Android head unit, FM antenna 'filter, rear SAM and an output to turn the antenna ON!

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New Member
Nov 8, 2023
R350 CDI
Hiya, fairly technical 1st question. 2012 R350CDI LWB W251. I'm new to Mercedes (years of Saabs) and MBWorld which I joined seems reluctant to provide a guru 🧔‍♀️

I swapped the Comand 2.5 for a (actually really good) cheap Aliexpress Android head unit. Everything is working except FM reception. I've traced this to what I believe is an output (actuator) on the rear SAM that looks like it powers 'off' the antenna via what is loosely described as 'antenna filter'. I think its a power switch as well as filter.

So, are SAM outputs open drain or open collector? Or do they mix them?! A temporary fix will be to apply 0V or 12V to the 'filter' pin3 I think, or can I be more correct and force a SAM output using some kind of CANBUS tool? GPS is working perfectly, but I'm not sure how that part (also in the roof) is powered.


The FM antenna 'amplifier' is usually powered from the HU, not the SAM.

The output on Chinese HU's is invariably blue,
but if you Ali express unit is designed to be plug and play for your model the ISO connector would take care of that.

These units are not unknown to arrive faulty. 1st for me would be to check the blue output becomes 12 volts (+ve) when the tuner is selected.
I would expect a blue 12 volts at the ISO, and at the additional blue, that would be part of one of the other plugs. Nowt wrong with using that to feed the amp if needed.
Otherwise ensure there is 12 volts at the amp when it's needed, but really if FM was good before it should be now.

PS. The GPS wouldn't be powered, so I think you're correct in thinking power is missing, but that would be at the amp.
Hi, thanks for the rapid reply! I checked the antenna power on the auxilliary head unit connector in the car and it doesn't exist. I only have 2 twisted pairs for the dual mics in my car instead. This is not the standard Comand 2.5 pinouts you see on the web, but is in WIS for the R class.

I'm using the head unit to power a twin FAKRA adaptor as I thought the FM amp got it's power through the co-ax. That isn't the case. But yes the head unit ant and amp aux power outputs do give 12V as expected when switched on, on the head unit side. I am fairly sure there is not a conventional head unit power on on the car side and it is sent via CAN to the rear SAM. N10/8 connector C 10 turns the amp on.
Yes, on a twin Fakra one side of the centre core will carry the 12V to the antenna amp.
Is one of those powered?
Yes, both powered and checked with multimeter. I am fairly convinced the rear SAM turns the FM amp off though.
Yes, both powered and checked with multimeter. I am fairly convinced the rear SAM turns the FM amp off though.
Is there a canbus connection to the FM amp?

While it's possible that it is SAM controlled I don't see a logical reason to that design.
Powered from the HU is a simple method.
For the HU to tell the SAM to control the amp seems ott, even for MB.
There have been instances where the centre core of a coax has broken, it being a single and brittle strand.
I guess eliminating that by confirming the 12 volts is present at the amp is reasonable.
Didn't fancy pulling the boot apart to get to the SAM today. Thought I'd do something much easier. Take the mag mount DAB antenna on an oven tray sat on the back seat (85 stations received) and put a DIY centre loaded dipole in the A pillar trim. The dipole is a bit of coax stripped back 345mm, the centre exposed and the copper folded back. 2 Turns of 50mm in the lead just at the junction. This picks up 95 stations inside the car! To be fair, the mag mount on the large steel roof externally does even better.

Getting the A Pillar trim off. OMG. The top clip is visible and strong, the other two are buried behind the air bag and its firing canister and INCREDIBLY STRONG! You shouldnt need crowbars to get this off. The little loop they give you to help pull it off helps, but what a game.

I've tweaked the metal clips so they don't hang on as hard and reglued the cloth edge that was partially bonded into the winscreen (by accident obviously). They make these cars strong and the trim itself would withstand being run over by a train I think.
You need a 'power insert' adapter that feds the power up the coax to feed the antenna amps because you had NTG2.5 (i.e you have a double white fakra for the FM/AM antenna - use the one with the locking tab out of the two, or one that does both. Such as this : Fakra to Male DIN aerial antenna adapter cable (With 12v phantom power) - InCarTec or VW Aerial adapter Fakra to din with phantom power - InCarTec
I have one of those and 12V measured on both centre pins. See pic below.

IMG_20231104_123135_906 (1).jpg

??? Why has the SAM got a connection (transistor shown) to the 'FM filter' (also shown with transistor) unless it is a switch? ???
The ML (and I assume the R class) can switch between rear window antenna and roof antenna pack, could be that, and equally could be wrong (i.e for the pre-facelift car that had separate power feed to turn on antenna amps). My memory is that the roof antenna pack does the actual switching, but I can't recall what tells it to switch.

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