Digital Aircon - Fan only runs on setting IIII

[dave_aber]

Hi All,

Just as we set off for our jaunt around Europe last month, I noticed that there was little if any flow from the A/C. When I set a temp of 18°C (i.e. absolute minimum) the AC system chooses speed IIII for the ran and it runs OK. If I select fan speed myself, it will only blow when I select IIII.

I also have a sneaking suspicion that the fan speed demanded by the AC is playing up as well. Going down in temp towards 18°C I'd expect it to try to blow faster at 19° or possibly 18.5° - not the case, Speed I indicated until IIII kicks in. This may be a red herring.

So, aware of the fact there was a factsheet, I sent off for the appropriate transistor chip, and have replaced it. Fault is still the same.

Visually checking, the fan is not doing anything on I, II or III, but runs flat out on IIII. The fan doesn't feel tight or binding in any way. There's very little evidence of crap in the system - even what I can see of the matrix through the hole left when I removed the speed controller module looks clean.

The PCB on the speed controller unit looked OK, with no obvious broken tracks, burn marks etc. The old transistor looked OK as well.

So, electronics / fan system gurus out there - any ideas?

[gmaczbongo]

It is a funny system and difficult to understand.
Min and max fan runs at decent speed.
In between, it seems to depend on how it is feeling on that day.
Press the clear screen button and it goes full blast and not even an araldited fly could hold on against the pressure.
It is either cold or roasting hot and not a lot in between.

[g8dhe]

The Max speed is achieved by a relay closing shorting out the transistor itself, so it does sound like something either driving the transistor or the transistor itself is the cause of the problem. However nobody appears to have the circuit of the actual A/C electronics module so it is difficult to suggest what might be the cause if it is the module itself.

Your best bet might be to measure the current flowing into the Base of the transistor and see if this is varying with the selected Fan speed, if it's zero then it might still be the transistor, but if it varies then it would suggest that the electronics module is sending the right sort of signal, not a 100% certainty, but it might help narrow down the likely cause.

[dave_aber]

Maybe you have the same fault as me!

Until about 4 weeks ago, it was pretty progressive. If you went to 18° you got an icy gale, and if you went to 32° you got cooked feet - but inbetween it would regulate up & down OK in auto sort of deciding what you need and setting itself accordingly.

And on manual, you could have any temp at any speed from any of the vents.

Now in manual, I can have any temp from any vent at max chat, or almost nothing (road speed induced wind only).

In Auto, I get very little airflow (fan stationary) unless I select Max or Min temp, which forces max. fan speed.

definitely a change from previously.

[dave_aber]

Geoff,

That was where I was thinking. I didn't look too closely at the circuit layout when I changed the transistor, but I guess the incoming connector has a drive level, and the other 2 terminals are in series with the motor.

Do you know where the relay is which shorts the tranny out to get the motor to max?

[Bongoplod]

The resistor on the switch is kaput,can be soldered,but likely to go again,best replaced,ive just had mine done after similar problems

Brian

[g8dhe]

Its relay G3-05, click to the next page for its location on the chassis itself;

[dave_aber]

Thanks for that Geoff.

I'll have a play around in the morning before I get on my travels again. Looks like a leisure battery out job.

[mister munkey]

Mine behaves like that when on Auto. Without the Auto on it pretty much behaves as it should.

I don't bother with the Auto anymore.

[dave_aber]

Woo Hoo!

Fixed it.

Decided that I would have another sniff around this problem.

Thinking about it, it can only be an issue in the speed regulation circuit or something seriously wrong with the controller. The speed regulation circuit has just the transistor module in it and a handful of connections. Ah, connections you say? I have moved my aircon controls to the left, swapped over with the stereo, so the wiring is a wee bit tight. Quick wiggle, and the left-hand connector on the back is now reseated and all's well again.

I love the cheap repairs!

(Pity I spent £7 on the transistor though!)

[Pete & Sue]

Our old Bong was behaving in much the same way. Took it to a local A/C man and had it checked, cleaned and recharged.
There was only 14.5 gms of gas left in the two units, It was recharged back to 950 gms + oil.
Now all is hunky-dory!