Mfts/guage Testing

[twoqu]

As you may be aware I don't have a sense of humour!
I am not joking. The vehicle in question is an A6 2.5 Tdi 5 cyl C4. As its a 10V it has a flange that the MFTS is mounted in much like the 10V turbo models. This flange is mounted to the cylinder head with 2 bolts and a gasket.
The clue was in the C4 references. If it had been on an S2/S4/S6 I would have mentioned it. I was only trying to establish whether you were able to test the MFTS itself.
The MFTS function is the same on all vehicles fitted with one thats why I was trying to provide information that might be useful across the range. This vehicle was fitted with the 3 Pin MFTS from new as was probably the S6 as well.
The factory test only eliminated the MFTS but used a resistance value which we, until recently, were unaware of. Thats why I was interested in the possibility of testing the MFTS directly.
In principle I cannot see why the 4 pin MFTS will not exhibit the same characteristic although its ouput may differ. As it is earthed through whatever housing it is screwed into (flange/water manifold) it relies on that component to have a good contact with the cylinder head. In my case the flange was not adequately earthed to the cylinder head a somewhat unusual if not unique outcome!

[UrS4boy]

As you may be aware I don't have a sense of humour! I am not joking. The vehicle in question is an A6 2.5 Tdi 5 cyl C4. As its a 10V it has a flange that the MFTS is mounted in much like the 10V turbo models. This flange is mounted to the cylinder head with 2 bolts and a gasket.

Sorry since this is an "S" forum, I assumed 3B/AAN/ABY/AAN. My bad. I found a diagram of the 5 cylinder diesel that shows the mounting of the 034919369C MFTS as you describe.

[twoqu]

An S forum, hmm....
Anyhow, if you ignore what its screwed into the info is valid for S Cars (particularly S6 with 3 pin). If someone with a 4 pin MFTS can post anything to compare with that would be useful. A comparsion reading at operating temperature would be good which should be achievable measuring between the T pin and the MFTS body.

[twoqu]

I know from looking at the SJM site that the VDO 4 pin Mfts was a lot more complicated internally than the supposed replacement, the BEHR 3 pin. See here for the pics. You will need to scroll down to the Mfts section:

http://www.sjmautotechnik.com/trouble_shooting/cooling.html

I have always wondered why the 4 pin Mfts needed a 12V supply unless it is used by the circuitary to generate the signal, which we know to be a resistance, back to the gauge. On one vehicle I had, the fuse supplying the Mfts had blown and indeed I had no coolant reading. So it does seem to support that theory.
Therefore perhaps it incorrect to expect the 4 pin Mfts to have the same characteristic as the 3 pin Mfts. It could be that the 4 pin cannot be tested in the same why as I have with the 3 pin as it would have to be supplied with 12V.
Would be interested to hear any comments on that or indeed a better explanation of how the 4 pin Mfts functions.
Below are a few pics of the two types. There are subtle differences. The first being that the body of the 4 pin is a regular hex, the 3 pin has a more complicated shape. Then there is the differences in both the width and depth of the bulb and consequently the total depth to the sealing washer, which itself is different.
The bulb on the 4 pin is @9.6mm W X 8.4mm D whereas the 3 pin is @10.8mm W X 12.0mm D. The 4 pin has a depth to the sealing washer of 17.6mm wheras the 3 pin is 20.5mm. The sealing washer is larger on the 3 pin at 19.2mm whereas the 4 pin is 17.75mm. Hopefully the pics highlight some of the other differences!

[UrS4boy]

Robin: This 4 pin vs 3 pin info is very interesting. However, you have to remember that there are no more 4 pin MFTS in the Audi parts bins, only the 3 pin. If a 4 pin fails, it will have to be replaced with a 3 pin (unless you find a good used one).

I imagine that when VAG switched from 4 to 3 pin, they gave the engineers a design brief that said:

1. No 12 V power to the new sender
2. Maintain the same functions (gauge sender, etc) and characteristics (resistance vs temp) as the powered 4 pin version.

Personally, I trust that the engineers were successful in completing the new design.

(I could be wrong).

[twoqu]

Yea I know! Just like to understand exactly how it works. If the 4 pin needs a 12V suppy to output that would make it difficult to test off the car and would explain why they appear dead to the MM.
It does seem however that the BEHR 3 pin and subsequent, perhaps lesser quality replacement do cause problems with the readings which sort of implies they must vary in the output of the resistance. Its not an issue for a like for like replacement but seems to occur regularly when replacing a 4 pin with a 3 pin. at the same temperature.
There are many variables I guess.

[twoqu]

Here is a nice cutaway. Unfortunately it doesen't seem to resemble either of the variants!

[UrS4boy]

Here is a nice cutaway. Unfortunately it doesen't seem to resemble either of the variants!

Nice diagram. All I ever found is this diagram from the VAG111Motronic self-study guide for the 3B 200 20VT.

Note the phrase: "...is an NTC resistor and an electronic evaluator circuit". I presume that is was this "evaluator circuit" that needed the 12V input (to create a reference signal, I presume).

[twoqu]

The problem is it does not look anything like the guts of the ones SJM dismembered!
I can see how that would work though with the resistor picking up the temperature through the body. Also explains the lag.
Not 100% confident in the accuracy of these SSP's. It appears to show a 4 pin with the innards of the 3 pin!

[Sleeper_Avant]

did we work out of the 4 pin MFTS could be checked with a multi meter against a rough scale of resistance lads?

i am giving a sensor to a forum member but would like to check its credability first

Sean

[Thuppu]

Thread lift. Would the cluster water temp gauge work with a normal temp sensor, like the one in the back of the head? My mfts is failing again and I don't wan't to buy a new one as I don't need the other functions it has. A normal sensor will propably also last longer.

[Error404]

I want to find a normal temp sensor to replace the piece of ****ing **** MFTS.
3 new ones in 12 months and now getting false over-temp boost cut
I've disconnected that ****ing pin now but the gauge always reads low at speed.

[UrS4boy]

F76 Multifunction Temperature Switch R&R Info

I've disconnected that ****ing pin now but the gauge always reads low at speed.

Do you have the rubber boot over the connector and the exposed end of the MFTS? If not, the airflow pass the connector will cool the sensor (somewhat) and give you lower readings. PN = 4A0971876 = "grommet"

F76 Multifunction Temperature Switch R&R Info (with Cross Links to G62 Coolant Temp Sensor):

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

[Error404]

Yes

[bertman]

I am with you Alex on finding a way to get rid of this unreliable POS,I have been through 4 in the past year and all have either failed or given totally inaccurate readings leading to me to come to the conclusion that they are not worth bothering with.
The last MFTS I put in seems to be behaving itself-for now.I bought a batch of 4 from a supplier who advertises on this forum and have not been impressed by the way they work or my calls to have them replaced.
If an alternative could be found that would be awesome