An interesting problem. Any ideas?

I hadn't thought of a chip. The two options I had considered were a reducing adaptor screwed into the cylinder head with the temperature sender supplied with the gauge screwed into the smaller female thread, or else a resistance bridge. I had got as far as measuring the current draw of the gauge (which varies a bit depending on engine speed and peaks at 50ma) but I hadn't got round to the Ohms Law calculations for the resistors.

The chip you identified has a fixed voltage output so is even better. It is also cheap enough to try as the first option, and I am pretty sure I have the bypass stability capacitors that the datasheet PDF recommends, somewhere in my handy bits box. The minimum input voltage of 13V should be OK with the engine running (my car still has its original dynamo!), and realistically I am not going to be interested in the gauge until it is. Anyway, I have still got the reducing adaptor option to keep in reserve for Plan B if the voltage drop doesn't fix the problem.

Thanks for your help.

Jim

Well ...

I thought the reducing adaptor thing might be a good idea anyway, but as you didnt seem too sure about either the male or female thread (I suspect they are probably BSF / BSW) I thought maybe the silicon solution would be easiest.

If you think the voltage is going to stray below 13v (I am familiar with dynamos) for any significant amount of time, then try using a fat electrolytic capacitor on the 10 volt side of it to buy yourself a bit of stability - fatness of capacitor obviously limited by available charge current through the regulator.

just out of interest .... I am assuming that your moggy is one of the models WITH a water pump ?

Happy Hunting

The main cause of a voltage continuously below 13V would be having headlights on in stop/start traffic, and I don't normally drive the car after dark. Parked unused for several days the resting voltage of the battery is likely to be 12.something V and climb to over 13V once I start driving.

I think the tapping in the head is an NPT thread, though I can't find any reference to what it should be. I found some pictures:

I did send for both the voltage regulator and the NPT reducer because they were both cheap enough to treat as disposable if I use one or the other successfully, though if I need both at least I won't have to wait for a new delivery.

It is a 1965 Moggy which I bought 19 years ago, so relatively late in the production series and it does retain the original dynamo and drum brakes. I can't see the point in choosing a classic car and then modernising it. The only modernising update I have done to it is to convert it from the positive Earth it had when I bought it, to negative Earth so that I could use a SatNav plugged into a cigarette lighter socket that I installed to power it. Negative Earth from the factory arrived only a couple of years after mine was built, so changing polarity was only an anticipatory alteration.

Now I wait for deliveries.

Jim

I changed my Mk2 Jag to neg earth mainly so that I could put a half decent radio in it, but never done anything that you could call a performance upgrade.

I did a bit more digging and apparently there is an electrical characteristic called a "sacrificial anode" where one piece of metal replaces the losses from another piece of metal in the same circuit, depending on polarity. A positive earth car sacrifices its body. A negative earth car gains tiny amounts of metal into the body from the other electrics in the car. So having changed polarity, we should both suffer a bit less from tinworm.

The other thing I looked up was what temperature sensor was available for a classic mini. Having found one that looked in its picture like the one I have in my cylinder head, I read the "suitable for" information and it said that it was suitable for A series engines with a voltage controller fitted. It looks as though your recommended chip, when it arrives, should solve the problem of my gauge reading too high.

Jim

To bring the saga to a completion, Having ordered both the reducer and the 1oV chip, the reducer arrived first. I had guessed at the description based on the measurement across the callipers put over the thread of the sensor in the head after I unscrewed a bit, then matching that size against a table of sizes I found on the internet to get the relevant Nominal Size to order. When it arrived the internal thread fitted my smaller sender, but when I put my callipers (which I had left set to the measurement of the thread on the existing sensor) on the item which had just arrived, the new arrival was wider than the callipers. That indicated that it wasn't going to fit the thread in the block. The reducer was cheap, so it wasn't worth sending back.

I did find another reducer online that might have fitted, but it was 3 times the price of the one I had just received. So I decided to test the sensor that came with the gauge to see if it might be worth it. I wired it up so that I could test it and also use the wire to keep the contact clear of the boiling water I was going to test it with. Obviously it would be cooling and I would have to delay reading the gauge until the hot water had brought the sensor up to its temperature. When I then tested the reading it did show on the gauge, but at about

The voltage control chips arrived a couple of days later. Physically they were a bit smaller than I had imagined. Among my handy electronics bits I did find a 16V 0.1mF capacitor. However the only 0.33mF capacitor I could find was a 400V job and it was enormous compared to the voltage regulator. As the capacitors were called optional I just used the 16V one. I found I had a circuit board with components on it that I had probably kept to rescue the components. The hole spacings matched the separation of the voltage controller connections. There was a section of the board which was under some jumper wires and had no components, so I cut out a bit and soldered the voltage controller to the tracks. Then I soldered in the capacitor, and finally I soldered in a length of 2-core (red and blue) for the input and output and a length of single core black for the earth connection.

In the car, I needed a screw to mount the chip on and the only one small enough was one that would give access to the internals of the heater, so I insulated my little connection board and the terminals of the voltage controller and when I screwed the item into place on the heater it fixed the insulation into position too. I then wired it in: Black to an earthing screw, red to the 12V ignition controlled supply that had previously fed the gauge and would now feed the voltage controller, and blue to feed the gauge with 10V instead of the previous 12V.

Then I started the engine and watched the temperature gauge gradually climb. When it had climbed as far as it was going to climb I took a note of the reading (in Fahrenheit) and when I converted that to Celsius it was almost exactly at the 85C where the handbook said the thermostat would be fully open. Job done!

Thanks for the suggestion of the voltage control chip. It turned out to be exactly what I needed.

Jim

another satisfied customer :-)

I assume "0.1mF" actually means o.1 microFarad ?

or 0.1 microFarad

Yes it did. I had no idea how to create the "mu" character.

I also do that now and again. Did you notice my 1oV?

You can copy µ from the MS Word symbol library - but the old trick of using ALT codes (ALT + 0181 in this case) doesnt seen to work any more

and its nowhere to be seen on the keyboard

doh .... yes it DOES - but only if you use the numeric keypad on the right !

and ... by the time you get there, it will have morphed in to ALT + 230 !!

I don't think I will bother. As someone without Word installed who is unlikely to need mu again for ages, I think I will stick with "m" because after all, you knew what I meant from the context.

I am impressed by your persistence though, leading to finding the answer.

Jim