Blower fan control
Posted: 09 Feb 2010
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.
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.