Water temp sensor.

[Johnno]

Hi all,help needed again!
Trying to fit Smiths water temp gauge to my ‘52.Now tried two sensors but neither work,seems I need to get a Smiths sensor? Can anyone help with this?
Thanks,John.

[geoberni]

A 'Smiths' Electrical Temp Gauge (or any other brand) circuit is basically a gauge mechanism that responds to a variable resistor to 'earth'.
You don't need to know how the gauge works, just that the change in resistance of the 'Sensor' with temperature will change where the gauge displays.

Have you got a good earth on the sensor?
Have you tried connecting the sensor wire to a good earth to see if the gauge responds?

There can be a mismatch between different sensors and gauge that will mean the reading isn't correct, but you should have something reading.

[Johnno]

Thanks for reply.
Sensor is correctly earthed and gauge goes to start point when ignition is on. As engine warms no other movement from gauge.

[geoberni]

and gauge goes to start point when ignition is on.

Really? As soon as you turn on?
That's not something I've ever seen before. In my experience, Temp gauges only move once the temp increases (modern car electronic ones excepted).

I'm wondering if it's connected correctly...

Have you got a multimeter? If not you really should get one for dealing with any electrical problems.

Ignition ON, disconnect the cable from the sensor and check the voltage. Should be battery voltage minus about 1/4 to 1/2 a volt.
Change to resistance and check Sensor connection to Earth, it's probably in the region of 1.6K ohm.
(Check the other sensor while you're at it and see if it's the same.)

Start the car and when it's warmed up for a couple of minutes check the sensor reading again, it should have dropped.

How old is the gauge?
Is it new or a similar vintage to the car?
Is it an actual Smiths one? (just making sure you're not using the name generically)

[Johnno]

Everything checks out and yes,it is genuine old Smiths.Tried swapping leads with same result,ie,goes to 30 on gauge.

[geoberni]

Everything checks out and yes,it is genuine old Smiths.Tried swapping leads with same result,ie,goes to 30 on gauge.

So I'm assuming that by 'everything checks out' you're saying that

You had a nominal 11.5 v at the connection to the sensor
The Sensor was around 1.6K ohm

I don't get why the gauge is going to 30 as soon as you switch on. The needle shouldn't move until the Bi-Metal strip inside has heated up for at least 45-60 seconds, even for that low a reading.
I wonder if there is some sort of short circuit within the gauge.

What exactly do you mean by 'Tried Swapping leads'?

[Johnno]

The ignition switch lead to opposite side of gauge.

[geoberni]

The ignition switch lead to opposite side of gauge.

That won't make a difference. it's just a wire wound heater warming up a Bi-metal strip.
I just don't understand why you are getting an immediate deflection of the needle as soon as you turn the IGN on. It doesn't make sense.
It's basically impossible.
These gauges require the Bi-Metal strip to warm up to move the needle.

LIGHT BULB MOMENT
The only thing I can think of is you have 2 problems :
A) a short to earth on the cable from the gauge to the sender, so as soon as you turn the IGN on, the gauge is warming up instantly and should be going full scale,
HOWEVER....
B) The gauge is so badly adjusted, 30 is the most deflection you can get on the needle

See here for the insides of a Smiths thermal gauge and an explanation of how it works. Water, Oil, Fuel, they're all the same design.
http://www.mgb-stuff.org.uk/fuel.htm

There is one flaw to the above Light Bulb Moment.
If you did check that there was some 11+volts on the lead you'd taken off the sender, then there can't be a short to earth If there were, you should have got no more than 1 or 2 volt reading, depending on the degree of short circuit.

[JOWETTJAVELIN]

It's also worth remembering that the electric gauges are polarity conscious, and some require a voltage stabiliser, so you would have to establish the type before condemning the temperature transmitter.

If you've established that, have wired it correctly and still have a fault, this is what you want.

https://www.ebay.co.uk/itm/SMITHS-TEMPE ... Sw925fGdEo

The reproduction ones just don't seem to work. The other option you have is to fit a thermometer with a capillary tube which would be more in keeping with a sidevalve car, and generally reliable.

[geoberni]

The basic Smiths gauges of the mid 60s onwards are NOT Polarity Conscious. They are just a heater element around a Bi-metal strip. They'll heat up no matter which way they are connected.

However, not knowing a lot about the earlier gauges I've done some research and pre-64 the gauges were apparently magnetic, not Thermally operated.

See this Mini Thread from a few years ago. The guy's initial problem is almost exactly as John describes.
http://www.theminiforum.co.uk/forums/to ... -problems/

it illustrates the back of the 2 different gauge types. So if John's doesn't have the 2 small holes and does have the 2 extra nuts, it's not compatible with latter sender.
But I dispute the need for a Voltage stabilizer. My Temp gauge is supplied straight from a fuse and doesn't suffer from fluctuating needle. Perhaps I'm just lucky.

[Johnno]

Ok,sorry for delay in replying but have a dose of c19.Over it now.
My gauge has the two adjusting nuts on the back so I am assuming I need an old sensor? Tried everything else,just will not work.John.

[geoberni]

Hi
I'm glad you've recovered, hopefully without any of the long lasting after-effects that some seem to suffer from.

If you have the 2 Nuts type, I'm afraid I've no personal experience of that one, only what I read on that link to the Mini Forum.

mga gas gauge.JPG (17.87 KiB) Viewed 3220 times

I'd never even heard of a magnetic temp gauge until I saw it mentioned on that forum, and had to look up how they worked.
Info here if anyone's interested.http://mgaguru.com/mgtech/electric/fg_01.htm


Unless anyone else has some reliable info, I would contact a supplier such as Bull Motive Mini Spares.
Their website lists 3 different Senders, covering 3 time periods, 59-63, 64-69 and 69-94.
https://www.minispares.net/product/all- ... 102/GTR102

I did note on one part of that Mini Forum Chat, one reply included

The early electric temperature gauges are not common so I am sure you could easily sell this gauge and use the money to buy a matching later gauge and sender.

I personally don't see the relevance to the voltage stabiliser for the Temp gauge.
They introduced the VS on latter Minors for the fuel gauge, which is fine, because it stops the fuel gauge bouncing around as the float bounces in the tank, but that's not really an issue with the Bi-Metal temp gauge, that is much slower to respond to the thermal effect of a changing thermistor.
I've got a latter gauge, the one they claim needs to have the VS and it works fine in Basil.

Your earlier comment about the gauge

everything checks out

doesn't hold up against the complexity of the early magnetic gauge. It's a complicated item and just because it gets some deflection, doesn't mean its serviceable.
It may just be your gauge is Duff; you could try some of the checks mentioned on the first link above, even if it is written about the equivalent fuel gauge. The readings may be a bit different, but it would give you an idea if all 3 coils in the gauge were intact.

[Johnno]

Thanks for your reply.I’ll have a look at Bull.
Lost two stone in five weeks,and it shows.Otherwise just tired all
the time.Thanks.