It may be one of the hottest days of the year but I have heat!!

I’ve had a bunch of posts over the years talking about heaters, some of them were planning, some were installing, some were testing, some were blowing up heaters.  I’ve finally planned, installed and tested what I hope to be the final version of my 944 heating system.  Surplus components from Think EV’s when they switched from the Mes-Dea RM3 fluid heaters to a ceramic or other PTC heater.  The combo includes a 200-450v RM3 heater along with a high quality bosch pump.  During the removal at Think the HV power wires had been trimmed rather short.  These wires were replaced in a previous post, ready for installation.

4kw heater 3


New wires above, heater installed below.


Like most other systems in the car, it’s very important to me to retain the stock look/feel/function of the original Porsche system.  I’ve talked about it before, but a big thing for me was to NOT have a dedicated non factory switch to turn on the heater.  Some people have an aftermarket switch with some sort of label saying “heater”, this just doesn’t do it for me.  In the stock heating system there is a temperature dial, it’s blue on one end, red on the other, and has numerical values in between.  The most intuitive method of turning on the heat would be to turn this knob from blue to red or some value between, we can all figure this out if we get into a new vehicle that we’ve never driven before.  Why should it be any different in my EV?  In addition to simple operation I also wanted to maintain the “automatic” climate controls that Porsche was using in 1986.  It’s a simple system, it’s mainly vacuum operated based on some simple analog electronics.  To make this happen I installed two microswitches, one in the heater control itself, it’s basically a NC (normally closed) switch that only activates (opens) in the cold position on the temperature dial.  The second switch was installed on the vacuum operated coolant valve that would normally restrict the flow of coolant from the engine.  I don’t actually use this valve to restrict coolant flow, but the microswitch serves to turn off the heater.



The vacuum valve with added micro switch, and installed view below.


vacuum switch

I haven’t found a good program to create schematics but the system is quite simple:

Switched ignition +12v is connected to the NC microswitch on the temperature dial.  In any position above blue (cold) this switch applies power to a 12v relay which turns on the pump that circulates roughly 2L of coolant through the 4kW heater and stock heater core.  The same 12v out of the relay that powers the pump then goes to the 2nd microswitch connected to the vacuum valve that would normally limit coolant flow.  If the cabin temperature is lower than the temperature setting on the dial the valve is open and the NC microswitch applies power to the 12v terminal on the heater.  If the cabin temperature rises above the preset value the vacuum valve closes turning off the switch and removing 12v power to the heater.  The heater itself also has built in thermal limits that attempts to heat the coolant to 70 degrees celsius.  Any of these various systems will turn off the heater which should reduce the amount of power wasted creating excess heat and allow me to dial in the amount of heat I need or want while driving.

coolant tank

With a 4000watt heater the draw on the main pack is 12-13A while it’s “on”.  Only time will tell as to what kind of duty cycle I will end up with as the portion of time the heater is on vs off.  This duty cycle which will vary both with ambient temperature and the desired amount of heat and will ultimately determine the impact it has on my range.

I was going to make a short video on the operation and the various systems that control the heater however the ambient temperature this afternoon is too hot for the heater to even turn on.  There will be plenty of chances to do so in the coming months as the temperatures drop.  We have had some chilly mornings so time permitting I will demonstrate the system in action soon.

I still need to create some sort of indicator to make sure I know the heater is “on” and don’t have it on needlessly wasting power if it’s not required.


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