2015 Audi S4. No crank, No Start, No Dashboard or MMI lights

[rogal]

Hi All, running into a weird situation this weekend. Came back from a drive and then after a while unable to start.
The only thing on the dash is a blinking red parking brake light. The rest of the dash is not turning on. I hear my secondary water pump kicking in when opening the door, lights are working, radio works but thats about it.
Checked fuses on both sides, removing glovebox, checked connections to the J533 box and they seem fine. VCDS only doors and hvac can be reached. Everything else either malfunction or cannot be reached.
Battery is fine, infact i charged it separately just to make sure.
Nothing else happens when inserting the key.
No work has been done on the car besides a tensionar and SC belt change recently.
Thoughts?

[Smac770]

That should be the in-tank fuel pump priming when you open the driver's door.

So you are able to lock/unlock the car, and you are able to turn on ignition and get audio system output.

With the car in the ignition on state (radio playing), did you check the fuse columns on each end of the dash to make sure all three are showing 12v? The black columns are term 15, only power with ignition on. Some of the fuses are term 87 (engine cranking or running), so check fuses near the middle of each column. The brown and red columns are term 30, they should always be reporting 12v. If the black columns are not showing any voltage, your term 15 relay is probably dead. Under the dash, behind the headlamp switch.

[rogal]

Thanks for reply.
I have an additional intercooler pump which turns on. But yea the intank fuel pump primes as well though barely hear it.

I can lock/unlcock the car via the door panel only, with the remote control i cant. Ignition doesnt turn on(at least nothing happens when key is in) but i can turn on the radio separately.
Only the parking light is blinking and the ebrake button is blinking. Dashboard and mmi unit is off.

I am going to check the V to the fuses now.

[rogal]

So completed some checks on the fuses. 12v on the brown and red columns. 0 on the black on both sides. So we are leaning toward the term 15 relay i suppose.

[rogal]

I swapped the relay with a working one, and unfortunately still same issue.

[Smac770]

Relay is managed by the J393. You also mention you cannot use the key fob to control the locks. The J393 is what's on the other end of that antenna. So I'd focus on the J393 next, is it powered.

That's going to be fuses 8, 10, and 11 in the right rear, in column "B" (black). This column is term 30, the fuses should always show hot.

[rogal]

Checked the fuses in the luggage area, column B black all of them are less than 3V. Column C brown are getting 12v

[Smac770]

So you probably have a wire issue from the battery positive post to that fuse column. The fuse column has a single wire running to it, then the fuse carrier has a metal bus bar to break out the connection to all the fuse slots.

Here's the diagram I made for another thread.

Audi B8.5 power distribution.jpg

So if column C is good voltage, then the voltage at the output of fuse SA1 is fine. If the voltage in column E is also good, then the issue seems specific to the wire leading to column B or column B itself. If the voltage on E is also bad, then it's probably the common wire from SA1 to the splice point for B and E. It could well be the splice point, with the column B connection being not well. I suspect E is fine, if your radio is playing without issue (unless you don't have MMI (navigation)).

[rogal]

So column E also has 12v. So then Column B is the focus.
I removed the fuse column, this is going to be a fun time tracing this cable as it seem to be going over the wheel first and hard to reach.

Diagram helps, many thanks.

[Smac770]

Alternative would be to not trace it down. Cut the wire where you can access, be sure to tape off/secure the source end as it'll still be somewhat hot, then run a new wire from the SA1 fuse output post to the fuse carrier wire.

[rogal]

Thats what I was thinking as a backup. But I still want to find the source of the problem.
Will look to doing it today and will report back.

[rogal]

Had this resolved before Christmas but didnt get a chance to upload the photos.
Anyways, there was an issue with the splice point deep in the right rear side with the cable leading to the battery and the other to column B.

Once I had this cut, cleaned and joined together the car started right up.

@Smac770, thanks very much, the info and diagrams helped, where to send the beer?

[Smac770]

Wow, that's some real corrosion there. Wonder if it got wet at some point somehow. Glad you were able to hunt it down.

[evanb]

@Smac770, thanks very much, the info and diagrams helped, where to send the beer?

Smac770 is one of the most knowledgeable and helpful members on this forum. Sir, you are greatly appreciated.