August 23, 2016 · webasto airtop st 2000 control rheostat

Inside the Webasto heater rheostat

The standard webasto rheostat

About Webasto auxiliary heaters

Auxiliary heaters can be quite a intriguing piece of hardware. They are simple enough to quickly understand their principles (fuel + air = warm and fuzzy car) and yet their daunting details puzzled many (self-proclaimed, me including) mechanics.

I have a 2015 Webasto Airtop 2000 ST, which is the smallest air-based aux heater. With its compact design and powerful output it is just the perfect setup for my 72' veedub bay window bus.

While the heater itself is a black-box, one has many options when it comes to the model of the heater (air-/water-based, capacity, car/truck/marine and so on...) and external controls like temperature sensors and control units.

There are many different types of controls: Some are quite sophisticated with displays and timer functions. Some are remote controlled and if you want to afford it, you can even control your heater via your phone. Because who doesn't want to send a signal on a round trip from the driver's seat to the next cell phone mast back into your car, all while you are charged for roaming.

Anyway, today we want to look at the easiest version of the heater control: The Webasto rheostat.


In case you haven't heard the word "rheostat" before: It's just an acient term for "potentiometer". If you have never heard that one before, you can look it up on Wikipedia.
Don't ask me why, but it is more common to refer to this device as a rheostat than a potentiometer, so I'll stick to that.

As you can see in the first picture, the rheostat is a pretty chunky device - built to last, designed by German Engineers, made in China. The rotary selector serves two purposes: First, turning on the heater, which is confirmed by a lightened LED. Secondly, you can select a temperature range or heat output, if you will.
To turn it on, you must overcome a rotary threshold, after that it's all smooth temperature selection with all that German engineering goodness.


The rheostat internals

Opening up that beast was easy. Yet, you should be careful, as the casing is fully made of plastic. There are four noses where the bottom part of the case is clipped into the top part. Take a little flat head screwdriver and push them aside (not in). Repeat with all noses on one side and the case will already begin to lift itself up. Now take on the other side, and the box will reveal its secrets.

I totally love how mechanical and massiv that whole switch is. You can really feel, how it was made to last in the hands of a tough truck driver after a long haul to Sweden and back.

basic electrical design

Basically it's a simple electrical design consisting of a push button, a potentiometer, an LED and some connectors.
Additionally you'll have two sprockets and a spring that give the switch its tactile sensation.

enabling the switch

When turning the switch, it will sink a bit inside its casing to give you some physical feedback as well. In the above picture you see the two sprockets detached from each other. When properly setup the sprockets are aligned and the spring is much closer to the push button. Rotating the switch will force the spring to budge under the sprocket and to engage the button, switching on the heater.

setting the temperature

All further rotation will alter the value of the potentiometer. Unfortunately no available datasheet of the heater will tell you the range of the potentiometer. This was my original motivation to open up the device: I wanted to know the resistance. Sure I could have measured right at the pins, but while we are at it...
I wanted to be sure, that there isn't any more complex installation inside.
The multi-meter read a maximum resistance of 2,2 kOhms.

connector pins

Just for the sake of completion: The heater connector has 4 numbered pins: 1 (blue) and 2 (white) connect to the internal potentiometer, while 3 (red) and 4 (black) go to the push button.
Pin 1 holds the actual resistance value, while Pin 2 goes to ground.
Pin 3 is some permanent 12V line that will turn on the heater when shorted against Pin 4.

Pin 3 and 4 are the only recommended place to turn the heater on/off. Cutting the connection between Pin 3 and 4 won't turn the heater off immediately. Instead it goes through a shutdown and cooling phase first. Cutting the main heater power instead, would prohibit so this. So please don't do that.

Hacking the Webasto heater

It's quite pretentious to call it hacking if you don't do nothing more than replacing a rheostat. In particular when there are real hacking projects available that are using the W-Bus (Webasto's CAN-like command bus).

But as long as the original rheostat costs nearly 100 bucks and all the other Webasto control units between 300 and 600 Euros, I consider each custom replacement for 10 euros, at least as price hacking.

My motivation for not using the original rheostat however has a different motivation. I don't want to disfigure the dashboard of my car. Instead I want to integrate it into some original levers that aren't used anymore in my setup. Modern technology with a vintage feel. But more on that later.

Wrapping it up

The Webasto Rheostat is a sturdy piece of hardware, no question. Yet it is simple enough to understand quickly and to realize that the heater itself in its basic variation, doesn't rely on too much complicated signaling. Of course this changes with the more complex controls. Still, if you can abstain from more fancy features like remote controlling or setting timers for heating, there is nothing holding you back from building your own controller.
In fact you can still have all the cool features Webasto offers. Take an Arduino or Raspberry Pi for example to control the 4 pins and the whole story changes.

Also looking into W-Bus hacking would be advisable. There is even a
library available for the Arduino.

For the moment I'll stick to an easier solution.

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