Firstly, this problem has been solved, but has left me with a question as to why things occurred as they did.
The airconditioner in my 1992 Daimler was not cooling and, suspecting low refrigerant, I took it yesterday to my aircond mechanic. There was still some pressure in the system because the clutch was engaging on the compressor. The morning was near freezing outside and I thought that lack of airconditioning wouldn’t matter to the climate control because all I needed was heating. However, there was no heat either. I tried many combinations of settings, auto or manual, econ mode, temp setting low or high, but nothing. It was quite cold! And an 80km journey.
I described the symptoms to the mechanic but told him to only address the aircond, which he did. There was a slight leak from a connector on the end of a hose which was recrimped, the system pressure tested and refilled. The aircond now works exactly as desired, AS DOES THE HEATER!
Now, was I conflating two independent problems and the heater somehow fixed itself, or is there some connection between the two?
Cheers
Gary
Hi Gary …
The way the heater system works in these cars is quite simple.
The hot water from the engine block always runs through the heater core whenever the engine is running, The amount of heat that reaches you is determined by the position of the two blend doors (they’re actually shaped like rolling pins but go figure).
Earlier XJ40’s came with a vacuum operated water valve that shut off water flow to the core when not needed. On later models you can easily install one in the intake hose going into the heater core and operate it manually (keep it shut off in the summer months). This is a very common modification on later cars, as is wiring the aux cooling fan to come on whenever the A/C clutch is energized.
The blend doors are shown in green
water valve.
Thanks Grooveman. It’s good to see a diagram. It looks to me that if the aircond is not cooling then the air would still be heated by passing through the heater coil. That is, unless the blend doors are redirecting the air flow around the heater coil. But why would it do that?
Cheers
Gary
Gary … Why indeed !
The climate control ECU (electronic control unit) has a variety of sensors (I believe it’s 5 but I wouldn’t swear to it in a court of law) that determine the position of the blend doors and the speed of the fans in the “auto”
mode of operation.
By going to “manual” you eliminate all but the evaporator temperature sensor and have a much better control over the system. Of course this does mean that every now and then you have to put down your glass of Champagne and lean forward to tell your driver Nigel “be a good chap and stoke the fire a bit”.
So to answer your original question could a failure of the A/C portion of the climate control system result in the loss of heater operation ? In “auto” mode I’d say maybe because of the ECU commanded position of the blend doors. In "manual’ mode my best guess would be no.
Thanks again Grooveman. My scientific brain puzzled how the system could produce the effects I observed, but I’ve concluded that I’m not using the car as it should be used. With everything set on auto it is just perfect. I will now give myself over completely to the car, and let it get on with what it does best.
Today we drove 500km interstate and the Daimler was a dream. I will keep spending money on it if it continues to reward me this way.
Gaz