Are my Ponton condenser/capacitor and ignition coil OK?

Left side of Mercedes-Benz Ponton M180 engine with ignition coil, condenser and distributor

From the second I got my 219 on the road I asked myself “the engine runs like a sack of potatoes, is my ignition system working as it should or what is wrong?” The same question often turns up on forums.

This blog post obviously does not cover the whole issue/system, only the two parts that affect the ignition coil and the condenser/capacitor. The ignition system consists of more parts, and error sources. Whenever I get time, how it should happen I have no idea, I plan to document the rest of my experiences around those. See this as the first version of this write up.

Note that I am “electrically handicapped”, so take the following “with a pinch of salt”, use it with your own judgement. On the other hand, when I do not understand something, I spend an enormous amount of time reading, testing, measuring as well as compiling and trying to understand what people with experience have to share. But again, “knowledge” I may have, but my experience using that knowledge is limited.

Basic literature

Beside the standard Ponton literature you should have, described in the article Restoration - Introduction, I can highly recommend the following literature. Since I’m fluent in German I go for the “original” language as often as I can. Hopefully, you’ll be able to find them in your language, or at least in English. If you have other recommendations, let me know so that I can publish/share it.

The first two are published by the German company Heel Verlag, one from Bosch (you can order it from them) and the other one is from the German magazine "Oldtimer Markt" and is a compilation of articles they have published in the magazine within the subject area. Then comes Bosch's old “Bosch Kraftfahrtechnisches Taschenbuch” finally three, older pamphlets from Bosch that describe battery ignition systems, interference suppression of the electrical system and finally one covering generators and voltage regulators. In German it would be "Batteriezündanlagen für Kraftfahrzeuge", "Nah-Entstörung der elektrischen Anlage in Kraftwagen mit eingebautem Empfänger" and "Lichtmaschinen und Reglerschalter für Kraftfahrzeuge". Very informative, almost everything you need to know about life to survive!

Some "musts" in your Ponton library!

Note, “Bosch Kraftfahrtechnisches Taschenbuch” (“Bosch Automotive Pocket Book”?) is of course not the same as the “Mercedes-Benz Tabellenbuch” (“Mercedes-Benz Technical Data Manual”), described in the link above, even though they look very similar. “Bosch Kraftfahrtechnisches Taschenbuch” is a wonderful little textbook on everything related to car technology which touches on, or forms the basis for, Bosch's products and work areas.

On top of that I have myriads of copies of materials from the 50ies and 60ies from different sources, mainly Mercedes and Bosch. Those are also very valuable.

Multimeter

I’m not the one to judge if a multimeter is good or bad. I’ve been using several over the years, with mixed result and measurements. Currently I’m using Würth’s automotive multimeter MM50/20 which has ranges and functions applicable to my Ponton needs (i.e. not high voltage).

My MM50/20 multimeter from Würth

I wouldn’t mind having the original, bulky, modular Bosch engine tester that covers everything including the questions below. The issue is however to find the complete set and having it fully functional and within specifications. Spare parts and knowledge for achieving that are scarce….

How do I know if my Ponton ignition coil is OK?

Diagram of Bosch's ignition coil, © Robert Bosch GmbH

My original coil was a TK 12 A 10, later number changed to 0 221 102 006. That one was later replaced by 0 221 119 031 (which comes with pre-resistor) or 0 221 119 030 (comes without pre-resistor/ballast resistor but needs at least 1.4 Ohm or it might get too hot and fail early according to Bosch).

I would guess that the coil I used was the original one. I don’t know if my father replaced it, but I have a feeling he didn’t. It "looked ok" so I decided to re-use it. However, during my test runs after the car was finally assembled in 2019, I had several issues with my ignition system (all the issues and their resolutions will be described as well, sometime… in the meantime you can look for my threads in the Ponton forum). If those came from the ignition coil or the regulator, or something else, I don’t know.

After some time, I could however determine that it was shot, firstly by an engine that first went from bad to worse until it wouldn’t run at all and secondly by measuring it, short circuit. I tested two other NOS ignition coils and their values and sparks were very unstable. Remember, the coils are filled/insulated with asphalt and after so many decades it has turned dry, hard and brittle. Your correct looking New Old Stock ignition coil may fall within its reference values and give you a good spark, but I would say that the risk that the old asphalt, probably sooner than later, will fail a give you a similar short circuit. And you know that it will happen when you least want it.  

Have a look at Mercedes-Benz Ponton Ignition Coil Replacement when it’s time to replace yours. According to that, and Bosch Classic’s confirmation, I replaced my old black coil with a "Red coil", 0 221 119 031, with the separate, huge and ugly 1,8 Ohm pre-resistor/ballast resistor. The latter I hid while keeping the old ballast resistor's metal box and its wires (though disconnected) for the looks. Sometimes I'm thinking about painting the "Red coil" semi-gloss black as the original, but I won't...

Also, my first "Red coil" got problems, first by showing erratic ignition behavior and a smoking hot pre-resistor and finally by failing - short circuit. My suspicions were now turned to the voltage regulator. See the blog post Voltage regulator woes.

If you measure a TK 12 A 10 ignition coil, the following reference values applies according to Mercedes-Benz’ “Engine test values - Test sheet 19b” for the 219a/220a:

• Primary winding (connection #1 to #15) – 2,0-2,5 Ohm
• Secondary winding (connection #15 to #4, i.e. the contact for the cable to ignition distributor) – ~8 kOhm
• Spark length – min mm 11
• Voltage without load – min kV 25
• Voltage with load (Bosch test equipment) – min kV 14-16

Note, if you go to the workshop manual “Type 190, Part 2” and work number "15-22" it says that the spark length should be 7mm. It also describes how you test the spark length. The “Mercedes-Benz Technical Data Manual” says 14mm for all Ponton models (as well as some others). It can be noted that early on in the process, summer -19, I did measure and test my old, black coil as per above and everything was within specifications (windings 2,1 Ohm and 7,7kOhm respectively).  

According to Bosch Classic, the measurements for the "Red coil" should show:

• Primary winding – 1,20-1,60 Ohm
• Secondary winding – 12,0-16,0 kOhm

Measuring my second "Red coil" when it arrived gave 1,4 Ohm and 13,45 kOhm respectively, so it was within range. 1,5 years later, i.e July -21, it measures 1,52 Ohm and 14,08 kOhm respectively. However, as I understand it, one should not stare blindly at these reference values, but focus on the spark, preferably when the coil is warm since that is when problems with coils normally arise.

How do I know if my Ponton condenser/capacitor is OK?

Contact controlled ignition system, © Robert Bosch GmbH

If you google “test condenser capacitor” you’ll find a lot of links and videos to help you further. One site I like, is MGA Guru's, partly due to its motto “This web site is dedicated to the proposition of having obscene amounts of fun with your Little British Car. Fortunately, this does not have to include spending obscene amounts of money. The less time and money you spend on tinkering, the more you have left for driving, so the philosophy is heavily loaded toward maintaining your own car”. Anyway, have a look at the article How to test a condenser (and also his other articles, e.g. around ignition, they are very good). I’ve taken the liberty to copy and paste its contents below, with the approval of the site owner, the Guru himself:

…

“For the record, "condenser" is an age old term, and "capacitor" is a newer term for the same device. Automotive ignition capacitors are still commonly called condensers.

Many people will replace a condenser with regular periodic maintenance, just because they have no way to tell how much longer it might last. But some new condensers might be bad right out of the box, or might fail very shortly after installation. My approach is to carry a known good condenser in the traveling tool kit, and don't replace the old one until it fails. For this to work you have to be prepared to change one at some inconvenient time, possibly on the side of the road, but you should be prepared for such a possibility at all times regardless.

For decades I have had very good luck with condensers, but in recent years there have been lots of reports of condensers that fail prematurely. In December 2013 I had one fail after 18 months and 9000 miles. The two months later another one failed after only 257 miles. Even regular replacement at reasonable intervals could not avoid these premature failures. This begs the question, how do you test a condenser to determine if it is good or bad? Well, even when you can test a condenser, this is still not the final solution, as a condenser might test good one day and fail the next day. But periodic testing of the condenser in the car might (sometimes) disclose a deteriorating condition before it actually fails. Testing can also verify a suspect condenser that might fail soon after installation. If testing reveals an apparently good condenser, then you can look elsewhere for a problem without having to replace the part.

"Bosch high-performance ignition coils TK 6 and TK 12 for high-speed engines and those with high compression; Number of cylinders 4, 6 and more. Case diameter 60mm. At 6 and 12 volts battery voltage, these ignition coils deliver an operating spark of 15mm length; the power consumption is 21 watts for the TK 6 and 19 watts for the TK 12 ignition coil (six-cylinder engine). Maximum number of sparks / min = 14000", © Robert Bosch GmbH

For anywhere from $20 to $200 you could buy a condenser tester. The real tester can apply a high voltage (500 to 600 volts) to test for leakage, and can also apply an AC current to actually measure the capacitance (storage capacity) of the device. These two tests more closely stress the capacitor in the same way actual operation does. But considering that a condenser is a cheap part, and you should always carry a known good spare anyway, the condenser tester seems a bit overboard for the average shade tree mechanic. Fortunately there is a way to do a rudimentary test with a common analog (moving needle) ohm meter.

1. Remove the condenser from the engine (or at least disconnect the lead wire). Note the small metal connector located on the end of the condenser. This connector is the "hot" or power connection. The metal case of the condenser is the grounding point. Discharge the condenser by shorting the lead wire to the case.
2. Switch the meter to the ohms position. Place the red lead into the "ohm" connector on the meter. Insert the black lead into the "com" or common connector on the meter. Set the resistance range to the highest available setting (if it is selectable). Connect the test leads together and zero the meter. If the meter won't zero replace the battery. (Yes, an ohm meter has a battery).
3. Touch the red lead to the hot connector on the condenser. Place the black lead to the metal case on the condenser. The meter's needle should jump slightly to the right (toward 0-ohms), then should drop back to the left towards infinite resistance). Hold the leads in place for 15 to 20 seconds. This action places charge in the condenser. If the test shows any reading other than infinity, the condenser is leaking and needs to be replaced.
4. Remove the leads and reverse the placement to the condenser. Move the red lead from the hot connector to the metal case, and move the black lead from the metal case to the hot connector. At the moment where both leads are touching the correct points, the meter should jump towards the right. The second time the needle may move twice as far, as this action discharges the condenser. Holding the leads in contact should again result in movement of the needle back toward infinite resistance.
5. Movement of the meter's needle indicates the condenser is good. If no movement was indicated on the meter in any circumstance, the condenser is bad and must be replaced. Retest the condenser several times for a consistent reading.

"Circuit diagram of a battery ignition system for a four-cylinder engine", © Robert Bosch GmbH

In operation the condenser will "ring" at up to 300 volts, so the condenser needs to be rated much higher, no less than 600 volts DC. The ignition will likely work with any capacitance value between 0.05 and 0.6 microfarad. Too high or too low value may eventually transfer metal from one side of the contact points to the other side leaving a pit and a point. Capacity of spark coil capacitors ranges from 0.2 microfarad to 0.33 microfarad. Almost all automotive coils use a 0.25-0.29 microfarad capacitor.

A capacitor may absorb moisture over a long period of time, and moisture can cause failure of the condenser. So it is possible that a condenser stored for 10 years or so might be bad or might fail prematurely in service. It is a good idea to check your traveling spare condenser occasionally.”

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To complement the above, my copy of Mercedes-Benz’ “Engine test values - Test sheet 19b” gives the following reference values for the 219a/220a's capacitor ZKO 29 Z 65 Z. If the values are the same for other Ponton models' Bosch capacitors I don’t know. But the range for this capacitor falls within the range that MBA Guru mentions:

• Insulation resistance – minimum 200 kOhm
• Capacitance value – 0,23 -0,32 µF

Do also see Electrical equipment, engine and the blog entry Voltage regulator woes!!