Temperature Sensor Location

[myquest]

Hello,

Looking for help from sages. I have a friend who has 1977 car which runs beautifully in all temperatures but the temperature gauge stays stubbornly towards the red. Even having tried to overheat it on the hottest day of the year it doesn't overheat and still runs well. He's concerned that if the temperature does rise he won't know about it. New sensors, new gauges have all been tried. Last resort might be that the location of the sensor, near the manifold might be wrong. Is this possible?

Thanks

[trunt]

It may be that many of the sensors available nowadays are not quite the correct specifications, many seem to read high. LD parts sell one that says it will read lower. The other things to check are the voltage stabiliser (especially if the fuel gauge also reads high) and the calibration of the gauge.

https://www.ldpart.co.uk/shop/shop.php?c=viewproduct&pid=191&cat=3&sid=sid12b7ea762987d093eba22f5abecac5a1

In my case I added a resistor to the circuit which dropped it to the middle of the gauge, but only after I had firmly established the actual temperature of the coolant.

[wilf]

Hottest part of the engine is usually where the original electric sensor was mounted - at the rear of the LH head. I don't hold much faith in automotive electrical gauges, especially ancient ones, and so swapped mine for a mechanical one, which I believe to be more accuate/responsive, plus I "calibrated" it by putting the sensing bulb into a pan of boiling water and seeing what the gauge read (It read 5degrees high).

I get a good "read" on what is happening with engine temperatures from that gauge (In fact I fitted a dual mechanical oil pressure/water temperature) to feed my Stag paranoia.

For your friend - if the car does not overheat, then the "normal" reading on his gauge is obviously with the needle pointing toward the red, if it moves away from there then and only then should he start worrying. These old electrical gauages are mere indicators, they don't give particularly accurate measurements of anything other than changes in whatever parameter they are reading. If he wants to try swapping the gauge for another he could try that, and no doubt a different gauge will have a different "normal" reading. Tony White (see "after 25 years of hoarding" thread in the for sale section), would no doubt be able to supply another gauge.

[richardthestag]

easy options first, the temp gauge needs to have a steady voltage. this is provided by the regulator on the back of the tacho iirc, may be the speedo. it is critical that whatever instrument is is attached to is earthed back to the body. if the earth wire has fallen off or broken then the fuel gauge will over read also.

next is the temp sender, has it got white insulation around the spade terminal or black. there is a sensor available new that fits the thread but has the wrong resistance curve and rather than read centre of the gauge at normal running temp, it reads 1mm to the left of the red sector.

ld part sells the correct sender

The thermostat must be fitted, and should have the foot on the end that blocks the bypass inside the inlet manifold. if it doesn't and looks like a normal car thermostat then the engine will always run hotter than usual.

does the car still have the yellow viscous fan fitted? with end off you shouldn't be able to spin it much more than about 1/4 of a rotation. much more and it is faulty and will not be cooling the radiator.

does the engine have a header* tank fitted. some will allow coolant to bypass that radiator and the engine will run hot.

what radiator is fitted, is there good flow through the radiator. is it hot across the whole matrix. an infrared thermometer is very useful to identify a bunged up radiator.

Good luck

[myquest]

Thank you, all. I will pass it on.

I know the car has an electric fan which is cutting in correctly and using an infrared thermometer he has checked the in and out temps of the radiator. I think I might go for the voltage stabiliser or earth.

[022Dave]

If you think it might be the instrument voltage regulator, buy an electronic version; they’re cheap as chips (geddit?). I got these from RS Components (535-9010 is their part number). Discard the centre pin, and earth through the bracket hole. Their website gives more details.

Dave

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[StagJonno]

Does the Fuel gauge indicate high as well? It is also supplied by the voltage regulator (VR). As Richard said, this device is fitted on the rear of either the Speedo or Tach. It's important that the VR has a good earth otherwise it'll just pass battery voltage, which will be between 2 - 4 volts higher than the VR average voltage. The original VR is.a bit of a fudge - it contains a bi-metallic strip surrounded by a heating coil. As the bi-metallic strip heats up, it starts to bend, switching the power supply off, then it cools and switches the power supply back on. It's set up so that the output averages 10V. The "modern" solution proposed by Dave will provide a steady 10V, and much easier to diagnose problems.

The fudge works because the gauges are also operated by bi-metallic strips, being thermally operated, they work fine on average voltages.

The following from the New Zealand Triumph Club might be helpful for fitting the solid state VR inside the can containing the original https://www.triumphclub.co.nz/?page_id=962

Otherwise, I agree with Wilf to look for changes in behaviour of the gauges, rather than considering them as instruments - they're indicators.