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Re: Spitfire Temp Guage

To: joseph@dgsys.com, Triumphs@Autox.Team.Net
Subject: Re: Spitfire Temp Guage
From: DANMAS <DANMAS@aol.com>
Date: Wed, 31 Dec 1997 18:28:43 EST
Organization: AOL (http://www.aol.com)
In a message dated 97-12-31 17:38:24 EST, joseph@dgsys.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:

50 ohms = full scale
100 ohms = slightly more than 1/2 scale*
200 ohms = approximately 1/4 scale

*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:

50 ohms = 7.0V 
100 ohms = 9.5V 
200 ohms = 11.5 V 

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

'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
                    http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74

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