DSG voltage reading ~4v intermittently - drops to Neutral while driving

[paulw91]

Howdy,

About a month ago my 2014 Multivan (DQ500 DSG) started occasionally dropping into Neutral while driving (revs climb and gear indicator on dash flashed on and off) and then it resumed driving after a few seconds. (Sometimes it would flash the spanner icon and I would roll to a stop and wait 5-20 minutes and it would come good).

As time went on, the issues became more frequent, and continued to be intermittent with seemingly no external factor influencing when it would trigger.

After a lot of diagnosis, it appears that somehow the "Always On Power (Terminal 30)" is intermittently dropping, we could watch it live and see voltage at 13volts, and then suddenly it would be sitting at 3v, climb to 4v, slowly move up to 6, and sit there for some time, with seemingly no external factors influencing this (although it could possibly relate to the car getting warmer/cooler over time).

There were times where I could drive for 45 minutes on the highway with no issues, then there are times from cold where you could drive for 2 minutes and the problem appeared.

Ultimately our understanding is concluding that it is either some sort of wiring issue, or possibly internal issue in the TCM (Transmission Control Module).

We replaced all the fuses, and also one of the relays under the battery, however no changes were observed.
My mechanic has wired up a bypass to have voltage go directly to the pin on the Mechatronics unit from the battery, and the issue still seems to persist.
We have also replaced the internal contactor PCB, as well as all the solenoids/valves inside, with no change.

The strange thing is that this data is being read out of the TCM - the TCM itself is telling us the voltage from the Always On Power is wandering around at lower voltages (3-6v) and then randomly normal at 13-14v.
The Switched-Ingition-On (Terminal15) voltage is reading a constant 14v in line with the battery.

All other modules in the car are working and no voltage issues reported. The battery is about 4 months old and appears normal, engine cranking speed is excellent etc. so I don't believe there is any sort of general power issue in the vehicle, just something specific to the DSG.

As far as we know, you can't just buy a new TCM, you have to replace the whole Mechatronics unit which has a cost price of around $4100, so I guess we want to be sure we have done all the low-cost items first, before leaping to this.

Seeking any other ideas on this, or particularly any insights into seeing wandering voltages like this occur and what could cause that...

At this point I feel the only option left is replace the mech which is quite expensive... perhaps I should get a 2nd hand mech, however it's likely it will have it's own issues

I know on the older mechatronics units/TCM's that internal repairs are now being done by workshops where solder joints failed, wondering if this could be a thing on the DQ500.

Open for any ideas and thoughts, many thanks in advance!!



Codes listed below from VCDS:

Address 02: Auto Trans (J743)       Labels:. 0BH-927-711.clb
   Part No SW: 0BT 300 045 P    HW: 0BH 927 711 C
   Component: DSG DQ500-7F  H07 2302  
   Revision: 00H07V01    CVN: D80E7252
   Coding: 0014


10810 - Function Restricted due to Insufficient Voltage 
          U1400 00 [00101100] - -


10821 - System Voltage 
          P0562 00 [00101100] - Too Low


10907 - Main Pressure Valve 
          P179C 00 [00101100] - Electrical Malfunction


10906 - Cooling Oil Valve 
          P179D 00 [00101111] - Electrical Malfunction


                Address 02: Auto Trans  (0BT 300 045 P)


10:02:29
  IDE00018   Voltage terminal 15 14.2 V
  IDE00019   Voltage terminal 30 4.9 V

[Syncrotek]

Hi,
Depending on where VCDS takes voltage value from, it would seem the terminal 30 voltage supply relay is ok, otherwise there would be grumblings from other controllers, you might well be on the right assumption about bad pin connections or similar, it brings to mind Greg Roles recent ABS unit repair. Would be too difficult to measure voltage losses across those pins while in operation, so would have to inspect carefully with a magnifying lens, & checking for looseness of the connections, assuming the J743 control unit is accessible enough. If you could get hold of a diagram it would help to identify the terminal 30 connection.
There is a 2 terminal connector, and a 16 terminal connector, I don't have a full pinout, maybe YouTube ?
Fuse SD24 - black/blue to 16pin connector T16m/pin 15
Fuse SD5 - red/yellow to 16pin connector T16m/pin 9
(Also to selector lever)
My phone has no way to post a pic in this reply, (pic of connector diagram).

[Syncrotek]

This is the connector pic



A - mechatronic unit
B - 16 pin connector
c - 2 pin connector

Not much info in these posts, but might give you a start.

Edit to add: I should remind to not cross connect (jump) any connections, and to not probe with positive or
negative any of the connections unless you are very sure of what you are doing, when measuring use a
high impedance DMM, or oscilloscope, avoid using old type test lights.

[DV52]

Syncrotek : Hi - good response to this odd problem!

I'm not sure what you mean by the "terminal 30 voltage supply relay"- but then I'm not familiar with how a Multivan is wired. Does this car have a T30 relay - or are you referring to the T15 relay (J329) that energizes the T15 rail voltage when the ignition key is moved from the OFF position?

Don

[Syncrotek]

Hi,

Terminal 30 relay J317 in E box under battery, supplies voltage to many fuses, it is switched on (neg) by engine control unit.
in the diagnostic "IDE00019 Voltage terminal 30 4.9 V " - terminal 30 relay switches battery voltage, should be more than 4.9V, but that 4.9V is being read from where ? If you can measure at connector T16m pin 15 you will get a better idea. (engine running).
You could check for any nearby earth connection as well, the earth terminal at the T16m is pin 16 as far as I can make out, that should be a brown wire.
My notes are a bit limited. I don't have diagram for the J329 Terminal 15 relay, which is under the dash fuses, but usually all engine controls are in the Ebox as I remember.

[MIG]

If the voltage has been tested at the connector and the grounds checked on the transmission, this could be a rare failure (I have not heard of it in the DQ500s) on the bond wires on the TCU board.

There are repair places that specialise in mechatronics repairs; if all other on-car tests don't prove conclusive, the unit can be tested. Otherwise, a TCU\Mech could be sourced and cloned.


Here are the bond wires on the DQ200. I could not find any pictures of the DQ500 open, as they rarely have issues.

[paulw91]

Thanks for all the replies!

This relay we tried swapping out with a new one, but it made no difference.


It seems possible to replicate the problem by grabbing the bunch of wires from the 16 pin connector and pressing on it... you can make the problem appear and go away.

My mechanic also measured voltage on the always-on-t30 wire at the box, which showed the same low voltage that the TCU itself was telling us through the OBD reader.

So that seems to narrow it down to most likely a short or broken wire in the bunch going to the Mech, rather than something internal in the mech itself. I don't think grounds have been checked, but the switched 15 power is showing perfect voltage all the time... which probably uses the same grounds, so maybe some sort of short.

My mechanic is going to pull out everything necessary to inspect the wires in more detail, because last thing we want to do is buy a mechatronics unit and the same problem occurs!!

Apparently for a vehicle/DSG this age, there is no VIN/security coding.

While my mechanic digs into wiring, seeing if we can find a 2nd hand DSG+Mech.