Guest Article: Electronic Ignition Upgrade for a D-Jet Mercedes-Benz

A practical guide for the 1972 W108 280SE 3.5 — with fewer rabbits to chase

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.