Xantia Climate Control Problems

BonceChops · Unread post by **BonceChops** » 28 Feb 2006, 22:48

I know "not this problem again" your all thinking.
This one is a little odd though. I am missing the 12V supply to the fan unit - as a temp fix I have connected a cable to the 12V fan supply so this is ok, it leaves fuse 20 but never gets to the fan. Anyone know of any connectors for this anywhere please? The original cable goes up towards the top of the dash then disappears from sight. When I start the car the fan will run slowly ( with setting on max ) for a couple of seconds then stop. After a while it will do the same thing again and again. I know the fan and speed controller works because I replaced the transistors with a couple of MJE2955's. I have a 1Kohm pot supplying base current to the speed controller and can vary the speed of the fan ok. I have tried another ECU unit and another heater control unit with the same thing happening.

I hope someone can help me as it is a little cold at the moment.

Unread post by **Peter.N.** » 01 Mar 2006, 10:07

Well done on your home made control unit! On the XM the supply comes from a fuse box just behind the battery, which developes a fairly common fault. Due to the ingress af salt water from the road, the connections corrode and the fuse contacts go almost open circuit, I say almost because they wont pass enough current to run the motor but will read 12v if the load is removed. I dont know if there is anything similar on the Xantia, but because of the relativly high current involved the connections should go straight to something fairly hefty.

Clogzz · Unread post by **Clogzz** » 01 Mar 2006, 11:34

Congratulations for the bodgie, I'm impressed !

The usual stunt is the ignition switch letting go.
Story here:

http://www.frenchcarforum.co.uk/forum/v ... hp?t=17322

BonceChops · Unread post by **BonceChops** » 01 Mar 2006, 11:57

Its definately not the ignition switch. The supply goes to the ignition switch then the fuse, I have supply on both sides of the fuse. It then goes from fuse 20 to the motor but in my case it doesn't. The odd thing is the cable leaving the motor goes up very high under the dash like it goes to the under bonnet fuses. I tested a mark 1 xantia in the breakers and the motor is fed from fuse 20, mine is a mark 2 so I am wondering if they are fed differently. I need to find out more about the climate control ECU. Like when does it give the speed control base current? After all the engine is running and the speed selector is on max. Next job might be to work out what the speed selector knob does and what signal it gives the ECU.

For now I have a temp supply from where fuse 20 should be so the fan only runs with the key turned and I have my poy sticking out of the ash tray

Clogzz · Unread post by **Clogzz** » 01 Mar 2006, 13:30

The Mk II’s from 1999 had the blower motor relay modification put in at the factory, and it may be near the fuse box under the dash.
No certainty about the location, my only experience is with my own Mk I.
The ECU regulates the base current on the ‘auto’ setting.
The 4 fixed speeds are set base current settings.

BonceChops · Unread post by **BonceChops** » 01 Mar 2006, 16:26

Clogzz wrote:The ECU regulates the base current on the ‘auto’ setting.
The 4 fixed speeds are set base current settings.

I am guessing that the fan speed slider knob gives a reference signal ( resistance or voltage) to the ECU and this in turn supplies the correct base current for the selected fan speed. I would not have thought the fan speed selector would supply base current directly to the transistors.

Clogzz · Unread post by **Clogzz** » 01 Mar 2006, 17:36

Speed 4 operates a bypass relay under the motor printed board that puts full voltage on the motor.
The transistors are not used for speed 4.
They are used for the 3 lower speeds, and that’s why when the output transistors go short, we have full speed on all speed settings.
The slider biases a first transistor which then biases driver transistors that operate the output ones.
For the 3 lower speeds, I think that the slider changes the resistance in the base of the first transistor, and varies its biasing to set 3 base current values.
The slider changes resistance only, and doesn’t supply any voltage or biasing current.
On the ‘auto’ setting, the ECU does the biasing by what it senses from the internal temperature sensor.

Hope I got this right.

BonceChops · Unread post by **BonceChops** » 01 Mar 2006, 19:18

That just about sounds like I expected. The driver transistors you mention must be in the ECU yes? The forth speed setting does not directly operate the relay I don't think.
As I twiddle my pot and the base current increases the fan speed increases also. When the base voltage increases enough the relay on the speed control board is energised and the fan runs at full voltage. This is achieved by the relay shorting the collector's and emmiter's of the 2 power transistors. As the base voltage reduces by turning the pot down the relay de-energises allowing the power transistors to control the fan voltage/speed. The relay's coil must be connected between the power transistors base and 12v to operate like this. There is a 2nd wire that connects to the fan from the ECU, I had assumed that this controlled the relay but can't do according to my testing, I don't yet know what this does, unless it monitors the fan motor voltage as a speed feedback to the ECU.

Clogzz · Unread post by **Clogzz** » 02 Mar 2006, 11:57

Bit of a late reply, we have very strange hours here, it’s about 10 PM, and I normally only get to play with the computer when I get to sit down for good after dark.

The driver transistor I would expect to be on the speed control module attached to the motor.
Haven’t been there for years, but there are quite a few components on the module.

It would make perfect sense that the 4th speed is set by the slider, and the bypass relay then closes when it senses the high current for the 4th speed.
That way, if there’s high current at a lower speed, caused by restricted airflow because of a blocked pollen filter, the relay still closes to protect the output transistors from excess current.

The second wire must indeed have to do with sensing whether the motor is spinning.
If the fan doesn’t spin, the air conditioning must be prevented from starting, otherwise the evaporator will frost up because there will be no drain of ‘cold’ from it.

ECU and slider faults seem to be rare.
Most instances of blower fan troubles are loss of supply, and shorting of the output transistors.

I don't see why you had to put a pot in as speed regulator.
Since you have bridged the missing supply, why wouldn't the slider work ?
Could the dark thin wire be something like a 'floating earth' to the ECU ?

rory_perrett · Unread post by **rory_perrett** » 02 Mar 2006, 15:16

Sorry for what may be a vague and possibly not relevant reply as it relates to a ’95 not aircon car.

Blower fan became intermittent and then pack up on our Xantia ’95 td. Couldn’t work it out as I appeared to have 12v in all the right places, including out to the fan but irtr wasn’t working. Assumed ignition switch wasn’t up to carrying the current required
and did the relay mod. Still no joy! Removed the carpet above the passenger foot well to have a look at the motor and found a green connector that plugged into fan motor was showing signs of over heating. There were 4 or 5 wires to this plug including a heavy purple one which I assumed this was the power supply to the motor and the other were to the slider for the speed control.

This is the half remembered bit, I seem to recall that with the plug removed, and therefore as I had assumed the main power supply, bridging across some of the contacts on the motor caused the fan to start working. I concluded that the power supplier to the motor was elsewhere and the wires to the connector I had removed were all out and back to the fan speed control. Cleaned up the plug and contacts and its working fine.

Rory