By John Tawadros

There is nothing quite like driving a smooth-running, high-compression 3.5-litre M116 V8 with a properly functioning Bosch D-Jetronic system. Conversely, driving one that isn’t running properly can be an exercise in frustration, despair and other distinctly first-world problems.

This is the story of how I diagnosed an ignition problem on my 1972 W108 280SE 3.5, discovered that its original transistorised ignition module had finally reached retirement age, and replaced the ageing switchgear with an electronic ignition system. The aim here isn’t to turn every D-Jet owner into an ignition specialist. It is to document what worked for me and, hopefully, save someone else from spending a small fortune chasing the wrong rabbit.

It started with a little miss

The trouble began innocently enough: a slight miss at idle.  Then it became rough running. Then the spark plugs started fouling randomly, and sometimes almost immediately after installation. As usual, I went through the obvious checks. Nothing immediately revealed itself as the culprit, and it was time to call in the professionals.

Diagnosing an ignition problem isn’t necessarily straightforward on any older car. On a D-Jet system, it can become particularly entertaining. I received a range of opinions, both professional and amateur, pointing in all sorts of directions:

• Vacuum leaks
• Failing injectors
• Fuel-delivery problems
• Intake-manifold leaks
• And, presumably, anything else within a 50-metre radius of the car

None of these diagnoses turned out to be valid to any meaningful extent. We’ve all experienced the pain of going down the rabbit hole, only to discover that we’ve been following the wrong rabbit. I certainly wasn’t keen to spend potentially thousands of dollars replacing unrelated components based on diagnoses I wasn’t convinced by.

The lesson: correct diagnosis is paramount.

There aren’t many genuine D-Jet experts around Sydney, but fortunately I know one. He diagnosed the problem quickly as an ignition issue and systematically checked the relevant components. The culprit turned out to be the Transistorised Ignition Module — the silver box mounted next to the radiator.

The spark being produced was erratic. That meant the plugs weren’t firing consistently — and as the problem became worse, the plugs began fouling. The ignition module still carried its original 1972 date stamp – at that point, I think we can safely say it had done its job. In fact, after more than five decades, I’d say it had earned the automotive equivalent of a gold watch.

These original ignition boxes are no longer readily available, so the logical next step was to have the unit rebuilt. I found someone who could do it. Unfortunately, there was a small problem.

Inside of Transistorised Ignition Module with protective coating.

Inside the module was a thick red factory coating. The coating was originally applied to protect the electronics from moisture — a sensible idea when the car was built. Unfortunately, it also makes accessing, testing and replacing individual electronic components extremely difficult. So, while I have always been a stickler for originality, there comes a point where originality and functionality have a serious disagreement. And functionality usually wins. With no module replacement available, I started looking at an electronic ignition conversion.

The route I took was to bypass the original ignition switchgear completely. That means bypassing:

• The transistorised ignition module.
• Both ballast resistors.

1) Ignition coil, 2) Ignition module, 3) 0.6 Ohm resistor, 4) 0.4 Ohm resistor.

I replaced them with an electronic ignition system consisting of:

• Pertronix Ignitor 1887LS: lobe-sensing ignition module for the Bosch V8 distributor.
• Pertronix Flamethrower 1.5 Ohm ignition coil.

This wasn’t actually my first choice. I’ll explain why later, because there is one particular Pertronix product that I strongly recommend you do not use for this application.

The D-Jet system is a little special, and information about installing electronic ignition on this particular Mercedes setup is surprisingly limited. So, here’s the procedure I followed.

Original Transistorised Ignition System diagram

Installation: step by step

1. Find the switched 12-volt supply (no. 15, Fig 1)

First, identify the switched 12-volt positive supply connected to the original 0.4 Ohm ballast-resistor.
Disconnect it from the original resistor arrangement.
This switched 12V supply will now connect to the positive (+) terminal of the new 1.5-Ohm ignition coil.

2. Isolate the starter-motor wire (no. 16, Fig 1)

Disconnect the positive wire coming from the starter motor from the same 0.4 Ohm ballast-resistor.
Isolate this wire securely, you won’t need it for the new setup.
Don’t leave old, unused electrical connections sitting around waiting for an opportunity to become an intermittent fault. Old Mercedes wiring has enough personality without giving it additional creative freedom.

3. Disconnect the transistorised ignition module

Disconnect the three-pin plug from the transistorised ignition module and isolate it.
Even though the original module will no longer be powered, it is important to disconnect it completely.
The reason is that the old module can still interfere with the D-Jet control system — and the last thing you want after completing an ignition conversion is for the dead ignition box to continue participating in the conversation.

4. Remove the points

Remove the original mechanical points from inside the distributor.
Also remove the wire associated with the points.
The Pertronix Ignitor will take over the triggering function.

5. Install the Pertronix Ignitor

Install the Ignitor 1887LS baseplate and module inside the distributor.
The 1887LS is a lobe-sensing system, so it does not require a magnetic ring on the distributor rotor.
Route the Ignitor’s positive and negative wires out of the distributor and connect them to the corresponding terminals
on the new ignition coil:

• Ignitor positive → coil positive (+)
• Ignitor negative → coil negative (−)

Make sure all wiring is properly routed and secured so that nothing can interfere with the distributor mechanism.

Pertronix electronic ignition module installation.

6. Reinstall the distributor components

Reinstall the rotor and distributor cap.
At this point, the physical installation is essentially complete. There is, however, one rather important step remaining.
The ignition timing will almost certainly need to be checked and adjusted.
Don’t assume that because the engine starts, the timing is correct.

The important lessons — and the mistakes to avoid

Don’t use the Pertronix Ignitor II

This is probably the most important part of the entire article. I initially tried the Ignitor II. It has more features and, on paper, appears to be the more sophisticated product. In this particular D-Jet application, however, it behaved poorly below approximately 3,000 rpm. I suspect the problem is related to the Ignitor II’s built-in features and how they behave in that particular operating range, but regardless of the precise cause, the result was the same: It wasn’t happy. And neither was I.  After hours of head scratching, troubleshooting and questioning various life choices, I went to the simpler Ignitor 1887LS. For this application, that’s the one I’d recommend.

Trust me: forget the Ignitor II and save yourself the hours of head scratching.

Use the correct ignition coil

The new system bypasses the original ballast resistors. That means the coil needs to be appropriate for a ballast-free setup. I used the Pertronix Flamethrower 1.5-Ohm coil. Don’t simply grab whatever coil happens to be sitting on the shelf because it looks suitably shiny. Don’t leave the ignition switched on The Ignitor module is susceptible to failure if the ignition is left switched ON for an extended period while the engine isn’t running. This is particularly important if the car is being worked on. Make sure your mechanic knows this too. A mechanic who is used to working on modern cars may quite reasonably leave the ignition switched on while testing something.

After installation: don’t skip the tune

If the original transistorised ignition system had been misbehaving for some time, there is a good chance that the car’s setup had been adjusted or tuned to compensate for the problem. Once the new ignition system is installed and working correctly, the engine should be properly tuned. This isn’t the time for guesswork. Have the ignition timing and overall engine setup checked by someone who understands D-Jet and knows these engines. A good electronic ignition system won’t magically fix an engine that has been tuned around a faulty ignition system for years.

A spare module is worth considering

One final practical consideration: carry a spare Ignitor module if you can. Depending on where you live, replacement modules may not be immediately available. In Australia, it’s worth checking availability with the local Pertronix agent before relying on the system as your only option. The whole point of this conversion is to replace one hard-to-find failure point with something more readily serviceable. There’s little benefit in achieving that and then discovering your replacement module has to be shipped halfway around the world while your W108 sits sadly in the garage.

What did I learn?

Despite the frustration of the original fault, I’m very happy with the outcome. The key wasn’t simply fitting an electronic ignition system. The important part was correctly diagnosing the problem first. There are plenty of potential causes of rough running on a D-Jet Mercedes, and replacing expensive components based on guesswork can get very expensive very quickly.

In my case, the problem was ultimately an ageing transistorised ignition module that had served faithfully since 1972. The electronic ignition conversion allowed me to bypass the obsolete switchgear while retaining the original distributor and keeping the car’s ignition system relatively straightforward. And, most importantly, I now get to drive and enjoy that glorious M116 V8 again. After all, the objective of owning a classic Mercedes isn’t to spend every weekend diagnosing it. It’s to spend every weekend driving it.

Author: John Tawadros.   John is a member of the Mercedes-Benz Club of NSW and the proud owner of a 1972 280SE 3.5 and a 1978 280CE.   You can follow his adventures with both of these cars at his Instagram feed Mrfrotop and his previous articles.  

The post Guest Article: Electronic Ignition Upgrade for a D-Jet Mercedes-Benz appeared first on Classic Jalopy.

In their care, the CE also underwent a full engine rebuild, which explains why it runs so well for a 445,000km car. I always felt that this little coupe was shown some love at some stage in its life, and now I know why.

I also learned that the first owner of the car was a dentist who picked the car up from the factory in Stuttgart.   From there, it went to a merchant seaman before being purchased by John and Jenny.

Today, I was able to finally meet John and Jenny and let them see the car they owned ten years ago is still going strong.   Thank you John and Jenny, it was a pleasure to meet you.

Author: John Tawadros.   John is a member of the Mercedes-Benz Club of NSW and the proud owner of a 1972 280SE 3.5 and now a 1978 280CE.   You can follow his adventures with both of these cars at his Instagram feed Mrfrotop. 

The post Guest Article Update: Mrfrotop’s 1978 280CE appeared first on Classic Jalopy.

I wanted a coupe I could drive regularly which had rear seats for our two toddlers, and a 450SLC seemed like the obvious choice. However, I struggled to find a C107 that I liked which didn’t have an unrealistic price tag. It’s not that the SLC prices were astronomical in 2019, but obviously the asking price should reflect the condition of the vehicle beyond a fancy respray. The C107 can have some issues which are skin deep. What might appear like an excellent exterior could have a project car lurking beneath. Perhaps I made the mistake of using Colin T’s beautiful white 450SLC as reference, and ultimately nothing I inspected came close to the condition of this car.

At this point, I wasn’t searching for a C123, but widening the Gumtree classifieds search slightly came up with a white ’78 280CE in Canberra. The W123 was one of the most successful Mercedes-Benz models ever, with over 2.7 million variants manufactured between 1975 and 1986. They were known for their unrivaled reliability and high mileage capability – which is a good thing, since this ’78 280CE for sale had over 430,000km on the clock! If you were buying anything made after 1990, perhaps this high mileage would be a deterrent. But this is a 123 we’re talking about here, and the 2.8(ish)L M110 twin overhead cam motor is known to be bulletproof. In any case the price was ridiculously low, and it even had 11 months registration left. A convincing argument emerged around driving this CE for a year, and if it dies after 11 months then at least we got to experience what a 123 was all about. My wife agreed, so that box was ticked and I arranged a bus ride to the ACT after an agreed conditional price with the owner.

Everyone has a set of non-negotiable conditions when buying a car, and some of these may differ from person to person. Perhaps an Australian delivered car is an important condition for some, or sub-200K mileage for another. For me, a good service history, an excellent interior and minimal rust are very important – the rest can be fixed and restored with time. During my brief inspection of the vehicle, I was blown away by the condition of the interior. It even had the original factory floor mats, which were in excellent condition. The dashboard was virtually crack free, and the burlwood highlights (featuring on the first series C123) were also excellent. It drove ok, but felt a bit soft in the rear (what I later discovered was Self Leveling Suspension or SLS). Overall mechanically it appeared to be good and frankly, the price didn’t justify a higher level of mechanical scrutiny. I was only going to drive it for year you see, until it dies or I find an SLC, whichever came first. With a straight body and excellent chrome work, it was obvious that this little Australian delivered coupe has been well looked after. Other than some minor rust in the bottom inside of the front doors, paintwork “patina” and some suspension squeaks, it appeared to be a good car. We did the deal and I drove her back on the 270km trek home. Perhaps that’s when the love story began. I just couldn’t believe that a high mileage car from 1978 could be so smooth, quiet and capable on the freeway. I love my W108 but the refinement of this little coupe is a generation ahead, which it actually is.

Out of the 2.7 million 123s produced, less than 100,000 were coupes, and a much smaller number were manufactured in RHD. Not a unicorn, but not available in abundance either. The more I read about the C123, the more I realise how sought after these were back in the day. The price tag in 1978 was AU$40,700 which was equivalent to a Jaguar XJ-S V12. Despite the exorbitant premium over the sedans, new 280CEs were fetching even higher prices overseas on the black market because they were so hard to get.

Other than a missing sunroof, my ’78 280CE coupe seems to have everything else. Power windows front and rear, power steering, disc brakes, air conditioning, cruise control and SLS – as featured in other early W123 delivered in Australia, perhaps to justify the hefty price tag at the time.

In the first few weeks, the tires brakes and suspension squeaks were sorted out and the AC was re-gassed. Later on, we replaced the fuel injectors and some minor electrical components. Changing all the engine and gearbox mounts made a huge difference to noise and vibration and totally transformed the car. The hood liner had turned to powder, and it was also replaced – we now have a clean engine bay which was free from hood liner dust. A fun “C19 lockdown” project was fixing the vacuum central locking system using some spare actuator diaphragms I had. Recently, I had the top of the rear seat (which was sun damaged) reupholstered in original MBtex. The colour being a relatively rare medium brown or “Tobacco” as per the MB colour chart. Paint wise, it turns out that a good cut and polish can do wonders. It’s now a comfy, reliable and pretty pillarless cruiser. It also seems to put the toddlers to sleep pretty quickly (thank you smooth riding SLS).

In the past year, I’ve added over 7,000km of reliable commuting on the clock. The original engine is still going strong and the transmission shifts smoothly and quietly. At the time this article was written, there was 438,755km on the odometer – truly impressive.

Recent enthusiast interest in the 280CE has shifted the prices upwards. It’s nice to know, but it really doesn’t matter. As you’ve probably guessed by now, I’m not getting rid of this pretty coupe any time soon. While I’m a bit precious about the W108 5-digit odometer ticking over to zero, I’m compelled to hit the 500,000km milestone in the CE. At least I can get to mount that mileage badge I bought from eBay on the front grill.

Author: John Tawadros.   John is a member of the Mercedes-Benz Club of NSW and the proud owner of a 1972 280SE 3.5 and now a 1978 280CE.   You can follow his adventures with both of these cars at his Instagram feed Mrfrotop. 

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