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Series III temperature gauge


Niko Saastamoinen
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I recently bought a Series III (-75 109" STW) for daily use (some of my friends did not understand this :blink: ). There has been lots of ugly things to fix due to the fact that there has been several "not so caring" owners during over 30 years :huh:

Yesterday I changed thermostat as the engine did not warm up well. Now the temperature rises nicely, but the temperature aggoring to the gauge looks pretty high. Otherwise everything seems to be perfectly fine (temperature does not rise from that, heater works fine etc.), so I guess it's harmless but annoying.

Hitsu-lampoa.jpg

Does anyone have a clue, why this happends?

And here's picture from my "Solihull collection" (RR -87 EFI & LR -75 SIII STW). :rolleyes:

Hitsu-IMG_4352.JPG

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did it do it before u changed the thermostat?

what temp thermostat did u fit?

Before changing the temperature was barely on the normal. And when driving on city in hot day it raised bit over mid poit. This made me think that the thermostat was stuck open.

Thermostat is 82 and the engine is 2.25 petrol.

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Before changing the temperature was barely on the normal. And when driving on city in hot day it raised bit over mid poit. This made me think that the thermostat was stuck open.

Thermostat is 82 and the engine is 2.25 petrol.

in that case i'd take the thermostat out and test it, just because its new doesn't mean its ok.

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Series temp gauge is correctly calibrated to show normal on a 74degree thermostat. Replace the 82 with a 74 and your temp gauge will rear correctly. It is correctly showing that the engine is running hotter.

Took me ages to suss this out when mine did exactly the same!!!

Jon

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Series temp gauge is correctly calibrated to show normal on a 74degree thermostat. Replace the 82 with a 74 and your temp gauge will rear correctly. It is correctly showing that the engine is running hotter.

Took me ages to suss this out when mine did exactly the same!!!

Jon

That's excellent. It's just the sort of thing that would happen to me.

Of course, I would have never figured it out and would have ended up having some ridiculous uber-cooling system built out of titanium/unobtainium, flubbed the installation and, finally, had to take it to one or t'other garage; only to have the old grease covered bloke break me the bad news that, yet again, I was an idiot.... :huh:

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You cant really put a resistor in it, as when the engine then really does overheat your gauge will still show normal! The series temp sender goes down to almost no resistance at all when it gets hot, so this is a bad idea. The only way to get a correct reading would be to either swap to an aftermarket gauage and sender unit, or to try one of the ex-military capillary type gauges.

Cheers

Jon

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  • 3 weeks later...

Right, I've just solved this on mine! On the back of the gauge cluster loose the "voltage conditioner" thats supplies "conditioned voltage" to the gauges. Replace with a 9V voltage regulator (mine was about 20p from RS). Mine temp gauge now reads smack in the middle of the gauge at normal operating temp, but if I short the sensor wire to ground the gauge still climbs to the end stops! Its also dropped the reading on my fuel gauge which is a bonus as that always over read too!

HTH

Jon

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No its "nominally" 10V. It consists of a bimetallic strip and set of contacts kind of like a set of points that open and close to give the nominal voltage. Horrible way of doing it. If you stick a voltmeter on it then you can see the voltage cutting in and out. I tried a 10V voltage regualtor first and it gave no change in the readings over the standard unit, but a 9v one gave the required drop.

Jon

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On my Landy I needed an oscilloscope to see the pulses, a digital voltmeter gave an unstable reading and an analogue meter just averaged the readings.

There are semiconductor regulators on the market, IIRC they have the same casing as the standard part but give a regulated output.

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  • 4 years later...

Few more questions.

1. So, if I want to replace an old bi-metallic type regulator with electronic one, should I opt for 1810 or 1809?

2. Speaking of engine temperature. How do I calibrate my temperature sensor on an old V8 engine? What's the normal working temperature for the engine and how do I measure it? Do I have to drain the cooling agent from the engine to remove the temperature sensor?

3. Tacho. Could anyone explain in detail how it works? According to the datagram, it seems to be connected to the same stabilized voltage source. I want to use analog-to-digital converter to log engine revs using my the car pc. Can I do that with the existing circuit (after voltage regulator replacement), or should I install my own sensors?

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My temperature gauge does not behave like that on my 109, and it has been fitted with 82o thermostats in a 2.25p and a 12J diesel, and now an 86o thermostat in a Tdi - the needle sits bang in the middle of the normal arc, and has done so with all three engines except when that have got too hot. My Lightweight's gauge indicates about one third of the way up the arc with its 74o thermostat.

You have a fault or a sender/gauge mismatch.

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  • 2 years later...

how can I test is my sensor is working? my gauge does nothing at all..

I read 68 Ohm on the sensor on a almost cooled down engine. I can trace this wire to the back of the gauge.

blue-green input to the gauge. I'd expect the green wire going to the top of the fuel and temp gauge to be the regulated 10~ volt... (will see if I can measure that from underneath the dash.....)

edit:

I hope you all don't mind i use the forum as a sort of notepad. Just in order to keep sharing (and not forgetting myself) what i've measured.

--

If i disconnect the cable from the sensor and place a 6Ohm resistance between that wire and mass the gauge comes all the way up.

sooo...

6 ohms reads hot (manual resistance)

68 ohms reads cold (sensor on engine)

2 options.

1. the sensor on the block is broken and should be lower than 68 ohms (didn't have a chance to measure on hot engine, but gauge doesn't come up....)

2. the thermostat is stuck open so that even driving 40 minutes in ~26 degrees outside the motor doesn't get to working temp...

How can I check my thermostat valve?

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how can I test is my sensor is working? my gauge does nothing at all..

I read 68 Ohm on the sensor on a almost cooled down engine. I can trace this wire to the back of the gauge.

blue-green input to the gauge. I'd expect the green wire going to the top of the fuel and temp gauge to be the regulated 10~ volt... (will see if I can measure that from underneath the dash.....)

edit:

I hope you all don't mind i use the forum as a sort of notepad. Just in order to keep sharing (and not forgetting myself) what i've measured.

--

If i disconnect the cable from the sensor and place a 6Ohm resistance between that wire and mass the gauge comes all the way up.

sooo...

6 ohms reads hot (manual resistance)

68 ohms reads cold (sensor on engine)

2 options.

1. the sensor on the block is broken and should be lower than 68 ohms (didn't have a chance to measure on hot engine, but gauge doesn't come up....)

2. the thermostat is stuck open so that even driving 40 minutes in ~26 degrees outside the motor doesn't get to working temp...

How can I check my thermostat valve?

From your description, it sounds like you have a sender fault, so just fit a genuine replacement (pattern parts are notoriously unreliable).

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