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.

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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)

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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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Hello!

So I got an IR-thermometer and my coolant temperature seems to be correct (82-85C). However, the temperature gauge is always starting at “l” when motor is cold and stabilizes somewhere right between “l” and H when motor is hot. I removed the cable from the temp sensor, grounded the cable and the needle went from “C” to “H”. My suspicion is that the coolant temp sensor is not doing its job and I will replace the old sensor with a new one that i bought.

Is there any way to test the new coolant temp sensor before installation? Pot of hot water?

When changing the temp sensor I assume I need to drain some of the coolant before removing the temp sensor (I have a S2), use some Teflon tape on the threads on the new sensor and use the seal that came with the new sensor. Any other recommendations or thoughts?

/Marc

I have not had to do this job but suspect if there are any test it will be detailed in the factory manual. Should be a resistance test, obviously changing with temperature over a stated range. I can have a look tonight when home if no one else comes back. Paul.

Common practice although, being a bit contrarian, I avoid it. I always wonder if the teflon might interfere with the grounding.

Cheers
DD

And the Series II cars did use a voltage stabilizer

I’ve long been suspicious of replacement senders. Not only in terms of quality but, also, incorrect or too-broad applications. When you see a single part number being sold as ‘correct’ for a very wide range of production models and years, one begins to wonder if it is a little bit correct for all of them but truly correct for none of them.

A few times over the years I’ve used an adjustable resistor to correct temp gauge readings when nothing else works.

Some here may remember discussions from 15-20 years ago regarding the additional resistor used on the temp sender on some Series III cars. I think we concluded that Jaguar used them on a car-by-car, ‘as needed’ basis.

Cheers
DD

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Normally, the gauge should be ‘off scale’ left with the engine cold (or the sensor wire disconnected), Marc - and off scale right with the sensor wire grounded…

Without temperature numbers marked on the gauge, as it is on mine, it’s a bit difficult. Straight up, 12 o’clock, is ‘usually’ around 90C - 80C around 11 - 100C around 1 o’clock - 120C at 2 o’clock…sort of.

Otherwise I suspect that you have a faulty gauge - or it is fed wrong voltage? Measure voltage at the detached sensor wire - it should read battery voltage.

In the meantime; read engine temp with the IR and make a mark, mental or physical, on the gauge. The main value of the gauge, which is hardly a precision instrument, is to have a ‘normal’ reference - and show whether the tem is rising or falling…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)
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Hello Paul!

I checked my Repair Operation Manual but did not find any information on temperature vs resistance. If you happen have this information it would be helpful. Thank you.

/Marc

@Doug_Dwyer

I will avoid the teflon tape as suggested. I got the replacement send from SNG Barratt so it should be correct. We will see I guess.

@Frank_Andersen

My gauge is at “l” when the engine is cold and then moves to 2’oclock when the engine is hot (confirmed 82-85C with IR-thermometer). When the sensor cable was disconnected the gauge was off scale to the left and when the sensor cable was grounded the gauge when off scale to the right.

Best regards,

Marc

I’ve just replaced my sensor, I did think about using Teflon tape but thought better of it and I can report no issues with leaks. My replacement sensor had a fibre type washer although the old one was a steel washer which I thought a bit odd.

I will have a look today and see what I might have. Paul.

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What does reading ‘I’ means in needle position, Mark - and 2 o’clock at 82/85C is certainly wrong…

Both readings imply that the resistance in the sensor is too low - giving too high readings both in cold and hot. Otherwise; your gauge itself is probably OK, given the disconnected/grounded response…

Measure the resistances of the sensor, hot and cold, against the actual temp readings. This may be a good reference - when testing a replacement. And it may ring some bells among us - for further advise?

A complication; different instrument were used, but their sensors were not compatible…

Frank
xj6 85 Sov Europe (UK/NZ)
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Hello Frank and thank you!

I mean that the needle position is pointing at the letter “l” in the word “normal”. Same as 1 o’clock.

Would you mind giving further instructions on how to do this? Would I just use a digital voltage meter to check the resistance between the connector on the water sensor and ground or would i test this at the gauge?

Best regards,

Marc