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Coolant temp sensor


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I have my coolant temperature sensor (just for the dash gauge - the ECU sensor is elsewhere) installed using an adaptor like this.

The gauge on the dash stopped working recently, so I started by just swapping out the gauge but that made no difference. I also changed the sensor, initially for the wrong one (lets gloss over that) but I now have the correct Smiths sensor and I have tested it on the bench - it correctly reads around 500 ohms when cold and 40 ohms when at 90 degC or so. I also confirmed this when it was installed in the car.

However, my gauge is still not working - the needle moves a small amount now (just about registers, showing 40 degC when it should be 90). If I short the sensor wire to that earth on the adaptor, the gauge needle does shoot round to maximum, which tells me the gauge, the earth and the wiring is OK?

I do have PTFE tape on the sensor on that adaptor - could that be causing the problem? Any other ideas?

Edited by nelmo
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PTFE is a good electrical insulator so you have probably found the problem.

The question is why it ever worked, I am guessing that the original PTFE did not fully cover the thread leaving a bit to just about provide an earth.

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6 hours ago, ozz said:

If you have the sender in the top of the adapter you could try rotating the adapter so that the sender is at the bottom and therefore always has water around it.

It always worked before and I don't think I'd have the space to do that and it would probably leak.

3 hours ago, IanS said:

If removing the tape did not help check the earth wire from the adaptor is still connected both ends.

I tested the gauge by connecting the sensor wire to the earth on the adaptor and the needle went round to max temp, so that suggests it is a good earth, no?

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When I had your symptoms on my zero, the adapter was in my top radiator hose at a high point in the system and a bubble of air formed in the top of the adapter which meant that the low reading was the temperature of the air trapped in the adapter. try squeezing the tube before the adapter to get some water in there and see if the gauge begins to read. My solution was to turn the adapter 180 degrees so that the sender was at the bottom. If you have tried everything else it might be worth trying this.


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So squeezing pipes didn't help BUT while I was squeezing, I noticed that coolant was dripping out underneath. I knew I had a slight leak but I hadn't been able to find the source, until now, so that's good. Leak is just because the jubilee clip seems to have stripped it's thread and won't tighten enough, so replacement on order. 

And maybe the leak at that point is somehow affecting the sensor? It's not a huge leak, so seems unlikely but we'll see... 

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