Center Temp Gauge (107) Restoration

[Strife]

I've recently completed disassembly of the center temperature gauge on a 107, mostly to paint the needle, but also to figure out exactly what is the thermistor's resistance curve. There have been previous attempts to find a replacement thermistor (MB assembly: 50+ dollars: Thermistor: About a buck) but common ones like at Radio Shack have neither the proper resistance nor the proper resistance curve.

Disassembly was very tough and delicate, because the unit was not designed to be disassembled. I got a functional, but shabby used one to try out, and it looks like new now. The lamp used is an extremely long-lived, underdriven "grain of wheat" type bulb, and I didn't replace it. Has anyone had theirs go out?

If anyone is interested, I did take pictures of disassembly, and would put up how it was done.

[DOCJBK]

My outside temp gauge works well but the lamp is out in it. Could you post pics.

[c107]

I managed to retrofit an e-bay acquired gauge in my '80 450SLC, works great with the right wiring, but the needle's yellow and I could not, for the life of me, figure out how to get the gauge open.

I can 'splain that one for anyone who's interested. Getting the gauge is sometimes hard, a new one is (was) $247.64 from Caliber, part # 107-542-00-27.

Steve

[Strife]

This is EXTREMELY difficult, because the gauge wasn't meant to be disassembled. Nonetheless, it is possible...but not for the faint of heart. Read on at entirely your own risk. WORK ON A SOFT PADDED SURFACE. A 3x or 5X jeweler's loupe is very helpful if you are over 25 years old. Go to Harbor Freight and get a cheapo set for $3.00. It will be among the best $3.00 you've spent.

The front bezel of the gauge is apparently crimped (permanently) on the gauge cup, which has a ridge around the front. It can, with a great deal of patience and effort, be lifted around the edge VERY SLOWLY with a small long flat, thin-bladed screwdriver with maybe a 1/8" blade, no more than 1/32" of an inch at a time. Do NOT distort the front of the bezel, and go around the circumference (several passes may be required, bend as little, and as often, as possible) until the top will just barely pop off. The goal here is to distort the metal as little as possible (NOT the angled front) so that the bezel can be removed and reinstalled with a "snap". The lip of the gauge cup is no more than 1/32" of an inch. Do not allow the screwdriver to slip!

After popping off the bezel, the glass can be removed flip over and let it fall on the carpet, towel, etc). Then, there is a light shield to be removed that may have been glued at the very, very edge. Remove the four screws on the back, then by LIGHTLY (LIGHTLY) tapping on the rear connector, about 1/2" above a carpet or bundle of cloth, the light shield and the "guts" of the gauge will come out.

From then on, painting the needle goes like the you would do a speedometer. MASK with GOOD, NEW masking tape under the needle. I didn't, and should have. The matte face of the gauge is easily damaged. I thought about changing the lightbulb but it looks like it's in good shape and appears to be a very long-lived, grain-of-wheat type bulb. It is soldered into the board, and replacement would require removal of the needle and face place (two screws and a very hairy pulling off of the needle). 12V bulbs like this are available in hobby stores. They are usually designed for long-life and may have a more reddish hue because they are "underdriven" at 12V, which contributes to their long life. Be aware that using a too-bright, and therefore, hot, bulb will make the gauge less accurate!

My gauge's bezel was rusted slightly and the paint was coming off. I sanded it with 320 wetordry and repainted with a matte-ish, wrought-iron paint. When sanding, keep in mind to minimize the areas that were bent during removal. The original paint was fairly glossy. I allowed the new paint to dry for TWO DAYS before proceeding. If you put your nose to paint and can still smell it, it isn't really dry. You will need to put a little pressure on this part to get it back together, and you don't want to ruin the paint (I suppose that you MIGHT be able to do this by masking after reassembly but I think it would be more difficult to do neatly).

While the bezel is drying, cover everything else up with a dust-free cloth.

When reassembling, cleanliness is CRUCIAL. NO dust is permissible, in the cup, in the mechanism, on the face, or on the glass. Hold everything up to a bright light before reassembly. Replace the four screws on the back first. There are guides that help position the gauge in the cup. Do not overtighten; you are screwing into plastic. Now replace the light shield and glass. No fingerprints inside the glass!

If you have removed the bezel properly, it should snap back on with your guiding it around the lip of the cup (using the loupe helps a lot here). There is a gasket inside of the bezel that seals the gauge and makes the glass rattle-free. Any marks or bends that were made from your removal behind the 45 degree part of the bezel should be very difficult or impossible to see when installed.

So, that's it: proceed at your own risk; removal of the bezel requires a touch and I'm not sure that most people could do this successfully. You may ask, "why do this"? Well, because the day will come when you can't buy a replacement, it's good to know, and I figure I spent $25 for the gauge, maybe $3.00 for the paint. I spent at most 3 hours on it, so I effectively paid myself $70 an hour, tax-free as opposed to buying a new one. Now I can buy more MB stuff that I can't restore!

[Strife]

Three additional notes:

1. Here, on a said accurate gauge, are my readings on temperature vs. resistance in kiloohms of the thermistor mounted on the license plate. I used a multiturn trimpot in place of a thermistor to take these readings. The trimpot had to be taken out of circuit each time in order to be measured; the gauge itself has some internal resistance which would cause erroneous readings.

120F 1.14
110F 1.34
100F 1.65
90F 2.11
80F 2.68
70F 3.30
60F 4.36
50F 5.94
40F 7.81
30F 9.67

Below this, the readings became somewhat non-repeatable. Thermistors (and the gauge) are nonlinear.

I took these readings fairly carefully; the gauge is damped and I allowed it the needle to go from 0 to the reading three times fairly repeatably. I tried to avoid any parallax effect.

I've found some thermistors in the Digikey catalog that match the 0/50 (approx) resistance ratio of approximately 8.5.

I'm fairly sure that a 0/50 C negative temperature coefficient thermistor, with 3K/25 degrees C nominal value, and a 0/50 resistance ratio of 8.5-9.1 would work fairly well in place of the MB unit.

2. Effect of Trimpot in Gauge

This seems to adjust the reading, not the curve. After I adjusted it, my (not accurate) gauge matched the one I got in fairly well on the above points. Supposedly, if you want to tune up the gauge, the magic figure to aim for is 3.00K ohms at for a 25 degrees C (about 77 F) reading.

3. Gauge Bezel

Upon closer examination, the bezel was found to be made of aluminum. I painted the needle on my old gauge, and took the slightly chipped paint off of the bezel in order to prepare for painting. Hmmm....it looked pretty nice under the paint. So I polished it on a buffer instead of painting, and reinstalled. Looks great with the other chrome accents around the vents!

[maheath]

In any event, all you need is the sensor itself which is an NTC thermistor. Units such as the Fenwal #192-302LET-A01 or Thermometrics #EC95F302W are more durable and accurate than the original spec. The Thermometrics unit, for example, lists for $3.75 and is available from electronics suppliers such as Newark and Allied.

This is from a 5 year-old post in the Tech Help forum. there's a lot more good information in the archives of that forum if you want to pursue it.

[nobby]

STRIFE....EH?
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That post made me dizzy......