VW Transporter T5 No Comms Wiring Problem / Diff Lock
Wiring Harness Corrosion
We often get Haldex Control units sent to us for Volkswagen Transporter T5 4Motion vehicles, part numbers are normally 09N907554A – 09N598554 – 09N598554A – 7H0907554 – 7H0907554A – 7H0907554B – which are showing this fault code on the ABS unit:-
01324 – Control Module for All Wheel Drive (J492): No Communications
When we test the unit it works fine, but it has fault codes for no comms stored on it for the Engine ECU, Cluster, ABS etc etc. Basically it is up and running but cannot talk to the rest of the vehicle.
They seem to be prone to harness corrosion problems underneath the back of the van, either above the spare wheel inside the plastic T piece where the harness splits off, or inside the connector where it plugs into the diff harness.
Obviously, problems with the Haldex ECU will cause the same fault code 01324, or even a blown fuse for the 4wd system would cause this no comms fault, so do not take this as definitive direction. The purpose of this page is to show people who have sent in non faulty units where to look for harness/wiring damage.
Diff Lock Switch
The next issue that sometimes occurs with the VW Transporters is the diff lock switch problem. Inside the rear diff, not only is there the Haldex coupling, there is also a mechanical diff lock which is operated by a vacuum diaphragm. There are 2 solenoids underneath the drivers seat which send vacuum to the diaphragm to either engage or disengage the mechanical diff lock within the crown/pinion gear set in the back.
There are 2 little magnetic reed switches inside the rear diff lock mechanism. When the system is either fully engaged or fully disengaged it triggers either the engaged or disengaged magnetic reed switch, which reports back to the Haldex ECU to show that it is either engaged or disengaged. If neither the engaged or disengaged switch is sensed, then it is assumed that the lock is “ajar”, e.g. not fully either way. This causes the diff light to flash.
Either the mechanics are not going either way fully, OR one of the reed switches is defective, OR the wiring is defective.
Here are a few diagrams to hopefully guide you though troubleshooting the system. The reed switch block and diaphragm is available with part number 02W525641A. Also within the measuring block using VCDS, you can actually read/see both the reed switch status, so it is ideal for diagnosing this problem. Notice the switch block shows it has pull up resistors inside, this is ideal for being able to test continuity without removing anything drastic. All the testing you need should be achievable from the connector to the Haldex ECU on Pin 3 and 4.
