If you own a newer Audi (2004+), you may have option called MMI (Multi Media Interface) which control all your media player and display on your dash 7″ screen. Some are built-in the dash or flip up like the A8 D3. The MMI is actually a very simple system, but quite confusing for Dealers or Repair shop to troubleshoot when things failed and consumers will normally end up paying for the wrong diagnosis and good parts until the problem is finally fixed, i.e, they replaced the right one after many other trials and errors. It’s good to understand the system and sometimes you can fix or find the problem yourself and save money. It communicates with MOST (Media Oriented Systems Transport) optical bus.
Most dealers don’t offer software update for MMI 2G – The latest version is 4610. Don’t update software unless you need to add AMI because you will need to get back the instrument cluster functions that’s lost in the upgrade. I have upgraded mine and . My 2008 Q7 still has the original 4140 which I’ve never updated. If it works, don’t fix it.
Since the optical bus is connected in SERIES, first device out to then second device input and so on until it gets back to the first device input to complete a loop. Think of it as some Christmas lights string, one burned out, the whole string is OFF until you replace the correct bulb to complete the loop.
So, one morning after a heavy rain over night and your car parked outside, the sunroof drains were clogged, the water entered the car, you turn on the MMI and the screen comes up with the MMI screen for a minute and goes black. You can hear the clicking in the rear of the back seat in the trunk, but nothing happens. Some devices on the MOST bus is malfunctioned (broken), the optical (red led) signal got stuck somewhere and didn’t get back to the head unit (first device) input. It could be one device or more. Before you get panic, you should check all the fuses, especially the one in the trunk, just above the battery, that’s the fuse for Radio, Amplifier, Navigation and TV tuner. If you’re lucky, the fuse is blown, replace it may fix the problem, but if it’s still intact, you need more troubleshooting to do.
It’s a mandatory for Audi owners with MMI system to have is the optical bypass loop (female), at least one as spare in case the system is down. Audi dealers do have the originals, but it’s not necessary, any aftermarket female loop would do.
Most of Audi savvy people would normally rely on the VAGCOM or VCDS to troubleshoot the system, but they will end up replacing some good parts instead. VCDS or VAGCOM will report very confusing data because when one device failed, all devices will show up on VCDS as broken which doesn’t help us to find out which one is the culprit. I’ve created a video to show how falsely VCDS can report failures. The incompetent dealers or Indies will use the VCDS output to justify for their replacing good parts mistakes on customers expense.
Here are some of the common troubleshooting techniques that I’ve seen so far. Every time we push a button on the control panel, it will wake the MMI system and try to select the device connected to the button, when the MMI wakes up and check the MOST bus by sending signals to all devices and got stuck due to some devices not responding, it will shut itself, hence the MMI screen displayed for a few seconds then goes black. The device query restarts when any button is pushed. Only if the head unit is functioning.
1. When you push any button on the control panel: NAV, CAR, SETUP, CD, AMFM… etc, the MMI screen comes up and then goes black, it normally indicates the Head Unit (most expensive part of the system) is working. If it doesn’t, then it may be dead or the control panel itself is broken.
2. Sometimes, people spilled drinks over the control panel by mistakes, the control panel no longer works, but still, we can check to see which part of the control panel is bad by pushing one button at a time to see which button is bad. If the MMI welcome screen never shows up, the head unit is dead.
3. It’s also known that if the device is faulty, there won’t be any LED signal passing through, so if we unplug the optical cable of one device and it’s not blinking when a button is pushed (MMI wakes) then we can assume the device before this is faulty, check the device loop picture to see components relative to the loop.
4. If you have to replace the HU (head unit-4E0035729-4L0), you do need to get CP (Component Protection) removed by a dealer or your sound won’t work or you can swap the old eeprom to the new one but you need electronic soldering skill.
This is my understanding of the MMI system, it’s only for references and not guaranteed 100% accurate, use it to troubleshoot at your own risk. If you see something that I missed, please comment below so I can add to help other Audi Enthusiasts.