Mfts/guage Testing

[twoqu]

Having finally (after 6 years!) getting round to solving an inoperative coolant gauge I was wondering if the way I tested the MFTS for a signal was not valid.
In my case the gauge itself was dead. The actual wire had detatched itself from the bolt that is used for the signal. These wires are very thin and appear to be glued on the top of the two bolts that are used to attach the gauge to the cluster. If you cannot get a resistance of @50 ohms across the bolts then there is a problem. I replaced the guts of the gauge with a brand new ebay find.
I had checked for continuity from the T terminal to the gauge cluster so the only other component in the system would be the actual MFTS. I had previously replaced the MFTS with another unknown one but of course with the gauge being damaged it would not have registered anyway.
Having decypted the VAG 1301 setting I was confident that simulating a resistance of @38 ohms should put the gauge between the two dots at the hot end of the scale and a resistance of @288 ohms should put it between the dots at the cold end.
Unfortunately I bought some 38K (0000's) resistors in error so could only find a 50 ohms resistor to use. This put the gauge at @90 C!
I decided to use a small jar and some boiling water to see if it was possible to measure the resistance of the MFTS. By contecting the MM to terminal T and the other probe to the threaded part of the MFTS I seemed to get a reading which was plausible. It was in the 40 ohms range when hot and when removed from the hot water it steadily decreased, which is consistant with how it should work.
So I was quite dissapointed to find that after installing it I still had a gauge that did not function! Perhaps I killed it installing it as it was the original. Or was I actually doing a valid test on it in the first place?

[UrS4boy]

F76 Multifunction Temperature Switch (MFTS) R&R Info

There might be some info here to help you:

http://forums.quattroworld.com/s4s6/msgs/22584.phtml

[twoqu]

There might be some info here to help you:

http://forums.quattroworld.com/s4s6/msgs/22584.phtml

Not really!
But feel free to update your info with some facts from mine. Just wanted to know whether testing resistance between the MFTS T terminal and the threaded part is a valid test. Guess I need one that works to try.

[UrS4boy]

Not really! But feel free to update your info with some facts from mine. Just wanted to know whether testing resistance between the MFTS T terminal and the threaded part is a valid test. Guess I need one that works to try.

Do you have access to a Bentley Manual? For the AAN, in Volume 3, pages. W90-58, -59 and -60, there is the following (my comments in parentheses)

"Connect the instrument/gauge tester VW 1301 to the T terminal of the thermal switch connector (not the thermal switch).

-Set the tester VW 1301 to 560 (ohms?). Switch the ignition ON.

- Coolant temperature gauge needle must lie with in the tolerance range - arrows "A" (shows just off zero to the end of the tight grouping of hash marks on the lower end of the gauge - which would be between the two lower sets of dots in the diagram below)

-Set VW 1301 to 58 (ohms?)

- Coolant temperature gauge needle must lie within tolerance range - arrows "B"
(which the two upper sets of dots around 110 C in the diagram below).

- Using a jumper wire, connect terminal C of the thermal switch connector to ground.
- Start engine
- Warning symbol must be displayed after approximately 30 seconds (warning chime sounds).

MFTS pin-out - note positions for the four pin harness connector.



Ignore the arrows for this case (they point to the normal operating range)

[Error404]

IIRC that is not the Ohms setting its a weird VAG code setting for the tester unit. I saw a website a while back where someone had worked out the actually ohms range that the codes corresponded to I'll have a look if i can find it

[Error404]

From memory I don't think this is it, but has some similar info

[andy10v]

IIRC that is not the Ohms setting its a weird VAG code setting for the tester unit. I saw a website a while back where someone had worked out the actually ohms range that the codes corresponded to I'll have a look if i can find it

That would be awesome, I have a few now and got them mixed up

Its annoying to test by fitting, and difficult if I am trying to trouble shoot another issue anyway as I think part of my loom may be fooked

[twoqu]

Again unfortunately that is of no help. Its basically the test for eliminating the MFTS completely and I am interested in actually testing the MFTS itself.
Additionally the setting on the VAG 1301 is not the actual ohms it puts out but a reference value perculiar to it. If you actually read my thread I state the actual ohms resistance need to actually to the test above as you are unlikely to see a VAG 1301 whereas you may be able to rig up a resistor of a particular value. The issue has always been what the setting on the VAG 1301 corresponded to.
Having mistakenly bought the wrong resistors to get the gauge into the higher calibration (38 ohms), I used a resistor of 50 ohms which equates to the 90 C range on the gauge. The lower the resistance to the gauge, the higher in temperature the reading.
As the VAG 1301 basically provides a fixed resistance at a particular setting between one connector and earth it duplicates the output of the MFTS at a particular temperature. Therefore I assume that the MFTS is effectively earthed through its threads to the engine and the resistance is transmitted through the terminal T back to the gauge.
But can you test that resistance out of the car by submerging the sensor element into very hot water and measuring between the terminal and the threaded portion? I thought I had and was therefore confident that the MFTS I replaced functioned, yet on the vehicle it does not function. I know the rest of the circuit is sound as I have tested it with the resistor.

[twoqu]

IIRC that is not the Ohms setting its a weird VAG code setting for the tester unit. I saw a website a while back where someone had worked out the actually ohms range that the codes corresponded to I'll have a look if i can find it

http://www.s2forum.com/forum/showthread.php?t=62503

[Error404]

I see, makes sense now
Well on the wiring schematic it shows it as earthing to the block

[twoqu]

Well I have just boiled another one and that looks dead as well!
Maybe the one I got a reading from was ok until I refitted it which illustrates how tempermental they are! Basically if you use near boiling water to test you should see a resistance of between 50 ohms @ 90C or less if the water is hotter.
The other functions would be difficult to check without the ability to heat a liquid up to @120C (cooking oil?) to see if the switches close. Again these can only close between the teminal and earth, which must be the threaded part of the MFTS!

[UrS4boy]

Again unfortunately that is of no help. Its basically the test for eliminating the MFTS completely and I am interested in actually testing the MFTS itself..

I am so sorry for attempting to help. I'll try not to do it again.

Good luck finding the info you need.

[twoqu]

I am so sorry for attempting to help. I'll try not to do it again.

Good luck finding the info you need.

Dave, can't you see that throwing information at people is not necessarily helpful however well intentioned. I have asked a specific question. I have researched the issue and have not found the answer anywhere and that includes all the (usual suspect) online resources.
There is to the best of my knowledge no definitive test for the MFTS function other than eliminating it and subsituting a fixed hitherto unknown resistance to the gauge.
If nothing else at least I have provided that information for others to use. If I can get the correct value resistors (which is not that easy) I could verify that they do indeed correspond to the calibration marks on the gauge. That test would be valid for the S2 and other C4 models as the VAG 1301 settings are the same.
For my own piece of mind I would like to make sure the MFTS works off the vehicle.

[Nuvo]

Why don't you use a linear potentiometer to create the desired resistance values for testing purposes. Just adjust the variable resistor to the value you want using a DMM.

If the MFTS is showing the expected results out of circuit using the boling water method you describe but not functioning when reinstalled in the car I would suspect a problem with either pin corrosion on the connector or a wire break somewhere.

[UrS4boy]

Dave, can't you see that throwing information at people is not necessarily helpful however well intentioned.

*IF* you take the time to re-read the info I provided *and* mesh it with the Bentley VW1301 info link that you provided (now "503"'d ), and reverse your thinking, I believe that you have your answer as to what resistances should be produced by the MFTS to generate a given temp signal.