Temperature Gauge Problem

[Fossil]

Hello Everyone,

Your thoughts and suggestions would be most welcome concerning the non-function of the temp gauge of my 61 SP250 B. This followed the complete rebuild of the engine in 2012 - 14, but having decided to tackle a complete retrim soon after that I didn't feel it necessary to seek your thoughts until now. With the end of the trim work approaching, the dash instruments and switches reassembled, and still no temp gauge function despite much attention, I've completely run out of ideas.

The following actions have been carried out so far: new sensor, thrice; new/refurbished gauge from BT; new sensor lead, twice; functional used gauge from other SP tried; fresh ignition power supply via voltage stabiliser, also used to supply fuel gauge which does work; instrument earthing replaced. The earthing of the sensor via the engine has also been verified. All electrical work carried out with new cable and "properly" crimped spade or bullet terminals.

The engine runs normally and the cooling system is healthy, the block having been cleared out during the rebuild. The radiator is aluminium with an electric fan, all works as it should having been checked many times with a digital thermometer.

The looming dealine is our proposed trip to Redhill in early June, which will be its first major Club outing since the 2009 Gaydon Anniversary, before the engine rebuild. If all else fails I'm resigned to going with a non-working gauge, the digital thermometer in the glove box, with a spare battery, and the regular use of the rad fan over-ride switch, but I'd really rather have the reassurance of a working temp reading, as well as the satisfaction of having sorted the problem. I have the uncomfortable feeling that something simple that I've missed is the cause, perhaps even hiding in plain sight, but I can't think or see what it is.

Thanks for reading this

Cheers

Geoff

[JohnM]

Weird indeed, Geoff - have you tried rigging up the gauge in the engine bay close to the sensor, using simple short wires to provide power and earth to the gauge, and connection to the sensor? (the gauges in the SP don't need a voltage regulator as they are "balanced bridge" types of course, so any live feed will do).
If the gauge then works, did you bypass the complete sensor wire from gauge to sender? if not, try the gauge in the dash with a separate new wire all the way to the sender - if it's not that, you may need an exorcist!
all the best,
John

[Fossil]

Thanks for your thoughts John.

I fitted the voltage regulator because the temp reading varied upwards when additional load such as wipers and headlights were switched on back in the days when it did work, often taking the needle well towards the 230 mark. I haven't set up the gauge in the engine bay with short leads but perhaps I will try that, the most awkward bit of the procedure is disconnecting and re-connecting the choke cable in order to pull the centre panel away from the dash, but even that is becoming simpler with repetition now that I have identified the correct spanners to deal with the outer and inner cable fixtures; and yes the complete sensor wire has been replaced twice to no avail. The only option left after all these would appear to be to replace the entire system with a different type of temperature gauge, but the gauge will then not match the others which will look horrible.

Cheers

Geoff

[theoldman]

Geoff,

The only thing which has not been mentioned is that has the gauge been earthed properly?

Neil

[A.N.Other]

Is the gauge earthed up properly and not just relying on it sitting in the panel ?
Is it shorting out to the panel light connection ?
Is the rubber seal. / insulators in the gauge OK and not causing a short ?
The best way forward is as suggested and do some testing with the gauge isolated
And some sort of testing of the sender unit. Even though you have renewed you can't rely on new bits, just thing of all those dodgy rotor arms out there.

[Fossil]

Neil, Colin, thank you.

The instruments have all had fresh case earth cables provided, as I noted above, although I think that this is principally required to enable the panel lights to operate, which they do. The gauge display circuit earths through the sensor the resistance of which varies with temperature of course Tomorrow morning I will set up the gauge close to the sensor as John suggested and see what emerges, with each of the new sensors tested in turn if necessary.

Cheers

Geoff

[theoldman]

Geoff,

The instrument case also needs a separate earth.

Neil

[Sydsmith]

The fact that you have experienced odd performance from the gauge in the past when other ancillaries are switched on would suggest that earthing is the most likely suspect, as has been suggested a separate earth lead to the gauge case is essential.

Other than that it is difficult to see what the problem is, except that off the top of my head and without thinking too much about the implications, has the battery polarity been changed in the past?

[Ian Slade]

Geoff, I can't remember if you had a new loom, however testing the gauge and sender is simple, remove both from the car, connect 12v across the gauge, it should go to full scale deflection, connect a PP9 battery across the gauge it should indicate 3/4 travel or 6v half way or 90deg all are approximate. The sender, if you measure across from the body to terminal there will be a high resistance on the Ohms setting over 2K Ohm, clip a wire to the body and and another to the terminal, connect the wires to the meter and immerse the sender in a pan and heat the resistance should decrease to about 100 Ohms. The next step is to connect the gauge to the sender with a length of wire (about a metre) connect the sender body to +ve of a 12v battery and -ve to the other side of the gauge, heat the sender either with boiling water or a lighter the gauge should start to indicate. If all this shows correct then you have a car wiring problem, suggest you then either see Bruce or ask my son Peter who is living in our house, if there is anything else you want, contact me on e-mail and I will give you internet contact numbers and we can talk.
Ian

[Fossil]

Hi everyone,

A brief update, but before doing so please note that I have earthed the instrument case, and have said so twice already! Actually that was the first thing I did in 2014 after the engine rebuild when the problem first reared its ugly head. And during the dash centre panel rewire just done I fitted a complete new case earth circuit for all the individual centre panel gauges as I've said.

This morning I did as John suggested and set up the gauge beside the engine with a power lead connected to the battery +ve, the other terminal connected direct to the sensor, and the case earthed to the battery -ve. Started, warmed up until rad fan switched on and then off: the needle rose approximately 4 or 5 mm above the minimum reading, or roughly one quarter of the way towards the centre reading of 185 degrees F.

Yes, the polarity was changed many years ago, long before this issue appeared. However to be sure I switched the two terminal leads, power and sensor, with the engine running and the fan also on, and there was no reading or needle movement at all, as opposed to the 5mm deflection present before the switch.

Next I fitted another four sensors in turn, including the original used item and three new items, making four new sensors tried altogether, with no clear difference or improvement.

I guess I should now speak to Barry again to see if he will swap the new gauge he sent me some time ago for another? Certainly worth a try. I can't re-fit the original gauge because it was returned to Barry in exchange.

Otherwise perhaps I need to find an exorcist as John suggested originally. Looks like we'll be driving south with a digital thermometer in the glove box after all.

Thanks everyone for your suggestions.

Cheers

Geoff