Ventilation fan has stopped

[HPsauce]

Originally Posted by moltuae

Attachment 16597

Very useful attachment which I've been studying. Thanks Mark (moltuae).

As far as I can see the main power wiring for the fan is "relatively" simple, though there are some additional thin wires to control units that are probably signalling/sensing in function.

Basically the Brown/Red Earth side goes to a major earthing point via the Blower Controller. Is that the "hedgehog" unit?
And the Red/Black Power side (up to 12v) comes via the fuse and solar panel relay.

Now, I have noticed the recirculation flaps are sometimes closed and reluctant to open, though haven't yet detected any pattern to this. Does this give any clues?
(I do normally use automatic recirculation to limit external fumes etc. but have now turned that off.)

All normal this morning though, fan ticking over gently in the weak sunlight.

[moltuae]

Originally Posted by HPsauce

As far as I can see the main power wiring for the fan is "relatively" simple, though there are some additional thin wires to control units that are probably signalling/sensing in function.

Basically the Brown/Red Earth side goes to a major earthing point via the Blower Controller. Is that the "hedgehog" unit?
And the Red/Black Power side (up to 12v) comes via the fuse and solar panel relay.

That sounds about right looking at the wiring diagrams. And the J126 control module does appear to have a hedgehog-like heatsink attached to it. Searching online, it seems the J126 control module is common to a lot of VAG cars, with some people reporting similar faults. The design and part numbers seem to vary from one vehicle to another but it's possible they share some internals (and therefore problems). I found this D2-specific post:
https://www.justanswer.com/audi/2jzi...sn-t-seem.html

Studying the diagram, it looks like the 12 volt feed comes via the J309 solar relay and the S25 fuse. The relay appears to be a change-over, switching between battery and solar power. I suspect it's activated when on battery power (supplying power to the blower via the normally open contacts) and deactivated on solar (supplying power to the blower via the normally closed contacts).

The J126 module appears to control the voltage to the blower (and therefore the speed) by introducing ground resistance. Judging by the size of the heatsink and the simple 3-wire connection, presumably it's little more than a high-power bipolar transistor. If that's where the fault lies, being intermittent in nature, I'd suspect a bad internal or external connection. Since it's dissipating a lot of power and probably getting hot-cold-hot-cold a lot, I'd suspect solder fatigue. If it comes apart easily, check for bad internal solder joints.

Another suspect would be that J309 relay. You could take it out and try bridging the N/O contact wires (red/green and red/black). Don't bridge them with the relay in-situ though, there's a danger that might send power back to the solar panel.

[HPsauce]

Thanks Mark, it's not the relay though; as noted in an earlier post I tested that by swapping it with an identical one that IIRC controls the horn.

Having looked at the wiring diagram I now suspect that the other (black, 3-pin) connector in that intake area is to the "hedgehog" (not the aux coolant pump) so I'll have a play with both in due course.

I'll have the cover off in a day or so when my replacement foam sealing strip arrives. That will be a good time to investigate; first test will be a direct 12V supply to the fan, bypassing that J126 module.

[HPsauce]

Originally Posted by moltuae

The J126 module appears to control the voltage to the blower (and therefore the speed) by introducing ground resistance. Judging by the size of the heatsink and the simple 3-wire connection, presumably it's little more than a high-power bipolar transistor. If that's where the fault lies, being intermittent in nature, I'd suspect a bad internal or external connection.

Part number 4D0 820 521A. New ones are silly money (hundreds of pounds).
Found one really cheap online in Europe and ordered it, if nothing else I can use it for testing by swapping the plugs over; no need to mount properly at first.

[moltuae]

You could also diagnose it with a volt meter. If you put the meter across the motor first, that'll rule in/out a motor problem. Running normally, I'd expect to see a voltage that changes with fan speed. When the fault occurs (ie fan not running when it should be), 12 volts would suggest a motor fault and 0 volts a controller/power fault.

If the first test suggest a controller/power fault, attach the meter's ground to a permanent ground connection (leaving the positive wire on the motor's supply connection). Running normally, you should have 12 volts. When the fault occurs if you still have 12 volts the problem is with the 'hedgehog' (or associated connections), if you have 0 volts, the fault it with the supply.

[HPsauce]

Thanks Mark, I like the logic, but it means hacking into cables or connectors to get readings and it's all weatherproof connectors in that area, so I'll try the "plug and play" option first.

I'm guessing that the thin wires running off the thick power feeds near the beige connector are probably to measure the voltages as part of the HVAC control system and the extra yellow/red wire is a control signal to the "hedgehog". I would presume it's fins are purely for cooling what is effectively a variable resistive load.

[moltuae]

That would be my guess too. The yellow/red is definitely a control wire. If the controller is a simple high-power bipolar transistor, you'll probably see a voltage on that wire that varies with fan speed (probably 0-5 volts). There does appear to be some feedback/sensing connections too which may account for the remaining thin wires.

[moltuae]

Originally Posted by HPsauce

but it means hacking into cables or connectors to get readings and it's all weatherproof connectors in that area, so I'll try the "plug and play" option first.

That's where the 'bed-of-nails' croc clips come in handy

[HPsauce]

Originally Posted by HPsauce

Basically the Brown/Red Earth side goes to a major earthing point via the Blower Controller. Is that the "hedgehog" unit?

Though studying the wiring a bit more there is a separate Earth to the solar panel wiring, meaning that (if my interpretation is correct) the "hedgehog" is bypassed when in "solar" mode. Logical.

That doesn't mean that the black connection in the air intake is irrelevant though, I don't know where that Earth lead splits off, it's still fairly big - 2.5 vs 5.0 for the main leads (minor ones are much smaller) and isn't in that connector.

[moltuae]

Originally Posted by HPsauce

Though studying the wiring a bit more there is a separate Earth to the solar panel wiring, meaning that (if my interpretation is correct) the "hedgehog" is bypassed when in "solar" mode. Logical.

Probably got a fixed high-wattage resistor in there somewhere if it does, to set the solar-powered fan speed. Since your fault occurs in normal running mode too though you can probably rule out a fault with that resistor or associated wiring.