The definitive heater modification

Frequently asked technical questions and common modifications/improvements
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Way2go
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Post by Way2go » Wed Mar 28, 2007 2:42 pm

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. :D
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?
1991 BX19GTi Auto

tom
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Post by tom » Wed Mar 28, 2007 3:22 pm

Yes.

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Way2go
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Post by Way2go » Thu Mar 29, 2007 2:49 am

tom wrote:Yes.
:? 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?
1991 BX19GTi Auto

gazpacho andulus
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heater modification

Post by gazpacho andulus » Sat May 26, 2007 12:07 pm

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.
now I'm listening. TZD Estate 1991

jeremy
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Post by jeremy » Sat May 26, 2007 12:46 pm

Have you plugged it in? - On mine the plug is accessible from the dashboard top tray.

gazpacho andulus
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heater fan modification

Post by gazpacho andulus » Sat May 26, 2007 5:46 pm

can't see anything to plug in. Have checked fuses. can you shine any more light for me? :?
now I'm listening. TZD Estate 1991

gazpacho andulus
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heater modification

Post by gazpacho andulus » Sat May 26, 2007 11:08 pm

found connector at the back was plugged in . how do I test to see if PCB is allright?
now I'm listening. TZD Estate 1991

gazpacho andulus
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heater modification

Post by gazpacho andulus » Mon May 28, 2007 12:13 pm

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 :?
now I'm listening. TZD Estate 1991

jeremy
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Post by jeremy » Mon May 28, 2007 2:46 pm

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.)

gazpacho andulus
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heater modification

Post by gazpacho andulus » Tue Jun 05, 2007 6:15 am

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. :D
now I'm listening. TZD Estate 1991

gazpacho andulus
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heater modification

Post by gazpacho andulus » Wed Jun 06, 2007 5:24 am

Will it do any harm to join wires 2&3 to achieve earth on 3?
now I'm listening. TZD Estate 1991

classic2cv
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Post by classic2cv » Fri Jul 06, 2007 12:49 am

I’ve just done mine not too much trouble (20 mins to take the heater panel off and mod it )

But it took a day and a half to find my soldering iron and solder (bought new solder in the end ) :x

Cleaned up the circuit board put in new bulbs and refitted it

Now I get more air and I’m able to see the heater controls in the dark, Result :mrgreen:
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tim leech

Post by tim leech » Fri Jul 06, 2007 9:24 am

My heater fan doesnt work on the TGS, the previous owner says its a relay? is that true and which one is it?

tom
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Post by tom » Fri Jul 06, 2007 10:41 am

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.

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Post by jeremy » Fri Jul 06, 2007 10:59 am

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.