Water Temperature Sender Unit

[emmb]

Can anyone tell me what is the differece between a black top sender unit and a red one?

[smiffy]

Not sure, but since fitting a new red-top one (from SNG Barratt) my temperature gauge only reads around 50 degrees with the engine warmed up, as opposed to around 70 degrees with the old black-top one. However, I've changed the carbs on my 2.4L from Solex to SU, and could not persuade the old sender unit to part company with the manifold, hence the change.
Best wishes, David

[sweller]

What's the actual temperature of the water? I'd stick a thermometer in the radiator to see if it's anything like the temperature shown on the gauge.

Simon

[stuarthardy]

Please be careful. Your cooling system is designed to operate under pressure in normal conditions. The increase in pressure leads to a decrease in coolant temperature (basic laws of thermodynamics). When you release the pressure by removing the cap, not only does the coolant expand but its temperature increases as well. Simply placing a thermometer in the radiator header tank will not really tell you anything because the cooling system will be at atmospheric pressure instead of at its normal operating pressure, whatever that is.

As for the temperature sender units, I know nothing about these but if the only thing that has changed is the sender and this is giving a different reading then I can only conclude that they are of different resistance ranges and are not really interchangeable. Unfortunately, there seems to be a trend within our hobby where people are happy to supply incorrect items on the basis that they can make more money out of them. Although I might be wrong in this instance.

Regards
Stuart

[vellis48]

A pedantic point maybe, but the reason for a pressurised cooling system is not to decrease the temperature of the coolant. For all practical purposes water is incompressible, and if any substantial change in pressure could be achieved (which it isn't) it would increase the temperature. What the pressurisation does is to increase the boiling point of the coolant. With a 7 psi radiator cap, pure water boils at 112 degree C. Works just like the old-fashioned pressure cookers (which of course wouldn’t be much use if they lowered the temperature of the cooking!).

Vince

[sweller]

As the mark 2s and S types have (as far as I'm aware) 4psi caps we're not dealing with huge pressures (but enough to give you a surprise if you release the cap quickly) - as long as you release the pressure slowly all should be well.

However, the point here is to confirm the accuracy (or otherwise) of the gauge/sender setup.

If the gauge shows 50 deg and the thermometer in the header tank is showing 70 degs then the sender is not sending the right temp!

If it's around the 50 the gauge is showing then there's some over cooling going on.

My money's on the sender/gauge at fault.

[stuarthardy]

I think the accuracy of the gauge/sender unit is always in question. My 420 doesn't have a calibrated temperaturte gauge, just a wide band of white 'Normal' temperature zone. This gets away from the question of absolute values of temperature.

Bearing in mind that a thermostat will typically operate at around 88 deg. C then whether the gauge is reading 50 or 70 C doesn't really matter, there is still an error somewhere. What is important is that you can monitor the temperature using the gauge to indicate potential problems.

You probably know all of this but you should make sure that you have a properly functioning thermostat of the correct construction. Again, a lot of aftermarket products are not suitable for the XK engine, despite having the correct temperature rating and seating diameter.

Some people remove the thermostat to get around an overheating problem, which is treating the symptoms, not the cause. This will cause the enging to run cold and increase wear and fuel consumption etc. Good quality anti freeze/inhibitor is also essential to aid better cooling as it circulates much better than water alone.

Regards
Stuart

[smiffy]

Good evening gentlemen
With the engine warmed up to working temperature, the radiator temperature is indicating a correct 75 deg C on my digital thermometer with the dashboard gauge indicating around 40 deg C.
Problem (partly) solved, as the red-top sender has a resistance of 1460 Ohm compared to the black-top unit of 510 Ohm at 8.5 deg C. The red-top unit supplied by Barratt is therefore not the correct part, as it will have incorrect resistance at the working temperature.
Remaining problem - where to get the correct sender, as my old one seems well and truly fused with the old manifold. I think that the Smiths part number is TT4801/00, or the Intermotor equivalent is 52770.
Best wishes, David

[smiffy]

The water is getting murkier! The Intermotor sender unit 52770 has the correct fitting for the Jag manifold, but a baseline resistance of around 1500 Ohm at room temperature. Does anyone have a black-top sender they do not want in return for some cash? If so, and it has a resistance around 500 Ohm at room temperature, please get in touch.
Best wishes, David

[smiffy]

Thanks to Pete Bell for encouragement, I managed to persuade my old temp sender to part from the 2.4L manifold (combination of penetrating oil, a blowtorch and moderate force did the trick). With the old sender installed in the new manifold, the temperature gauge registers the correct temperature with the engine hot. There must be more than one sender/gauge combination that were used in the Mk2 range.
Best wishes, David