After doing the heater matrix and o-rings on our Vogue SE in January 2020 and having to bypass the matrix again early in 2021, I’ve got the dashboard out doing the whole job again.

This time I’ve opted for an Audi heater matrix and have removed the metal pipes through the bulkhead and replaced them with silicon pipes connected directly to the T’s in the engine bay where the LPG comes off the heater hoses.

After finishing I decided to fill it with coolant with the dashboard, airbags and HEVAC panel connected so that I could get it pressurised and check for leaks.

All was going well at this point, no leaks visible and everything seeming to be tight.

I then moved the flaps on the heater box and am a bit confused now… On the Windscreen setting I’m getting heat on both sides, however, on Face/Lap and Feet I’m only getting warm air on the drivers side. On the passengers side I get cold air.

I had to split the heater box so that I could trim it slightly to get the Audi core in and am wondering if there’s a chance I’ve got something in the wrong place.

Open to ideas on best things to check though :)

David.

As well as the cog that the distribution blend motor drives, there's a lever that operates the flaps for the vents and a hexagonal rod that runs through from one side to the other. The lever hooks onto a cam thingy on the cog, so on the drivers side, while the rod pushes in from the passenger side with a plastic plug to stop it coming out. When I had Nigelbb's car apart for a new matrix, I found that rod had slid out so wasn't doing anything. Not sure how it would affect temperature from side to side though.

Perhaps remove the blend motor on the passenger side and check everything it drives moves freely.

That's my next plan of attack I think.

I can feel the flap moving through the side of it but something just isn't right. I've got a little inspection camera that I'll feed in and see if things are moving.

Had a proper look at this today. The heat coming out of the passengers side when set to the windscreen was actually coming from the drivers side. The passengers side blend motor wasn’t moving and was on cold.

I stripped the motor and checked it over. When trying to calibrate the motors using Testbook this one wasn’t trying to turn at all. I put power directly to the motor and stupidly forgot it was connected to the HEVAC panel… it didn’t like that as it promptly dispersed a puff of smoke.

I had a spare panel in the workshop that the fascia PCB was faulty on so swapped the main PCB from one to the other and reassembled it in the car. The blend motor then start started turning as you’d expect so i reassembled it and refitted it and it’s working great after a recalibration so it looks like the PCB was the cause of the fault before I fried it further.

When I press AUTO now I get a fault code stored “Compressor Clutch Control Circuit Fault (O/C or S/C 0v)” am wondering if this could be a compatibility issue with the panel as it came from a 1996 car and is now in a 2002 car or if it’s because there’s no refrigerant in the A/C. The last panel was on the car with no gas in it and it never flagged a fault but am unsure if it’s maybe a difference in the firmware.

Time for a bit of Googling I think.

David.

Been there, done that. Putting 12V onto the blend motor with the HEVAC still connected causes a small puff of blue smoke and the unmistakable smell of burning silicon. There's 4 small 8 legged LM272M op amps, and they don't like it up 'em as they used to say on Dad's Army.

It is a difference in the firmware but the reason for the error is that the early HEVAC would have driven the compressor clutch directly but on a later car it drives a relay to pull in the clutch. The early ones monitored the amount of current being drawn so will log a fault when driving a relay as it doesn't draw enough current.. Once it detects the fault it won't try to engage the clutch again. You can jumper the relay so it drives the clutch directly or add a resistor so it draws more current and the fault will go away.

I saw the melted LM272 chip when I stripped it. I wondered about replacing it but am not sure if there’s another fault somewhere on the board as it wasn’t driving the blend motor before I blew it up.

Might try and find a later control panel so that I can keep it original.

Replacement heater panel sourced from a local breakers yard today for £20 :) Going back at the weekend when it’s not blowing a gale and chucking it down for a few more odds n sods.

Ordered 5 LM272’s from eBay so will repair the old panel when they arrive too as you never know when a spare might come in handy.

David.