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willysmjeeps.com :: View topic - M38A1 engine diagnostic tests
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M38A1 engine diagnostic tests
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JeepdaddyRC
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Joined: Jan 10, 2020
Posts: 170

PostPosted: Tue Feb 09, 2021 12:41 pm    Post subject: Reply with quote

Can a military 24-volt coil cause a weak yellow spark?
I've experienced no spark conditions in CJs (caused by a bad coil from an ignition switch left on).
Does the military coil either work or not work? Is there anything in between? Please excuse my unfamiliarity with military 24-volt waterproof ignition system. Appreciate your patience.
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RonD2
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Location: South Carolina, Dorchester County

PostPosted: Tue Feb 09, 2021 6:59 pm    Post subject: Reply with quote

Hey Daddy (LOL)
Yes, there's no difference between military 24-volt or civilian 12-volt coils --- both can do weak yellow spark. Lots of electrical components including coils get weak before they fail altogether. And some get hot bright just before they fail. Some fail open and some fail shorted, both clean fails and easy to spot. Get out your multi-meter and measure the resistance (ohms) of the primary and secondary windings (and also between the two) --- you might get a good indication if it's in spec or not.

Failed open or failed shorted is obviously easy to spot (no spark at all). Any readings close, or more or less than spec are a little harder to spot sometimes, but can still be suspect (especially intermittent or heat related impending failures). There are plenty of threads in this forum to search that say how to do meter checks, as well as using your tech manual.

Best and simple check is replace it with your known-good spare, or at least compare meter readings between the known good and suspect coils.

I won't mention how useful a megohmmeter can be for testing insulation resistance breakdown of and between coil windings. A regular multimeter only goes so far when checking coils.

My humble opinion is that every ignition coil, no matter the rated input voltage, should produce a bright and strong blue spark. There's no mistaking it. Anything less is a problem.

Good luck!

P.S. >>>> even a good coil can't produce a bright strong blue spark if the input voltage is faulty. Better make certain your batteries are fully charged and the charging system when the motor is running is working correctly. And crappy electrical connections and rotten wires will get you running in circles....
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Ron D.
1951 M38 Unknown Serial Number
1951 M100 Dunbar Kapple 01169903 dod 5-51

“The only good sports car that America ever made was the Jeep."
--- Enzo Ferrari

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JeepdaddyRC
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PostPosted: Tue Feb 09, 2021 8:49 pm    Post subject: Reply with quote

Tested coil per distributor photo in Wes's album.
Primary (negative to positive post) = 6.6 ohms
Secondary (negative post to coil center) = 12,800 ohms
These values appear to indicate coil is ok.
Still trouble shooting yellow spark at plug gap.
Plugs black and wet. Trying some carb cleaner to clean electrodes.


Last edited by JeepdaddyRC on Tue Feb 09, 2021 9:15 pm; edited 1 time in total
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RonD2
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PostPosted: Tue Feb 09, 2021 9:10 pm    Post subject: Reply with quote

I tend to agree the coil seems ok, but ohms testing ain't the same test as with voltage applied. With the engine running, what input voltage is on the primary?

If you plug the words "coil test" into the search button above, and click "search all terms" you'll get a flood of knowledge to digest.

Quit messing around and install your brand new set of spare spark plugs, correctly gapped of course. Cleaning the electrodes on your plugs isn't going to fix weak yellow spark. Hot blue spark will clean the electrodes for you.

Your earlier post said timing is set to 8 degrees BTDC. Isn't that a little retarded? What about adjusting the valves? Done that yet?
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Ron D.
1951 M38 Unknown Serial Number
1951 M100 Dunbar Kapple 01169903 dod 5-51

“The only good sports car that America ever made was the Jeep."
--- Enzo Ferrari

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JeepdaddyRC
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PostPosted: Tue Feb 09, 2021 10:06 pm    Post subject: Reply with quote

Yes, valves adjusted per specs. I was surprised by the valve rotators. #3 exhaust valve rotator was down against the tappet screw. I had to push it up with my finger to get the feeler gauge in there.
Cleaned engine to chassis ground.
The lead from positive coil post (that disappears below the plate the points are mounted on) was frayed and bare. Not sure where that goes or how easy it is to replace. Tried to re-insulate with electrical tape.
Timing was increased from 5 degrees to about 8 as suggested for modern octane fuels.
Wish I had a new set of spark plugs. $45 each. Yikes!
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RonD2
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PostPosted: Tue Feb 09, 2021 10:17 pm    Post subject: Reply with quote

Quote:
The lead from positive coil post (that disappears below the plate the points are mounted on) was frayed and bare. Not sure where that goes or how easy it is to replace. Tried to re-insulate with electrical tape.

Electrical tape isn't going to cut it. Asking for trouble if you don't already have it. Replace the wire. And any others that look or smell like it. Quality automotive wire isn't expensive. Get mil-spec if you can. I can point you to source if you like.

Quote:
Timing was increased from 5 degrees to about 8 as suggested for modern octane fuels.

Timing terminology is usually either "advanced" or "retarded" not "increased". Further "advanced", or further "retarded" --- with TDC being the reference mark. If you went from the factory spec 5 degrees BTDC to 8 degrees BTDC then you went further retarded. My opinion is that modern fuel ain't that big a deal, especially while you sort out your engine issues. Recommend you put it back to the factory tech manual spec of 5 degrees BTDC. And unless I'm mistaken, the conventional wisdom advice is to "advance" (not retard) timing slightly to compensate for modern fuel. But you need to carefully tune the engine and carb vacuum and RPM. Better yet, just find a gas station that sells ethanol free gas and forget about it.

Add some ZDDP to your crankcase oil.

You need the voltage measurement on the coil primary to see if it's getting the proper voltage so it can make a bright blue spark.

Military spark plug prices? LOL. Wait till you see what everything else costs. If you have to ask what things cost, you can't afford this hobby. Laughing Welcome to the club!

Just my 2 cents.....
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Ron D.
1951 M38 Unknown Serial Number
1951 M100 Dunbar Kapple 01169903 dod 5-51

“The only good sports car that America ever made was the Jeep."
--- Enzo Ferrari

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wesk
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Location: Wisconsin

PostPosted: Tue Feb 09, 2021 11:39 pm    Post subject: Reply with quote

Ron D said:
Quote:
Your earlier post said timing is set to 8 degrees BTDC. Isn't that a little retarded?


RonD said:
Quote:
Timing terminology is usually either "advanced" or "retarded" not "increased". Further "advanced", or further "retarded" --- with TDC being the reference mark. If you went from the factory spec 5 degrees BTDC to 8 degrees BTDC then you went further retarded.


Ron, You are in over your head here! BTC = Before top dead center. This means advanced timing and when timing is advanced it falls before TDC. After TDC is retarded timing which = later, past TDC, or retarded timing of spark.

Jeepdaddy,
You did just fine advancing your timing from the standard advance setting of 5 Deg BTDC to 8 for better performance with modern ethanol blends as long as it doesn't reduce the cranking RPM during start.

The basic operations of DC ignition coils has changed very little since the 1920's. The biggest changes were the upgrading of voltage over the years from 6 to 12 in the early 50s including the military switch to 24V in 1948 and the modernizing into the computer age which brought us the tiny individual cylinder coils. Believe it or not they are all just the 1920's 6V coils with added resistors (both internal and/or external) based on voltage increases.

Coils should be tested for Ohms at primary & secondary circuits and amp draw at the specified voltage. These readings need to be taken with the coil cold and hot.





And do not overlook testing the governor advance curve per the correct curve based on which distributor you have:





When you are checking engine grounds do not overlook the distributor body to block grounding.
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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RonD2
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PostPosted: Tue Feb 09, 2021 11:43 pm    Post subject: Reply with quote

Thanks Wes. I'm usually a$$ backwards about something or another.

My apology to Daddy!
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Ron D.
1951 M38 Unknown Serial Number
1951 M100 Dunbar Kapple 01169903 dod 5-51

“The only good sports car that America ever made was the Jeep."
--- Enzo Ferrari

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JeepdaddyRC
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Posts: 170

PostPosted: Wed Feb 10, 2021 9:37 am    Post subject: Radio Noise Filter Capacitor Wire Reply with quote

I discovered the wire at the + terminal ( A ) of the coil is badly frayed and bare. I read it is about 5" long and goes to the Radio Noise Filter Capacitor in it's housing center front of the distributor.

How does one repair or replace this wire? It disappears under the plate holding the points (down into the timing advance spring area). Would liquid electrical tape or heat shrink suffice to repair the frayed insulation near the positive coil post?

I see Midwest Military carries this part. Just not clear how to access it or replace.

Could this lead to a weak spark?
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wesk
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PostPosted: Wed Feb 10, 2021 12:38 pm    Post subject: Reply with quote

I would suggest you have someone more qualified remove the distributor and give it a thorough tear down and inspection. At the same time this would be a good time to have things tested properly. That distributor has two capacitors in it. They can only be properly tested with a capacitance meter. The top one you can see is called a condenser and it's purpose is to help reduce flashover at the contact points. The radio noise filter is inside the body where wire #12 enters the distributor. My advice is to eliminate that noise filter which has no real purpose with modern radios.

Once the distributor passes a bench check by a pro make sure it is reinstalled correctly by first checking for proper indexing of the oil pump gears which drive the distributor then properly indexing the distributor. Again try to get more experience persons involved with you.

Still waiting for your leak down test results.
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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JeepdaddyRC
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PostPosted: Wed Feb 10, 2021 12:47 pm    Post subject: Reply with quote

Thanks Wes. Leak down test results coming soon. My son and I hope to do it in next few days.
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JeepdaddyRC
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PostPosted: Wed Feb 10, 2021 9:08 pm    Post subject: Reply with quote

Cylinder Leak Down Test Results:
Reminder - compression test showed cylinders 3 and 4 needed further evaluation:
1. 125 dry, 125 wet
2. 122 dry, 125 wet
3. 95 dry, 100 wet
4. 85 dry, 95 wet

Cylinder #3 leak down 25-30% at 60psi
Cylinder #4 leak down 30% at 60psi
For both cylinders no bubbles in radiator, no sound at exhaust pipe, no sound at carb, no air escaping from adjacent cylinder.
The escaping air sound came from the oil filler (crankcase).
If I pushed the incoming air to 60-80 psi, the engine turned and, while in reverse and with parking brake on, the vehicle started to move!
Please note these readings were taken in our cold garage (at 20 degrees) and engine was not at operating temperature.
Looking forward to your expert interpretation.
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wesk
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PostPosted: Thu Feb 11, 2021 12:04 am    Post subject: Reply with quote

When you do a leak down test the norm is to pump 80 psi into the cylinder with the piston TDC on compression and record the resultant PSI differential reading IE 1 - 80/60, 2 - 80/75, 3 - 80/70, 4 - 60/55. These readings would indicate that 1 & 4 have issues. They are read as #1 cyl 80 PSI over 60 PSI or 80 PSI in and 60 PSI out. 65 to 75 out are generally acceptable readings.

Then we determine where the loosing pressure is exiting the cylinder. IE 1 - E, 2 - R, 3 - C, 4 - I

1 = Exhaust, 2 = rings, 3 = coolant, 4 = Intake

Ring/piston skirt leakage is considered dynamic leakage.
Dynamic leakage is usually an acceptable leakage so long as the out PSI reading is in the 65-75 range.

Exhaust, Intake, and Coolant leaks are considered static leakage.
No static leakage is allowed. Leakage at these locations must be fixed.

When keeping piston still becomes impossible then you may leave the piston at Bottom center throw with both valves close and record those readings, but keep in mind that the cylinder is usually snugger at the lower end so readings will be higher than norm usually.

Your results indicate 3 & 4 have extremely worn rings or rings that are corroded and stuck in the grooves or cylinder bores that are excessively worn at the top of the cylinder.
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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JeepdaddyRC
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PostPosted: Thu Feb 11, 2021 10:15 am    Post subject: Reply with quote

This is the type of gauge we used. Incoming shop air was 60-80psi, needle in green at 25%. Interesting this gauge indicates 25 is in "low" leak down range.

May try to repeat the test with different gauge and try to keep piston from moving at 80psi. Is a breaker bar on crankshaft nut acceptable?

If the test confirms we are more than 10% leakage at rings/cylinder walls, is a rebuild our only option?



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wesk
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PostPosted: Thu Feb 11, 2021 11:10 am    Post subject: Reply with quote

I learned to use the leak down tester years ago on aircraft piston engines. I have always used a test set that has two identical pressure gauges. Maintaining accuracy & consistency is very important. Avoid terms like close, almost, variable (60-80) etc as much as you can. It should not be difficult to take all 4 readings at exactly 80 PSI in so all four readings mean exactly the same differential basis.

Quote:
Interesting this gauge indicates 25 is in "low" leak down range.

If you had exactly 80 PSI in and your gauge read 25% leakage rate then that 25% is = to 60 PSI Out. The cutoff between acceptable and unacceptable readings varies with type of engine being tested. The closer tolerances an engine is built to the higher the PSI out reading should be. These old 4 bangers are usually good to go with 80 in and 60 or more out.

The only leakage type you are permitted to make a judgement call on is ring/bore wear (Called dynamic sealing). Static seal leakage permitted is ZERO (0.0). So any air blowing in the Intake, exhaust, between cylinders or thru the water jacket is not permissible period.

Quote:
Is a breaker bar on crankshaft nut acceptable?


Yes, as long as the nut has the proper torque on it.

Quote:
If the test confirms we are more than 10% leakage at rings/cylinder walls, is a rebuild our only option?


Just the leakage rate alone is not the sole basis for what to do and when to do it. First it is not uncommon for that particular piston to have all of it's ring gaps in line which will result in a high leakage rate. The solution to that is to run the engine several times and rechecking that cylinder's leak down rate again each time. No improvement usually indicates the issue is ring to bore clearance. But you still do not have a final answer. It may just be that your rings may be stuck in their groove in the piston and not able to make full contact with the cylinder wall. These possibilities take you to the next step in the troubleshooting procedure. Soak rings & Piston with a penetrating lubricant for several days. Repeating application each day. The making sure the engine rotates freely with plugs out by hand put the plugs in and run it at 1600-2000 RPM for 20 minutes or so. Repeat the leak down test. If still not joy then it is time to pull the head and oil pan and remove the offending piston. Check the rings while on the piston to make sure they are free to move. Mic the piston diameter, mic the bore diameter and see where your readings lay in the limitations specs page. Remove the rings from the piston and place them in the bore and check their end caps. Compare that reading to the specs & limitations page. Inspect the bore surface for corrosion, scratches and pits. Quite often a simple re-honing of the bore solves the issue. Or a set of new rings or new oversize set can solve the issue.

The point here is one should first determine positively what problem exists and then thoroughly inspect the condition of all parts then make a decision.
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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