Blower control (twin blower for A/C) - 19D

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white exec
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Blower control (twin blower for A/C) - 19D

Post by white exec »

Have today had some erratic behaviour of blower speed, and am needing to chase through the controls.
Blower fans both working, but running at much reduced speed across the range — except on maximum, which is ok.

Have identified all the components in the A/C electrical circuit, with the exception of two: #270, and #788.
Can anyone identify what these two are? I don't have a components code listing that matches this circuit.
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Re: Blower control (twin blower for A/C) - 19D

Post by Jaba »

Good that max speed is working. Two things that play up a lot are the power regulator transistor connections, the earth strap gets corrosion and the carbon track of the speed regulator.

Those two items: #270 is a microswitch at the top of the speed dial for selecting full speed relay and #788 is a series resistor located in the plenum somewhere. I am not convinced it does anything as I used to have a BX with a Diavia a/c which did not have that resistor. The twin blowers worked fine without it but I did once connect one in to see what happened and noticed no difference whatsoever.
Others have cured blower speed problems by connecting an extra earth to one of the control board connections. You will have to do a search for more info on that , actually its coming back, I think it was to cure a no fans blowing failure. Worth looking at anyway.
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Re: Blower control (twin blower for A/C) - 19D

Post by white exec »

That's really helpful info, John - so thanks for the quick reply.
My guess for the series resistor is to limit the current surge of both blowers being suddenly turned on at full speed. Looks like a 'power resistor' by the way Citroen draw it, and air-cooled by being in the plenum. Will try to find it.

Component numbering for BX is very inconsistent, with EWD component numbers being dependent on which model of BX you're dealing with! For XM, component numbers remained the same once allocated, with new components (for any model) simply adding to the full list. Much less confusing! Hey, ho...
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Re: Blower control (twin blower for A/C) - 19D

Post by xantia_v6 »

From examining the diagram above, I would say that the purpose of resistor 788 depends on whether the transistor shown is actually a Darlington pair, or just a single power transistor.

If it is just a single transistor, the current gain will be fairly low, and there would be enough current flowing via the transistor Base electrode for the variable resistor to give some semblance of speed control. 788 would, at most, improve the speed stability a little.

If it is a Darlington, then the transistor would have enough gain that the Base current would be negligible (and unpredictable) and 788 would be needed to form a stable voltage divider. It is not clear why a high power resistor would be needed for this.

I would suspect that the power transistor is dying of old age.

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Re: Blower control (twin blower for A/C) - 19D

Post by white exec »

Hi Mike,

As you were writing that, I was looking again at the circuit, and 788 is obviously in the variable side of the blower supply, and not the direct (full speed) side. So you're right, that 788 forms part of a potential divider for the rotary control, rather than have it operate as a less stable series resistance.

Will try to open up 658, and see what's in there. At least LHD gives better access to some of the behind-dash electrical items, when the top glove compartment has been lifted out.

The blower speed yesterday misbehaved for the first time, under very hot ambient conditions (32°C outside), but was ok a couple of hours later when things had cooled off.
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Re: Blower control (twin blower for A/C) - 19D

Post by white exec »

During yesterday's wobble with the blower speed, I was surprised at just how little cooling was delivered when the speed went low, only to recover noticeably when the blower was run at full speed (something I normally avoid doing out of respect for somewhat elderly parts). Normally, under less demanding outside temperature conditions, the air from the face-level vents is beautifully cold, even at blower half speed, or even less, but not on that day. Cooling was marginal, to say the least.

While reading up on AC compressor sapped engine power, I fell over this Wikipedia article, which contains some interesting detail. It states that evaporator cooling is directly influenced by the air flow through it, and that maximum 'second stage' evaporative cooling is only achieved when adequately ventilated. Could explain the noticeable fall-off in cooling — slow fans speed, and 32°C air entering ventilation system. (The reason for not switching to Recirc was that we had four boxed-up cats in the car, en return from cattery, and we wanted as much cool air to flow through the car as possible. Maybe Recirc would have been the cooler option.)

Lots of good description in the Wiki article:
https://en.wikipedia.org/wiki/Automobil ... nditioning
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Re: Blower control (twin blower for A/C) - 19D

Post by white exec »

Jaba wrote:
Tue Aug 03, 2021 10:28 pm
. . . Others have cured blower speed problems by connecting an extra earth to one of the control board connections. You will have to do a search for more info on that, actually its coming back, I think it was to cure a no fans blowing failure. Worth looking at anyway.
Have found the thread, from 2014 - "Heater fan not working!".
Contains all sorts of really useful info, including a missing/intermittent control panel earth, importance of having the max.speed relay properly in circuit, and repair procedure for the Darlington transistor module. Applicable to both single- and dual-fan (AC) models.
Also refers to fan speed intermittently falling off, which was the problem here.

Thanks for the prompt, John. Spot on.
Chris