Multi-Function Temperature Sensor (MFTS) Diagnostic

[Marc Boucher]

What would be a diagnostic procedure to differentiate between the MFTS, the 
gauge, and the thermostat, without removing the thermostat and or MFTS?

The sjmautotechnik website gives a procedure for testing the gauge: "unplug 
the connector from the sensor and ground the corresponding terminal 1/T 
(BR/Y wire) in the connector, the coolant temperature gauge in the 
instrument cluster should go to a max temp reading with the ign. key on". 
Since the gauge moves quite a bit, this test probably wouldn't be much help.

The other tests described on that site are specific to turbo models (MFST 
malfunction causing ECU to reduce boost).  Since I've got a NF engine, 
non-turbo, its not applicable.

I've got my doubts that the thermostat could be the cause since the heating 
is blowing hot air and the oil temperature gauge is just under 100 when the 
guage usually drops to zero.

I'm wondering if there's a test whereby you test the ohms across the MFTS 
(or the mA through it) as it heats up to see if there's a dead spot 
somewhere.



--------------------------------------------------
From: "Huw Powell" <audi at humanspeakers.com>
Sent: Friday, November 25, 2011 3:49 AM
To: "Marc Boucher" <mboucher70 at hotmail.com>
Cc: <quattro at audifans.com>
Subject: Re: Multi-Function Temperature Sensor (MFTS)  Diagnostic

>
>
> On 11/25/2011 3:04 AM, Marc Boucher wrote:
>> Could anyone recommend a diagnostic procedure on the Multi-Function 
>> Temperature Sensor (1990 Audi 100 NF engine)?
>>
>> My temperature gauge steadily rises as the car warms up but at some point 
>> it reverses, heads back to cold and stays there for 5-10 minutes.  At 
>> some point when the engine reaches operating temperature it once again 
>> rises until it hits the normal range, and then it usually (possibly 
>> always?)stays there for the duration of the drive.
>
> Wild shot, but could be thermostat issue and the sensor/gauge are fine?
>
>> I realize that the MFTS is a failure point, but its possible that its 
>> also the temperature gauge or the wiring.  Thus if there's a diagnostic 
>> that I can do before replacing it, I'd like to try.
>>
>> Should replacement be necessary, does it simply unscrew from the bottom 
>> of the coolant outlet, or is it necessary/easier to remove the coolant 
>> outlet first?
>
> Yes, unplug, unscrew, replace, reconnect.
>
>> Thanks,
>> MC
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>