Follow up to temperature gauge question

[Theo Law]

Hi again
My initial thread is archived at the following url -

https://www.ncrs.org/forum/archive27.cgi?review=236703

Now I'm back at home I've had a chance to read your responses properly. I'm interested in the debate about whether you should/should not add a trim resistor to the sender unit. It's clear that adding a fixed resistor to the sender is going to shift its transfer function up by constant amount across the operating range. However I know from previous experience that each of the gauges has a color coded cermamic calibration resistor attached to it, so what about changing the value of this? This would surely be the "correct" way to do it? (in the absence of the correct original sender unit) Which of the two coils does this calibration resistor connect to? I know it goes to ground, but how it connects to the gauge internals, I'm not sure.
As I said in the previous posts, I'm happy now that my gauge is over-reading, rather than my engine overheating, so given that I have a few other jobs to do to the instrument cluster, I think it can wait a few months and become a winter project for me to pop the cluster out. Then I can actually get at the gauge, hook it up to a power supply and variable resistor, and see exactly what's going on.

Cheers
Theo

[Joe C.]

Re: Follow up to temperature gauge question

Theo,

It's just a matter of your preference. No matter what you decide to do, just remember that the relationship between actual temperature, and angular movement of the indicator is not linear. The needle moves more rapidly in response to temp change at low temperatures, and more quickly at high temps. This relationship will not, of course, change if you decide to add trim resistance, but will merely shift the entire range either upscale or downscale. ADDITIONALLY, note how the scale on the gauge face becomes more compressed as you move to higher temps (i.e.: 180-100=80* ;240-180=60*.........it will be helpful to [theoretically]divide the scale into quadrants, and extrapolate this relationship before making any changes)
Most important, record the measured temps and indicated temps versus trim resistance as you proceed.

Joe

[Les Jacobs]

Re: Follow up to temperature gauge question

Theo I took a trial and error approach to an over reading C2 gauge. I purchased a several different resistors (10, 22, 39 ohm) from Radio Shack, and installed one in series with the sender via alligator clips. Next raised the coolant temp to 210 or so (monitored via thermometer) and checked the gauge. It was now under reading, so I added a smaller resistor and retested. By tinkering with different series, parallel combinations I got a gauge reading correctly @ 210. Next, I checked the gauge reading at 180 and it was right on. I never tested higher than 210 and would expect it to under report at some temp..just don't know where. (I was happy with the results..at 210 I have problems) After the test, I purchased a resistor that was closest to the test value, and spliced it into the line. It worked for me, but if you require a "large " compensating resistor, your high end readings may be too compressed on the gauge. The good news is you can do a cheap test to how it works w/o altering anything...then decide. Les

[Theo Law]

Re: Follow up to temperature gauge question

Hi folks, thanks for the replies, all really useful information. Now that I at least know that the "problem" is an inaccurate gauge rather than an engine approaching meltdown, I can tackle this relatively at my leisure.
I know from previous tinkering that there is a shunt resistor on the back of the gauge that is picked from a range of values to give the right calibration.
I also know the temp/resistance characteristic for the sender, from dipping it in an oil bath and measuring temp with a thermocouple. One thing I noticed is that the sender had a lot of hysteresis, that it, it takes quite a while for the resistance to settle when at a particular fixed temperature - for this reason the gauge must be pretty inaccurate as the engine is actually warming up, and we should only put faith in steady state readings.
When the car's resting for the winter, I have a couple of jobs to do in the dial cluster, so will set the whole thing up on the bench, with a power supply to mimic the regulated 14V and just have a play with it. Changing the value of the shunt resistor appears to be the "correct" way of doing it with these later model gauges. Clearly adding a resistor at the sender will have an effect too, but I figure if I'm in there I might as well do it the factory way...

Cheers
Theo

[Joe C.]

EDIT!

The following appears in my post, above:

"The needle moves more rapidly in response to temp change at low temperatures, and more quickly at high temps."

Change the word "quickly" to "slowly".

[theo law]

Re: EDIT!

Hehe, thanks Joe! I'd kinda figured that's what you meant ;-)