The heater on my 1996 x300 does not work. The pump works when I bridge the relay, but the relay does not get turned on by the climate control unit (wich I have already replaced by a know working one). I am not sure what else could be wrong, so I thought I would check the temp sensor while I have taken the steering column apart (wiper stalk needed replacing). But I can’t find the sensor.
The other thing that is funny is, the heated rear window does not work. Could that be a clue as to why the climate control does not turn on the heater pump?
My first impression on why anything doesn’t work on an X300 is “check the fuse”. After finding the correct fuse panel or inline device bured behind the interior trim.
IIRC the sensor was mounted behind a louver in the valence panel under the steering wheel. (or was that the X308?) So, a hunt may be on for the bit of harness that connects to the (now missing) sensor.
Try running the temperature setting to maximum warm (over 80F?) that forced the system to ignore the temp sensor.
I have checked all the fuses in the fuse boxes. All are ok. I have also tried setting the climate control to “HI”. That didn’t work either (engine definitively warm enough). So are you saying there are any in line fuses under the dash board?
Since I owned a 1997 X300 and a 2001 X308, my recollections of exactly which one was more annoying to fix are a bit muddled.
I definitely remember an inline fuse behind the trim panel to the right of the center console. Whether it was part of the heater or radio… and why did I have that part of the car apart?!?
At this point, finding the sensor (or where it is supposed to connect) is second priority.
Getting more info on how the ragsnapfruffing computer is supposed to behave is more important.
You mentioned bypassing a relay fires up the pump. At least the mechanicals are intact. But you have to know where Hot and Ground come from to drive the relay. And “Is the relay any good?”
Does the relay coil get Hot from the ignition circuit and the controller provide ground or vice-versa?
Start with a pinout of the relay and and a test light. See what you’ve got vs. Ground. Then check vs. Hot.
You could get lucky and only have a bad ground or an open coil in the relay.
Or the (expensive) HVAC computer is faulty.
The relay works as well. The climate control is supposed to switch the ground to the relay coil, wich it doesn’t do. I will go through the wiring diagrams again and see if there is anything else that could cause the problems.
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What you call the HVAC computer is the climate control, isn’t it? I have replaced that with a known for one. And still both the heated rear window and the heater pump stayed inoperative…
the first thing to check is whether there is gas in the system.
2nd thing, is does the tripple switch in the AC line think there is gas in the system!
don’t worry about temp sensors, If you set temp to Lo AC should run.
Cooling works fine, heating doesn’t. If is the water pump that pumps water for the heater that does not get turned on.
PM conversation sent
Nothing in my inbox 
Yeah. Way too many restrictions. I got the “email failed” messages.
Here’s what I sent:
Just to confirm: the relay coil Hot is provided by the ignition/accessory circuit. Relay coil ground is provided by the heater control unit. And when you manually connect relay coil ground to directly to ground, the relay trips and the pump runs. If this is the case, you found you have the expensive proper fix (heater control unit is faulty).
A band-aid solution would be to wire a manual switch to ground the relay as you see necessary in the cockpit.
Hope this helps a bit.
Yes, correct.
The only thing is, I have already replaced the control unit and the know good unit behaves exactly the same. I have also checked the wire from the control unit to the relay. It also is OK. So I was thinking something must stop the control unit from turning on the pump…I just don’t know what.
Arh. That pump.
Yeah check the relay coil side has a + supply.
The V8’s have a common problem with the alarm sounder blowing fuse and front foglights and heater pump not working. x300 electrics are similar in places
Maybe the panel is not getting the engine running signal ?
That could be the sort of thing I am looking for!
Where does that signal come from? Would it still run the AC though? AC works, just heating doesn’t.
I always have doubts about ‘new’ or ‘guaranteed good’ electronics. I’ve had way too many out-of-a-sealed-box failures.
Unless I’ve personally tested -recently- it is not “known good”.
Get your test light or DVM right on the back of the connector to confirm whether or not the HCU is providing the ground signal the relay needs to the car. If it is, a wire is broken in a harness, bypass it. If not, the connector or HCU is bad.
I’ve had connectors fail too.
Sorry I don’t know off hand.
I only have x308 diagrams
I’ll let you know if I ever solve this riddle.
the x308 has an engine speed input to climate control
pin 31-6
I’ll check the diagram for the x300 and see if I can locate the signal. This sounds like a very plausible problem.