The definitive heater modification

[Kitch]

Worth pointing out that his is a mk1, if it's a 19GT. Totally different arrangement.

[white exec]

Just before go and (finally) do this, it dawns on me that there must be a reason Citroen never did it! I'm about to modify a brand new panel which was allegedly made in Jan 1994, and isn't wired to max fans.

I begin to wonder, if the reason it was never done, was to prolong the life of the PMOS / blower motor? Now if i had the technical spec for the motor it would be easy to figure out, but i don't, sad.

Just reading this now, I wonder whether Vanny has a point about Citroen building in max speed protection for the control circuit, and maybe the blower motors as well.

It's interesting that later cars (eg Xantia, XM, ZX...) also used (a pair of Darlington) transistors for blower speed control, but also included a "max speed relay" (on the p.c.b.) which cuts in only when max speed is selected. On these cars, which use a single blower, the running current on max speed is in the region of 25A, and starting current (if you're careless enough to turn the key with the blower set to max) higher than that. The relay makes sure that the transistors do not have do pass this high current either continuously, or on fast-start-up.

Now the BX has twin fans, operating in parallel, and the combined current looks quite modest*** from the pair here, but I could understand why Citroen might have limited how max blower speed might operate, perhaps with a less rugged transistor stage.
*** Edit: Found to be 15A each, from a subsequent bench test here, so not that small.
________

The black stripe on the Eledasa p.c.b. links pins 4 & 5 on the module connector. I'm fairly sure this is a printed-on resistance strip, which would correspond to the small resistor (next to the diode*) on the circuit diagram.
* Not sure where the diode is mounted (back of p.c.b.?) but probably there to protect the control transistor, when relay 745 switches off.
.

[tom]

If it was not intended to be used, there wouldn't be a relay fitted.