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The temperature gauge on the P38 has been going up into the red area on the gauge with the light coming on. The Nanocom shows the temperature at 80 degrees.

I've put a new sensor in and have the same result.

Resistance is .758 Kohms at 18 degrees and 58 ohms at 80 degrees

Voltage is 4.39 at 18 degrees and .2.85V at 80 degrees

There is the same voltage at pin 19 on the BECM so continuity is OK

Should I be looking at the BECM, the wiring or the gauge as the next suspect or are those readings off?

Many thanks as always.

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Just been out and tried on the Ascot which is early enough to have the same single connector sensor as you have. At 11 degrees (according to the HEVAC), the resistance of the sensor is 0.908 kOhms and the gauge, as you would expect, reads cold. Unplugged the wire from the sensor and connected a 62 Ohm (the closest I could find in my box of assorted resistors) between the alternator body, which should give a decent ground and the connector. With the ignition on the gauge read just below the mid point. Which suggests it isn't the sensor but something odd happening in the AD conversion within the BeCM or the DA conversion back in the instrument cluster.

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Thanks Richard. I suppose I could try a secondhand cluster to eliminate that.

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If you've got one. I've got one that I swapped the circuit board over as the driver chips for some of the warning lights died so I had no oil pressure light (amongst others). Just be careful though as swapping the whole cluster can affect the recorded mileage.

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Just had a thought. If you've got 758 Ohms when cold and 58 when hot, presumably the gauge reads correctly when cold? In that case, if you were to put a resistor in series with the sensor, it should bring the gauge reading down. I'd start with something like a 200 Ohm resistor (so giving 258 in total) and see what the gauge reads then.

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That's inspired Richard. I'll get down to the resistor shop in the morning and have a play.

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So, with a collection of resistors I got these results.

disconnecting the green sensor wire and sending direct to earth via a 22 ohm resistor the gauge read high with light on.
With a 68 ohm resistor the gauge read 1/4 to 1/2 full
With a 120 ohm resistor the gauge read just above cold.

Whilst providing a direct earth to the body of the sensor via the alternator body and reconnecting the wire it took a lot longer for the gauge to hit the highest point. Maybe why the later sensors were two pin.

I'm going to pick up a genuine LR sensor tomorrow having had a little lie down after seeing the cost to see if there is any benefit.

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Checking the earth path from the sender to the battery there was a 6.1 ohm resistance. Remade the joint between the chassis earth and the earth cable to te alternator and it's looking like it has sorted it.

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Nice one, I suspect you are correct and that is why they went over to the two pin sensor on later cars. Wouldn't have expected 6.1 Ohms to have made that much difference though.

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You are probably right Richard. I took it out to get the tracking done and the gauge still wavers about too high relative to the Nanocom readings.

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Mine, with the 2 pin sensor, also reads differently to how I would expect it to. It has always sat very slightly above vertical, suggesting the temperature is higher than the Nanocom (and two other generic OBD readers) tell me it actually is. My sensor died so I kept the Nano plugged in to keep an eye on the temperature only to find that it runs very cool, around 85 degrees C. As they are no longer available, I got a second hand one from a breaker and fitted that. Initially it read below the centre point, exactly where I would expect it to be with the lower running temperature. Over a period though it has crept up and now sits where it always used to, very slightly higher than vertical. On a long downhill when the temperature would drop, it does, but under normal running, it stays just above what would be considered as normal despite the actual temperature being below normal. Considering the analogue voltage from the sensor doesn't drive the gauge directly, but reports to the BeCM which sends a data stream to the instrument cluster which then converts it back to an analogue signal to drive the gauge, I suspect there is some sort of adjustment going on so the gauge reads normal irrespective of actual running temperature.

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If cleaning up the earth sorted it (at least of awhile) then I would look at either swapping the sensor for the later 2 wire one, and adding in an earth wire to maybe one of the other sensor ground wires in the loom (or even back to the battery or engine ecu directly), or somehow add a ground to the original sensor and again ground that back to somewhere reliable.

The temp gauge is heavily damped so I presume that it will not wander around the centre of the gauge when driving normally and the thermostat opens/closes etc, and I remember when mine got super hot and blew the heater core that it stayed central for a LONG time and then suddenly shot up into the red before it went pop.

There was a known issue with the 1 wire temp gauges and their reliability and I think the remedy was to upgrade to the 2 wire version.

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Apologies for the delayed response Marty.

I keep coming back to the temperature gauge like a loose tooth.

Running it up again yesterday the Nanocom sat steadily at 91 degrees and the gauge sat central for a few minutes and would then edge up a notch and then come down again for a minute and then edge up a bit more next time and then edge back down but less until it finally got into the red.

Testing the temperature gauge in diagnsotics it would happily go to low mid and high points when asked. Checking the fuel gauge though got no response at all.

I was thinking that my original solution of finding a higher than normal resistance in the earth from the alternator to the chassis was flawed as the resistance drops the higher the temeprature. It should therefore read low rather than high. Then I thought that maye it was just a hot spot around the sensor and I hadn't got all the air out but then it is next to the other temperature sensor so that doesn't really make sense.

I'll give the two wire sensor a try.

Thanks for all the advice.

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Hi,
I have a similar issue, my temp gauge is also fluctuating around quite a bit and mostly shows cold when the Nanocom shows a steady 89 C.
Cleaned up the earths an connenctions but didn´t make it better.
But since the 2-wire sensor AMR5929 does not seem to be available anywhere at all and any similar replacement I can find here in Germany has metric threads, I decided to try the older 1-wire Sensor AMR 1425, which works great. Fits physically and shows correct readings.
I connected the earth from the 2-wire plug to an earth by the engine and the other to the sensor, but at the moment only with small crocodile test leads to see if that works.
Does anyone know where I can get the 1-wire plug from for that sensor ? I couln´t find it anywhere and looks like a smaller version of a spark plug connector....

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It's just a push-on bullet connector so should be easy enough to find. Something like this https://www.autoelectricsupplies.co.uk/product/31/category/6 is what you need although you could probably even use one of these https://www.autoelectricsupplies.co.uk/product/58.

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Thanks, I found one of these and they fit well.
https://postimg.cc/rKNV3s0S

Now it looks like this, found a ground nearby and protected the wires with fabriccoated fuel hose...
https://postimg.cc/nMQMRnGk

Edit: Inserted different new links to the photos that now work....