Temperature gauge reading with cold engine

Hello!

My 1974 S2 seems to always run a bit hot according to the gauge. Normally the gauge stabilizes at the “l” in “normal” during regular driving. I have not had any issues with lost or boiling coolant and the car runs fine.

The other day I turned the ignition key to the “ACC” position and listed to the fuel pumps working and noticed that the temperature gauge rose to the “m” in “normal”. This was with a cold engine (20C) that wasn’t even running. I drove away, and as usual the temperature stabilized at “l”.

Do you believe that the gauge stabilizing at “m” with a cold, non-running engine could be an indication of a faulty temperature sender or is it incorrect to read the temperature on the gauge when the engine is not running?

/Marc

My '72 S1 regulates itself around the “a” and the “l” in “normal”. Dead cold it’s just off the stop with the ignition in “on”. Occasionally it will go straight to full scale when switching on - and I find the sender lead has fallen off and earthed. You may have a fault developing in your sender or perhaps an odd parallel path in your wiring that presents and clears - possible, unlikely but worth a look. Paul.

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The gauge is not a precision instrument, Marc - it’s more an indicator for temperature changes…

I would suggest that you get an infrared thermometer to verify actual ‘engine temp’ - coolant temp at the base of the sensor. This will reveal an eventual gauge misread or whatever, which is useful - ‘…worth a look…’ - as Paul expresses it.

To work on a temp/display problem without knowing the actual temp at the sensor is pointless.

With ign ‘on’ the gauge should read actual engine temp, with or without the engine running. The gauge is powered from the ign key - and the sensor provides the relevant resistance for a correct gauge reading.

The gauge is also powered in ‘crank’, but there is a lot of ‘disturbances’ during actual cranking - instruments showing something that is not true…

Frank
xj6 85 Sov Europe (UK/NZ)
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A while ago, my S1 would read high ( in the red) when idling, although finger in the filler neck suggested temp was normal.Bought a new thermostat, but when I tested the old one it was fine so reinstalled it. ( the devil you know) Replaced temp gauge sender unit ( only a few $$) and reading is normal as to be expected. Suggest you start there.

Marc,

if you disconnect the green (?) wire to the water temp sensor with the ignition on the guage needle should be fully left. If you then ground the wire the needle should move full right. Then the guage is o.k. and it is worth spending a few quid, bucks, eurons on a new sender.

Good luck

Jochen

75 XJ6L 4.2 auto (UK spec)

Thank you all for your replies. Perhaps I am over analyzing the issue and should just to get an IR-thermometer to be able to sleep better at night :slight_smile: If the sender does not seem to work as it should it an easy thing to get a replacement.

Safe driving!

/Marc

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Indeed, Marc; an adequate infrared thermometer is cheaper than sleeping pills - and much more useful for a variety of cooling problems…:slight_smile:

Frank
xj 85 Sov Europe (UK/NZ)
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I have the same issue on a different car, got a thermocouple type temperature reader and that confirmed actual temperature was spot on at 180F but gauge showed 210.

I think there’s a weird earthing problem going on with that car but it is proving a pain to diagnose

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It may just be the sensor not suited to the gauge - or a faulty sensor…

The usual way of working, the lower the temp; the higher the resistance - and vice versa. Gauge reading too high - too low resistance in the sensor/ground connection. Alternatively; if a voltage stabilizer is incorporated - it may deliver too high voltage…?

One way of testing is to connect a variable resistor between the disconnected sensor wire and ground. Then adjust the variable resistance until the gauge reads 180F - and measuring the sensor’s resistance at that temp. Then compare the two resistances…

If the resistance difference varies smoothly with temp; the sensor is likely the wrong one for the instrument. If erratic; the sensor is likely faulty…

Frank
xj6 85 Sov Europe (UK/NZ)
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