120k 05 tdi passat possible CAN problems

I just acquired an 05 that has some serious electrical ossues. The cluster is lit up like a Christmas tree and of course the performane is suffering. The tranny module and ecu aren't communicating regularly. the problem is imtermittent to and seems to be good warm /bad cold day. I normally don't buy cars like this but have a complete 05 donor that everything worked right electrical wise before I robbed the tranny and it's just sitting. My question is what would be the logical steps to get this car back to good electrical health? I'll check the connection pins on all the modules and ecu first and check grounds but then what? Car hasn't been in a flood either. Way too nice and low mileage car to junk. Engine sounds good.

hookup the vag-com clear all your codes and see what returns, check for water under the passenger seat pull the carpet

The carpet is out of the car-previous owner replaced the tcu before he gave up on it. I will connect VAG to get a baseline reading. I have a spare convenience module also. These cars have reliable electronics for the most part from my experience so I'm hoping the problem will be obvious like a ground.

This car does have a wiring problem. Should I start at the ecu and check all the connections? The guy I got it from said the tcu was wet and he replaced it. My bet is when the tcu got wet, it's connection may also now be corroded. I've plugged 2 known good tcu's in and same fault:
18034 - Powertrain Data Bus
P1626 - 000 - Missing Message from TCU
Freeze Frame:
RPM: 0 /min
Torque: 204.0 Nm
Speed: 0.0 km/h
Load: 0.0 %
Voltage: 12.01 V
Bin. Bits: 00001000
Bin. Bits: 00000000
Bin. Bits: 10000100

P1626 is theft deterrent fuel enable. Doesn't make sense as car always starts

you will fine some corroded wires in the harness and pay particular attention to splices and grounds

I need a wiring diagram of the gateway between the ecu and the tcm. Surely there's a fuse somewhere protecting the tcm. I see no obvious corrosion anywhere.

Would be good to get a baseline of what on the powertrain CAN is being recognized in live data. ECM, CAN Gateway, Cluster and a couple others should have it viewable in MVB 125. If it's actively talking to a particular module, it will have a 1 next to it, 0 if not currently recognized.

That having been said, I've also experienced a few ABS modules absolutely scramble communication on the powertrain CAN before. If you don't have an oscilloscope to see what the CAN high and low signals look like, using that live data is the next best thing.

Never hurts to at least unplug the ABS module and see if communication to the TCM restores. I know that sounds a little backwards, but it's a little easier than tracing wires at least and again, ABS modules seem to be the most common cause of powertrain CAN communication crashes.

The final solution to this one was the TCU. Prior owner had snipped the passenger side sunroof drain duckbill a little to aggressively and pulled the grommet loose from the hard plastic A pillar drain. With a little fishing with a .1 weedwhacker string, I was able to get it back on. No more floorboard water. He cooked 2 TCU's before I found a third and plugged it in and no more TCU to ecu communication errors.

If you open up the tcm harness about 12" you will find a splice with 3 brown wires(grounds) solder those to eliminate ground problem. they are not visible without cutting the harness wrap. In the floor area and kickouts are several ground trees, steel stud, brass eyelets, aluminum acorn nuts, perfect electrolysis nests. clean and apply no-alox to prevent further trouble. Although you may think you have fixed it now, these items will continue to cause problems unless addressed. I bought a flood car, up to mid cluster high water mark, fixed it and drove for 5 years.

It seems that keeping both the sunroof drains free and clear of debris as well as the cowl drains under the battery/booster area are key to keep a B5 platform car happy and healthy. These are things I routinely do to my own and countless customers' cars that come in here.

I actually just finished replacing [yet another] EA888 engine, this one in a 2012 Q5 which is based on the Audi A4, so not that much different in general layout than our B5 Passats, and its cowl area was PACKED with tree debris. And I mean packed, maple tree whirlybirds, acorns, sticks, twigs, seeds, leaves, you name it, it was down in there. Nasty rotting smell, too.

Thanks for the help guys. thundershorts , I did remove and thoroughly clean those ground straps. Long live the B5.5 tdi's. Far and away the best tdi platform ever! I have four and just bought a manual wagon for FIVE.