2.7T Reverse only works with MAF unplugged

[RRobertz]

Hello good people of Audizine forums. I have scoured the interwebs trying to
find an answer to this very specific problem.

I have a 2001 Allroad 2.7T. Its problem is that reverse will engage only for
a few seconds at a time. Then, as I'm rolling, reverse disengages and I roll to a
stop. If I select neutral and then reverse again, I get another 2 seconds of
reverse. This only happens when the engine is hot. And get this: If I unplug
/ disconnect the MAF plug, reverse is fine.

Has anyone had this weird issue?

Extra info:
I've checked voltages and there is 12 volt, 5 volt ref and ground and also a
~1.6V (I think it was) fluctuating on the signal pin at idle, which indicates
there is no break in the wiring. I've taken the TCU out from under the
passenger side carpet, and the unit was dry and no corrosion on the
connector. The car had the transmission rebuilt ~30.000 miles ago for
~3500usd with the reverse notorious clutch pack replaced. Reverse is strong
with MAF unplugged anyways. No fault codes when scanned with OBDII
scanner. All rear lights are functioning fine including back up lights. The
problem appeared after the car sat for 2 years, I bought it a couple of
months ago.

Thanks.

[Grisho]

Wow, pretty interesting challenge mate! When you say OBDII scanner, I assume you have not checked it with a VAG-COM or similar which can scan also intermittent and pretty much all codes which are not always in the regular OBD2 scan. Can you scan it with it?

[RRobertz]

It's a (real) ELM327 bluetooth module (not a cheap china clone) although it won't support VAG-COM, unfortunately.

As the previous owner had the cars sat for two years, the drain holes were clogged with leaves under the battery tray.
I figure maybe something else has short-circuited somewhere (as the TCU was dry).

- Are there drain holes any other places besides under the battery tray?

Running out of ideas here, but I think I will solder in switches to each of the 4 MAF plug wires and switch them off one by one
and see whether this will yield some sort of clue as to whether there is a fault in one of the wires or whether the MAF needs
to be completely out of the picture for the reverse to work, i.e. signal wire. The circuit is impaired somehow..

- Any insight into which ECU pin / where the signal wire from the MAF goes to? Not finding wiring diagrams online :/

[seanf86]

MAF unplugged the TCU requests full line pressure, reverse gear requires higher line pressure than other gears, I would start by checking the fluid and making sure the correct stuff is in there and its full and that the filter is sealed tight against the valve body.

These trans have some issues with pressure regulator valve if you say the reverse drum was already replaced I would look into that, there are repair kits that allow you to repair it yourself without too much hassle.

[RRobertz]

MAF unplugged the TCU requests full line pressure, reverse gear requires higher line pressure than other gears, I would start by checking the fluid and making sure the correct stuff is in there and its full and that the filter is sealed tight against the valve body.

These trans have some issues with pressure regulator valve if you say the reverse drum was already replaced I would look into that, there are repair kits that allow you to repair it yourself without too much hassle.

Thanks for that info! I'll definitely check my oil.

-You say the TCU requests full line pressure, is there then perhaps a wire going from the TCU which one can monitor/manipulate the voltage on to check whether this is the case?

[seanf86]

-You say the TCU requests full line pressure, is there then perhaps a wire going from the TCU which one can monitor/manipulate the voltage on to check whether this is the case?

Thanks for your info

You need vag com and figure out which measuring blocks to check in the TCU, with the maf unplugged you'll have a code in the TCU for unable to calculate load when the car can't calculate engine load full line pressure is applied to keep the trans from slipping.

I'm sure there is a pressure sensor in the valve body you can read with vag com, unfortunately I haven't had to dig too far in to my auto besides axle seals, fluid change and a torque converter so I can't tell you where to look, a Bentley manual may also have the information you're looking for.

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