BXProject

[Vanny]

So, finally i figure it out!

There was at one point some talk about moding the PCB on the blower fan control board because the PCB was 'wrong'. Now the mod (cut a track and joing another) always seemed a little odd, why the hell would Citroen allow a supplier to spec a control board that didn't work? This is pure insanity surely?

Well i think i might of stumbled accross the reason. A different PMOS!

PMOS was relatively new automotive technology 20 years ago, and the FET's themselves where inherently unstable. The whole point of having the FET is so that the blower speed doesn't change with battery voltage, in theory if you set the fan to a speed it should stay at that speed regardless of engine revs etc, in theory. Around the time the Mk2 came out PMOS where still to a greater or lesser extent still in there automotive infancy, and i would reckon that during the design/overhaul for the Mk2 a PMOS was specced that was found to be insufficient/incapable of performing the job. The result was a change of PMOS without the change of board.

The PCB that came with the Mk2 would probably only achieve maybe 90% of the max blower, which is still rather a lot and probably considerably quieter. Move on 20 years and with all the resistances increased in the wiring harness and the PCB tracks being warn then i suspect a lot of people are experiencing maybe 70% of max blower, so carrying out the mod to effectively short out the PMOS on max blower seems like a win win.

So why the difference? Well in the 80's, automotive FET's couldn't bridge themselves 100%, so if you had say 12.8v at the battery, and a 0.4v drop in the wiring harness and another 2.4v drop accross the FET then you would be down to 10v at the blower! If this was the case with the job intent PMOS then it would have made sence to have a 'MAX FAN' position, and hence the extra track on the PCB. Role through a few months later and there is a better FET available with more stable voltage control but it is self bridging, this FET can give 12v+ at the blower but if you short it out then it reduces the life expectancy, so the supply reqorks the PCB.

Okay so this would mean that everyone carrying out this mod would blow there PMOS right? Probably not. PMOS work hardest at around 6v, this is where they generate the most heat and thus have the most need of the heatsink. I would suspect the BX PMOS is fused (in that little black housing) and the reason the PMOS life should be reduced is down to the fuse over loading. But in the UK with cooler climates, and fans on max if its not cool, then the PMOS survives being shorted, and little more is said on the matter.

I will take a look at the BX PMOS if i get chance, but i will likely go ahead and do the PCB mod myself, twin fans running max out, a dream for cooling you down on those unusual summer evenings.

[Bx Bandit]

Is the fet/pmos thing the transistor jobby that fits within the fan ducting? Where be the fuse thinks you?
I've done the mod and yes it works, but I know the transistor has failed as the fan won't go down to low speed..........BUT, it will still go to the max when the pcb modification brings the fan relay in. That's on the 16v with no aircon, but on the TZD, there is an extra micro switch operated by the temp knob in the cold position - this also brings in the max fan speed relay when the fan knob is turned to max. If you nudge the heat up a fraction, the micro switch contact is broken and the relay cuts out.

[Vanny]

There is no transistor in the fan ducting.

The 'fan' is the collection of blades that mount on the motor to make a blower. Don't know why the terminology is so specific, but to save confusion if i'm talking about the blower then i'm talking about what normal people would just call a fan.

PMOS is the abbreviation of p-type MOSFET (denotes how the semiconductor juntions are arranged), MOSFET is the abbreviation for metal–oxide–semiconductor field-effect transistor.

So yes, there is a field effect transistor (FET) in the air intake ducting. But do not confuse the opperation of a FET with that of a common transistor, they have some critically different capabilities, specifically power capacity.

http://en.wikipedia.org/wiki/MOSFET

[mat_the_cat]

I don't know if this is relevant to your musings, but on mine (originally non AC) I had the board which didn't operate the full speed relay. So, as advised (which made sense apart from why did Citroen supply the car in that configuration) only to find that the relay did not click in on high speed.
On tracing it further I found that there was a microswitch behind the temperature control with had dirty contacts. Once cleaned this enabled the high speed relay to work, but only on the cold setting. My brain is a little fuddled this evening, but can anybody see the logic in this? I mentioned it to Tom, and he hadn't seen the microswitch on other BXs.
I hadn't realised that you could get a transistor to operate without a voltage drop over it, as I think you're saying (the Wikipedia page will have to wait till another day). I don't remember a fuse in the transistor housing BTW, but I could be mistaken.

I've just typed this out and only now noticed Bandit is saying exactly the same thing - oh well, I'll post it anyway so he knows that he's not alone...

[Vanny]

No a transistor wont work without a voltage over it, but when you connect the source to the drain you get 12v either side. I'm not talking about shorting it out internally but physically connecting the In and Out of the FET by adding the bridge to the max speed on the PCB. While for many components, connecting both sides to 12v isn't a problem, but on a transistor your driving current through the gate, thus inducing a reverse current and destroying the FET all together.

I'm not talking about a current overload fuse here but a thermal fuse, these can be tiny, not much bigger than a resistor. Have Mat or Bandit taken the black cover off the back of the PMOS that goes into the ducting? IF there is a fuse, then that is likely where it will be. It will be a single use non replaceable fuse designed to stop a thermal event ie the motor setting the car on fire in a specific failure mode.

In the case of BX AC the rules of engagement are different!

[Philhod]

Never had a problem with mine. It works fine, full blast, hot or cold.....

[mat_the_cat]

Yes, I've taken the cover off to replace the transistor (I did wonder if it had failed so replaced it with a heavier duty version (something like 30A IIRC). I did wonder afterwards whether you meant a thermal fuse, and I've got to say I can't remember one way or another.

The first paragraph I'm very interested in but my brain is off line at the moment and is having trouble with thinking. If you had a protection diode pointing towards the gate would that work in preventing reverse current flowing?

[Vanny]

you need twin zenners between gate and drain. I think its advantageous to have a diode between source and drain, though it would mean if you wire it in backwards then you get the blower running all the time.