temp gauge testing

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type53b
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temp gauge testing

Post by type53b »

My car seems to run a bit hot at the moment. The gauge is sitting about 3/4 of the way up after about 20 minutes driving but I am not fully convinced it is overheating. I fitted a new gti rad, flushed the cooling system, filled it up with g12 and as far as I can tell I bled it correctly. It doesn't lose or leak coolant. The level never varies. There is no oil in the coolant tank. Could it be, that she is at normal temperature but the gauge is reading high? If so how do I test the gauge and sensor ?


remember, if it's got tit's or tyre's, sooner or later it's gonna give you trouble.
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scrappy1382
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Re: temp gauge testing

Post by scrappy1382 »

hi matey, there is a alternative to your problem if you wish for your fan to cut in earlier ?

1 you can buy a thermostat with a lower open/close temp
2 a earlier temp switch rigged into your cooling system that allows your fan/fans to kick in earlier.

my storm always had weird temp readings including the oil.

i will send over the link to the earlier fan switch system

http://www.ebay.co.uk/itm/Mishimoto-Adj ... SwpLNX7gUC

something like this


Scirocco Shane
84 cosmos blue storm - mafia car choice 2015 rip :-(
84 Havanna Brown Storm - 8-)
mark1gls
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Re: temp gauge testing

Post by mark1gls »

The sender should have writing on the side with the temperature range on it, what does it say?

The gauge should sit at 1/2, when stuck in traffic rise to 3/4 then fan kicks in and lowers it back to 1/2.

Is your bottom radiator hose getting hot?

Try starting the car with the expansion tank cap off and see if there is a flow of coolant into the tank from the thin hose, it should increase the flow if the revs are increased.

What colour expansion tank cap do you have? If it's black change it for a blue one, you can have problems with the black caps.


Mk1 78 Scirocco GLS 1.6 FR, weekend toy.
Mk1 88 Golf GTi cabriolet 1.8 DX. Daily drive.
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ThomasD
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Re: temp gauge testing

Post by ThomasD »

type53b wrote:My car seems to run a bit hot at the moment. The gauge is sitting about 3/4 of the way up after about 20 minutes driving but I am not fully convinced it is overheating. I fitted a new gti rad, flushed the cooling system, filled it up with g12 and as far as I can tell I bled it correctly. It doesn't lose or leak coolant. The level never varies. There is no oil in the coolant tank. Could it be, that she is at normal temperature but the gauge is reading high? If so how do I test the gauge and sensor ?

The gauge can be tested. If you disconnect the wire at the sensor, and connect the wire to earth through a variable resistor, here is what you should see:
With a resistance of 40 ohms, you should be at the full hot side of the gauge.
With a resistance of 258 ohms you should be at the full cold side.
The service manual allows for one needle width of deviation from full hot or full code with these resistances.
These measurements are made with the engine running.

VW has a simple tester for this, but you can use also use a variable resistor or potentiometer. That being said, I'd be surprised if the gauge it bad. I think it would be more common for the gauge to not work at all. I'm not sure if the senders themselves go bad - again I would sort of think if they fail, they fail altogether. The service manual says if the gauge is good, but your reading is bad, replace the sender.


Mk2 Scirocco Storm
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type53b
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Re: temp gauge testing

Post by type53b »

Hi all, thank you for all the advice. I have figured out that the engine is running at normal temperature but the gauge is reading high. I done this by unplugging the wire off the oil temp sender at the top of the oil filter housing and connecting it to the water Temp sender and then reading the temp off the oil temp clock above the radio. I gave the car a good blast for 20 mins and it held steady at 90 degrees on the oil temp clock but read 3/4 on the water clock. I then left it idling for half an hour and the temp climbed up to 100 degrees on the oil clock, then the fan kicked in and dropped it back to 90. I kept swopping the wires back and forward to see what each gauge was reading and it stayed the same , oil gauge said 90 , water gauge said 3/4. The oil gauge read 100 ,the water gauge was nearly all the way to the top and the fan cut in every time and cooled her down to 90. Thank god!!! So the water gauge must be bogey.


remember, if it's got tit's or tyre's, sooner or later it's gonna give you trouble.
mark1gls
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Re: temp gauge testing

Post by mark1gls »

Could be you have the wrong sensor fitted there is a couple of types which look the same but have different readings-operating ranges.
What's on the side of your sender?


Mk1 78 Scirocco GLS 1.6 FR, weekend toy.
Mk1 88 Golf GTi cabriolet 1.8 DX. Daily drive.
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type53b
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Re: temp gauge testing

Post by type53b »

I will have to check tomorrow when I am back in work. The car is stored in a shed there. Should there be a temp range stamped on it?


remember, if it's got tit's or tyre's, sooner or later it's gonna give you trouble.
mark1gls
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Re: temp gauge testing

Post by mark1gls »

Yes there should be a stamp on it if it's a VW one, may not if aftermarket one. I think it should say 120 on it but I will check mine tomorrow to see what's stamped on mine.


Mk1 78 Scirocco GLS 1.6 FR, weekend toy.
Mk1 88 Golf GTi cabriolet 1.8 DX. Daily drive.
Membership No. 323
type53b
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Re: temp gauge testing

Post by type53b »

Ok thanks mate!


remember, if it's got tit's or tyre's, sooner or later it's gonna give you trouble.
mk2daz
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Re: temp gauge testing

Post by mk2daz »

Take out the temp gauge sender plug, bridge the two pins on the plug together with a jumper wire then turn ignition on, if gauge goes to max then your gauge is working fine and it's most likely an incorrect sensor as has been said above :)


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type53b
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Re: temp gauge testing

Post by type53b »

I don't have a 2 pin connection on the temp gauge sender. It is just the single nail head connection with one spade terminal slid onto it. I already took this off and grounded it off the engine block and the needle went straight to the top of the gauge. Is this the same thing???


remember, if it's got tit's or tyre's, sooner or later it's gonna give you trouble.
mark1gls
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Re: temp gauge testing

Post by mark1gls »

My VW sensor has 12v and 125 written on it, it does have a part number but it's at the back near the head and I can't really read it!


Mk1 78 Scirocco GLS 1.6 FR, weekend toy.
Mk1 88 Golf GTi cabriolet 1.8 DX. Daily drive.
Membership No. 323
mark1gls
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Re: temp gauge testing

Post by mark1gls »



Mk1 78 Scirocco GLS 1.6 FR, weekend toy.
Mk1 88 Golf GTi cabriolet 1.8 DX. Daily drive.
Membership No. 323
type53b
Posts: 175
Joined: Tue Jul 07, 2009 11:12 pm
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Re: temp gauge testing

Post by type53b »

I can't decipher what is written on mine . I think I will have to remove it and clean it up to figure it out.


remember, if it's got tit's or tyre's, sooner or later it's gonna give you trouble.
msh209
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Re: temp gauge testing

Post by msh209 »

Some info about those two different (but similar) temperature sender sensors mentioned in a post here.

I just had to replace mine to fix my temp gauge and of course I ended up with the wrong part causing plenty of confusion.

Wrong sensor: The sensor with the blue cap 025-906-041/A has a resistance of about 400 ohms at 80 deg C and when fitted showed overheating when the car clearly was not overheating.

Correct sensor: The one with the black cap 251-919-501/D has a resistance of about 130 ohms at 80 deg C and works fine on my scirocco. It's clearly the correct one but for some reason the parts catalogue for VW at the auto shop points towards the blue one.

It seems that the higher resistance makes the dial on the temp gauge move more to the hot side, I guess it must just put a larger voltage across the dial

I took readings for both of these sensors throughout the temperature range with a saucepan/multimeter and the black 251-919-501/D has roughly 0.5-0.6 of the resistance of the blue one over 20-90 deg C temp range. But great it works on my car :pilot:

Hope this is useful to someone. BTW My car is a 1992 mk2 and I got the working one from Heritage car parts and its listed as :Coolant Temperature Sensor, Black/Green 2 Pin 20mm: T25 1985-1991, Golf Mk2 1988-1991, T4 1991-2003. I've only been working on the scirocco for a while but it seems to me that VW use different parts for the same thing quite a lot. I wouldnt be surprised if the blue cap one was correct for some sciroccos. The Haynes manual wasnt mush help on this either.

BW