Water temp sensor.

for those with Series MM sidevalve cars produced between September 1948 and February 1953
Johnno
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Water temp sensor.

Postby Johnno » Thu Nov 05, 2020 8:44 am

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.
Power to your Pistons,Johnnydoome.

geoberni
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Re: Water temp sensor.

Postby geoberni » Thu Nov 05, 2020 9:18 am

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. :-?
Basil the 1955 series II

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Johnno
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Re: Water temp sensor.

Postby Johnno » Thu Nov 05, 2020 9:58 am

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.
Power to your Pistons,Johnnydoome.

geoberni
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Re: Water temp sensor.

Postby geoberni » Thu Nov 05, 2020 12:43 pm

Johnno wrote:
Thu Nov 05, 2020 9:58 am
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) :wink:
Basil the 1955 series II

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Johnno
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Re: Water temp sensor.

Postby Johnno » Thu Nov 05, 2020 12:55 pm

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

geoberni
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Re: Water temp sensor.

Postby geoberni » Thu Nov 05, 2020 1:47 pm

Johnno wrote:
Thu Nov 05, 2020 12:55 pm
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'?
Basil the 1955 series II

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Johnno
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Re: Water temp sensor.

Postby Johnno » Thu Nov 05, 2020 2:16 pm

The ignition switch lead to opposite side of gauge.
Power to your Pistons,Johnnydoome.

geoberni
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Re: Water temp sensor.

Postby geoberni » Thu Nov 05, 2020 4:47 pm

Johnno wrote:
Thu Nov 05, 2020 2:16 pm
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. :roll:
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 :evil: If there were, you should have got no more than 1 or 2 volt reading, depending on the degree of short circuit.
Basil the 1955 series II

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JOWETTJAVELIN
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Re: Water temp sensor.

Postby JOWETTJAVELIN » Thu Nov 05, 2020 6:15 pm

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.
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Basil

geoberni
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Re: Water temp sensor.

Postby geoberni » Thu Nov 05, 2020 7:24 pm

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.
Basil the 1955 series II

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