Climate Control

I’d read in a magazine story on buying these cars secondhand that the climate control used a whole bunch of servo motors, which sounded complex. But before thinking about motors, there was an even more obvious problem. The climate control selection (air con, vent outlets, recirc, etc) is made by means of a row of pushbuttons. You press one in and another mechanically pops out, so that only one at a time can be engaged. Except in the case of this car, none of the buttons would stay pushed down.

I pulled this bit of gear out of the dashboard and found that the switch portion of the module could be separated. Then, heart in my mouth, I disassembled the switch and its associated printed circuit board. Inside were six knobs, seven springs, five semi-triangular pieces of plastic and a channelled track in which triangular bits of plastic sat. The pushbutton assemblies made contact with metal conductors actually on the PC board – basically, the whole assembly was the switch.

And I had no idea how it all worked.

I puzzled over it for about an hour before getting the hang of how when each switch was pushed in, the triangles of plastic were displaced laterally to disengage any other switch that at the time was depressed. Theory in my head, I then tried to reassemble the switch, with only limited success. The trouble was that it all kept popping apart. Another pair of hands were needed and with those made available, we put the switch back together.

Only to find that the problem remained as it had before – no button would stay locked down.

Further effort and experimentation found an easy answer, though – it just needed a stronger lateral spring so that the system would overcome the friction of years of work. So if you now show me an E23 BMW climate control switch assembly where the buttons won’t stay pushed-in, I’ll be able to fix it, even without pulling it apart. (The helper spring can be added externally… I know that now!)

But would this make the climate control system work? I reconnected the giant plugs from the wiring loom, started the car and pressed the buttons. And although the buttons now stayed clicked-in, the climate control continued to do nothing but blow air-conditioned air over the inside of the windscreen…

That was enough for the day (earlier, I’d also fixed the electric seats) so I left it. The next day, though, I was back at it. As is often the case, a pause also brought me back to the job with a fresh perspective.

Vacuum Hoses

Hmm, I thought, so if the switch is now working, perhaps it’s whatever it is switching that has the problem? The article had been wrong – rather than servo motors, the BMW uses six solenoids that control the action of engine vacuum that is switched to actuators. Press the right button on the climate control and perhaps three solenoids have power switched to them, in turn feeding engine vacuum to the three correct actuators.

That much I could work out by simply looking at the system. The row of solenoids was clearly in view, a different coloured hose leaving each associated valve. These hoses disappeared off into the dash, presumably going to the different actuators. But there had to be a common source of vacuum, and I noticed a rubber manifold-style tube that connected all of the secondary ports of the solenoids. I also could see that this tube had some splits in it where the assembly pushed over the solenoid nipples, so I used some glue to repair these vacuum leaks.

But still the system refused to work.

Another hmmm. Obviously the manifold connecting the solenoid valves had to be fed a source of engine vacuum, so I tried tracing this hose. But it disappeared behind the instrument panel, lost completely from view. Even with the instrument cluster out (yes, the inside of the car now looked like a bomb had hit it), the hose was inaccessible. But looking under the bonnet revealed what (I hoped) was the other end of this hose, going via a T-piece to the bottom of the intake manifold plenum chamber.

So perhaps this hose had a leak in it? That would prevent any vacuum getting to the solenoid assembly. And yes, when I pulled off the hose from the solenoid assembly and started the engine, no vacuum could be felt at the cabin end of this hose. To get a quick and easy answer I ran a new temporary hose straight from the solenoids to the engine intake plenum.

And, finally, the climate control started to work! Now I could get air to the face-level vents, switch air to the windscreen – you name it.

Rather than try to locate the full length of the original hose (which would have required the whole dash to come out, I think), I ran a new hose to provide the required vacuum to the climate control solenoids.

More Air?

While cycling the system through its functions I’d found some of the actuators a bit sticky in their operation, so I idled the car for about 15 minutes while I went through the different climate control functions.

And then I noticed another thing. The labelling on the system is a bit odd, and without manuals (workshop or owners!) I was unsure if the system was meant to operate quite as it did. Because, with the air conditioner on, the amount of air coming out of the face-level vents was way less than when you had some of the other buttons pressed – the ‘Auto’ button under the ‘Heating’ heading, for example.

The buttons look like this:

But what if you wanted ‘Recirculate’ with the air conditioning on? I couldn’t work it out. Perhaps more than one button was supposed to be able to be pressed simultaneously? I looked in that magazine article at the pic of the climate control system and – in that car at least – only one button at a time was pressed. So perhaps we’d swapped some of the buttons when the switch gear was apart? But we’d been very careful to observe the correct order and make sure that each switch had a matching number of contacts to engage with on the PCB…

To get a feel for which solenoid was operating when which button was pressed, I then made lots of measurements and drew up a truth table like this:

…where an ‘O’ indicates the solenoid was open and a ‘X” indicates that it was shut

It was then easy to see why when the system wasn’t working, it defaulted to window defrost – that’s the safest failure mode, which is achieved with all solenoids off (and/or no vacuum being available). But solenoid #5 seemed a bit of a dud – it appeared to never work! (In fact later I discovered that it controlled the airflow to the rear seat vents.)

Looking at which solenoids were on and which were off in different conditions – and then pulling the wire off each solenoid to see which function was disabled – soon confirmed a few theories. Solenoid #1, it appeared, was the fresh air/recirculate controller – and the system wanted (in these weather conditions at least) to switch to recirc whenever the air con was turned on. Trouble is, the volume of airflow decreased a huge amount when the system was in recirc mode – and it wanted to go into this mode nearly all the time!

Modifications…

Disabling Solenoid #1 (by the simple expedient of pulling of its power supply) switched the system permanently into fresh air (ie non-recirc) mode. With the air con running in either bi-level or auto modes, this gave plenty of cold air from the face-level vents – but of course the recirculation button then ceased to work. It would be easy enough to rig a switch to turn on the recirc solenoid manually, and if I was super enthusiastic I could always pull apart the climate control pushbuttons and manually organise things so the recirc function was activated only when the recirc button was pressed. But the thought of going back inside that switch sent me straight to the cab sav bottle…

In the warm climate in which we live it’s far more important to have a good flow of air-conditioned air to the face than pretty well any other function. Looking at the truth table it appeared that I could wire a relay, operated by the power feed to solenoid #4. The relay could be configured so that when #4 was activated, the power supply to #1 was stopped. That way, in Bi-Level air con and Up/Down heat modes, the system would change from the current status of being in recirc mode to being in fresh-air mode. But in Recirc, Auto Heating and Auto Air Conditioning modes the system would still work in recirc mode as it currently did.

I installed the relay (arrowed) and the system worked just as I wanted it to.

Hell, as well as repairing them, now I am into modifying climate control systems…