Hello. I need help fitting a new electric temp sensor and gauge. My non original "bulb" type sender and gauge stopped working just before Rylstone and we did the trip with sensors connected out from under the bonnet and in through the quarter vent window to an electronic gauge. ( The car ran perfectly.)
I bought a new sender and gauge and calibrated them (with a 10 volt supply) to 100 degrees in boiling water BUT need some inside info before I connect them.
1. If I unscrew the bulb sender will all of the (cold) fluid run out before my wife can block the hole with a (gloved) finger? I would prefer to not drain and refill if possible.
2. Will I be able to easily screw in the new sender (no teflon tape)?
3. Do I have to worry about the depth that the new sensor screws in?
I will use an electronic gauge for the next few months and after New Year will attack the actual gauge replacement. I'm sure that I will have to ask a lots more questions then.
Thanks Dave D

You can use a suitable container to catch the coolant from the sender hole (ice cream contaiers work well). Then pour back into the rad when finished. Remove the rad cap so no airlock.
Do not use teflon on the senders.
Depth willl be common sense once you have the old one out.

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Cheers Dave.

Have been doing similar things with a non-functioning temperature gauge recently too:

The standard thermister sender seems to be on a tapered thread and tightens up long before it bottoms out.
Under normal circumstances you will spill very little coolant.
Not sure how long it takes to remove a bulb-type sender but I doubt you'll need the services of an attractive assistant unless you are very slow.
The 'sender' earths to the head so no teflon or threadseal.

The gauge in your photo looks the same as the Caerbont/Smiths gauge I have. Would be interested to know the GTR part number that matches that gauge.

Thanks for all of the help, but common sense is not one of my strong points.
The attractive assistant, her comments and small dextrous hands are always welcome when working on the Mini.
It is a Smiths (Caerbont?) gauge. I bought it from Howard Instruments in Melbourne as I assumed they could match it to the correct sender. (GTR ?) One of their staff owns a few Minis. I have attached the receipt with part numbers. The prices were similar to other suppliers.
Did the original Mk1 S have a red or white hot segment?
Thanks again Dave D

Thanks for the part number of the sender. Does the sender you were supplied have a coloured plastic insulation ring between the body and the male terminal for the wiring harness? And, is it black?

Regarding original temperature gauges on early Mk1 S, there seems to be several. Some were like the BT2204 we have bought (no colour; no Made in UK either!). There are others with coloured sections of blue at the cold end and red at the hot end of the scale. Different UK cars had similar but slight variations on this pattern.
Also see for examples:
viewtopic.php?f=2&t=101758

The temperature sender that I bought has a black insulation ring but I would not use that to decide which sender you have. Howards have many Smiths senders in their catalogue: the one that they sent to me wasn't actually the one labelled for Minis. It has 0300A stamped on one spanner face and 01 - 15 on the next.
That was why I rang them and got them to match the right sender for the gauge AND I did the stove top test to calibrate the gauge with the sender in boiling water just in case. Interestingly I found that there is a long time delay (about two minutes) before the gauge reads the actual water temp. The sender and gauge both take a while, to warm up or cool down, to a changing water temp.
Removing the tester from the block and testing in a cup of hot water will only tell you that it and the gauge in the car are working. The temp reading will not be at all significant because the water will be cooling while the sender and gauge heat up.
I will ignore the coloured Hot "flag" and pretend that it came as white.
Thanks Dave

From my experience no 2 senders or guages are equal and the confusion around which goes with what is all around.
The only sure way, as Dave suggests, is to mix and match to get your own preferred outcome.

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Cheers Dave.

And from the factory they used different senders to match the different thermostats, so the Normal point was in a suitable position

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I use the original 1970 Matic gauge, a red sender and a trim pot so it is in the middle with the thermostat housing at 82C and engine running. Voltage stabiliser is electronic.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

I went about it a similar but different way. I wired a 5K pot in and set so the gauge was where I wnted it at temp. Then I picked a sender from my collection that matched.

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Cheers Dave.

I used a variable resistor to simulate the sender values so that I could adjust my very, very low current electronic gauge to read through the same range as the Smith's Gauge.
BUT: the Smith's gauge and sender current is nearly 100mA at 100 degrees. This is a very high current to run through most carbon film variable resistors. "Moving" the gauge by only 10 degrees would dissipate about 0.1 watts. The Jaycar catalogue shows a number of variable resistors with rated wattage as 0.05watts, 0.02watts, 0.125Watts and trimpots as 0.1watts. A wire wound variable resistor however would be quite OK.
Don't let the blue smoke out!
Dave D

You could use a spare dash light dimmer.

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Cheers Dave.

Temperature Gauge update:
The gauge/sender when fitted, read (with 90 head temp) so close to the H that it was nearly useless.

I sent the gauge back to Howard's, who after three weeks, had not even bothered to get back to me.
I messaged Caerbont (Smiths) in Wales, who got back to me in less than an hour, to say that the sender unit was wrong. They messaged Howards who then sent me the correct TT4806-00 sender for the BT2204-11 gauge. (No apology!!!!!)
It gives a wider spread but I had to fit a 5 ohm resistor (actually two 10 ohm, 1 watt resistors in parallel) between the 7810 voltage regulator (10.14 volts) and the gauge, to move the 82 thermostat point to N on the gauge. The resistor has little effect (about 1%) on the cold readings, but lowers the hot readings by more (about 5%) as the sender resistance drops from 500 (cold) to 37 (at 100 degrees).The white paint marks are 100 degrees; the highest temp that I could calibrate the gauge. When I'm confident that it works perfectly I will fit it into the dash.
Thanks for all the help.


I took some IR photos that at least showed that the thermostat is working

Wow, love the photos. Doesn’t matter how useful they are in diagnosing, they just look good.

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ex-NSW Police 1970 MK II Cooper S
VMCI #43

Very cool pics. Amazes me that you get 5c difference within such a small distance. Almost every part of the head runs at a different temp.

I think even my mechanical gauge is 5-8c lower to what the max readings off my head were

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1966 Mini Deluxe - “Gabby” 34 years under restoration and counting...
1970 MGBGT
1959 MGA