Ignition failure - switchgear transistorized control unit

Hello,

I am looking for advice on my 1971 Mercedes 350SL. The car starts and drives normally for short journeys, although the last two attempts at extended driving (2-4 hours) the car has broken down and will not restart, or will restart briefly with rough running. When the car refuses to start I have confirmed there is no spark, and then suddenly there will be spark and the car runs.

The following have been renewed recently: leads, plugs (without resistor), cap. After the first breakdown a new coil and ballast resistors were installed. Points are in good condition and gapped to spec. Trigger points are also clean and appear to be in reasonable condition.

I believe the transistorised control unit has is failing under extended driving due to heat and load. Is there any way to confirm this and what are the options for repair/upgrade. I have removed the unit, part number is 0227051012

I have looked at the 123ignition distributor upgrade which would also eliminate the trigger points, but this seems overkill at this moment until I confirm the cause of failure. Although I am keen to hear anyones experience with this unit.

Alternatively, electronic ignition conversion such as Pertronix. I hear mixed reviews, and still do not have clarity if this would eliminate the switchgear control unit. I believe it would make the unit redundant and on paper seems like a reasonable solution and upgrade, but I would like to hear input on this. 

I look forward to the discussion.
Kind regards,

Jmew

I forgot to mention, the engine rpm counter stopped working shortly before the first breakdown. It remained broken even when the car was being driven for a week after. I noticed it only started working again after extended motorway driving where engine speed is around 3000rpm. When slow driving resumes, such as city driving, the rpm counter stops working again.

I mention this as I know the engine speed signal is taken from the electrical junction where the switchgear output joins (green yellow wire). My thoughts are that the unit is operating at a lower capacity and only with high rpm is the signal strong enough to engage the rpm counter. This would further suggest the transistorized control unit is failing?

Hi Unknown - I guess that Jmew ist not your first name ?

You write that you used ignition leads without resistor. That seems okay. But spark plug connectors have resistor? You should measure 1kOhm on all of leads from distributor to spark plug connector.

A hot candidate for such a failure is green cable from distributor to connector block where also TD (rpm) signal is taken. Your rpm counter takes its signal from there. A running engine and no rpm to be counted there is impossible. So you seem to have another issue with rpm counter. I would disconnect it for the time being. It is suspicious as it works at high rpm and not at low rpm. It could even influence signals going to ignition module.

Your ignition module can fail of course. Mostly they are subject to water entering their housing. You can test statically. But a longer test on a test stand would be preferable if you have failures only in some cases.

Also wiring and power resistors can fail.

Thank you for your input.

The spark plugs are without resistor, not the leads. IIRC I fitted BP6ES.

That is interesting regarding the green cable from distributor. Is this cable anything special, or can two cables of similar gauge be installed as replacement? I know the outer green sheathing is quite hard and brittle.

I have inspected the ignition module for corrosion, and thus far after removing the cover there is no evidence of water ingress, albeit the aluminium case is corroded. What advice do you have to test statically, and is there anywhere I may send the unit for testing/repair?

Thank you again for the advice. 🙂

Jason

Hi,
green cable is such is nothing special. Best version is the shielded version. But you need to use the plastic part where it goes into distribtutor. And that you normally only get by buying a cable with capacitor. However availability is 0.

If there is no water inside ignition module, it should work, You can do a test of your ignition system as described above in 107 workshop manual which covers ignition und engine combustion.