M30 points ignition problem - BMW Forum - BimmerWerkz.com
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#1 (permalink) Old 03-12-2013, 03:04 PM
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M30 points ignition problem

Here's one for the old school. The car is actually a 2800CS but the old M30 motors in the 5 series all had old fashioned distributors with points ignition systems.
The car has stood for ages while the body was restored ,but the engine ran fine before it was taken off the road. New battery, points and condensor. With ignition on ,flicking the points gives a nice fat blue spark at the high tension lead out of the coil. But when I crank the motor on the starter, the spark is tiny and useless, won't even fire a plug. As soon as I stop cranking ,and while the engine is still turning slightly under inertia, the spark is fat and blue again.
Voltage to the coil is well over 12v and only dips to about 11v when cranking.Tried a new coil, no difference.
What am I missing??
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#2 (permalink) Old 04-05-2013, 07:20 PM
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No Spark

The better you define the problem, the easier to define the solution.

From what you say, everything seems to work well UNTIL you engage the starter.
So, start from there. Check the starter relay. Check that the starter electrical connections are clean, shiny and tight.
Make sure your grounds are all clean, shiny and tight, also. All of them.

There's not just an issue with volts, it's amps, also. If there's not enough amps flowing thru the starting routine you'll get a lesser voltage drop.
The amps are getting used up by all the bad connections.
Remember the issues with bullet fuses, when bad connections would cause dim lights AND hot fuse holders, yet not burn the fuses? Same kinda thing.

Mine started slow but OK, with a 9 volt reading at startup, when I first got it.
The main ground from the battery to the frame was a sorta loose rusty greasy blob.
Fixing that also fixed a lot of other problems I was getting used to having.
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#3 (permalink) Old 04-11-2013, 02:51 PM
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M30 points ignition problem

Problem solved , but in a very embarrasing way. The clue was that the problem only occurred while the engine was cranked.
There is a positive 12v feed from the starter solenoid to the coil, in addition to the feed from the ignition switch. By mistake, I attached the solenoid feed to the negative side of the coil ,instead of the positive. My only excuse is the wire was the same color as the other negative side wire ,which signals the rev counter.
Switching the solenoid feed to the pos side of the coil restored a fat blue spark ,even while cranking. No surprise there!
I'll put the pillow back over my head now.
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