1978 Chrysler 140 Magnapower CDI bad again...

[Cbrettsky]

Hi everyone, I wanted to see if anyone had any info about a 1978 Chrysler 140 CDI module that continues to go bad. I picked up this motor a year ago with the original style CD module on it that had the large black box and separate coil on it. After some inconsistent engine running I traced it back to this module which I replaced with the newer CDI module 115-3301. Ran great after that for a few months. After layup this winter and a full boat overhaul including rewiring the electrical system the boat ran perfectly for one day then after some minor electrical work (rewiring a stern light) the engine demonstrated the same characteristics of the first bad CD module. I am not a mechanic by any means but what happens is when the engine is started in neutral fast idle it cranks and revs to 2800 RPMS(normal).......When I notice there is a problem when it runs slightly rough and only gets about 2100 RPMS.....After repetitive starts this is consistent. It also will not stay running at idle. My (Not so smart) diagnostic check included pulling spark plug leads one at a time and noticing a change. However when this is happening pulling the number 3 lead will result in no change. (No spark on #3)
This is exactly what was happening when I changed the first CD module and after it ran perfect. However this time after pulling the lead it would not fire up at all. DEAD.
Soooo, after replacing with a new CDI module the engine immediately fires up and has been running perfect now for a week.
My question is does anyone know what could be causing these CDI units to fail?? I know they are sensitive to electrical loads. I shorted out the stern light when re wiring it after the last one failed. It didn't blow the fuse but I definitely had a red to a black for a few mins with power on and the key powering the CDI module.
Could this have done it?

Sorry if this is not to clear in my explanation but again I am not a mechanic.
Thanks!
Chris

 

[jerryjerry05]

Don't really know that much about the Mag Power.
What kind of battery are you using?
A deep cycle? that can kill your system.
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[Cbrettsky]

I'm using a new hybrid dual purpose battery. There are some clear warnings on the CDI shipping paperwork that says the unit can be damaged with a dry cell battery but nothing about a deep cycle. Thanks!

 

[Nordin]

I have read through your post a couple of times and I am not quite sure the CDI is bad.

As the Magna Power I and the aftermarket 115-3301 is a battery feed system with a beltdriven distributor and you have spark at one or two plugs but not all three.
If there is spark at one or two plugs the CD module works and if there are no spark at #3 the issue is the trigger or the distributor.

From my experience the module do not like reversing the battery poles, it will go bad if you change the minus and plus poles.

Maybe I missunderstand your post as I am in Sweden and my English is not perfect.

 

[Cbrettsky]

Thank you for the reply Nordin. So far it has been running well daily for two weeks. Initially after I changed the cdi unit it ran perfect leading me to believe it was in fact a bad CDI. I don't know if a distributor or trigger problem in the distributor will slowly start to cause my problem again.
Thanks!