Incorrect temperature readings

chrisgt

Active member
Joined
Apr 29, 2021
Location
Maine
TDI
1998 Jetta TDI
My coolant temp readings in vagcom are incorrect. After the car sits overnight, the report engine temperature is between 95C and 100C, this is clearly wrong.
The temp value slowly wanders up and down.

If I unplug the black 4-pin sensor on the top coolant hose that's right on the engine outlet, the temp reading slowly drops to about 15C and kind of wanders up and down slowly.

This behavior seems extremely weird to me, is it indicative of a sensor failure, or some problem in the wiring?
I have spent a good deal of time searching, but have been unable to find anything quite like this.

EDIT: I should also note that it doesn't matter if the engine is running or stopped (key on), the same thing happens. Also 98 jetta TDI.
 
Last edited:

chrisgt

Active member
Joined
Apr 29, 2021
Location
Maine
TDI
1998 Jetta TDI
It's been raining a lot so I looked to make sure no water got in the ECU, it's dry. Checked all the wiring between the sensor and the ECU, looks fine. I have no idea but a new sensor is really cheap from idparts... I'll just put a new one in there and see what happens ... I would expect a different behaviour for both a failed sensor and for unplugging the sensor, but I can't think of anything else it could be.
 

TooRoundTDI

Skunk Rocker
Joined
Jun 8, 2005
Location
Broad Brook, CT. I have vag-com, PM me.
TDI
1998 Jetta
Man that's weird but maybe there is some kind of buffer for the sensor in the ECU, even a simple filter cap that may be on its way out if it's electrolytic. Total guess but makes sense that it drops when unplugged but the wandering, no idea. Interested in seeing what it ends up being.

Iirc there's three temp sensors, fan, ecu and gauge maybe they share a circuit somewhere. Did it happen before you swapped the radiator sensor? I'll have to look at the Bentley unless I gave it up with the car and don't have another one.
 

Nevada_TDI

Top Post Dawg
Joined
Aug 17, 2008
Location
Reno, sort of...
TDI
2001 Jetta TDI
I recently had a CTS issue myself. The side that goes to the gauge died so I used Torque to keep a watch on the actual temperature as trying to look at Vag_Com can be dangerous. Right before I swapped the old CTS out, the gauge in the dash started going up and down quite erratically. At first the gauge went to the maximum right side whenever I turned the key, and then it went to the left, and then as I said above, back and forth. The night before last I installed the new one, and all seems to be well.
 

Steve Addy

Top Post Dawg
Joined
Aug 7, 2002
Location
Iowa
TDI
97 Mk3
My coolant temp readings in vagcom are incorrect. After the car sits overnight, the report engine temperature is between 95C and 100C, this is clearly wrong.
The temp value slowly wanders up and down.

If I unplug the black 4-pin sensor on the top coolant hose that's right on the engine outlet, the temp reading slowly drops to about 15C and kind of wanders up and down slowly.

This behavior seems extremely weird to me, is it indicative of a sensor failure, or some problem in the wiring?
I have spent a good deal of time searching, but have been unable to find anything quite like this.

EDIT: I should also note that it doesn't matter if the engine is running or stopped (key on), the same thing happens. Also 98 jetta TDI.
It was always my understanding that this particular sensor was for AC control in the event of overheating etc. It might also have been for fan control too, but I don't recall that specifically.

I would think it might be time to replace the cts that controls the gauge (and I think provides input to ECU for GP's too). I think that's the one in the flange that connects to the hose you're talking about. Replace or at least test it if possible, which means digging out the Bentley manual for that one maybe.

Steve
 

chrisgt

Active member
Joined
Apr 29, 2021
Location
Maine
TDI
1998 Jetta TDI
unplugging the 4-pin sensor seems to affect both the temp gauge and the ECU, it has two elements in it. And yes, it was reporting something like 140 degrees to the ECU and the A/C wouldn't work.
The GPs also wouldn't cycle when it was 30 degrees out this weekend, fortunately the TDI is an amazing motor and lights right off anyway :) The OM617 in my mercedes wouldn't do that...

Anyway, I got the new sensor and it fixes all the issues, everything back to normal.
The old sensor had a very hard sealing o-ring that was just barely leaking, the internal seal to the sensor was very hard and the thing had coolant in it (i cut it open). It was clocked in the hole with the element to the gauge up, so it had not yet shorted out, thus the gauge worked but the ECU was reading garbage.

I think what really threw me off here is the fact that unplugging the sensor didn't either rail high or low (depending on PTC or NTC), or go to some safe default.
 

Steve Addy

Top Post Dawg
Joined
Aug 7, 2002
Location
Iowa
TDI
97 Mk3
My coolant temp readings in vagcom are incorrect. After the car sits overnight, the report engine temperature is between 95C and 100C, this is clearly wrong.
The temp value slowly wanders up and down.

If I unplug the black 4-pin sensor on the top coolant hose that's right on the engine outlet, the temp reading slowly drops to about 15C and kind of wanders up and down slowly.

This behavior seems extremely weird to me, is it indicative of a sensor failure, or some problem in the wiring?
I have spent a good deal of time searching, but have been unable to find anything quite like this.

EDIT: I should also note that it doesn't matter if the engine is running or stopped (key on), the same thing happens. Also 98 jetta TDI.
I'm sorry, I misread the above and thought you were dealing with the sensor in the upper radiator hose, not the one in the flange.

IIRC there's a test procedure for it in the Bentley, if it passes then I'd be checking for continuity in the wiring harness and checking for any chafing that might have causes a problem in the wiring out near the sensor.

Steve
 

chrisgt

Active member
Joined
Apr 29, 2021
Location
Maine
TDI
1998 Jetta TDI
I'm sorry, I misread the above and thought you were dealing with the sensor in the upper radiator hose, not the one in the flange.

IIRC there's a test procedure for it in the Bentley, if it passes then I'd be checking for continuity in the wiring harness and checking for any chafing that might have causes a problem in the wiring out near the sensor.

Steve
I did check all the wiring to the ECU and it was fine, the check pointed to a bad sensor. A new one fixed it. In fact, an autopsy of the old one revealed it was filling up with coolant.
 
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