jeremy wrote:The speed control on BX heater fans is achieved by the use of transistors which do a very good job but always have a residual resistance. The system is designed to remove this residual resistance by simply bypassing the transistors to enable full power to be delivered to the motor - and they did it rather nicely using a relay rather than a big switch on the dashboard. The only shame is that someone got the circuitboard wrong!
As for motor life - its designed to work in this way. Its not compulsory to run the thing flat out all the time - and you would probably get too hot anyway! Full speed is only achieved when the knob is rotated fully clockwise, the variable slower speeds remain undisturbed.
Theory seems spot on Jeremy.
A question though, in the absence of the full circuit diagram. Is this modification applicable when you have factory aircon and thus 2 blowers that already pump out a lot of air or only in the case of cars with single blowers?
ie is the relay referred to fitted when you have factory aircon and does it do the same job?
My Cars: 2003- Passe-Partout 1.9 TGD estate 2005 Grolliffe Tizzydee turbo estate and sundry other BXs and Grace, a CX TRD. 2008 to 2023 - all sorts of stuff, some interesting 2024. TxD 1.9D estate. 'Wheelybin'
Are you answering "yes" to the first part or the second part
Way2go wrote:Is this modification applicable when you have factory aircon and thus 2 blowers that already pump out a lot of air or only in the case of cars with single blowers?
Fantastic.!! I managed to follow the instructions and expose the cicuit board. Trouble I've got is that the blower nor lights show any sign of life. any ideas would be gratefully accepted.
the brown connector has 5 wires going into it I earthed a tester on the car body and spiked 1 2 and 3 with no result. When I spiked 4 I heard a click behind the dash and the tester lit up. when I spiked 5 the heater started to blow(the PCB was not connected) I plugged the connector to the PCB and got the same result when I made contact with the two left hand terminals at the top of the PCB but nothing worked without me shorting iy out. I think the PCB is knacked. any comments to shed more light on this
The circuit board is in the earth return circuit - ie the circuit is through the ignition switch, through the component (ie blower motor etc) through the plug into the circuit board thence to earth.
So you have earthed the control coil of the max speed relay (click) and the blower motor - so that part of the car wiring is OK. Working from the right on the circuit board terminals 2 and 3 are connected together on the board and are earthed - terminal 1 is the panel light.
If none of the 3 functions work (panel lights, max speed and blower motor) then there may be a general board fault but this is unlikely as the whole thing is so simple.
If the panel lights only work then the chances are that there is a problem with the small 'wiper' situated under the rotating disc has been lost or displaced. Its function is to join 2 segments under the rotating part of the switch together and complete the circuit. If this is missing I'd still expect the panel lights to work subject to what I said in my earlier post (namely with good bulbs etc.)
terminal 3 wasnt earthing I got new bulbs which worked and thus illuminated I put a screw driver across 2 & 3 and it started to blow. I rotated the white dial and It increased to full power. IT WAS A PROBLEM WITH THE EARTH. Thanks for your help.
My Cars: 2003- Passe-Partout 1.9 TGD estate 2005 Grolliffe Tizzydee turbo estate and sundry other BXs and Grace, a CX TRD. 2008 to 2023 - all sorts of stuff, some interesting 2024. TxD 1.9D estate. 'Wheelybin'
x 12
Postby tom »
Possibly. There is a relay under the column, right hand side, single 8mm bolt. There are three thick wires. pull out the relay and bridge any two.
There is one inert combination (nothing happens) and two active ones. One will cause the fan to run flat out, the other will cause it to run on the speed control. If none work then the answer is no, it is not true.
Tim Try the motor connection first - I found that the motor failed due to intense corrosion there - I must have had about 10 goes at cleaning it - and was about to go and get the kit to chop off the connectors and substitute something else when it sprang into life.
I'd check the motor first - independent feed and earth. If you can't get the motor working searching for relays etc will be a waste of time.
The relay is only used for maximum speed - all other speeds are controlled by the transistors. So if it is a control issue I'd expect the knob or the transistors to stop it working on slow speeds if they fail.
If you take the plug to the circuit board you should be able to induce full speed using the relay if you make the appropriate connection to the plug. I say that as you should be able to get access to the plug through the clock tray.
Much to my surprise the rheostat on the back of the knob stands up to use rather well. I would have thought it was prone to dirt on its track (or burning) and softening of the wiper (rotating) contact spring (provided by its shape). If this happened its likely to do something somewhere round its rotation (audible sparking, slow speed only - that sort of thing) - and will probably respond to cleaning.
Are you answering "yes" to the first part or the second part 
^^bx>