EndlessCorners

Backstory: The only thing that works on the unit are the backlights when the headlights are on. Otherwise it is completely unresponsive. Trying to get into the On-Board Diagnostics does not work. The dash exterior temperature display only shows " - - - ".


I verified that it is getting 12vDC on the correct pins.

I verified that its grounded.

I verified that any unit you place in the car behaves the same way.

I verified that the entire unit is dead because it does not show up on VAG-COM.


I don't even know what shop to take it to. It seems that the private shops (in the bay area) are drivetrain specialists, and the last time I went to a dealership I was told my S4 doesn't exist. As the weather gets colder the car is becoming completely undriveable due to window fogging.

js0ne

I would have to say you most likely have a wiring issue. You need to check for more than just 12v.

With a wiring diagram you'll need to 'pin out' the terminals at each connector one by one.

EndlessCorners

if the head unit is getting power and has a ground shouldn't it at least power on so I can get diagnostic info?

Turbo_Anarchy

under the hood and in the dash.

EndlessCorners

Checked them all with a Ohm meter.

Besides that I have had all the relay and connector panels open and have wiggled everything as well as checked for anything out of the ordinary.

I also swapped out the load reduction relay to verify that is working.


The single weird thing I did find was a 4 pin connector with two conductors that wasn't plugged into anything behind the passenger a-pillar kick panel. I couldn't find anything to plug it into. It appears to be part of a bundle going to a green box above the HVAC floor vent hose.

Turbo_Anarchy

28 years as a mechanic and I still dread electrical problems. They are fun to find and rewarding to fix when you find them quickly. But they suck when they are elusive. Sounds like you are eliminating the possible causes properly. It is probably a connector somewhere. The Bentley should show the locations of the connectors. Good luck with it bud.

EndlessCorners

Thanks. I work with dense data and control wiring for a living and stuff like this even frightens me.

Im trying to avoid checking every single pin. I know certain things don't affect the operation of the HVAC head. I have had the headliner temp sensor unplugged for the past 4 months and that hasn't affected it at all.

Im hoping there is somebody on here with a ton of experience (maybe a dealer tech) with these things that can say "if it sees an open circuit to such and such it wont power on". Then I can save myself hours and make a direct attack on the problem.

jjm's quattro

cluster - the wiring is related - my dead HU turned out to be dependent on where the inst cluster dimmer was set. HU wouldn't work unless the dial was turned to full bright. In the end it was the HU that had failed and swapping it cured the problem - but still worth a shot.

Joey T

unless you can verify voltage and ground while the unit is plugged in. You may find a bad ground that shows ground with a test light or MM, but not when a higher amperage load is introduced.

Tomorrow I will try to get and post the wire diagrams for you. What year is it?

EndlessCorners

How do you check the resistance of the ground under load?

Its a 93. I have been using the diagrams here: https://forums.audiworld.com/s4s6/msgs/243113.phtml + Alldata. If you have something better I would greatly appreciate it.

Unless it first wants to see input from one of the devices it controls I don't understand why it wont power on at least. I can't imagine the AC head wanting so much current that a weak + or - lead would keep it from working.