Heater blowers on full speed

[adamskibx]

Hi All

Has anyone had this before? It used to work when it felt like it but now it runs full speed regardless of switch position. I did some reading on another website about this happening to a Xantia but cannot find any mention of this problem on a BX.. Also, if anyone has any tips about removing the facia panel so I can get to the switch that would be handy. I cannot figure it out.

Thanks,

Adam

[adamskibx]

I've figured out how to remove the fascia to access the control panel. Even removing the connector to it leaves the fan on full speed. I read on the BX DIY website that there is a full speed relay under the steering columb. Removing that also has no effect. I guess I can put up with it but it's a pain as it will make the alternator work harder wasting fuel.

[Jaba]

With the high speed relay removed you remove the earth path for the blower motor so I dont think you found the correct relay.

The first suspect must be the power regulator transistor in the blower housing.

[adamskibx]

Thanks Jaba

It must have been the wrong relay then. It was a big white one attached to the inside of the lower shroud I found. I'll have a look for another, but if the power module in the motor is suspect I suppose I might as well go straight to that.

Thanks for the pointer.

Adam

[rutter123]

sounds as tho the speed regulator has gone, in which case it will only run on full speed, these are still available tho pricey, try one from another car first if you can before spending out on a new one, maybe someone can offer advice on repair of the old unit?

[adamskibx]

Yes - I removed the speed control unit, which is cooled by the passing air, and tested it after having cleaned off some corrosion between terminals. There was no change. My theory is that the full speed relay has also failed, meaning the unit was taking all the load while the fan was on full. I think I'll try to track down a second hand replacement and wire in a simple on/off switch for now.

Thanks,

Adam

[mat_fenwick]

sounds as tho the speed regulator has gone

Not sure which bit you mean by this, as he's removed the transistor module which sits in the blower housing and the problem remains. On most cars, if a heater control resistor fails, only full speed will work but the BX is a little different! My best late at night guess without checking the circuit diagram, is that the full speed relay contacts have stuck in the Normally Open position and bypassed the transistor module.

The large white relay shaped unit sounds like the lights-on warning unit.

maybe someone can offer advice on repair of the old unit?

There may even be a thread on this very forum But doubt that's the problem in this case.

[Brian]

Hi, Pull the connector off from the thermo sensor, it's located at the bottom of the radiator drivers side.

If that stops the fan, then you have found the culprit.

Good luck

[mat_fenwick]

There's no link between the radiator fan switch and the heater blower though...

The blower motor is fed via an ignition switched supply, and earthed either directly via the full speed relay (3 thick blue wires and two thinner green ones IIRC), or the transistor in the blower housing. If you've removed the transistor module that only leaves a faulty relay, or a short to earth in the wiring to the relay. Unless I'm missing a trick, which is always possible

[Brian]

Hi Mat,

Must remember to read the thread complete so that it sinks in to my ageing brain.
I was Confused, thinking it was the cooling and not the heating fan...

But in the summer, it is infact a cooling fan, so not that far out

Sorry

[Thread Bear]

I have still to resolve the problem with mine. The run out from BX DIY is only so helpful as it is not at all clear to a first time investigator where or what to look at. The only relay I can find near the steering column is the one attached to the lower cover and would be on the left. But are LHD a mirror image? It does not tell me if position three on the circuit board for the rheostat is live, earth or something else. My assumption is this is the live feed in from the relay. Without a wiring diagram I am unclear quite how that functions with a relay. Why not a switch creating a simpler more reliable item? There is something else going on I am not aware of as it then talks about a 6 pin white plug. Where? There is a 4 pin plug on the relay on the heater motor. I do not really understand what that is for. Seems a complicated way to do something simple. There is insufficient info here to be able to resolve this wiring issue unless the fault is one of three things. Mine seems to be a fourth problem as I can make the circuit board work onward. I can prove the relays. I can prove an earth but there is no function on the relay. I know the motor end works. That would seem to be an impossible failure unless this is the wrong relay. If so were, in fact, is the relay?

Secondly I have replaced the two bulbs on the circuit board. This now light up when the ignition is on. I thought they were supposed to come on with the light switch. I am assuming this plug is idiot proof as its handed but can in fact go in the other way round?

[mat_fenwick]

Thread Bear: Forget the relay for the time being if you have a complete failure of blower motor. All the relay does is bypass the transistor module in the blower housing.

Have you tried running an additional earth cable to terminal 3 on the heater dial PCB? This may well cure the problem without needing to understand further. Alternatively, you can link to the other brown cable (sorry, can't remember which terminal it goes to) as you've proved that earth is OK as the lights work - for some reason the earth for the lights is separate. The lights should come on with the ignition, again for reasons unknown to us!

In a nutshell, the function of the transistor module is to control the amount of current allowed to flow to earth from the motor. The speed control dial allows a smaller amount of current to flow (via terminal 3) to earth which in turn controls a larger amount of current flowing through the transistor. Think of it as an infinitely variable relay! It's more efficient than the normal resistor pack, with little energy being wasted as heat which I guess is the reason Citroen designed it this way.

[Defender110]

As Matt has said, link the 2 earths, read throught this hread.

http://www.bxclub.co.uk/forum/viewtopic.php?f=15&t=5515" onclick="window.open(this.href);return false;

[Thread Bear]

That is what I do not understand. The blower, having been changed for a working one in and out of the car, it should work, to my head layout. If I cross the PCB directly it works at the differing speeds. The scraper is working. That leaves the relay on the blower. Not sure how to test that but if the motor runs jumped to the PCB it must be working. Indeed I drove last night with a jump cable onto the PCB contact.

So terminal 3 is a dedicated earth, not a component, as such. I imagine its not linked to the lights in case it dumps through the bulbs and knocks them out. Different fuse too I expect. These bulbs are linked to the dimmer switch.

Proved the relay is not the door open warning one last night.

I will try the routes suggested forward. It would seem that there is a issue with the proportional control or the resister unless the relay wiring is defect like the indicator. However bypassing it should prove the rest of the system, which so far has not been the case.

I have my spare blower relay/resister here. There is a soldered in strap on this that seems to not be soldered to the back of the resister but just pressed against it. It has a loose end. What is this, a crude over-heat off. Is this not likely to get polluted with corrosion. If its a connection then this could be a cause of failure or poor performance.

The problem here is that without a wiring diagram it is not possible to check each wire, the next component, next wire, next component. This ultimately is the only way to chase out faults when people have been fiddling. I know its been apart once (?) already as the screws are messed up. Probably wrong tool used as they are these nasty star things. It is the bone headed way I work because it is effective in chasing out intermittent and silly faults. Never trust what someone else has done. Never trust crappy looking bits, even if correct fitting. This methodology starts to fail when you get to moderns as the wiring becomes too complex and interconnected. This problem above is nudging the point of not being simply testable and I am missing something.
I fear if it is not going to work today the snippers will have to come out and a simpler solution found. I have not time to muck about and a house to put together. As it is Homer is a still born project due to the time taken to sort this car's appalling state of fittings out. If I had known it had been maintained so badly I would not have bought it, but its a risk you take. Its nearly a good car now. I just hope the rest of the fleet are not similar! As it is the other Turbo Estate has been effectively scrapped by the wrong mounting bolts being fitted. Buying a BX is certainly an adventure in botched maintenance engineering but I fear this is really common to any older car, in truth.

[Defender110]

As said link the 2 earths.