Electrifying an R129 SL - are EV conversions the key to modern classic longevity?

Electrifying an R129 SL - are EV conversions the key to modern classic longevity?

Posted by Harry on 3rd Feb 2023

Hello R129ers, wishing you all a very belated Happy New Year. Today’s topic is a real fan favourite; the imminent demise of the internal combustion engine! Yes we know, sigh… But it’s not all doom and gloom (actually, it kind of is if we don’t get our act together folks).

Some of you may have read about our friends over at The SL Shop and their foray into the red hot sector of electrification - retrofitting classic Mercs with EV tech. Check out this brilliant article on their debut attempt, the R107 SL SportLine ZERO, the world's first fully electric and emissions free Mercedes-Benz R107 SL:

The R107 SportLine ZERO. The SL Shop

What a pretty little car. It must have been a mind bogglingly extensive, not least expensive R&D undertaking to mate cutting edge EV powertrain technology (including not 1, 2 or 3 - but 10 Tesla batteries) with a decades-old chassis. But by all accounts, they’ve knocked it out of the park - albeit with the usual EV caveats we’ve come to expect within the current paradigm… for example, a fairly mediocre range of 150 miles. Popping down to the pub or into town? Yes please. A road trip to the South of France? No thank you, we’ve got that covered…

There she blows. Hagerty

Ah. That’s more like it. This brings me conveniently onto our main topic. Has anybody out there heard of, or come across, any brave souls attempting to convert their R129 SL into a fully electric, emissions-free (perhaps emissions ‘lean’ is the better term) machine? The only bona fide example we can find is this ‘93 500 SL over in the USA with a measly 50 mile-ish range, 
listed on eBay in 2012 at 89,000 miles for $14,000.

And batteries sourced from a… wait, is that right?

2012 Nissan Leaf, in all it's glory. 
James Martin / CNET

Yes, batteries sourced from a Nissan Leaf; that once majestic M119 V8 powered autobahn devourer can just about top 90 mph! Though, in fairness, this conversion was first completed more than a decade ago in 2012 with a battery refresh in 2021, the specs would suggest something of a downgrade, and our assumption is that a downgrade is not particularly appealing to the discerning R129er.

Elsewhere, we found another whiff of an electrification attempt on an old, dark and dingy corner of the internet over at the DIYElectricCar.com forum, where it appears one courageous cowboy down in New Zealand was giving it a go in 2009. But alas, the thread appears to have died quite some time ago, and with no news from the brave scout known as ‘Kwikkiwi’ - we think it’s safe to assume the project has failed.

We’ve also found some interesting nuggets in this BenzWorld forum thread concerning the closely related W140 S Class.

But no cigar.

Our presumption is that any undertaking is being quickly thwarted by the mammoth levels of R&D that would be necessary for replacing or repurposing the complicated web of electronic management systems which make the R129 and it’s myriad technologies work - and it’s those modern, largely electronically-governed innovations that really differentiate the R129 from it’s agricultural predecessors (including the R107 SL) and most other classic cars for that matter.

Electric AC Conversion Kit. Classic Retrofit

Modern cars are complex machines, with ancillary power intricately derived from their internal combustion engines. Many EV conversions therefore require sacrifices, or some very clever engineering to retain everything the car was originally capable of - power steering, air conditioning, infotainment, fancy folding roofs.. you name it, solutions are required, and they can be pricey to say the least.

We spoke to Jon Peck from Electric Classic Cars, the world's largest classic car to EV conversion company. Headed up by Richard Morgan, the company run a popular YouTube channel and have a TV show called ‘Vintage Voltage’ airing on Quest TV.

Vintage Voltage on Quest TV.

Interestingly, they have converted some cars manufactured as recently as 2016; but they’re very selective indeed. That particular 2016 example was a Land Rover Defender, a staple of their conversion lineup… but as Jon explained:

‘The simple reason is that they’re almost identical to the original Defender, there aren’t really any modern gizmos, the chassis is very basic, it pretty much is a classic car’. 

They’ve also tackled a Series 4 Alfa Romeo Spider from the early 90s - a pretty, but somewhat rudimentary Italian automobile, a far cry from what the Germans were up to at the time.

A Mercedes-Benz W140 S-Class. FavCars

They’re now developing a complete bolt-in, plug and play conversion kit for the Ferrari Testerossa amongst other titans of the classics market (for certified installers - not DIYers!). I quizzed Jon on the technical stumbling blocks when tackling modern cars… 

‘The biggest one is the ECU, modern ECUs are highly complex, taking readings from numerous sensors to start the engine, keep it running, and communicate with all of the various driver aids we’ve gotten used to in the modern era’.

Jon was briefly interrupted at this point needing to be on deck for some logistics in getting one of their Fiat 500 conversion kits over to LA via Chicago. Busy bees! Jon continues… 

‘If you’re replacing every aspect of the management systems that keep the car alive, you’re basically building a new car using the old shell, and that’s something we don’t do, not least because of the regulatory hurdles involved in making said new vehicle road legal’.

‘Another key issue is how to deal with the cabling or wiring. Everything has to be bolt-in, you can’t alter the original chassis, you can’t make structural alterations via drill holes to make room for cabling or new brackets and clips. Modern cars like the R129 have a monocoque chassis as opposed to the more traditional ladder chassis with a separate body, so any alterations would mean going for IVA (Individual Vehicle Approval) testing and registering for Q-plates.’

Jon estimates that tackling an R129 electric conversion would easily climb into the six figure realm, and at that point (let’s be honest) you’re probably going to be keener on putting your money into something like this:

A Mercedes-Benz R129 SL 73 AMG. Secret Classics

In many ways the modern era began with trailblazers like the R129 SL. It’s no wonder that companies converting ‘classic’ classic cars seem to be sprouting up everywhere, and those specializing in ‘modern classics’ or modern cars full stop seem to be a rarity.

Mating electric motors to the cars original transmission. Moment Motor Company

Hit us with your thoughts on electrification, we’d love to hear from any EV experts out there that are passionate about classic Mercs. Would you convert your R129 SL into an EV if money was no object? Or is the entire notion complete sacrilege and the EV fad a con destined to fail due to dwindling finite lithium reserves?

How much of your money (or your children’s inheritance) would you be willing to spend to convert your R129 into an EV? Or.. if you are already electrifying your R129 or other classic Mercedes-Benz motor, what resources are you using to do so, and how is the process going?

Here's to the future.. whatever that may mean.

Happy motoring! Harry.