Heater CCU help required please - Tom?

[Donkychobo]

Hi, tried posting on another forum but no replies. Got introduced to this one so fingers crossed.

I was fault finding heating on a 944 my son has just bought. 1988 standard 2.5 UK car. His dream car. I had a couple myself when he was a small boy.

Came to the conclusion a previous owner has swapped out the control unit for climate control unit 944652021-03 (non aircon) when I believe the car should have a 2100 basic CCU fitted. No aircon or climate.

The reason I think this is because there are no temperature sensors fitted to this car and the build label shows no option code for the automatic heating.

So I assume the CCU we have here is the wrong one and looking for a resistance from the three temperature sensors when there are none fitted?

My question is, how do the CCUs differ, can I alter this one to take account of there being no sensors or does he have to find the money for a used replacement basic type CCU.

They look the same other than the temp dial being in numbers rather than blue/red.

Maybe the basic one just has some fixed resistors fitted rather than the varying ones from the sensors. Or is the circuit very different?

I did see a write up where someone had figured out what the CCU does in respect of when to open the mixer flap and heater valve solenoid. All to do with differing temp readings between the three sensors. But how does the basic car heater decide this?

|Everything works on the heating system except these two functions and I am guessing it is due to the car not having temp sensors.

Trying to save him some much needed funds for other improvements required.

So the re circ flaps operate, the sliders change the airflow, the fan works. Changing the temp dial does not move the mixer flap motor or open the heater valve solenoid.

Tested the motor and it works with direct voltage, have vacuum into the solenoid but it does not switch to open the line to heater valve. I could not pick up a voltage.

Tested the voltages at the mixer motor plug, the 10v does not flip and some of the other voltages are not as per the workshop manual. It was at this point I decided to test the temp sensors and realised they are not fitted

It blows warm air but there is no control over temperature.

Thanks Rob

[Tom]

Hi Rob -- so glad you found us!

It sounds like you may have already discovered our HVAC diagnostic charts, located here:

https://carpokes.com/viewtopic.php?t=357

I'd start by running all of those diagnostics and see what's working and what's not. If the only problem is that the vents have heated air all the time, then 'typically' the most likely culprit is the heater clip on the side of the heater core console.

See more about that here:

viewtopic.php?t=371

Now, regarding you climate control unit. I think you have a typo in the part number above. The UK version of the Porsche Pet shows these possibilities for 1988:

944 653 021 02 Control switch 1
944 653 021 02 01C Black
944 653 021 03 Control switch 1 M424
944 653 021 03 01C Black
944 653 021 04 Control switch

I assume the basic 2100 unit you think may have come with the car is the first on the list above, like the one in this link:

https://www.rosepassion.com/en/diagrams ... 01c-442479

I don't know a lot about those units. My question, however, is how you concluded your car has no temp sensors? I don't think M424 is likely to show up on the option label on your car. Note, for example, that Rose Passion lists cars without automatic heating adjustment as Option code M424Z. So apparently M242 means you have automatic heating, and M242Z means you do not. Do either code show on your options list? If not, is there any other reason you think the climate control unit has been swapped out for the wrong version? If you pull the vents, do you have this sensor?

sensor.webp (1.23 MiB) Viewed 955 times

[Donkychobo]

Tom, thank you for replying.

Those codes are not on the build label.

Heater clip is good. Motor / flap moves if an external 12v is applied, linkage all good.

All 3 sensors are not present, no indication they ever were. Under bonnet sensor socket is tie wrapped along the loom.
No air feed pipe or sensor in place for the small round internal vent.
Just the grey clip inside the larger dash vent, no sensor or fly lead.

When I pulled the CCU it had electrical tape around it so its not all original from day one. Someone has been in there messing.

Also, when we got the car home to investigate zero heat, found a T had been fitted and a direct constant vacuum pipe had been fitted to the heater valve holding it shut.

Re routed so vacuum going into the system entry pipe plus reservoir. Reconnected the solenoid to heater valve vacuum pipe.

Everything works now but.
1. 10v does not alternate on motor plug when turning the temp knob. I'll paste what I get there in a minute.

2. Heater valve solenoid has vacuum going in but never switches for vacuum out. Heater valve remains open now.

Yes, the picture is of the CCU type I am assuming should be fitted. I see a couple on ebay with the part number ending 00 instead of 03. I will add a link.

My guess is the heating was not working, someone fitted a used CCU but for climate.
It did not work so bodged the vacuum lines to turn off heat.

I could be wrong, as trying to both fix this and fathom out what has been messed with.

I am thinking I have two options.
Fit a non climate CCU
Or
Fit three sensors and hope the existing then works.

My expectation is the two units differ in that one is uses the variation in sensor resistances to switch the internal op amp that feeds the motor control ICs while the non climate version will use the resistance from the temp knob alone.

[Donkychobo]

testing at the heater flap motor plug.
service manual says. Temp set to cold
Term 1 & 2 = 6v yes ok
Term 1 & 3 = 2.5v no, just getting milivots
Term 1 & 4 = 10v no, 0.1 v

Set temp to full
Term 1 & 4 = 0.1v yes
Term 1 & 5 = 10v yes
Set back to cold and should go from 10v to 0.1v no, no change

[Donkychobo]

https://www.ebay.co.uk/itm/386257453681 ... media=COPY

But I also see one that looks the same with part code ending 02

[Tom]

Well, based on all that, it sure sounds like you car came with the red/blue control unit. Adding sensors to retro-fit that unit sounds like a fairly big/risky effort. Before doing that, I'd be tempted to patch in 3 simple resistors in place of the sensors with values that make sense -- something like 20K for outside temp (50F) and 10k for the others (77F) to see if the unit will try to reduce the heat when set to the coldest position (or if at least the unit passes all the tests on the chart that way). Or even patch in potentiometers to simulate various inside/outside/mixer temps.

Have you tested the heater solenoid as shown in my charts? Worth checking, but based on your tests, it sounds like your control unit is bad or can't function properly when hooked up to a car without sensors (and whatever other differences might exist under the dash?).

The ccu you currently have uses now obsolete motor-driver chips with a servo feedback signal from the motors, which makes me wonder if they used cheaper "dumb" motors on cars without the automatic climate control (i.e., motors with no position sensor). Hardly seems worth the effort to make and sell the red/blue unit if the only difference is a few missing chips on a PCB.... If you are flexible enough, check the part number printed on the heater flap motor, just to see if it's the same as the one used on the automatic climate control cars.

You can see the theory of operation here if interested:

viewtopic.php?t=1007#p10632

I'd be inclined to try cheap resistors in lieu of temp sensors and see if that's the only thing wrong with your current set up. If you get super lucky and that works, then you can add sensors without risk (see my earlier links for relatively cheap sensors). If it still doesn't work (and if that solenoid checks out), then I'd probably just bite the bullet and get a known-good blue/red CCU. Just too many moving variable otherwise.

[usury]

If you decide to go the "install the temp sensors" route, this is the thermistor I used when repairing my climate control system...

https://www.digikey.com/en/products/det ... 04/5967514

Also, if for some reason the temp potentiometer on the CCU is flaky, this one will work. It's the closest I could find from a reputable supplier of new components. It has 300° of rotation (vs the factory 270° pot), it's plastic, and the shaft is too long. The shaft is easily trimmed and filed to accommodate the CCU knob.

https://www.digikey.com/en/products/det ... A/16028651

[Donkychobo]

Thanks Tom / Usary,

I have not checked the feedback pots on the motors yet or the solenoid coil but will do next just to be 100%.

I did think of wiring in 3 x 50k pots in place of the sensors to see if there was a middle ground where the temp knob alone could move the flap / close the water.

From the tests so far, assuming the CCU is a good one I can see no difference other than no sensors. I'll check the interior loom has a the places to attach them.

Seen a motor fan sensor on ebay, as you say the other two could be made.

Its this or buy a basic CCU (hoping its a good one) then try and sell the other to recoupe funds for him.

With UK car sales there has always being a popular game by German car manufacturers of pricing for the basic car then charging lots extra for every possible option.
It probably saves the manufactures nothing by having to 'downgrade' a car during production but its a profitable system.

[Donkychobo]

Update.

So today I made up three leads with 50k pots and attached to the empty sensor plugs.

Set all to 10k

On with ignition and the mixer flap did a little nod.

Set temp dial to full cold and the flap moved to max and the heater valve closed.

Set dial to max temp and the flap moved to around half and the heater valve opened.

So adjusted the interior sensor pot until the flap was all the way.

Turned temp to halfway and the flap did the same.

My lad will need to try it now over a few outdoor days but looks to be working as you'd expect a non climate car to.

I bet the only difference between the two controllers is a few fixed resistors across the temp sensor terminals.

Might be worth sorting out sensors now as everything else is there and working for climate.

Thanks Rob

[Tom]

Update.

So today I made up three leads with 50k pots and attached to the empty sensor plugs.

Set all to 10k

On with ignition and the mixer flap did a little nod.

Set temp dial to full cold and the flap moved to max and the heater valve closed.

Set dial to max temp and the flap moved to around half and the heater valve opened.

So adjusted the interior sensor pot until the flap was all the way.

Turned temp to halfway and the flap did the same.

My lad will need to try it now over a few outdoor days but looks to be working as you'd expect a non climate car to.

I bet the only difference between the two controllers is a few fixed resistors across the temp sensor terminals.

Might be worth sorting out sensors now as everything else is there and working for climate.

Thanks Rob

That's great information, thank you for posting it! Crazy that Porsche went to all the trouble to deactivate the climate control, but as you say it was probably most about justifying an upcharge than cutting costs on a cheaper version. I'd be inclined to wire in 3 cheap resistors and call it a day. In 25 years with my car, I don't think I ever actually used the climate control to maintain a certain temp. On the other hand, there's something nice about having things work as they were designed, and those thermistors are cheap, so that might be a fun project at some point. Either way, I'm thrilled to learn something new -- great info!