W126 handsfree solution with original Becker

My one owner 560SEL still has its original Becker CD player.   Australian delivered 560SELs and SECs came standard with the CD player, which also has a control box in the boot.   Mine is still working well.  My goal was to find a good W126 handsfree solution with the original Becker.   While a modern radio and speakers would probably sound better, the Becker is a part of the original character of the car and worth keeping.

This era of Becker do not have an aux input like the 60s models do.   Therefore, the only solution is to have the unit modified with the line in, or use an FM transmitter.   Becker cassette owners can also use one of those cassette adapters if their cassette player is still working.  At some point, I will have my Becker modified for a line in.    In the meantime, I am using an FM transmitter for my W126 handsfree solution.

My needs are pretty simple.   I want to play music off a USB drive, and I want to be able to make hands free phone calls from my phone via bluetooth.   Music streaming from the phone is only nice to have.

My first solution used a Kogan FM transmitter.   I was unimpressed with this unit.    I will write a separate review some other time, as it left a lot to be desired.   Not specific to the Kogan unit was the requirement that I keep the ashtray open at all times.    The W126 interior is really nice, but let down if the ashtray is always open.

As I needed to replace the Kogan FM transmitter, I started looking for one I could modify so it fit inside the ashtray.    I found the best selection on AliExpress.   In the end I chose a model where the main housing was a small rectangular unit.    It was easily small enough to fit into a W126 ashtray insert.    I bought two of them.    One to modify for the ashtray and the other as a backup, If I found my modifications were fatal to the unit.   I have been using the backup plugged into the ashtray and I have been happy with its functionality.    It also has a USB charging port for my phone, an added bonus.

I didn’t want to modify my original ashtray insert, so had planned on buying one from eBay.   None for a reasonable price had come up, but then I purchased my 420SEL parts car.   The ashtray was in good condition.

The first step was to see if my AliExpress FM transmitter could work without its appendage that goes into the cigarette lighter.   I carefully removed it.   Inside the appendage was a small transformer that took 12V and ground from the car and supplied 12V, 5V and ground to the FM transmitter.   The main FM transmitter ran on 5V, but it also had a battery monitor function that needed 12v.  I was able to ascertain all this by applying 12v and ground to the old cigarette lighter connections and using a multimeter to verify the voltages coming out the back.   Red was 5V and Yellow was 12V.

Instead of trying to build my own housing for the existing transformer, I purchased a separate one online that came in its own housing and would fit in the back of the cigarette lighter insert.   I wasn’t sure of the amperage requiremnets, so I built in some fat and got one that could do 5A.    Since it has to power the phone charger as well, I wanted some head room.

Next step was to modify the 420SEL ashtray insert.   It had two features to aid smokers.   Around the rim, there was a cigarette holder that would have prevented the FM transmitter from being fully visible.    There was also a bump on the bottom, presumably to stop a cigarette falling in if it is resting there.   That made the button on my FM transmitter too high.    I used a dremel tool to cut off the cigarette lighter holder and a hammer to bash out the bump.

From there, I did a quick test fit to see if the modified insert still went into the ashtray housing.   It did.   I also did a test fit of my transformer and FM transmitter.   They worked too.    My W126 handsfree solution was taking shape.   It was a pretty simple task to wire it all up.   I planned to use the cigarette ligher feed, as I would no longer need it and did not want to modify the car.    I would need two 12v wires. One for the transformer and one for the 12v of the FM transmitter, plus a ground.   Confusingly, the output of the transformer was a 5V yellow wire which then connected to the 5V red wire of the transmitter.   Similarly, the red 12V wire from the car connected to yellow on the transmitter.

I had originally planned to send the wires out via a hole in the clear plastic insert on the right hand side which allows light to get in.   I soon abandoned this approach as I could not re-fit the ashtray insert to the housing with the wires sticking out this way.   In the end,  I drilled a hole in the back which worked.   It probably would have been better to drill it about 2cm to the left as there is a gap in a plastic molding that would have made the exit easier.   To stop my components rattling around I used some velcro.   As a future improvement, I should put a rubber grommet around the hole for the wires so they do not chafe.   The two 12v then connected to a single spade connector, as did ground.

Next step was to remove the ashtray housing from the 560SEL.   This was only necessary to unplug the cigarette lighter.   Its a two minute job, with two screws visible once the insert is removed and the ashtray is fully extended.    The factory plug worked fine with my spade connectors, so my W126 handsfree solution can be removed at any time.

Important to the design was to leave enough slack in the wires so the insert could be removed while installing the ashtray housing.   This is essential as the housing cannot be screwed in with the insert installed.

Obviously, before I screwed it all together, I tested the solution.   Success!  Everything seemed to work fine.   I then properly installed the ashtray housing and the insert.     At some point, if I am still happy with this solution I will create a cover for the wires on the right, and provide an on/off switch.    I’ll also look at a retractable charging cable for the left side, so its not hanging around when not in use.

The ashtray only needs to be open if I am speaking on the phone, or if I want to change track on my music.    I had hoped the remote control that came with the FM transmitter would even allow me to go to the next track without opening the ashtray, but unfortunately that was not to be.    It works quite well though.  When driving home I got a call, and all I had to do was open the ashtray and press the round button.   For most of the time, the ashtray remains shut.   The final photo shows the normal look. Unless you open the ashtray you would never known there is a separate transmitter in there.

This particular unit also has a line out.  I will be able to use its USB and telephone features even after I have a aux line-in installed on the Becker.   This will improve sound quality as the FM transmitter is ok, but not perfect.   And while the remote control does not work with the ashtray closed, the bluetooth does.   So far, I am pretty happy with my W126 handsfree solution.   With a few modifications, it can be even better.