Coolant Temp sensor

[jenkiam 0]

My dash temp gauge refuses to go much above the 70 degree min temp, so i suspect that the coolant sensor is faulty. But, i cant seem to see where the bl**dy thing is under the bonnet! Anyone got a engine bay pic of the sensor to help this old fool out?!

 

cheers

Jenks

[Dox 23]

Which engine?

[jenkiam 0]

doh! its the 2.9 VR6.

[g0ldf1ng3r 15]

19 minutes ago, jenkiam said:

My dash temp gauge refuses to go much above the 70 degree min temp, so i suspect that the coolant sensor is faulty. But, i cant seem to see where the bl**dy thing is under the bonnet! Anyone got a engine bay pic of the sensor to help this old fool out?!

 

cheers

Jenks

it could well be that it just has a very old thermostat in it

old stats would always sit at 70

whereas a more modern stat is designed to sit at 90 by the engineers

do you have any overheating issues?

[jenkiam 0]

nope, no issues apart from the fact the needle doesnt rise! I expected it to move into the middle of the gauge to indicate normal running.

[g0ldf1ng3r 15]

34 minutes ago, jenkiam said:

nope, no issues apart from the fact the needle doesnt rise! I expected it to move into the middle of the gauge to indicate normal running.

i wouldnt worry about it then m8

& id hedge my bets it is an old stat as i say

my 16v is the same, the stat is fine but the dash gauge always sits around 70

the VR6 dash gauge however sits at 90 & i know it was replaced with a new stat when it had a head rebuild.  prior to that work the gauge would sit at 70

ive been told that VW engineered the stats this way to reduce calls from general public about low temps

[1xshaunx1 27]

Thermostat might be stuck open. Does it take an age to get to 70

[fendervg 32]

It's not the stats they engineered, but the dash circuits - they evened out the fluctuations on more modern cars with voltage stabilisers etc. so as not to panic people. My VR will change quite a lot depending on being stuck in traffic or on the open road with plenty of airflow. The only difference with the stats would be the rated temperatures at which they open and close, but that could affect a lot of other things such as th engine not getting hot enough to go into normal fuel map.

The sensor is in a part of the thermostat housing, facing the front, often covered by a little black plastic flap - it is at the front of the block, to the right, and there will be three sensors in a row - I think you would be looking for a yellow one for the dash coolant gauge. If you look and follow some of the wiring from near the distributor or coil pack, three parts of the loom will go to this housing.

Checking that your existing stat is functioning properly or even better just replacing it is also a good idea - but a lot more pain that swapping out the senders!

The blue one is for the ECU coolant temp, and there will also be a black/brown for the fan controller. There's a thread on here with all the details, including the low temp senders and thermostats - found it:

 

Thermostat housing on the right - where you can see the holes for the 3 senders.

Edited March 7, 2022 by fendervg

[seanl82 23]

I have a new (well its about 6 years old now) stat and it still sits at 70 though. Its stamped on the thermostat what temp it opens at - usually 87. 

I'd first check if it raises above 70 when sat idling then drops down when cruising. As Shaun above says, if this along with taking an age to get to 70 is apparent, its likely thermostat.

[Philly-R6 0]

There are two coolant temperature sensors - one for the gauge in the dash and one for the ECU.

If you have VCDS see if you can see what temperature is being reported to the ECU? if they are similar the temperature is probably correct.

[Dox 23]

1 hour ago, fendervg said:

It's not the stats they engineered, but the dash circuits - they evened out the fluctuations on more modern cars with voltage stabilisers etc.

 

Thermostat housing on the right - where you can see the holes for the 3 senders.

MK1s and 2s had voltage stabilisers, if you didn't have them the temp and fuel gauges would read higher engine running than off due to higher voltage from the alternator, these gauges need a V S to deliver 9 / 10v constant.

OP

KEEP AWAY FROM THE RAD FANS WHEN CHECKING BELOW - LOOSE CLOTHING AND DIGITS TOO!

If the stat is stuck open or opening early you'll feel it by compareing top and bottom rad hose temps, same temps from cold stat stuck open, bottom cold until 70 degrees its opening early.

Use a thermometer to confirm the gauge is reading correctly

[fendervg 32]

Good safety advice there, and yeah, an IR thermometer is very handy to get spot temps in the engine bay.

In my experience the dash coolant temp gaiuges on cars of this era are just not that accurate, and it's easy to stress over them - if the sensors and wiring are old, they will also not read accurately. Using VCDS to check the temp signal going to the ECU from the blue sender works well, and then comparing to the dash gauge, but I've often seen a 20% discrepancy between the two, and also in the rate of change, so go figure. I think if you renew the coolant system and related components and make sure that your fans operate as described in the coolant thread, you should be fine.

I tend to think of the dash needle a more of an "oh sh*t, pull over" warning device rather than an accurate display of coolant temperature. The MFA oil temp is a lot more useful.