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MG MGB Technical - Temp sending unit

About 4 years ago I had my 1971 MGB cylinder head reworked and during the process a new sending unit from Moss was installed. It reads too hot by a lot. I checked the real temperature and everything looks good. I have read in several places that the senders are not good. Does anybody know if Moss has addressed this? or of a reliable source for a replacement?
G Nicholas

Is it an electric gauge? If so,
there are two possibilities you should check out:
voltage stabiliser (is one fitted is it connected, is it working) which reduces the voltage to a nominal 10V. Without it the gauge will over read.
Is the sender the correct one? There is more than one type and they look the same.
Paul Walbran

Not sure what the Canadian spec was for '71. My UK '72 originally had the capillary dual gauge and no voltage stabiliser. Now it's got a Smiths electrical gauge and a sender that works without a vs. These gauges tended to be fitted to early BMC Minis and the like and use a now very hard to get sender. From about '64 Minis and probably everything else were fitted with electrical gauges that worked with a vs. These use a sender that is more readily available. As Paul says, the senders look very similar (maybe a different colour code on the plastic bit?) and whilst physically interchangeable aren't compatible with the other gauges (which also look similar!). If you Google temp senders they've got part numbers like GTR102 (early, no vs)and GTR104 (slightly later with vs). Check if you've a voltage stabiliser, and then go from there. If the sender worked fine before, then it might be wonky now. If it's never worked properly, then you might have an incompatible sender. Bear in mind there's a third type (GTR101?) that was fitted to rubber bumper MGs. That might be the one Moss sent you.
Peter Allen

It's a Smiths electric gauge which I think is on the same circuit as the fuel gauge which works perfectly. The problem started when the shop converted the head to no lead and replaced the perfectly fine sender with a different one. Maybe my best bet is to talk to Moss.
Glen
G Nicholas

There were three gauges and three senders used at various times, and if you get the wrong combination they will either read high or low. From what I've been able to glean chrome bumper should have a red insulator, and 77 model year on should have black. Suppliers vary as to what is needed for early rubber bumper - some say red and some black, but as the gauge doesn't seem to have changed between the start of the 1971 and 1977 model years the implication is that it didn't change from red to black until the start of the 77 model year.

However if your gauge is reading high because the red has a lower resistance than the black another implication is that you should have a black but have a red. Then again there are other colours that given an even lower resistance.

As said the voltage stabiliser could be stuck, but that is easy to check from the driving seat. Turn on the ignition and let the fuel gauge stop rising. Then start the engine and rev the engine to start it charging. If the fuel gauge stays the same the stabiliser is fine. If it is faulty it can easily go up by more than a quarter-tank.
paulh4

That's interesting, PaulH. I've seen green as well! Were they fitting electrically operated water temp gauges at the same time as the 'mechanical' dual water/oil gauges? As said, my '72 UK market Roadster originally had the dual gauge, but I subsequently changed it for separate gauges sourced from autojumbles. It hasn't got a voltage stabiliser. Mr Nicholas' '71 has an electrical gauge, so probably has a voltage stabiliser. I always thought the changeover was when they went to rubber bumpers. In the past I've bought the wrong sender eg Intermotor do one with the same part number, but they must have changed the spec in the meantime. I wish there was a definitive visual identifier.
Peter Allen

North America got separate electric oil and temp gauges with the Mk2. The oil gauge reverted to capillary for 1972 but with the same physical appearance as before, and that's how they stayed. The UK didn't get separate temp (electric) and oil (capillary) gauges unto the 1977 model year.

Up to 1964 no MGBs had stabilisers as the fuel gauge and sender operated in a completely different way which made it self-stabilising. All MGBs had stabilisers from the factory after 1964. Without a stabiliser for the fuel gauge a 1972 car (any post-64 car) would read all over the place. At that time it was high up behind the dash, above the wiper motor and almost out of sight even when you know where it is, my 1973 attached. My 1975 is low down below the wiper motor, just where the bulkhead steps forward to form the heater shelf and very accessible.



paulh4

As they say, Paul, everyday's a school day. I've had this particular B 35 years and never realised it had a voltage stabiliser, but have now found it! I've assumed the non-original gauge I put in was a pre-vs gauge, and it could well be if I bypassed the vs, being unaware of its existence. On the other hand, it may well be a post vs unit as I've a red sender. Gauge works fine, so I need to check my wiring to know what I've got.

By the way, in my '50 Riley, that too has been converted from a capillary water temp to electrical gauge and that definitely hasn't got a vs, but the sender is black.
Peter Allen

A little word of caution with the electric sender units as well---Don't overtighten them when screwing them in ,it can alter their output signal if they're overtightened
William Revit

For 1968 - 1974 cars, typical sender room temp.resistance (@ 68*F) is 800-880 ohms
--Normal running---190*F is 31-35 ohms
William Revit

On the face of it because the senders work on the basis of a reducing resistance as the temperature goes up (as William indicates) i.. a negative temperature coefficient, which means the current goes up as well, the gauge is basically acting just an ammeter, using just a bi-metal strip inside the gauge.

For that reason as the voltage varies with revs and electrical load the current will vary even though the temperature of the coolant and hence the resistance of the sender remains the same, and that will cause the gauge to go up and down as well. Hence the need for a stabiliser for this system.

The pre-stabiliser fuel gauge system worked the other way round in that as the sender resistance went up the gauge went up. But this used a complicated system of coils and a resistor in the gauge which acted like Wheatstone Bridge and operated magnetically, which made it independent of voltage.

FWIW my tests of red, white and black senders gave different figures to William:

C F Red White Black
0 32 1605 1848 2283
10 50 997 1055 1420
20 68 640 650 928
30 86 440 410 594
40 104 303 264 392
50 113 208 174 263
60 140 150 122 184
70 158 108 82.5 129
80 176 87.0 68.0 100
85 185 70.0 54.0 82.0
90 194 58.0 43.8 69.0
100+ 212+ 47.0 35.1 58.3

All those have a negative temperature coefficient so would normally be used with a stabiliser and a bi-metal gauge.

I see the GTR102 has a green insulator, what the characteristics of that and its gauge are, and the fuel gauge and sender used on those cars, I don't know. I do remember the fuel gauge was the undamped type and flapped about all over the place when under way, the same as the unstabilised MGB fuel gauges, so maybe they used the same principle and the sender has a positive temperature coefficient. I see lots of questions on Google asking about temperature/resistance characteristics but no answers, although one thread that contains pictures of the pre and post 64 Mini gauges does confirm that the earlier one is magnetic like the early MGB fuel gauge.
paulh4

My 1970 B has a clear plastic sleeve. One from Moss was red and read high. I suggest looking for a sender with a clear plastic sleeve.

The electronic stabilizers available now read high too. Wire a variable voltage regulator between the stabilizer and the fuel and temp gauges, fill the gas tank to the brim and set the needle for F (not beyond). This will render an accurate temperature reading as well. these gauges will calibrate perfectly if the voltage is properly regulated.
Glenn Mallory

If you are fitting a variable voltage regulator it can replace the existing stabiliser, you won't need both.

Far more important to get the calibration of the gauge correct at E than F. My first replacement ending up running me out as it was showing nearly a quarter when empty. I realised something was wrong as it had stopped moving up and down slightly going round curves, but stopped about a mile short of the next filling station. For calibration I deliberately ran out, put a gallon in, and only then calibrated for E to give me something of a 'reserve'. That failed soon after, as did the first and second replacements on the other MGB. Only with the fourth one did I not have to recalibrate yet again. Judging by what people have reported about the 'modern' replacements they seem to be equally as variable, which seems odd given their physical construction i.e. they don't use turns of fine wire on a former. But then they are all being measured with different ohmmeters.
paulh4

Something happened to my fuel gauge/sender where full reads full, it gets to about half full then drops to empty. Like any relationship when you've lost trust, you look elsewhere, and I now fill up every 200 miles, or sooner.
Peter Allen

This happens when the winding (assuming the original style sender) has broken about half-way along. BT, DT, which led to each of my four replacements.
paulh4

This thread was discussed between 26/12/2019 and 06/01/2020

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