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Date: Wed, 31 Dec 1997 18:28:43 EST
From: Dan Masters
Subject: Re: Spitfire Temp Guage
In a message dated 97-12-31 17:38:24 EST, joseph@xxx.com writes:
> My temp guage appears to be not working properly. >>
snip<< Do any
> Spitfire owners have a clue as to what is the reasonable range
of resistance
> values for the temp sensor (cold resistance and hot running
resistance) ?
Joseph:
I just run some tests on the Temp gauges in my TR6 in response to a request received from a lister. Since his request was not sent to the list, I answered off the list, but I will repeat my answer here in case it is of help to you. I think the TR6 gauges are probably the same as the Spitfire, but I'm not sure.
Anyway, here was my answer:
*************************************************
I recommended using a 100 ohm resister [connected between the meter and ground for testing purposes] just to be on the safe side. I don't know if it would hurt the gauges to connect them directly to ground, but I would not want to take a chance. Now that I have ran a few tests, I would be even more concerned about grounding them.
I measured the resistance of the temp gauge, the fuel gauge, and the voltmeter from a '75 TR6. Both the fuel and temperature gauges read 62 ohms cold. The voltmeter read 120 ohms cold.
Next, I connected the fuel and temp gauges to a benchtop power supply, with an output voltage of 14.8, just a little bit higher than the output of a good alternator, using 50, 100, and 200 ohm resisters. As you might expect, both gauges read the same, even though the markings are different. Readings were as follows:
*1/2 scale when changing from 200 to 100 ohms, about 5/8 scale when changing from 50 to 100 ohms. Had I left them connected longer, they might have stabilized at the same value either way.
I also measured the voltage drop across the resisters, with the following results:
The 100 ohm 1/4 watt resister got quite hot after a while. I would recommend a 1/2 or a 1 watt resister if you run the tests for any length of time.
As for adjusting them, the following info was posted to the MG list just this morning. I have no experience of this, but it might be worth looking into.
If you take the gauge out you will see a circular cutout in the back of the case by each terminal post, they may have a cork disk in them. Behind the cutout there is a thin, shiny metal piece with a slot cut in it. Using a suitably sized implement to twist in each slot will adjust the high and low 'resting' positions of the gauge, they can be quite stiff. I would advise shaping an old screwdriver to fit the shape of the slot as close as possible, it is easy to 'round out' the slot (I practised on an old gauge first and discovered this). I adjusted the low position on the gauge and the high position using the voltage 'stabiliser' as I only have one electric instrument.Good luck,
PaulH.
I hope this is of some value to you. You could run the same tests on the Spit gauges, and compare the two sets of data. Whatever resistance it takes to get, for example, a reading of 1'2 scale would be the same resistance of the sender for that same reading.
Dan Masters,
Alcoa, TN
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