Old School
Pierre Hedary
Transistorized Ignition Systems
Between Points and Computers
From 1969, Mercedes-Benz decided that normal points, condenser, and coil were simply not enough. For its prestigious 6- and 8-cylinder engines, a more advanced, powerful ignition system was needed. After the dust settled, Bosch developed a transistorized system with Daimler-Benz that delivered more spark at a higher engine speed. This system is a staple on any Mercedes-Benz made during the summer of 1969 and up to 1975 (8-cylinder models with D-Jetronic injection), 1976 (6-cylinder models with D-Jet or carburetion), and, in the U.S., 1978 (on the 4-cylinder 230 sedan).
My primary purpose in writing this article is to help people understand how transistorized ignition systems (T.I.) can be diagnosed and maintained. I also wish to debunk some rumors about these systems that are floating around. I won’t step into the hot debate between traditional points ignition and electronic ignition; however, I tend to advocate traditional Mercedes-Benz systems. My intent here is to help you understand T.I., and to help you enjoy your Mercedes as the authentic, wonderful, well-engineered car it has been from day one.
First, There Was Battery/Points Ignition
After the days of magnetos had passed, and Daimler-Benz began making larger numbers of cars after the war, component development and reliability took a front seat, and it was necessary that ignition systems be as well-engineered as the rest of the car.
Daimler-Benz made reliable, well-engineered ignition systems partnering with Robert Bosch. By 1965, the typical Mercedes-Benz ignition system on a 6-cylinder model (with the exception of the M198 and M189 6-cylinder models) consisted of:
- A red 1.4-ohm ballast resistor
- An ignition condenser
- An ignition coil
- A set of Bosch or Beru nonresistor spark plugs
- A distributor cap
- Ignition points and rotor
- A distributor spark-plug wire cover
- A set of copper plug wires
That means that if you bought a 1958 220S sedan, you should not expect any ignition problems, with the exception of having to replace the points, a condenser, spark plugs, a distributor cap, and a rotor every 10,000 miles or so. While this all sounds wonderfully simple, there are a number of reasons why these systems often fail to work.
The point gap on your distributor (arrows), 0.3mm and 0.4mm on all Mercedes-Benz cars, is so small that you can’t just set it by eye. Point gap should be examined before you let a shop throw a $250 electronic ignition system at your car. 
If you have an emergency, you can set a brand new pair of points with a feeler gauge, as shown here, the car will run reasonably well if you gap them in this range until you can use a dwell meter to set them perfectly.
The most common one seems to be that people become stingy and refuse to order their parts from Mercedes-Benz. I have seen many a 230SL with a blue ballast resistor on its ignition system – and a burned set of points that were not part-number matched to a distributor. If people do go to a Benz dealer, they may be put off by the slightly higher prices of the parts.
Heed my warning: Replace the points, condenser, plugs, cap, rotor, and ballast resistor all at once. If the spark-plug wires look old, and their insulation is bad, a new set from Beru Systems and a nice set of plug wires, fabricated using all the original leads and ends, can be sourced through Mercedes-Benz.
A resistance test of the wires should give you a reading of 1,000 ohms, hot or cold. Test them under both conditions. If you take shortcuts with a traditional system, then a short cut on a transistorized system will punish you severely.
How does Transistorized Ignition Differ from Points Ignition?
An easy way to answer this question is to compare a Mercedes-Benz with points ignition with a car that has T.I. As an example, let’s use a 1968 Mercedes 280SL versus a 1970 280SL. These cars are very similar, but the 1970 has T.I.
In the front left fender, you will see the traditional coil. The original coil would have been black on the 1968 280SL, and on the positive lead, you would see a black wire. This wire originates from the red 1.4-ohm resistor on the inside of the fender. This resistor reduces the voltage to the coil from 12 volts to just over 6 volts. The negative lead from the coil is tied to a wire that goes to the distributor. This wire is linked to the points and the condenser. The distributor has a high-tension lead coming from the coil that supplies spark to each plug wire. This covers all the bases on traditional ignition.
This ancient Bosch Blue coil and 0.6 ohm resistor (under clip on right) still work, but they could be operating at diminished capacity. A failing resistor also could cause some problems with a car’s ignition that some people blame on the T.I. control unit.
A T.I.-equipped car is similar, but there are some changes in the equipment and wiring. The coil would now be blue (or, if your car has a new coil, it would be silver and labeled “transistor”). The power supply to the coil (labeled “15,” indicating that it derives power with the ignition on) would originate through a silver 0.6-ohm resistor, which takes its power from a small silver box under the battery tray. This is the heart of the T.I. system – the transistorized ignition control unit. This lovely little device takes its power from the main harness through a blue 0.4-ohm resistor (by the hood support). The 0.4-ohm unit has a bypass from the ignition that temporarily supplies the points with 12 volts from the battery during cranking. A junction block next to the T.I. unit corresponds with the points and condenser. Meanwhile, back at the coil, the negative lead goes to ground.
This 0.4ohm resistor is an original unit, installed at the factory in 1970. It should probably be replaced with a new one. 
The Transistorized Ignition control unit is always mounted on the inside of the front left fender, with four Phillips head screws. On 113SLs it is directly under the battery tray. On 108, 109 and 111 cars, it is directly below the inner headlight protrusion, on the inside of the fender.
Why do we need all this extra equipment and all this wiring modification?
T.I. ensures three things:
- It demands less maintenance. The ignition points don’t have to handle continuous high amperage, so they last a lot longer. The T.I. unit controls how many amps the points receive, based on engine speed, and how many times the points are opening and closing.
- It helps facilitate smoother running, and a more powerful car. You will notice a difference in a Mercedes-Benz equipped with transistorized ignition – it will be smoother, more powerful, and start more easily under all conditions.
- It is extremely reliable when set up properly. These systems are actually very dependable, especially after they were refined. Rarely does the owner of a properly maintained 1972 280SEL 4.5 call me and say, “I am having an ignition issue.”
From 1969 to 1972, the T.I. control units had a finned case (left). This unit is from a 280SE 3.5. From 1973, they had a smooth case (right). The unit is from a 1974 450SL.
Due to differences in the wiring harness, early (left) and late(right) T.I. control units cannot be exchanged. The only exception is that 1972 4.5liter cars have a plug style that will accept the 1973 and later units”.
What Goes Wrong with T.I.?
Before we begin troubleshooting, let’s review two basic tenets that must be met before we service our ignition systems. If the parts may be old, replace them. If other systems are giving trouble, they should be fixed first.
If it’s old, or you don’t even know how old it is, replace it. If you have an older Mercedes-Benz with points ignition, whether it has T.I. or not, and everything is over 10 years old, replace all the parts discussed earlier.
If you have a 10-year-old coil, and spark plugs with 8,000 miles on them, examine your ballast resistors, plug wires, wiring connections, and so forth. If they have been on the car forever, and they certainly have black carbon, broken insulation, or crumbling ceramic coatings on the resistors, don’t bother troubleshooting. Replace them.
And when you do replace ignition parts, use Mercedes-Benz original equipment that matches your vehicle identification number (VIN), distributor number, or engine number. This will rule out pending equipment failures.
How is your engine’s health? How does your engine run? Does it get good fuel economy? Does it burn excessive amounts of oil? Does it have a shoddy mechanical history of botched engine rebuilds or poor service? Are your carburetors or injection system noticeably dysfunctional? While we shouldn’t try to tamper with the fuel system until the ignition is set up properly, and we shouldn’t try to rebuild the engine if we have some normal wear and tear and a little oil usage, we should still take problems in other areas of the engine into account when troubleshooting the ignition.
Now we can review basic areas of the ignition system to highlight the kinds of problems people run into. As a side note, I won’t cover setting dwell and timing, but if you feel the need to learn more about the process, visit SL113.org. The wonderful Pagoda SL group outlines the setting of timing and dwell in several posts by insightful members, and the information is relevant to most 4- and 6-cylinder Mercedes-Benz models with T.I.
Help! My Points Are Cooking
A small black carbon “eye” on the ignition points is nothing to fret about too much, but if that carbon eye grows quickly and your car begins to develop an intermittent miss, after cleaning your points, you may need to check your resistors. The ballast resistors can fail because they have a big job – reducing voltage by approximately 50 percent each time. They get hot and, even in as little as two years, a new set can become overworked. If the resistors cannot reduce voltage sufficiently, then the ignition points will receive that extra voltage and burn up slowly.
If, for some reason, you know your resistors are flawless, then the ignition condenser could be the culprit. This little guy acts as a sponge to absorb excessive voltage. If it is not absorbing the voltage, then the points will have to take up the slack. Oddly enough, Mercedes-Benz has eliminated the condenser on V-8 cars with M116/M117 motors. The excessive voltage is now channeled away by a thick, green wire with a specific resistance that bolts up to a junction on the frame.
Once you have eliminated these problems, check your point gap. The point gap is widest where the peak of each distributor cam lobe opens the gap of the points to its maximum width. The width between the contacts should be between 0.30 and 0.40 millimeters. If you must replace the points, checking dwell is a good idea. However, if you do not have a dwell meter handy and you need to replace the points, if you gap them properly, your dwell really shouldn’t change much, if at all. I have replaced a few sets of points in tricky situations with no dwell meter or timing light, and I later discovered that as long as my point gap was okay, my dwell was pretty good, too. Don’t worry so much about dwell – the point gap is far more critical.
If your distributor has a vacuum advance or retard mechanism, check the point gap with said mechanism engaged. This ensures that when your mounting plate for the points rotates under vacuum, it does not significantly alter the point gap. After installing new points, check them several times in the first few hundred miles. Apply a little dielectric grease on the distributor cam to keep the cam from wearing.
Bosch platinum plug with just 1,000 miles on it is from a healthy engine. Note the black carbon color on the point of the plug. These plugs are a big no-no, as they are both platinum and resistor type. Make sure your Bosch plugs have neither an “R” for resistor nor a “P” for platinum in their designation. In any and all cases, your correct plugs must come from Mercedes-Benz or NGK. Mercedes clearly forbids the use of either platinum or resistor plugs, but many shops install these because they don’t know any better.
Help! My Spark Plugs Are Fouling
Bosch and Beru are the original equipment suppliers of Mercedes-Benz spark plugs. As a Mercedes-Benz owner, you should know that the spark plugs for your car cannot be purchased as aftermarket parts. Mercedes-Benz has exclusive distribution rights to Bosch W5DCO, W7DCO, and W9DCO spark plugs. The only substitute really worth anything from another supplier is the NGK BP6ES. This plug will substitute for the Bosch W7DCO.
So what do we do now? These numbers are still rather useless. To help you understand which spark plugs to use, let’s start with two things to avoid when replacing your plugs.
- Do not install platinum spark plugs in any Mercedes-Benz with a conventional distributor. This applies to all cars with points, and all cars with a cap and rotor, whether it is a 1990 560SEL or a 1971 280SE. No platinum plugs. Why not? Well, that tiny platinum tip – even if there are four electrodes below it – can and will blow off, and will foul easily because it is so tiny.
- Do not install resistor spark plugs in your car. In a plug designation, the letter “R” indicates a resistor plug. A Bosch WR9DCO is not correct for your 1973 450SE (or any older Mercedes). A Bosch W9DCO is correct. The same can be said for Bosch WR7DCO, NGK BPR6ES, and similar resistor plugs – they only belong in the wastebasket if you run a reputable service facility for Mercedes-Benz.
As a rule, avoid Champion plugs. Also, if your Mercedes dealer sells you Beru plugs, they are just as good as Bosch.
Which Cars Take Which Plugs?
As a rule, cars that do not have T.I. usually take a Bosch W5DCO plug. A W7DCO can be substituted in hotter climates, and the NGK BP6ES is also a good plug. If you have a rich-running car with an old Zenith or Solex carb on it, and a carb rebuild isn’t in your budget, the Bosch W9DCO will help you burn off some extra fuel. Cars with T.I. are usually happiest with a W7DCO or NGK BP6ES. However, in hot climates, the Bosch W9DCO is beneficial. Cars with the later electronic ignition systems seem to like the W9DCO the best. On all models, the spark plug gap is 0.032 inch. All plugs provided by Mercedes Benz come pre-gapped.
Some Common Problems
My car still won’t start. Is my T.I. unit defective?
This unit rarely fails. I know some people speak about the gradual failure of the transistorized unit, but it really is a simple switching mechanism.
If you think you have a bad control unit, get an ammeter. Turn on the ignition and check the amperage between the negative and positive poles of the coil. If you are getting nine amps at the positive terminal, your T.I. unit is doing what it needs to do.
If you have a no-start situation, and you have an old coil, old points, and your original T.I. unit, the points need to be replaced and gapped, even if they are fairly new. Sometimes even new things fail, and all items are suspect, new or old.
If you continue to experience problems, try swapping in another coil before you conclude that your T.I. unit is defective. A break in the primary or secondary winding of the coil can interrupt your nine-amp reading. Your coil also may be worn out.
If you decide you need to replace the T.I. unit, remember that the T.I. unit on most 6-cylinder cars up to 1972 is the same. V-8 cars from 1970 and 1971 use the same unit as 6-cylinder cars. V-8 units from 1972 to 1975 are interchangeable.
My car Is hard to start, misses going around corners, and doesn’t have any power in the higher RPM ranges.
Assuming the point gap and dwell are healthy, I have observed ignition coil failures in the past that caused missing and power loss in hard-right turns, premature power loss at high RPMs, and weak starting.
The substitution method is best here. Remember that your old Bosch blue coil is 40 years old, and it may not be perfect anymore. A new coil from Mercedes-Benz costs approximately $160. A tow-truck ride costs about the same.
My distributor seems to be worn out. My car is missing and the timing is erratic.
Don’t worry – you can get Mercedes distributors rebuilt. For distributor rebuilding services, feel free to e-mail me and I will put you in touch with the right parties. Companies such as Metric Motors and Black Forest can assist you in this department.
I do find that the rumor of oil misting on the ignition points is largely untrue. In my years with these wonderful cars, the distributors have been excellent.
The End of the Tour: Enjoying T.I.
Your older Mercedes-Benz with transistorized ignition was built to a standard. Many individuals will want to sell you an electronic ignition system to fix something that isn’t broken. There are also many individuals who will tell you that a problem is lurking in the background with your T.I. unit when it is not. One note here: If you do decide to install an aftermarket electronic ignition, be sure to disconnent your T.I. to avoid sparking and destroying the new components.
With my experience as a Mercedes-Benz technician, I assure you that if you follow the tenets in this article, you are not likely to experience serious problems with your original transistorized ignition system, and you can always contact me by phone or e-mail if you do.
Happy motoring!
Pierre Hedary owns and operates a vintage Mercedes-Benz repair shop in Titusville, Florida. Hedary, a second-generation Mercedes-Benz technician, enjoys answering questions about older M-Bs. Contact [email protected], (407) 765-2867.