Jamiegreen said:

If my d2 can hold temp with heaters on in minus conditions I would have thought a d3 would have no problem. Not to say I wont be fitting one though because she takes her time getting there.

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I have found D1/2 and 3 all take roughly the same mileage to get to max temp, 4 to 5 miles and will hold it whilst working, but start to run light, and they will cool down, the std dash gauge will not show you that info, only a proper temp gauge will.
I have see a 200 drop back down to 70 degc, after the stat has opened due to running light for about 3 miles.

After much searching, I debated using an Arduino to produce the W bus signal, but quite honestly that's getting a bit beyond my electrical knowledge, swapping the circuit board to a tt c is an option.. but not cheap and not sure it would run right. Finally I've found a seller on eBay who specialises in making w bus converters and makes them specifically for d3s, which will turn a standard 12v signal from a switch/timer/remote fob or anything else into a w bus signal the heater will accept. Not too expensive either.. could also be handy if you were fitting a d3 unit into a d2.

https://www.ebay.com/itm/152931507581

lynall said:

I have found D1/2 and 3 all take roughly the same mileage to get to max temp, 4 to 5 miles and will hold it whilst working, but start to run light, and they will cool down, the std dash gauge will not show you that info, only a proper temp gauge will.
I have see a 200 drop back down to 70 degc, after the stat has opened due to running light for about 3 miles.

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That's why I remove my viscous fan in the winter.. certainly helps, and my stock electric one works so that should kick in if it ever did get too hot (I think!) Not that it ever has.