Coil interchangeability?

This is the place to get help with technical matters concerning your M151 jeep

Moderators: rickf, moose53, Mr. Recovery, raymond

mutt and jeff
Staff Sergeant
Staff Sergeant
Posts: 147
Joined: July 4th, 2020, 10:18 am
Location: Winston, GA

Coil interchangeability?

Post by mutt and jeff » November 20th, 2020, 5:36 pm

As if we don't have enough to talk about with these coils, I've got another question. While screwing up the ignition on my A1, I've got the original coil with the asphalt disc on the bottom and it may or may not be pooping it's pants. I have a Chinese coil and they are plug and play as far as fit. Operational differences are yet to be seen.
But I have a third coil thown into the mix that came out of an Autolite 24v Ignitor distributor off the parts shelf. This was a points distributor and not an EI one. The mounting "legs" on the Autolite coil are different than the Prestolite coil and there is no provision for the wire retaining clip.

So, the question is . . . if I have to, do you think I can use this Autolite coil in my Prestolite EI distributor?
coils 1.JPG
coils 2.JPG

rickf
General
General
Posts: 17367
Joined: November 26th, 2007, 1:28 pm
Location: Pemberton, NJ.

Re: Coil interchangeability?

Post by rickf » November 20th, 2020, 8:54 pm

That Prestolite coil looks like one that has the external resistor that sits underneath the coil. Do you have the resistor? If you need to know what it looks like just look at the pics of the coil that TJ just sold. His was one of those old units. If you do not have the resistor for the coil then no, you cannot use it.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

lpcoating
Colonel
Colonel
Posts: 1676
Joined: February 19th, 2013, 11:38 pm
Location: Mansfield, PA
Contact:

Re: Coil interchangeability?

Post by lpcoating » November 20th, 2020, 9:18 pm

Like this one?
20201120_201730.jpg
Guy
M151 Body Panels - http://www.m151bodypanels.com/

'68 A1 - Under full resto

rickf
General
General
Posts: 17367
Joined: November 26th, 2007, 1:28 pm
Location: Pemberton, NJ.

Re: Coil interchangeability?

Post by rickf » November 20th, 2020, 9:34 pm

1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

User avatar
muttguru
Colonel
Colonel
Posts: 1869
Joined: November 26th, 2007, 12:18 pm
Location: Li'l ol' England.....

Re: Coil interchangeability?

Post by muttguru » November 21st, 2020, 7:27 am

The coil to the right in the picture was the original type. After the Mil received lots of complaints about the plastic breaking where the "ears" met the coil, the redesigned coil was fitted the metal mounting ears.

Ken

Image
Kind regards....
Ken

Always wanted - Details and pictures of M416 Trailer data plates & M151 data plates & body-tags for my research. Thanks!

Contact address - - muttguru@aol.com

Note for 2019..... Ken...get out more....

rickf
General
General
Posts: 17367
Joined: November 26th, 2007, 1:28 pm
Location: Pemberton, NJ.

Re: Coil interchangeability?

Post by rickf » November 21st, 2020, 9:36 am

Ken, note how much shorter that coil is under the mount, isn't that also the external resistor coil?
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

mutt and jeff
Staff Sergeant
Staff Sergeant
Posts: 147
Joined: July 4th, 2020, 10:18 am
Location: Winston, GA

Re: Coil interchangeability?

Post by mutt and jeff » November 21st, 2020, 10:50 am

It is my understanding that the ballast would be at the bottom of this coil and nothing came out when I disassembled it from the early distributor. I will take a pic of the top, but I'm only feeling a condenser and points. When I stack the two coils next to each other, they certainly appear the same hight, so I don't know how an additional part would fit at the bottom of the original coil. But, I'll take a pic of the entire top of the distributor.

Hambone
Colonel
Colonel
Posts: 1921
Joined: October 1st, 2010, 12:19 am
Location: El Dorado, Arkansas

Re: Coil interchangeability?

Post by Hambone » November 21st, 2020, 3:08 pm

Looking at the bottom of the mounting ears, the original coils is shorter, so looks like there would be space to install resistor, without resistor the coil wouldn't bottom out in distributor, maybe that's why the ears broke???

User avatar
muttguru
Colonel
Colonel
Posts: 1869
Joined: November 26th, 2007, 12:18 pm
Location: Li'l ol' England.....

Re: Coil interchangeability?

Post by muttguru » November 21st, 2020, 6:25 pm

A Brief History of Ignition Coils.
When the M151 was introduced in May 1960, the ignition coil listed in the -20P was not the same as listed in the M38 and M38A1 parts lists. This came as a surprise (to me) because the coil was supposedly interchangeable between both distributors….as well as the M37 and various gas-fuelled trucks.
The M38 and M38A1 coils are listed as Part Number 7374882 and were designated by the manufacturer Electric AutoLite as model CT-4002. At a guess, I would say that throughout the years of operation of those two Jeep models, it would seem that the CT-4002 (made by AutoLite) was the Military’s item of preference, I couldn’t find any other manufacturer so it would be safe to assume that coils made by AutoLite were perfect.

The M151 started off with a new coil reference and NSN reference.
These were P/N 8328027 and NSN 2920-324-0371. The picture of the coil in the May 1960 -20P looks identical to the coil used by the M38 and M38A1. Here again, I have to guess that from the introduction of the M151, some pre-mutt parts beginning with “7” as used on the M38 and M38A1 were given a new number beginning with an “8”. Presumably the Army accountant-types wanted to account separately for items used by vehicles and equipment other than M151s.

Note that the 7374882 coil and the 8328027 coil both used the metal mounting “ears” and they certainly look identical.

Two years later, the December 1963 -20P listed different details for the coil.
The Part Number was now 8712399 and the NSN had become 2920-076-8993…but the photo in the parts list was the same as in the 1960 -20P.

With the release of the April 1968 -20P, there was a slight change. The coil part number remained the same (8712399) and so did the NSN (2920-076-8933) but the description of the coil included the words “Coil and resistor.” Surprisingly (or maybe not), the photo in the TM remained as before…..a plain coil with metal “ears”.
However, in Change-1 to the 1968 Parts List, issued in February 1969, it showed a picture of the coil plus the resistor.

The Change-3 to the 1968 Parts List, issued in August 1971 showed a complete change of references for the coil. These were announced as Part Number 11663066 and NSN 2920-00-152-2264 with the words “Replacement coil, Ignition. There was no further mention of the resistor.
From then onward through to the end of production, this part number and its NSN remained unchanged.

So what do we make of all this? Here is my view, for what it’s worth.

1. The coils used for the M151-series up to mid-to-late 1963 were the same as those for the M38 and M38A1, except with different reference numbers.
2. From late 1963 onwards, the plastic-composite type coils with the ballast resistors (and the plastic mounting “ears” were introduced.
3. By the time that PS Magazine warning about broken mounting ears and coil cracks was issued (December 1970) the game was up for the coils with the ballast resistor. The Military was already going back to the good old days of the M38 and M38A1-type coils with the metal mountings.

Since then, we’ve seen the unreliable chrome coils initially sweep the market…until owners discovered the short life-span these things had.
One day, some enterprising company will see that there’s a market for a coil that meets Mil-Specs and take action to meet that demand.
Let’s hope it won’t be too long to wait.
Ken
Kind regards....
Ken

Always wanted - Details and pictures of M416 Trailer data plates & M151 data plates & body-tags for my research. Thanks!

Contact address - - muttguru@aol.com

Note for 2019..... Ken...get out more....

User avatar
m3a1
Brigadier General
Brigadier General
Posts: 2699
Joined: August 7th, 2014, 6:36 pm

Re: Coil interchangeability?

Post by m3a1 » November 21st, 2020, 11:25 pm

Might the short lifespan of the Chinese coil be based solely upon their understanding of the specs of the original coils (absent the ballast resistor?). Stranger things have happened. Could very well be that we are running too much juice through these Chinese coils!

Thoughts?

Cheers,
TJ

User avatar
muttguru
Colonel
Colonel
Posts: 1869
Joined: November 26th, 2007, 12:18 pm
Location: Li'l ol' England.....

Re: Coil interchangeability?

Post by muttguru » November 22nd, 2020, 7:04 am

TJ....
that is a real possibility. But to test that theory, there would have to be a way to put the resistor outside the distributor housing..... the Chicom coils are 'full size' so even if you had the original ballast....it wouldn't fit under the new coil.

Back in the early 70's I had a Vauxhall (GM) Viva car. It had an externally-mounted standard 12v coil...like most other cars of the era. But attached to one terminal of the coil was a small capsule-shaped thingy about the size of a £1 coin. I remember our local car mechanic telling me it was there to "reduce the amount of power" going into the coil. I accepted what he told me and didn't question him further.

Tj....did you see any markings or values on that resistor that was with the coil you sold recently?

Time to ask our electrical Guru, George Ramselaar.
Ken
Kind regards....
Ken

Always wanted - Details and pictures of M416 Trailer data plates & M151 data plates & body-tags for my research. Thanks!

Contact address - - muttguru@aol.com

Note for 2019..... Ken...get out more....

rickf
General
General
Posts: 17367
Joined: November 26th, 2007, 1:28 pm
Location: Pemberton, NJ.

Re: Coil interchangeability?

Post by rickf » November 22nd, 2020, 8:08 am

Did you guys see Louies testing of the coils? This was very comprehensive and revealing. He also noted that the Chinese coils are substantially lighter in weight than the US made coils which means less copper inside. It is his opinion that there may be a resistor built into the coil.

https://g838.org/viewtopic.php?f=2&t=12808&start=15
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

12Bravo
Sergeant
Sergeant
Posts: 96
Joined: May 15th, 2018, 7:49 am

Re: Coil interchangeability?

Post by 12Bravo » November 22nd, 2020, 9:18 am

Jeep ran a ballast resistor on the fire wall Jeep vehicles 68 to 71. Napa has them.
Attachments
s-l500.jpg

rickf
General
General
Posts: 17367
Joined: November 26th, 2007, 1:28 pm
Location: Pemberton, NJ.

Re: Coil interchangeability?

Post by rickf » November 22nd, 2020, 9:43 am

So did Chrysler< Ford and Gm over the years but without knowing the values and doing the math you will not know what the secondary voltage is coming out of the coil. I can tell you that Louie tested the OEM coils on 12 volts and got 20,000 volts out, the same output as the Chinese coils on 24 volts. But putting 12 volts into the Chinese coil also got 20,000 out which is why he suspects an internal resistor. This is above my pay grade when getting this far into electronics but I am thinking the Chinese coil will far no better on 12 volt than on 24 volt from the sound of it and if you run a ballast resistor you will be dropping the voltage down to roughly 9 volts. Personally I am looking at the other end of the scale and looking at Capacitive discharge ignition. I run an MSD 6AL on my CJ and it has worked well and kept it running even when on it's side with gas pouring in the carb. Power to the unit would be through a 12 volt converter and the trigger would be good old fashioned points. And the points will never wear since they are only a trigger and do not carry a load. The coil is fired by a much higher voltage but lower amperage so coil temps will be down but secondary voltages will be up. Wiring would be two wires running in through the same place as the current power wire runs now and if the wires were pained black or sleeved you would never know it was not stock.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

User avatar
muttguru
Colonel
Colonel
Posts: 1869
Joined: November 26th, 2007, 12:18 pm
Location: Li'l ol' England.....

Re: Coil interchangeability?

Post by muttguru » November 22nd, 2020, 12:43 pm

I spent the morning rummaging through the garage with some success. I found my old 'ballast' coil AND the resistor.
The details are on a paper tab glued to the body and it says... USE EXTERNAL RESISTOR UNDER COIL. It was manufactured by Prestolite Division - Eltra Corporation - Toledo, Ohio.
Part Number 8712400. Manufacturer's Part No. 200735.
I know the coil is kaput...and there is a hairline crack but no leakage. The main body of the coil looks like it was assembled as two distinct halves of the plastic shell and what appear to be four tiny holes on each half of the shell....it looks like there are screws or plastic pegs holding the two halves together with the two side-seams and the top block sealed by tar or black glue.
The resistor is a pressed steel dish with a red and black wire going into it and from the top of the dish, it appears to have been filled with a grey plaster-like substance. There are cracks in the plaster, like old dried out boot-polish.

If I knew what I was doing, I'd test the resistor to see what I get (Ohms??) but will the cracks in the plaster affect any readings? And by the way...is this plaster/potting material hazardous? :D

Ken

Image

Image
Kind regards....
Ken

Always wanted - Details and pictures of M416 Trailer data plates & M151 data plates & body-tags for my research. Thanks!

Contact address - - muttguru@aol.com

Note for 2019..... Ken...get out more....

Post Reply