Hooking the auxiliary heater to the keyfob remote
The Webasto heater is suplied with a fairly basic timer control module to allow programmed heating of the car. This is probably perfect if you know in advance when you'll need the heater each day. However, since I use my truck for my daily commute, and I rarely know in advance what time I will leave each day, even to within an hour. Webasto sell a remote start unit, but since my alarm keyfob already has several unused control channels, it seemed a more convenient solution to use one of these.
My suspicion was that the telestart would interface to the heater/timer with a fairly simple scheme. Since the heater can be programmed only in 10 min steps from the remote, it seemed likely that something as simple as a pulse counting scheme would work. However, when I investigated the timer I I found it wasn't quite so straightforward. The timer runs from a 12V supply, and gives a 5V logic output HIGH when heat is required, LOW when off. There is an additional unused wire labelled 'w-bus' which seems to be a 1-wire bus, and sends a serial word when the timer switches on or off. Presumably the Telestart sends a serial word into the timer to command the heater on or off.
The upshot is that the easiest way to control the Webasto remotely is to override the timer control output, adding another 5V control line. The alarm output is a switched 12V signal open collector output, which can be timed (only to a max. of 255 seconds) or latched. For the first iteration I will use a latched output, limited inverted and pulled high to 5V. This is (I think) unlikely to cause problems if wired in parallel with the timer output.
In the event I settled on an even simpler scheme. The timer module has an unused connection which I originally thought might be used by the 'Telestart' remote unit. The timer sends out a burst of serial data on this unconnected pin whenever it switches on or off, but I do not know the purpose of this. I disconnected this output and used the pin to provide external access to the 'burn' button on the timer controls. Simply connecting this to the alarm remote means that a single button press lights the auxiliary heater for 30minutes under timer control. A second press switches off the timer at any point. This seems to work really well. I added some filtering in case of the heater being activated by electrical noise, and have experienced no problems so far.
Update : The scheme above worked really well, and I had no problems whatsoever. However over the summer, the immobiliser threw a fit and left me stranded blocking both directions of a major road, with a 16ft plant trailer on the back. Happy, I was not! An hour later, the immobiliser (and associated alarm) was sitting on the scrap pile in my workshop. It has since been replaced with a new system, but one which does not have useable remote control channel for accessories. As winter approaches, I realised this left me without a remote control for the heater system, and I purchased a 3ch remote unit from RS (or one of those) for very reasonable cost. However, a friend pointed out two very interesting facts...
- The Webasto 'Telestart' system is a two way device, with long range. This means you can turn the heater on while in the pub/cinema/meeting from up to 600m away, and know that the heater has started. This is a serious advantage over any system I could put together without spending a lot of time on it.
- He happened to have a Telestart unit that he was prepared to sell at a very reasonable cost. :-)
So... the result is that I now have a Telestart unit fitted to the Ibex... and very nice it is too!