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MG MGB Technical - Temperature gauge repaired
|I thought I'd post this in case it helps someone else in the same boat I found myself in over the past week. I couldn't find anything in the archives here or elsewhere about this. What I did find related to gauges with a capillary tube rather than an electric gauge. |
I pulled my MGB from storage last week, and noticed almost immediately that the temperature gauge was not registering at all. It was staying to the left of the C mark, save for the occasional wild jiggling from hitting all the potholes this horrible winter has left us with this spring.
I did find in my internet searches that the sender on the front end of the head varies resistance, and that at higher temperatures, the resistance decreases, allowing more current to pass through the gauge. I measured the resistance in the sender while cold and at running temperature, and found that it was in the thousands of ohms when cold, but only a few hundred ohms when warm. Apparently if the sender shows about 35 ohms, you're almost ready to boil over. So that led me to believe that the sender was functioning (I assume as intended).
I then stuffed my hand up behind the dash, sometimes from the drivers seat with arm reaching through a gap in the steering wheel, and other times from jamming myself and a trouble light into the footwell. I managed to remove the leads to the gauge. I also saw that one wire leads from the fuel guage, which itself comes from the voltage regulator. I tested continuity between the gauge and the sender, and it was fine. I checked for voltage from the fuel gauge (which is really coming from the voltage regulator) and found the readings to be wildly variable, which is to be expected when using a digital multimeter for this. I figured the voltage regulator is functioning from this.
I read somewhere that the gauge itself should read 58 ohms with the leads removed. I managed to get two leads with alligator clips attached to the gauge way up under the dash, and the gauge read infinite ohms. The fact that everything else in the system plus infinite resistance in the gauge lead me to believe the gauge was suspect.
Again from within the footwell at times, and from the drivers seat at times, I strained and struggled and finally got the small knurled nut off the screw. It was on very tight, and turned out to be the most difficult part of the entire job. I also managed to remove a few ounces of skin and maybe a pint of blood from my hand, from scratching against what seemed like countless very sharp edges in the vicinity. I withdrew the retaining bracket and the gauge came free. I pulled the light bulb from the back of the gauge and had the gauge out of the car.
By turning the chrome ring counterclockwise, it came off, along with the glass. I was then able to carefully remove the black face of the gauge using a knife to lever out the thin metal face. Then the same for the black background, and carefully pulled it past the needle, and I was able to see into the workings of the gauge:
|It turns out that there is a fine heating wire wound around a bimetallic strip which as more current passes through, heats up and makes the needle pass further to the right. This small wire appears to be attached to the metal pins with a blue glue, to which the leads attach on the back side. One end of the wire had come off (the pin at the top of the photo). I hooked a 9V battery to this fine wire and to the other pin, and the needle moved. I then attempted to solder (not really expecting to be successful) but had no luck. My solution was to put a couple of curls in the fine wire, wrap it around the pin, and put heat shrink tubing around it to hold it there. I had to use tweezers to manipulate the fine wire, its pretty delicate. I tested with 9V battery again and again the needle moved. Here is what it looked like when I was done:
|I have no idea if this will be a long lasting solution to this, maybe somebody knows what was originally used to glue this wire to the pins. But during a test drive with my 5 year old, it is registering just to the left of the N, where it always has in the past. |
My understanding is that the fuel gauge is exactly the same, except that the faces are different. Hopefully this could be helpful for someone who needs a fuel gauge repair. As far as difficulty goes, this wasn't that difficult. On a scale of 0 to heater core replacement (ie 10), I'd give this a 5. Be prepared to squeeze your hand into a nest of razors:)
|"I checked for voltage from the fuel gauge (which is really coming from the voltage regulator) and found the readings to be wildly variable"|
This is nothing to do with the digital meter. If you have the factory 'stabiliser' this actually functions by turning the system voltage (which could be anywhere between 10v and 14.5v) to the gauges on and off about once per second. The stabiliser has a heating coil on a bi-metallic strip just as the gauge does, but instead of moving a pointer it opens and closes a contact, which connects and disconnects system voltage to/from the gauges. The higher the system voltage the more heat in the heating coil, so it moves faster and opens sooner. But it cools down again at a consistent rate regardless of system voltage. The effect is that at high system voltages you get a short on period compared to low system voltages, and over time both situations (and all those in between) average about 10v. This causes the thermal gauges, which only move relatively slowly compared to the rate at which the voltage is being switched on and off, to give a relatively constant reading. Very clever, like much of the pre-electronic systems in cars of this era.
I totally agree with you. As of last week I just finished changing a complete wiring harness on an early 1974 MGB. Only to find that the starter would not work. Up in the top center just behind and below the hole for the radio there was an Ignition immobilizer with about 8-10 wires on it. These little goodies are NLA. I had to unplug it and remove the RED with white stripe from it along with a yellow purple striped and join the two to bypass that little BAT@#$%%.
My arms are a total wreck of bruises and cuts.
As for North Bay I was stationed there in the early 50's with the RCAF.
It was the best trout fishing that I ever had even better than Bagotville.
Now in Florida
|Paul - Thats probably the best explanation of the workings of the voltage stabilizer I've read to date. I'm not exactly sure how the digital multimeter works, but my guess is that it samples the voltage over time, so with the regulator switching on and off at whatever rate it does, it causes the reading on the display to vary considerably. The fact that it wasn't steady at 12V or 0V suggested to me that it was functioning, likely as it is intended.|
Any idea what they used to attach those fine wires to the posts? Someone suggested to me to try conductive epoxy if my repair doesn't hold.
Sandy, North Bay has likely changed quite a bit since the 50s. Still has plenty of great fishing, with several clean and beautiful lakes surrounding us.
|A friend of mine repaired a 'hot-wire' MAF sensor on a Suzuki Swift with silver-loaded epoxy.|
|Probably originally spot-welded, but there seem to be several different types of conductive adhesives around.|
This thread was discussed between 04/05/2014 and 09/05/2014
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