How do you attach an Oscilliscope to check ignition system - PeachParts Mercedes-Benz Forum

Meza · #1 07-23-2003, 06:57 PM

I have a slight miss that I believe is ignition related. I was looking at the MB manual for 103 engine under "Electrical system, engine" and it shows different images for graphs that represent Voltage versus time. By analyzing the image/signal, a problem can be identified or narrowed down to certain things. I do have access to a 4 channel oscilliscope and would like to know what things do I need (like current probe, ..and so on) and where to hook things up in the engine. Any help would be appreciated. I would like to test the car this coming weekend?
Thanks,
Meza

erubin · #2 07-24-2003, 04:01 PM

Old HP, etc oscilloscopes are real cheap. I got one for free and would also like to know if an electronics type oscilloscope can be used for analysing an automobile's ignition system.

I had always assumed that these oscilloscopes would not work with high voltage ign systems but maybe there is a way?

Anyone out there know?

#3 07-25-2003, 08:31 AM

I wrote a lengthy reply to this thread yesterday, but I guess I screwed up somehow and it didn't get posted.

erubin is right. You can't use a general purpose oscope on the secondary side of the engine.

In the days of point ignitions, the ignition scope was very necessary. Now with the flame thrower ignitions and 100,000 mile spark plug life, these things are either in the junkyard, or like mine, is in the corner gathering dust. and being used for storage.

You could obtain or fabricate a high voltage probe to step down the 50KV or higher secondary ignition voltage to be viewed on a general purpose scope, but I feel that this would be a waste of time. The occasion for the need of an ignition scope these days is EXTREMELY rare.

Hope this helps,

stevebfl · #4 07-25-2003, 11:08 AM

The simplest test of ignition which can be done with any scope is to view the primary side of the system.

Virtually everything that takes place in secondary is mirrored in primary.

But just like your EKG looking at a scope pattern has no more value than your knowlegde of the system and what it all means.

Meza · #5 08-01-2003, 02:08 AM

I guess I would have to use a digital multimeter and check the resistance. I looked at the scope and the max voltage input is limited to 250V. Not to mention that no voltage probe is available. Steve has a point about what he stated. Anyways, I found the problem. It was a bad spark plug wire going to cylinder #3. I couldn't believe it because I bought the ignition wires new a little over 2 years ago and what's funny my #3 spark plug fouled almost every two months for the past 2 years to the point that I bought several plugs and put them in my trunk. A new set of Bosch ignition wires should be here tomorrow. Thanks for the input from everyone.