IAT (G42) Wiring Problem

[martinjs]

Can anyone tell me where the wiring for this sensor comes from and goes to, particularly the 2x white and black wires.
My ecu is throwing a short to ground code. I've had this problem for over a year now since i did an engine swap.
I've tried 3 different sensors. I've ruled this out as i never had this problem on my old engine.
I have tried running a fresh wire to ground and also one straight from the sensor to pin 44 then checking the temp in vag-com. The reading looks ok for the first 2 mins or so, -49deg at idle and rising when reving the engine then is jumps to the default 39deg and stays there.
The only other thing i noticed was that my after-run pump runs constantly when pin 44 was connected straight to the IAT sensor. This has never worked even when bridging the 2x connectors on the water manifold, i presumed it was the relay.
Im losing my patience with this now, its a pain to drive, you can feel the ecu retarding the timing and my mpg is ****.
Any suggestions what to do from here?

Thanks

[twoqu]

Quick stab! I beleieve its the after run pump that has the connctor with three wires! The air temp just has two, the signal and the earth! You haven't swopped those connectors have you?

[martinjs]

No i haven't swapped them. In vag-com it is showing that it is getting a signal from the sensor, the temp reading is changing as the throttle is blipped until it throws the code.

[twoqu]

Pretty sure there is no connection between the after run system and the engine ecu. After all it only works with the engine off! So whatever you are doing with that wire is at a guess is either earthing or powering the after run pump.
Can you get a pic of the connector to both the IAT and the after run pump, particularly the wiring.
As it happened after a engine swop, its pretty easy to mix things up, especailly when those components are close together.
Well have to check but I recall the after run pump having three wires to it. The IAT has only two, one sensor and one earth back to the ecu. Get the colour codes from the IAT wires while you are at it!

[twoqu]

Well i have cheked an S4 loom. I am assuming that what you have!
The after run pump has a black connector with two white/black wires and one brown/black. The IAT connector should be tan with a green/blue a brown/blue wire.

[martinjs]

I think you are right Robin, dont worry the person who did the engine swap will be getting both barrels.

[UrS4boy]

Quick stab! I believe its the after run pump that has the connector with three wires! The air temp just has two, the signal and the earth! You haven't swopped those connectors have you?

There are three 2 pin connector that could be possible to switch around by mistake. All three are possible on an AAN but only two are likely to be swappable on an ABY:

(Note: The following is based on an AAN, not sure about ABY wire colours):

1. The G42 intake air temp sensor (IAT) is a two pin. On an AAN, the wire colours are Brown/Blue from the T55/44 (ECU pin 44) and a Green/Blk wire that goes to ground (somewhere).

2. The N71 Idle speed control valve (ISV) is a two pin. It has a white/yellow wire from T55/4 (to control the ISV location) and a red power wire that comes through the S75 fuse.

3. The after-run pump. It has three wires but a 2 pin connector. A white/blk and a blk/white wire (presumably two 12V relayed power sources) are siamesed into one pin and a third wire, brown/black (presumably a ground) in the second pin.

On an AAN and I would guess on an ABY to swap the 2 pin connector on the ISV and the IAT. They are the same size and the wire lengths make it possible to make this minor, but important, mistake. It looks potentially possible to swap the after-run pump into the wrong position in an AAN but I doubt that it is physically possible in an ABY situation.

Hope that helps.

[twoqu]

Experts eh!
The after run pump connector is on the same branch of loom as the TPS, MFTS and the after run switch wires and the alternator blue wire where the IAT is a branch on its own, if that makes sense!

[UrS4boy]

Well i have cheked an S4 loom. I am assuming that what you have! The after run pump has a black connector with two white/black wires and one brown/black. The IAT connector should be tan with a green/blue a brown/blue wire.

I checked an AAN loom too. The one in my car.

You posted while I was outside checking and then checking Bentley. At least we confirm the AAN wire colours. I couldn't find the after-run pump in the Bentley (so far) but I can confirm the control of the pump has nothing to do with the ECU. http://forums.quattroworld.com/s4s6/msgs/21503.phtml

Dave F.

[martinjs]

There are three 2 pin connector that could be possible to switch around by mistake.All three are possible on an AAN but only two are likely to be swappable on an ABY:

(Note: The following is base on an AAN, not sure about ABY wire colours):

1. The G42 intake air temp sensor (IAT) is a two pin. On an AAN, the wire colours are Brown/Blue from the T55/44 (ECU pin 44) and a Green/Blk wire that goes to ground (somewhere).

2. The N71 Idle speed control valve (ISV) is a two pin. It has a white/yellow wire from T55/4 (to control the ISV location) and a red power wire that comes through the S75 fuse.

3. The after-run pump. I has three wires but a 2 pin connector. A white/blk and a blk/white wire (presumably two 12V relayed power sources) are siamesed into one pin and a third wire, brown/black (presumably a ground) in the second pin.

On an AAN and I would guess on an ABY to swap the 2 pin connector on the ISV and the IAT. They are the same size and the wire lengths make it possible to make this minor, but important, mistake. It looks potentially possible to swap the after-run pump into the wrong position in an AAN but I doubt that it is physically possible in an ABY situation.

Hope that helps.

Yes my wires are as you say Dave in points 1 and 3, just swapped them around and took it for a spin. It revs right through now but doesn't spool as fast for some reason, peaks at 1.3bar whereas before it was 1.5bar.
The loom is a bit stretched to reach the IAT now though. AAN btw.

[UrS4boy]

Yes my wires are as you say Dave in points 1 and 3, just swapped them around and took it for a spin. It revs right through now but doesn't spool as fast for some reason, peaks at 1.3bar whereas before it was 1.5bar.
The loom is a bit stretched to reach the IAT now though. AAN btw.

I might not be understanding you, but the intent of me and twoqu checking the IAT, ISV and After-run pump connections for wire colours was not so you would swap the connectors around but to confirm that you have the correct connector going to the correct device. Pull back the rubber boot on the connectors and confirm the wire colours and then plug the appropriate connector into the correct device.

[martinjs]

Yeah i know i did check the wire colours. Im not colour blind. Read my first post, its pretty obvious the after-run pump connector was in the wrong place. I realised this just after i posted post #3.

[UrS4boy]

Yeah i know i did check the wire colours. Im not colour blind. Read my first post, its pretty obvious the after-run pump connector was in the wrong place. I realised this just after i posted post #3.

Then I guess our help wasn't needed.

Happy you have it all sorted.

[martinjs]

The penny dropped after Robins second post.
TBH its an easy mistake to do on the AAN due to the close proximity of the IAT and after-run pump, the looms for both sensors are similar in length, actually on my car the IAT loom is slightly shorter yet it has further to travel
Admittedly i should have caught this earlier. Most if not all of my experience with these engines has been on S2's.

One to remember in future. Case closed. Thanks.

[twoqu]

Might be worth yanking the ecu fuse in case it has adapted somehow to using the after run pump as an IAT sensor!