Temp Gauge reading

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sandyman
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Temp Gauge reading

Post by sandyman »

I've got a 1.9 TD in my T25. Was wondering where the needle should normally be on the temp gauge, when engine is hot and running normally.

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Mocki
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Re: Temp Gauge reading

Post by Mocki »

on a factory fitted engine, according to vw, in the handbook, anywhere from 1/4 to 3/4 , usually a needles width either side of the red light , but frankly where-ever it sits normally is its normal position, any change is time to investigate
no idea with a alternative engine, rather depends on the resistance of the sender and wiring loom
Steve
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sandyman
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Re: Temp Gauge reading

Post by sandyman »

Thanks Mocki yeah I guess it depends on a few things. Thing is I've just had the head gasket done after overheating issue (guess why...yes because the temp gauge had never worked and water hose split!) and my mechanic had done a good job but said to keep an eye on the coolant level. Haven't had the opportunity to take it for a proper spin because need to do a couple of repairs to get it through MOT. But on a fairly brief run to the shops noticed the needle was creeping past red light- assuming if it had run more the fan would've come in but can't be sure. Do the MOT people leave engine running until fan belt comes on when doing the emissions test?
Anyway mechanic coming back soon to do work for MOT so after it passes should be able to give it a good run!

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ZsZ
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Re: Temp Gauge reading

Post by ZsZ »

The temp sender is a thermistor (heat dependent variable resistance) wired to measure resistance to ground. The needle shows this resistance.
The resistance values are different even in same partnr senders and there are a lot of different senders in different VWs, and the ground contact quality has a big impact on these values too. The voltage and the voltage regulator of the instrument cluster also can effect the position of the needle.
VW designed the temp gauge like the needle sits in the middle with the designed operating temperature of the coolant (regulated by the thermostat) but 1/4 range needle movement to each direction is normal as the coolant heats up a bit when you push hard on climbing to a hill and cools down if you feather down.
To have factory like coolant temp indication the main thing is to use T25 senders, T25 thermostat and have a good ground connection and good wire connection both at the engine bay and at the gauges.
To measure the exact coolant temperature to decide if it is in the designed range, you need a thermometer, preferably an analogue one.
Zoltan
1986 Multivan ex-Caravelle. Van since 2006, running mTDi 1Z since 2008 with Fiat Croma 1.9 TDid pump 2008-2019, custom pump since 2019
5spd custom box 4.57 diff + 0.74 5th

lefty67
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Re: Temp Gauge reading

Post by lefty67 »

To give you an idea, on my 1.9 the needle generally sits just before the light but Will rise to touch the light when going up a hill and will sit on the light if sitting in traffic and riding the clutch. When cruising at about 3200rpm with no load on the engine the needle can drop to just above the quarter position on the gauge.
Only had the light come on once when the fan didn't kick in after the van had been left running for ages on the mot ramp. Found the radiator was full of air and not coolant. I hadn't bled the system properly after a coolant change :)
1.9td AAZ

T25 - an adventure before dementia

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