TF Coolant temperature gauge

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Fraser
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TF Coolant temperature gauge

Post by Fraser » Sat Sep 02, 2017 8:33 pm

Well we're just back after a lovely day out in Lancashire and what used to be Westmoreland. Top down on the car, (after we'd got off the M6 which had a massive traffic jam near Preston), a nice lunch at the village of Brindle. We finally ended up at Arnside, very picturesque and had a cup of tea.

It was on approaching Silverdale on our return journey that I noticed the coolant temperature light was on, (red), and looking at the gauge, it showed maximum, which on my TF is 130C. Yet the car was driving perfectly normally, no symptoms of overheating. A brief stop showed coolant level correct, with not signs of overheating. The oil temperature was at its usual position, (about 100). Very odd ! So before joining the M6 for the run south to Sandbach, I checked again. Everything looked normal so off we went. A break at Forton showed nothing untoward so we made a fast run back home with the gauge still on maximum.

So my question is - are these cars prone to failure of the coolant temperature gauge, and what component fails ? I would suspect the thermistor, but with modern cars it may be something else.

Fraser
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Re: TF Coolant temperature gauge

Post by Fraser » Sun Sep 03, 2017 1:13 pm

Update
Switched on ignition today with a cold engine, and gauge showed three bars (its normal reading when hot). Started car and after a few seconds the gauge showed max temperature. So clearly something has failed. As the engine runs perfectly OK, the coolant sensor for the ECU is OK, so I am assuming the sensor for the temperature gauge is what has failed. As this sensor can only be accessed from underneath, it looks like a trip to my local mechanic.

What a pain !!

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talkingcars
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Re: TF Coolant temperature gauge

Post by talkingcars » Sun Sep 03, 2017 4:35 pm

The blue coolant temp sensor for the gauge is accessible from above with the engine cover removed, I changed mine a few years ago.

If you are quick you can swap one out and the other in with very little loss of coolant thus avoiding the need to bleed the system.
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Fraser
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Re: TF Coolant temperature gauge

Post by Fraser » Sun Sep 03, 2017 10:28 pm

Thanks for this info. I suspected the engine cover had to come off. I assume by blue, you mean the colour of the plastic socket where the wiring connects to it.

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Charless
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Re: TF Coolant temperature gauge

Post by Charless » Mon Sep 04, 2017 7:46 am

Yes - early ones had blue which was later replaced by black but the sensor range is just the same.

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talkingcars
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Re: TF Coolant temperature gauge

Post by talkingcars » Mon Sep 04, 2017 9:43 pm

Fraser wrote:I assume by blue, you mean the colour of the plastic socket where the wiring connects to it.
The gauge sensor has a blue plastic cover.
It has a plug with one wire.

The ECU sensor has 2 wires and is either brown or black depending upon which ECU is fitted.
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AlexBarwell
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Re: TF Coolant temperature gauge

Post by AlexBarwell » Wed Sep 13, 2017 1:25 pm

Yes, single wire blue plug and makes earth connection through the engine. ECU temp is 2-wire brown plug, both off the same housing. Plugs are referred to as Junior Power Timer, however, the ECU plug is special with an offset keyway that is hard to source.
Had a similar issue on my '04 reg 160 after previous owner had been taking it to lobotomized grease monkeys. From ignition on, all fans would then decide to kick in regardless of temperature, and could here ECU staying a bit chokey. Temp gauge varied from nothing to normal - found the gauge plug had slipped out as the monkeys had pinged the retaining clip off, and the ECU temp wires were badly strained when they did the clutch slave (which wasn't a problem - it was the clutch shaft that was stiffening a bit).
For both they are temperature dependent resistors, so should get a significant change in resistance from room temp to when immersed in boiling water, all good, but as these sensors are in the vicinity of moisture, condensation/moisture can form on the contacts and give a bad connection, so silicone grease (not rubber compound!)to displace water on the contacts and a good jiggle - lovely! Emmissions were perfect.
Had it before on another car where a weak sensor connection could be interpreted as 'failed' so ECU may fail-safe and run the fans regardless.

Fraser
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Re: TF Coolant temperature gauge

Post by Fraser » Tue Sep 26, 2017 5:10 pm

Well, bought a new sensor off Rimmers, and had it fitted yesterday. Gauge now working ! I checked the resistance of the new sensor, and this was about 50 ohms when cold. The old one had almost no resistance when I checked it after removal.

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