Webasto Thermo Top with CAN Bus: Installation & Guide

Hello everyone!

I recently won an auction for a Webasto Thermo Top Z/C…
Everything is connected according to the wiring diagram, but nothing is working.
The device was installed as an original equipment manufacturer (OEM) product by Mercedes.
Does anyone know if the heating is not normally controlled via pin 1 (+12V switching signal)?
"...but is controlled via the CAN bus? The wiring diagram (connection diagram) from Webasto states..."
"Anyway, there's nothing to worry about; it's designed for retrofitting kits."

Here's the translation:

"The fact is: The heating system's control electronics contain a TJA1054 bus driver and a CAN filter, suggesting that the heating system is likely CAN-bus compatible. However, the bus driver might also be capable of single-wire communication!"

One pin of the CAN throttle connects to the diagnostic line (pin 2 on the connector), while the other connects to pin 3, which, according to the installation instructions for the Thermotop C, is an outside temperature sensor.

I have already tried using the Thermo Test Windows program from Webasto to establish a connection with the heating system for diagnostics and control, but unfortunately without success.
First, I used a cheap OBD interface (connecting the K-line to the diagnostic line), then with an RS-232 level converter and a differential (RS-485) driver connected to the two CAN lines of the heating system, and finally with a proper CAN dongle... That took me the entire weekend.

Conclusion: The heating can be turned on in single-wire mode via the diagnostic connection (the DC converter provides power to the electronics, and that's all), and it also transmits some data over the line, but the diagnostic software cannot make sense of this data.

Does anyone know which interface is actually used for the diagnostics, or can the heating system only communicate via CAN bus?

"And then there's this 'W-Bus', do you have any idea what that is?"


I hope you have some tips for me!

Sure, no problem. Here's the translation:

"Thank you very much in advance."
Greetings,
Paul.

Wow, it seems like you've made a lucky find .

Unfortunately, I don't have a truly satisfactory idea either. But how about trying to figure out which car model they should be installed in and then asking Mercedes for wiring diagrams for that specific model? That would at least allow you to simulate the original wiring. Perhaps there is a suitable Mercedes CAN timer available...

Alternativ einen Hilferuf an Webasto senden, die sind nach meinen Erfahrungen auch immer hilfsbereit und kompetent. Ob sie allerdings bei deinem Problem helfen können bzw dürfen? KA.

Regards,
Sure.

Here is the translation of the text from German to English:

"Guste" means "gust" in English. It can also be used as a verb, meaning "to gust."

hello rammelsberg

To control a Webasto Termotop C, you need a Webasto control unit that supports the W-bus, such as a timer (model 1533) or a Termocall 90T. I had it retrofitted by a Bosch service center, and the technician said that applying 12V to pin 1 of the Termotop would make it work. However, that was incorrect; it only works when controlled via the W-bus. The timer (model 1533) has a W-bus output and a static 12V output.

best regards, frank

Hey, that's some good news!

I recently won an auction for a 1533 early voting machine, and it should be arriving soon.
I'm already looking forward to not having to scratch anymore .
Very happy.

Well, there's still a long way to go until then. This time, I want to make sure everything complies with the German Road Traffic Regulations (STVZO). I contacted Dekra about it, and they said that if the heater has an E-number, I can simply install it without needing a separate inspection. However, Webasto told me that I would need an EC type approval certificate, which the installation workshop needs to fill out and stamp.
I guess I don't need to ask if I can fill this out and sign it myself.
Wink.

"No, I don't own or work in my own car repair shop."
Do I still have to go to the TÜV (technical inspection) then?

Well, let's see. First of all, I need to make sure the heating system actually works at all, and then I'll decide what to do next.

If I make any progress, I'll let you know!

Best regards and happy holidays.

Paul.

Hello fellow mechanics!

Frank was right; the heating system is controlled via W-Bus. With the new timer switch 1533, it worked perfectly from the start. The W-Bus connection of the timer switch needs to be connected to pin 1 of the heater unit, where normally a 12V switching pulse is applied; then it will work. I installed the heating system in my Passat 35i, and despite having air conditioning and ABS, there was no space issue. It also works to draw fuel from the return line.
Unfortunately, the heater has a small problem: Sometimes, when turned on, it enters an error state after only a few seconds. The glow plug turns off, and the fan runs at full speed. The clock display then shows "F --". Sometimes, the heater also takes 20 minutes to start heating before it stops supplying fuel and shuts down in the error state.
My initial guess would be a faulty igniter (since it also serves as the flame sensor).

What do you think about that?

Greetings,
Paul.

The Thermo-top is usually diagnosable via the K-Line; you should try that (or have it tried). Hopefully, this still works with your specific model. It's also possible that the heater shuts off due to overheating, for example, because of incomplete bleeding.

Hi!

Diagnosis is a good keyword. Does anyone have experience with the WebaSTO software and the KKL interface from dieselschrauber.de?
I don't think it's the ventilation; sometimes the heating system even starts showing an error message during the initial startup process (when the fan slowly increases speed and the fuel pump starts making noise).
Shouldn't I also receive an error message via the clock (W-bus)?
I'm going to try replacing the combustion chamber (with glow plug) based on suspicion; I have an older Thermo Top lying around.
Perhaps it would help me troubleshoot the problem if I measure the voltage at the glow plug during startup using an oscilloscope; I'll try that sometime this week.

I'll let you know if I find out anything new!

Greetings,
Paul.

Hello,

I would guess it's related to fuel supply.

I recently installed a Thermo Top C heating system in my Audi 80 B4 TDI as well.
Initially, I connected the fuel supply to the feed line, and I always had the problem that the heating system would only work intermittently.
After I had installed the original fuel line from the STH (presumably a specific device) with an inner diameter of only 2 mm all the way to the tank connections, I no longer experienced any problems.

It's probably because the pump is too weak for the thick hoses that are already installed in the vehicle.

'For me, the installation was relatively straightforward because there was a free hole in the rubber mounts for the fuel lines in the middle that fit perfectly.'

best regards
sLindi

Hello everyone!

Here's a brief status update:

The fuel supply isn't the issue; the return suction works perfectly. The problem lies with the control unit. Once the engine or coolant reaches a certain temperature, the control unit triggers an error and shuts down. I suspected it was related to the unconnected relay for the interior fan, so I finally installed that relay. Now, the heating system doesn't turn on at all; it immediately goes into fault mode. The relay briefly engages when this happens. I suspect the CAN bus is causing problems again. It's possible that an attempt is being made to send a message via the bus to the comfort control unit for the interior fan (which I don't have...). Since the message isn't received and acknowledged, an error is generated.

With a different control unit that doesn't use the CAN bus, the heating system works perfectly fine.
Well, I think I'll focus on the diagnosis again, or try to simulate a CAN bus.

Best regards,
Paul.