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The new 3 pins sensors earth through the sensor/water manifold/........
Could explain a lot of the issues people have had.
So sort the water manifold earthing, or add and earth to the sensor body?
Not quite!
The 4 pin had a 12V supply which was omitted from the 3 pin. They all earth through whatever they are screwed into. However as that is usually something that is attached to the engine there is scope for it not to be earthed as well!
As a matter of interest, what make was the sender ?
I've been having trouble with the gauge reading since the original 4 pin sender died.
Meyle sender - gauge reading 110 degrees once warm
Behr sender - sits on 90 degrees at idle. Once on the move it drops down. New thermostat didn't make any difference
I have used an ancient s/h OE BEHR one being a cheapskate! Wasn't really sure what the problem was and was unwilling to goose a decent new one. The p/n will be 034 919 369C.
The 4 pin ones I have are made by VDO.
I'm going to do some testing. Not sure if there is a bad earth or a dodgy sender. Pretty sure the Meyle senders are just crap. The fuel economy has dropped so much it's worthwhile getting another sender.
As prj mentioned the MFTS has (mostly) nothing to do with the ECU and the engine. The temp sender that the ECU "listens" to is the G62 at the back of the head, as discussed here:
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There does seem to be a lot of reports of 3 pin replacements of a 4 pin causing over reading. I have also heard that 4 pin ones do not work in cars originally equiped with 3 pin!
I checked whatever information I could find regarding the VAG 1301 settings for the gauge calibration on vehicles fitted with a MFTS. The only exception to the 560/58 settings was the T44 where the setting was 600/50! Whether that is down to a difference in the cluster circuitry or a different output from what would be a 4 pin MFTS is debatable. It looked like most early to mid 90's Audi models(100/A6/80) using a MFTS had the same settings so I assumed that as the 3 pin was a mid 90's introduction that the temperature/resistance output were all the same.
I don't think that over reading would be earth related. It would have the opposite effect. Taking my example if the gauge needs 130 ohms to read just below 90C, mine was registering over 300 Ohms which is not adequate to reach the 288 ohms needed to register 50C.
It relatively easy to test up to say 90C in the kitchen which is as close to operating temperatue as we can simulate. I got about 140 ohms against the 130 ohms on the vehicle. I am still not clear why when I tested with a 50 ohms resistor I got a similar reading.
Since fitting the Behr sender I've lost about 60 miles per tank.
As the other have said, the MFTS should have no effect on fueling as it is used to control the temperature gauge and the other pins are for overheat warnings.
Might be worth using Vagcom to monitor engine temperature in case its the actual coolant sensor that is at fault.
Since fitting the Behr sender I've lost about 60 miles per tank.
Beg to differ all you want, a simple glance at the electrical wiring diagram of the car will show that the MFTS has little to do with the ECU apart from the thermal switch used for the boost cutoff in ABY at high temp, which does not have any effect on fuel economy (on contrary, it would improve it, if it was faulty).
There may be something affecting your fuel economy, but the MFTS sure as hell is not the culprit.
Update.
Well it does seem that it was the condition of the bolts/washers/housing that were the problem.
After cleaning them up and checking the resistance was the same from the MFTS T terminal and the housing and the engine itself, the gauge now indicates water temperature.
There was no sealant on the bolts but the washers were pretty grubby and the housing had some corrosion.
Moral of the story is not to overlook the little things.
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Probably thats its on a C4 model you didn't get over there!
Obviously the water manifold is attached to the cylinder head by bolts, so in theory, however unlikely the possibility still exists of a bad contact to earth. I doubt whether its much of an issue of vehicles with O rings as there will be direct contact between the two components. If there is a gasket then it might be an issue.
I was trying to make the information generic rather than specific.
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