Prestolite Control Module Potting Repair

1959 - 1978, M151, M151A1, M151A2, Technical questions and discussions, regarding anything related to the M151.
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goodeboy22
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Prestolite Control Module Potting Repair

Post by goodeboy22 » Sun Apr 09, 2017 5:37 pm

Hello All,

Has anyone tired to repair the Prestolite control modules by removing the old potting that is prone to failing and replacing with some other material to act as an insulator? I ask because while trying to figure out why my Jeep died this weekend ('43 GPW with M151A1) engine I noticed the potting in the control module has started to degrade and coated the whole distributor with this nasty sticky film. I believe I found the reason the Jeep died (bolt holding the distributor came loose and I lost time) as I was still getting spark at the end of the day so I don't think my control module shorted out yet. I'm thinking of removing all of the old potting material, cleaning the electronic board with alcohol and filling the module with silicon caulk. Does anybody have any other suggestions?

Thanks and Regards,

David
1943 GPW


Rickf
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Re: Prestolite Control Module Potting Repair

Post by Rickf » Mon Apr 10, 2017 4:40 am

Silicone caulk is acetic acid and that would be the end of the wiring. You would be better off filling it with epoxy resin. Chances are if it is gooey that you are not going to be able to save it but I guess it is worth a try as long as it is still working. A stall out with a gooey Prestolite module is pretty much a dead giveaway of a shorted module.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

goodeboy22
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Re: Prestolite Control Module Potting Repair

Post by goodeboy22 » Tue May 30, 2017 6:04 pm

I want to follow up on this topic as I was able to successfully re-pot my Presotlite module and got the M151 engine purring like a kitten again. First off, I was suspicious the ignition module was really bad as I was still getting spark (although weak) when the vehicle died. I removed the ignition module, coil and a spark plug wire and was able to bench tested the entire ignition circuit by hooking a 24v power supply to the positive primary post on the coil and triggering the module by running a screw driver next to the trigger switch. On the bench I was getting a bright blue spark and the module never failed to trigger/fire so this confirmed my suspicion the module was still good. The coil was also ohmmed out and found to read 6 Ohms on the primary and 15 Kilo-ohms secondary both readings within spec and thus confirming the coil is still good.

Once the module was confirmed to be good Isopropyl alcohol and a toothpick were used to remove what was left of the bad potting. Once the majority of the packing was cleaned out compressed air was used to dry the board.

Rickf is completely correct about the concern with acetic acid in silicon caulk with electronics. Doing some researching online I found you need to verify the caulk doesn't contain acetic acid and several electronics hobbyist have used GE silicone II caulk with much success over the years. I bought a tube of the black GE Silcone II which can be found at your local Home Depot store. The distributor was then completely dissembled and cleaned to remove any of the old potting residue that had gummed up the mechanical advance.

Root cause of my problem turned out to be three things.

1) Bad gas that had gummed up my carburetor. Solution was to drain the tank and rebuild the carburetor. During the rebuild discovered someone had installed the spring on the wrong side of the accelerator pump diaphragm which wasn't helping my problems.

2) Number two spark plug was dead. This was the cause of the sputtering I had noticed while driving. A quick test was to screw in a regular spark plug (I discovered an Autlolite 85 I had laying around my shop worked just fine) and use a jumper wire made from Tractor spark plug wire.

3) I had a short between the primary and secondary circuits in my disturber because of how the module wires were dressed. This turned out to be the real culprit. After putting in new gas, fresh batteries and confirming I had four good spark plugs the Jeep started and was running decently albeit still hard to start cold. During a rainy Saturday (and of course while I'm at a show) the Jeep died on me again and completely lost spark. While diagnosing the problem this time I was turning the engine over with the distributor cap off and I saw a massive blue spark arking between secondary lead on the coil and one of the positive leads of the ignition module. Redressing the leads to what was suggested by the TM fixed the problem and the Jeep has been starting better then ever and purring like a kitten!

I hope this write up may help anyone who is in a similar situation. The point is it may be possible to repair your old Prestolite module with a simple tube of caulk (assuming it hasn't shorted out completely.) In the meantime I'm back to spending money on the Go-Devil that is going to replace this M151 engine :D .

Best Regards,

Dave
1943 GPW

Rickf
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Re: Prestolite Control Module Potting Repair

Post by Rickf » Wed May 31, 2017 4:30 am

Very well written and a great follow up. Thanks. It is nice to get a resolution on a subject which does not happen very often. Also, great diagnosing on your part.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone


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