Help please - Xantia II heater blower

[rmunns]

My car is a year 2000 Xantia Hdi90 Forte saloon, 70000miles.

During a journey last night I switched the heater fan on. I have never had any problems with this - the auto function worked fine, could get three or four various fixed speeds as well.

But last night the fan went to full speed and would not change - couldn't turn it off or change the speed from full.

Had a big journey (600km) in front of us, so in the end I stopped the car and, after a bit of detective work, removed fuse 21. This stopped the fan and didn't seem to have any other effect.

But can someone please let me know what I should do?

[Xaccers]

The transistors need replacing.
Can't remember their details though.

[Peter.N.]

MJ 11015

[MPV]

It's a common problem, do a search on the forum, there's a long thread about this somewhere.

[Clogzz]

http://www.frenchcarforum.co.uk/forum/v ... 12&t=22398

http://www.frenchcarforum.co.uk/forum/v ... hp?t=22348

[CitroJim]

Thanks Clogzz, I was just about to post the same links

Please note that due to pressure of other activities I'm not currently able to carry out modle repairs as stated in the linked threads Sorry...

However, all the guidance is there and the transistors themselves are available from Farnell Electronics at a good price.

You MUST replace them as a matched pair; a matched pair being a pair that carry the same batch numbers.

[rmunns]

I found this on another forum and will try it before going looking for transistors.

Heater blower problem now solved! It wasn't the diodes (output transistors?) defective. It was just a dirty contact on the back of the heater motor circuit board.

There's a small spring contact which goes through the circuit board and presses on a loose button (looks like a tiny "fridge magnet") which sits in a recess in the main heatsink casting.

I blew all the dust out of this with an airline and then, with light pressure on the clip, moved the button about a bit to create a good contact again.

Reassembled everything and much to my surprise everything worked properly again !

Hope this helps someone else.


Thanks for the replies, I'll let you know what transpires.................

[addo]

I'm curious as to why they die.

Is it a case of a marginal situation for heat dissipation (as in, they run too hot all the time) or simply that the switching cycles of an aircon unit on "Auto" take them into MTBF territory?

There seem to be similar Darlington trannies in the TO3 package with higher stated heat dissipation - would these tolerate a heat stressed environment better?

[Peter.N.]

The first pair I had fail I replaced with ordinary PNP transistors, I found the heaviest current ones I could, I didn't know they were darlingtons having no data - they were still working alright whan I sold the car.

Peter

[CitroJim]

In my experience they die because the circuit is actually rather crudely designed and there is no effective control of current sharing between the two darlingtons. This causes one to pass more current than the other and eventually one goes open-circuit. Then with only one of the pair working, it is overloaded and eventually goes short-circuit. All this just to save a couple of low value emitter resistors which would have helped balance current sharing enormously...

Hence why they must both be replaced as a matched pair - only then can you be reasonably sure they're sharing the load more or less equally. This can be roughly checked after repair by running the module in the open air by dangling it off its leads and feeling the two transistors after a few seconds of running to ensure they are equally warm. Careful as extended running out of the cooling fan breeze can cause the transistors to get hot enough to burn you...

In the above post the 'small button' and spring contact is in fact a thermistor. It measures heatsink temperature and help prevent a condition known as 'thermal runaway' Basically, as the thing gets hotter and hotter more and more current will flow until it cascades out of control. The thermistor and associated circuitry prevents this and has the effect of maintaining fan speed with varying heatsink temperatures...

[addo]

Thermal runaway sounds a bit like when a diesel starts feeding on its own blowby oil...

[CitroJim]

Thermal runaway sounds a bit like when a diesel starts feeding on its own blowby oil...

Yep, that's it Adam Same thing in a different context...

[Peter.N.]

...a very common one with early germanium transtors.

Peter

[rmunns]

For the first time I took advice from another Citroen site and it has been a disaster!

I disassembled the heater (not a trivial task - particularly difficult to force out the main connecting plug), then took off the module with the transistors on it.

I used a compressed air can to blow out the contact as described in my earlier post and fiddled about with the spring and the place where it is supposed to contact.

On reassembly and putting back in car - now I have no fan at all under any settings.

So I've shot myself in the foot. Apologies of course to all on this forum - I've belonged for a long time and have gained a lot from it. Why was I so silly as to take advice from another forum?

So, can anyone please suggest what now? Can I just put 12V across the motor to check motor itself is ok now? I have seen the ebay listing and would happily pay for the transistors and fit them, but now I think I have messed things up completely.

[rmunns]

Impressive how this forum automatically put the two dots on Citroen!