engine.

 

 

 

 

 

 

 

 

Symptoms :

  • The car was running fine
  • No signs of loosing water
  • No steam from the radiator
  • Radiator not warm
  • Electric fan not coming on

These 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.

 

 

 

 

 

 

 

 

In the end 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 actually, the Saab gauge was over-reading.

After-market Temperature Gauge

 

 

 

 

 

 

 

 

So the Saab gauge was disassembled for investigation.

Back in the '60s and 70's many cars used gauges which had a wire wound coil, moving inside a permanent magnet to give a reading, but this relied on a very stable 12v supply to get a good reading, which involved many regulators etc in the circuit

It appeared that the Saab gauge used a wire wound coil for both the moving and fixed parts of the gauge. This meant that as the voltage of the car moved around both the needle and fixed magnet would change by the same amount, giving a stable reading.

Saab Gauge Workings

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.

 

 

 

 

 

 

 

 

In this particular gauge 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 original sensor and were swapped in. 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 replace with 30 Ohm and 8.2 Ohm resistors in series.

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