My Yellow W123 gets a Remap!

Page may contain affiliate links. Please see terms for details.


Authorised Forum Sponsor
Nov 29, 2002
Nuneaton Warwickshire
Lots of Mercedes!
Hi All,

Not having much time over the last couple years, I haven’t posted much about what I’ve been up to.
Work, is the boring answer, it has been extremely busy here at Mercland.

Here is one project that has been ongoing since 2013.

I know my W123 is marmite… it’s fun though, something a little different.












It’s been in the family since the early 80’s and so it’s really a part of me. I grew up with that car around me every day.

I could never imagine life without it. It was my first car at 17 and most people who know me, know my yellow “banana boat” or “the flying custard”…
I had a Morris Marina of a very similar colour.

Wasn't quite as nice as your Merc! :thumb:
Since this thread

I got talking about a V8 conversion with a Mercedes Club member Martyn Marrocco who had done a couple of these conversions in conjunction with a chap called Nigel Ball.

Nigel is known for his ludicrous conversions. He has done lots of V8 g wagens and will pretty much experiment with whatever you want.

I used to own one of his converted G wagens and that’s how this all came about.

He also helped with this one belonging to motoring journalist Matt Jones:

Nigels web page: Home - M-B V8conversions

There have been quite a few conversions if you search the net.
However most, if not all, have been with standard automatic gearboxes.

I really wanted a 5 speed manual mated to a 560 V8 M117 engine. How cool would that be?!

I asked Martyn to supply the parts and I really wanted to do this myself. My main concern was engine mounts and gearbox mating plate. We got talking, but Martyn did not want to sell the parts separately as he and Nigel had put years into the design and perfection of some of the custom parts needed. Fair enough.

So I let him have my car to do the conversion. This was back in 2013.

I drove the car up and in the following weeks, he stripped out the old 200 carb engine.

The engine bay was prepared for the V8.
My engine bay has always been a bit of a mess. The paint was stripped and improved.

The cross member has to be customised. A W126 brake servo has to be fitted.

Then I went up with the donor car a few months later.
A 560SEC that has been sitting in my garden for years.
I bought it ages ago at a car auction.
The body was tatty but the engine was like new. Around 80K miles and it had a brand new Bosch genuine fuel distributor and lots of other new parts fitted.
It was a good engine!

Martyn removed the engine.. cleaned and prepared it beautifully.



The Getrag Manual Gearbox
The subsequent months/years were taken up with the following:


All New W126 Brakes including callipers, discs, pads, shoes etc.
W126 Brake Servo has to be modified to fit as mentioned.
Servo Vacuum pipe is custom made.

Steering and Suspension

Lowered ride height.
Sporty shocks will follow.

Power Steering Pipes all have to be custom made. This was done.


Rear Differential has to be changed to an early W126 item.
We sourced a 2.65 diff from a later SEC as I recall, it may even have come from the donor SEC.
Martyn transplanted those internals into an early W126 diff casing.
The early diff casings have the same mounting points as a W123. Later ones don’t.

The propshaft has to be customised in terms of length and is a W126 item.

Engine mounts are custom designed. Martyns secret!

We used a standard 5 Speed Getrag GL275B (MB Code 717.401) transmission from my W123.
These are strong boxes and will easily handle the M117.

Many custom conversions often favour Getrag 265 box. This is a great box also, but Martyn has used the standard 5 speed before and had developed the custom parts required.

The flywheel and ring gear is a modified 230E item and a 230E clutch is used.

A 230E Crank Spigot bearing has to be modified to fit the M117 Crankshaft.

The gearbox is mounted using a custom made adaptor plate. This was the source of the main delay and why this project took so long.

Nigel, who supplies many of these custom parts was out of action for a long time. This meant the adaptor plate and other smaller parts held up this conversion.

Got the part in the end and it was fitted.

The starter motor is a CLK W208 item. Its smaller and works with the flywheel.

Radiator mountings have to be modified to suit W126 item.

A W126 header tank has to be installed with mounts from the W126.

The engine and gearbox were eventually fitted into the bay.

Gear selector rods have to be customised.

The throttle linkages have to be adapted to work.


My car was a carburettor model. This meant no electric fuel pumps at all.
So a 280E tank was installed and the w126 dual fuel pumps have to be fitted. Wiring ill come back to.

The pipework all has to be routed up to the engine bay and I replaced it all when the car was back with me recently.

Martyn fitted braided fuel hoses to the engine, which look lovely!

The above took just shy of 4 years.

Martyn is always very busy with his restoration business, however the main delay was awaiting parts from his partner in this business.

The wiring required to make all of this work is another custom item that we were waiting on.
In the end this never materialised.

Martyn is a fantastic restorer and fabricator but not an electrician.

So I suggested I finish the car off myself.

I have messed around with W123’s and W126’s since I was in nappies, so I was confident I could sort it out.

The car was then collected and brought back to sunny Nuneaton.


I needed a completely running donor car, so that I could be sure all of the required items were working.

I have mountains of W126 parts here, but there was no way I was going to use bits and pieces and then scratch my head when it didn’t work.

I found a 420SEC which had been mostly stripped.. but the car was still running and driving.

The 420 is essentially identical to the 560 apart from the engine block which is bored out for the 5.6.

So I set about splicing all the wiring to figure out what I needed to make the engine run. This was not fun. I stripped the entire car to get to all the wiring.

Being a scrap car I just cut the car out around the wiring in many places!

The amount of wires that are required for the engine is tremendous.

The wiring looms contain wires that you don’t need and so these all have to be traced back to where they come from one at a time. For example the engine bay loom includes wires for items such as headlamps, horns etc.
Inside the car the loom contains spurs for the seats, a/c etc etc.

To cut this long story short, after a few days with my head torch, sticky labels, wire cutter and multi meter I had a loom that I thought would make the car run….

This was then transplanted into the W123.

The W123 has very little room in the engine bay compared to a W126.
So I lengthened the loom in some places so I could mount all of the relays and control boxes under the passenger dash area. I took feeds from the original W123 fuse box to provide the power required.

The wiring to the fuel pumps was also stolen from the 420 SEC. It had to be lengthened, but that was no drama.

In short I tried to make it as A V8 W123 would have left the factory in term of wiring. Cannot get it perfect as the W126 engine bay and dimensions are different to a W123, but it worked ok.


The rev counter has to be adapted.
You take the W126 rev counter workings and transplant it onto the W123 face plate.
A little fiddly but just some soldering iron work.

The speedo works, as it’s the original gearbox. However due to diff ratio change, it reads 40 when im doing a lot more than 40 ;).
This will need to be addressed using a special gear box on the bowden cable.


No room for the original viscous fan.. An electric fan has been fitted which is controlled by an aftermarket thermo switch. I could have used the W126 switches, but the aftermarket one looked a better solution to me.

I fitted a W126 A/C switch and this is a manual override for the cooling fan should it not cut in.



Using the 230E flywheel meant that the M117 engine was now missing its crank sensor trigger.

Martyn uses an early W126 distributor to get around this.

Crank sensors were only used after 1985. The pre 1985 distributor does not rely on a crank sensor to feed back to the Ignition control unit.

However upon startup I found the early distributor was difficult to get right in terms of ignition timing.

I wanted to stick to the later distributor as it’s a lot more reliable.
It adjusts timing itself based on information from crank position sensor.

So I needed to replicate the rear flywheel pickups somehow.
Removing the 230E flywheel and having a custom one made with pickup points was an option, but something I didn’t really fancy.

Luckily an old contact of mine makes and sells custom pickup rings that are used for aftermarket fuelling/ignition systems such as megasquirt. Those rings are different to what I needed but they are mounted to the fron crank pulley.

The W126 flywheel has four pickups on the flywheel. These tell the ignition control unit where the crank is. Its pretty basic but works.
So I asked him to make me a front mounted crank pulley wheel to mimic the rear flywheel, only scaled down.

And he did:

He also suppled a crank sensor mount that cleverly bolts to the front of the engine.

After trial and error with positioning of the ring, it was welded in place and painted black.

You would never know its there now.

Thanks to Andrei from Belarus for this, he was really on board with the idea!

The cost was only around 70 EUR. Most would have told me to bugger off and stop wasting their time.

So this pickup ring allowed me to use the original distributor for this engine.

To finish off I fitted some nice leads supplied by my buddy Phil over at Magnecor Leads.

With wiring complete I changed all the fluids, bled the brakes, changed the front to back fuel pipes and we fired her up.

What a glorious sound with no exhaust. My ears were ringing.



Last edited:

With the engine running, it was time for an exhaust.

Contacted my friends at Custom Chrome in Nuneaton who got me in before Christmas 2016.

They made a beautiful system which took them 2 days.
I specified that I wanted it quiet… Nothing like the widebody SEC please!

There are 4 downpipes, two centre boxes and one rear silencer.

A nice subtle downwards facing tailpipe, the same shape as the original W123 item… just a little larger and more reflective.


Things to do now

• A vibration when taking up the clutch needs to be addressed. Maybe some alignment issue here.
• A slight hesitation at part throttle once warm needs to be addressed. Hopefully nothing major.
• Suspension needs to be slightly better. Martyn lowered the car but I think it’s a little too low as its catching sometimes. Some stiffer shocks and I may fit a set of H&R springs.
• Car requires a full repaint. It was done back in 2012 but being sat for all those years has affected some panels. So car will be off to our bodyshop in coming weeks.

Whats it like?

Wow. Mercedes should really have made a V8 W123.
Martyn is convinced they had planned to, as there are some clues to this. For example, the W126 wiring grommet is unique and the W123 has the hole marked out in its bodyshell..

I guess in the end Mercedes wanted to separate the W123/W126 models by engine size, two distinct models. Not like the never ending overlapping ranges of models we have now.

It’s fast. Not 63 AMG or old 55 AMG fast.
It’s a different feeling. It just gets up to speed very quickly but doesn’t necessarily feel like you are hooning it. At motorway speeds it feels like it could pull and pull.

I haven’t pushed it too hard yet, as lets face it, it’s a 37 year old shell with 3 times as much BHP as it should have. So ill get used to its characteristics for a while I think.

It would need some more suspension work to make it stable if I were to track it.. But that’s not my plan.

I wanted a unique car and a Manual V8.... I have got one and it’s fantastic!

It’s now a flying banana!

There will be some issues to iron out, however the quality of Martyn’s fabrication and the care he has taken in his work has meant there are very few issues.

Normally projects like this either get abandoned or just don’t end up working as people lose steam or money.

There is a lot more than just throwing in an engine involved..

I’ve probably missed lots of steps, but the above is what I can recall and what I kept notes on.

In the time it has taken to put this project together, I have had two children!

Now they can get share the fun in our “ellow car”.

So thanks to Martyn, his better half Susie, Nigel and anyone else who helped!
We got there in the end.

Here is a video of it running now:

I know its not as fast or exciting as a C63 with weistec bits on it, but I enjoyed the process!

What next? I feel I need a V8 W123 estate at some time ;)
Last edited:
Fantastic Jay! Great to hear it's finally done. I really look forward to seeing it.
That is brilliant.
Well done for sticking with it until the end.
It looked like you could get sucked into the carb on that first video.
No wonder the petrol warning light is on :D

Great thread, bet it puts a smile on your face :)

There is something quite delicious about "the unexpected" and you certainly have a unique car there. Well done.
Do you recall the paint code Jay? It's such a distinctive colour!
Love it Jay especially when you did you're own wiring. There's nothing more satisfying when it comes together and works. Well done :thumb:
Fantastic job Jay. Great result. I admire the ingenuity, sheer persistence nay endurance demanded by such a project. Its perhaps worth commenting this conversion was essentially all based on contemporary 80's technology. Couple that to the area where you seemed to be left to your own devices --- namely the wiring/electrics--- I personally would welcome a few comments about just how viable a similar project with more modern vehicle technology would play out. I suspect with todays electronic laden cars it would be a non starter even if the car was "retro engineered" . I bring this up simply because there might be some keen young chap reading your epic thread while looking at his 2litre diesel W204 parked in the drive thinking I could pop a V8 in there! ;)

Again congrats on a marvelous achievement with the ' ellow car :thumb:
Fantastic car and a brilliant writeup - many thanks for sharing!

It sounded rather nice with no exhaust :D
Do you recall the paint code Jay? It's such a distinctive colour!

673 Sahara Gelb, original colour!

Its perhaps worth commenting this conversion was essentially all based on contemporary 80's technology. Couple that to the area where you seemed to be left to your own devices --- namely the wiring/electrics--- I personally would welcome a few comments about just how viable a similar project with more modern vehicle technology would play out. I suspect with todays electronic laden cars it would be a non starter even if the car was "retro engineered" . I bring this up simply because there might be some keen young chap reading your epic thread while looking at his 2litre diesel W204 parked in the drive thinking I could pop a V8 in there! ;)

Yes we sometimes get phonecalls here at Mercland along the lines of, can you stick a V12 into my C Class?

Its very difficult due to the electronic systems involved in just about every component in the car. Seat belts to radio, I cant think of any part of the car that would work if the ECU were removed!
Not something we would attempt for any amount of money!

The idea with this idea was we wanted to make something that MB could have made. So its an engine and gearbox from the same era and using as many Mercedes Parts as possible.
I think the only non W126 item is the cooling fan and controller, which I will probably re engineer in time.
Nice "remap"...certainly not one that could be done online!!

Users who are viewing this thread

Top Bottom