Author Topic: How to test a blue temp sender  (Read 5422 times)

Offline PeteG40

  • Administrators
  • *****
  • Posts: 4008
  • Housewives heart throb
How to test a blue temp sender
« on: March 12, 2008, 10:36:13 pm »
Here's something I got off the polo gt owners club back in the day and i keep it in my multimeter just in case.

These things can go at any time and to my  knowledge some have even been faulty as new from VAG. Most are marked with a date (such as 10.90) on the rim. If they go it can mean excess fuel consumption and rough running.

It can be found on top of the thermostat housing to the right of the head with a blue electrical plug.

Simply measure resistance across the two poles
3.2k ohms - 4.2k ohms at 10 degrees
2.1k ohms - 2.9k ohms at 20 degrees
1k ohms - 1.4k ohms at 40 degrees
520 ohms - 670 ohms at 60 degrees
280 ohms - 370 ohms at 80 degrees

If faulty you could chance a second hand one but they are about £10 new from VW. Part number 025 906 041 A and N 903 168 02 for the seal. Be sure to make sure your car is cool or you might get covered in hot coolant!!!

Offline Andy

  • Traders
  • *****
  • Posts: 1495
Re: How to test a blue temp sender
« Reply #1 on: March 14, 2008, 09:00:10 am »
Worth adding that on G40s the air temp sender (pins 2 & 3 on CO pot) seems to use the same temp vs resistance coefficient.

Here are some measured values from the senders I measured up for my Emerald install, both are standard Bosch curves:

Coolant temp
12.2kΩ @ -7°C
2.46kΩ @ 20°C
220Ω @ 95°C

Air temp
11.68kΩ @ -7°C
2.56kΩ @ 20°C
220Ω @ 95°C

Offline supercharged spaniel

  • Members
  • *****
  • Posts: 1034
  • GRRRRRRR
Re: Re: How to test a blue temp sender
« Reply #2 on: March 14, 2008, 10:37:13 am »
with the CO pot.  how do you know what to adjust it to? mines been set at 550 ohms but its obvioulsy adjustable so is it worth adjusting dependant on car?  and what characteristics determine how its set?

Offline hayesey

  • Administrators
  • *****
  • Posts: 5683
  • The CV joint killer
    • ClubG40
Re: Re: Re: How to test a blue temp sender
« Reply #3 on: March 14, 2008, 12:34:55 pm »
needs to be set at idle with a gas analyser in the exhaust, set it so the emissions are OK.

Offline supercharged spaniel

  • Members
  • *****
  • Posts: 1034
  • GRRRRRRR
Re: Re: Re: Re: How to test a blue temp sender
« Reply #4 on: March 14, 2008, 03:03:14 pm »
[quote pid=362 author=hayesey date=1205498095]needs to be set at idle with a gas analyser in the exhaust, set it so the emissions are OK.[/quote]

ah right.  thanks for that.

Offline dannyb.G40

  • Members
  • *****
  • Posts: 126
Re: Re: Re: Re: Re: How to test a blue temp sender
« Reply #5 on: May 04, 2008, 10:44:31 pm »
With regards to testing:

Does the engine need to be running?
How do you know how hot the coolant is?
Does the sensor have to be removed to get to the poles?

(I study an electronics module at uni and still dont understand how it works). Any help much appreciated as usual!

Offline Alexiskayak_7

  • Members
  • *****
  • Posts: 625
Re: Re: Re: Re: Re: Re: How to test a blue temp sender
« Reply #6 on: May 04, 2008, 11:45:06 pm »
Look... I personally take off the sensor and rapidly tap the hole so coolant won't run off. Then i take the sensor i put it in a cup of water and you start to boil the water (while you have placed a thermometer in the water and off course the ohmmeter  in the sensor) and you start to count


In the other hand you can start your engine put a thermometer at the coolant pot, then you take of the sensors plug and you start counting...


The 2nd way is much more easier but the engine's Air/Fuel Ratio runs a little bit rich...

Offline Andy

  • Traders
  • *****
  • Posts: 1495
Re: Re: Re: Re: Re: Re: Re: How to test a blue temp sender
« Reply #7 on: May 07, 2008, 06:34:49 pm »
[quote pid=1199 author=dannyb.G40 date=1209937471]With regards to testing:

Does the engine need to be running?
How do you know how hot the coolant is?
Does the sensor have to be removed to get to the poles?

(I study an electronics module at uni and still dont understand how it works). Any help much appreciated as usual![/quote]
To get the figures I quoted above I removed the sensors from the car, plugged in some flying leads to them, and stuck them in to a bag with a type K thermocouple.

I measured the resistance first at room temp (20°C), then stuck them in the freezer for a few hours to give them time to soak (-7°C). After that I stuck them in a collander above some boiling water (95°C). I nicked Yoof's multimeter with a type K thermocouple input so I could measure the temperature as well as the resistance.

The above method's not hugely accurate - but the sensors vary hugely and the tolerances on the input are so large that it doesn't matter. It's more than sufficient to see if you sensors are working or not.