VW T4 Auxiliary Heater: Diagnosis & Troubleshooting

Hi,

"The auxiliary heater in our T4 isn't working anymore. The display is okay, but we were told by a workshop that "the control unit" is faulty, and the repair cost would be around 400-500 euros."

Now that I'm doing this professionally and I have VAG-COM, I wanted to try it out.

Does anyone know more? Is this device diagnostic? If so, where is the control unit located in the T4?

(T4 2.5 TDI Syncro, 1999)

If it's an Eberspächer unit, the control module (SG) is directly integrated into the heating unit (SH), and it needs to be removed. I suspect the same applies to Webasto units, but I'm not certain.
It can't be repaired; it's beyond saving.
First, you need to determine the type and device number. Bosch can read the error codes using the Eberspächer software. Some can also be read using VAG and VAG-Com, but the diagnostic depth is five times greater with Eberspächer.
Eberspächer's hotline is excellent.

With the older Eberspächer pre-selection timer, the auxiliary heater can be diagnosed via the buttons on the timer itself, without having to remove the control unit or the heater.
It then displays error codes.
At most, it might be necessary to run some cables.

The 1999 model probably already has the blue-lit clock, right?
It doesn't work with that.

Yep.

"It already has the newer interior with blue lighting... hmm, so the workshop was probably right with their 500€ estimate. I was hoping that only the control board behind the timer would be damaged."

With the approximately 260,000 kilometers it has now, the €500 is definitely not worth it for those 2 or 3 winters .

I don't know which workshop diagnosed the problem, but I suspect that not many workshops actually have a good understanding of the subject matter.

"In fact, some of the control units in the heating systems sometimes fail (I suspect a D5WS unit is installed). However, often the control unit is simply locked due to operation outside of its specified parameters."
Try having it unlocked at a Bosch service center, or get an old pre-selection clock and do it yourself.


1. Is it an air heater?
2. Check the battery and the relay.

I believe that most auxiliary heating control units have a diagnostic lead (usually green). However, this lead is rarely connected during installation. There is a specific and dedicated pin in the OBD socket for the auxiliary heater.

Best regards.
Thomsch.

There are parking heaters where you can read out error codes. These heaters have a clock that can be set. There is a separate device for reading the error memory.
Heating systems that don't have a clock cannot be read out, or at least, not a specific model range.

All heaters can be read out - at least those from Eberspächer - but not all with VAG, but rather with the Eberspächer tool.