No fat blue spark at coil

JeepdaddyRC · Post by **JeepdaddyRC** » Fri Feb 26, 2021 10:02 pm

With ignition on and following the coil test pictured below, there is no fat blue spark when using civy spark plug near block and opening points.
Could this be due to:
1) bad primary capacitor?
2) bad coil?
All I get is a small white spark or no spark at all.

Post by **wesk** » Sat Feb 27, 2021 9:52 pm

Did you take primary and secondary coil resistance readings both hot and cold? (It pays to disconnect the two wires for this test) They will help answer your question very quickly.

What was coil's input voltage during your spark test? This can be read a wire #12 at the coil or terminal "A" in illustration below.

Disconnect wire from terminal be during ohm test.

JeepdaddyRC · Post by **JeepdaddyRC** » Sun Feb 28, 2021 11:12 am

The input voltage (measured at coil terminal A and ground) with ignition switch on = 24.8 volts.

Primary resistance readings (measured at coil terminals A and B) = 6.6 ohms

A few weeks ago this coil's secondary resistance reading (negative post to coil center) = 12,800 ohms. Today I can't measure any secondary resistance reading (at each ohm setting).

Bad coil?

This coil does not appear to be original (no Autolite or made in USA markings). It has a black top with silver sides.

I hear the offshore repo coils are hit or miss. DeBella has "an improved" repo. There are NOS available at very high prices. Is there a risk with NOS coil that has been sitting for many years?

Post by **wesk** » Sun Feb 28, 2021 11:36 am

First readings all appear normal. That today reading on the secondary is extremely odd. Time to try a different meter, check quality of grounds and etc.

Under your spark test results from the other day I suspect ground issues or excessive gap. They make a spark check tool that is adjustable for the gap.

I doubt shelf life is a serious issue with an OEM truly NOS coil.

JeepdaddyRC · Post by **JeepdaddyRC** » Sun Feb 28, 2021 12:24 pm

Here are the corrected readings:

The input voltage (measured at coil terminal A and ground) with ignition switch on = 24.8 volts. Does this mean primary radio capacitor is OK?

Primary resistance readings (measured at coil terminals A and B) = 6.5 ohms (within specs)

Secondary resistance reading (negative post to coil center with wires reconnected) = 13,140 ohms. Higher than the 10,000-11,000 Prestolite spec (tested today at 40 degrees garage temp).

Still no fat blue spark using civy plug from coil center to ground when opening points. We got a little spark at points when opening them to check for fat blue spark.

This vehicle is very hard to start. Try new coil?

Post by **wesk** » Sun Feb 28, 2021 2:17 pm

The input voltage (measured at coil terminal A and ground) with ignition switch on = 24.8 volts. Does this mean primary radio capacitor is OK?

It means the Radio Noise Filter is not shorted to ground or open. It's capacitance function can only be determined with a capacitance tester.

Primary resistance readings (measured at coil terminals A and B) = 6.5 ohms (within specs)

Close enough but I believe the coil's temp was too low during your test.

Secondary resistance reading (negative post to coil center) = 13,140 ohms. Higher than the 10,000-11,000 Prestolite spec (tested today at 40 degrees garage temp).

As above, I believe the coil's temp was too low.

Still no fat blue spark using civy plug from coil center to ground when opening points. We got a little spark at points when opening them to check for fat blue spark.

Have you ensured the engine block ground is correctly installed at the right front mount? Have you tried this spark test with a correct properly gapped spark plug installed in the stock plug lead with the distributor properly set up and fully assembled with all four plugs removed from the engine and left attached to their leads?

Note the Army didn't include this resistance check in their version of meter testing coils. They only used the current draw test. I suspect because the current draw test would normally heat the coil close to it's normal operating temp and the resistance checks must be done without DC power applied so the coil is not heated.

When you review the MIL-spec for the 5.5mm ign wire you'll note this resistance chart:

3.4.4 Capacitance. The dielectric constant of the insulation used on the completed cable shall be
such that the capacitance is not more than the values shown in table III.

TABLE III. Capacitance.
Capacitance per foot of cable
picofarads (pf)

Cable diameter designation - 5
Copper conductor 19 strands - 46 picofarads (pf), 64 Ohms/1,000Ft. or 0.0064 Ohms/foot
Copper conductor 37 strands - NA
Steel conductor 7 strands -37 picofarads (pf), 68 Ohms/1,000 Ft. or 0.0068 Ohms/Foot

Picfarads is a difficult dimensional measurement that I am not totally familiar with.

JeepdaddyRC · Post by **JeepdaddyRC** » Sun Feb 28, 2021 5:07 pm

Have you ensured the engine block ground is correctly installed at the right front mount?

Yes. Removed ground strap, cleaned and sanded contact area, then reattached.

Have you tried this spark test with a correct properly gapped spark plug installed in the stock plug lead with the distributor properly set up and fully assembled with all four plugs removed from the engine and left attached to their leads?

Yes, early on we did spark test with correct Autolite spark plugs gapped to .030 installed in the stock plug lead with the distributor assembled with all four plugs removed from the engine and left attached to their leads. The spark looked weak (small, yellow, sometimes blue) and inconsistent.

Thought we had it figured out due to locating a frayed primary wire to positive coil terminal (from radio noise filter). Re-insulated the wire and looks we are getting power to coil based on the 24.8 input voltage.

I am no electrician and this is new to me. I don't understand why we are not getting the fat blue spark in Wes's test from coil secondary to ground with civy spark plugs when points open.

Have learned lots and appreciate your patience, experience and good guidance.

RonD2 · Post by **RonD2** » Sun Feb 28, 2021 6:07 pm

You mentioned that "bad wire" in an early post. "Re-insulating" a defective wire isn't something that an electrician or mechanic would do (ok, maybe just to limp home in a pinch). In the shop it deserves nothing but replacement with brand new wire......along with every other wire of unknown old age showing cracked, frayed, or burnt evidence. Have you taken a close look at your wires (disassembling what you can't see)? If not, bad wire can cause a lot of symptoms that can keep you chasing your tail for a long time. Some fixes aren't as easy as you'd like them to be. Good luck!

A "pico-farad" is one-trillionth of a farad. Pretty small number. The farad is a measure of capacitance, not dimension. I believe the reference is to a specification for wire or cable (per foot) to call out insulation resistance breakdown or interelectrode capacitance between conductors, or between a conductor and a shield. It's supposed to be a very small number. And nobody without very specialized test equipment can even think about measuring that. Most electricians or mechanics just replace every suspect bad wire....much cheaper in the long run....

In other words, the U.S. National Debt is 28 trillion dollars. That's $28,000,000,000,000 (if my zero count is right). We're talking 28 bucks, or "28 pico-dollars" here.

Post by **wesk** » Sun Feb 28, 2021 6:35 pm

While we are discussing wires & spark plug shielded leads have we made sure we have all the correct components, properly installed in each lead?

Large file size here: http://willysmjeeps.com/v2/modules/gall ... Plug_1.jpg

Large file here: http://willysmjeeps.com/v2/modules/gall ... Plug_2.jpg

Not only do these leads need to provide an open path for the full spark voltage but it must be a clean, corrosion free path. Make sure there is not a corroded contact spring or corroded contact down inside the barrel of the plug. Make darn sure that spring on the ends of each plug lead actually reach and press securely at the contact in the base of that spark plug's barrel. Nothing more embarrassing then hooking up a wire that never contacts anything!

That fat blue spark must be traveling smoothly to the tip of plug. A worn distributor shaft bushing/bearing can create enough wobble to force that spark to jump a wider air gap then it's used to between the rotor and the cap. Worn tower pins in the cap can force a wider air gap. Burned rotor tips, and tower pins can increase resistance for the spark voltage. Worse yet carbon tracking inside the distributor cap can steal a lot of your spark or cause misfiring.

Frayed and damage shielding allows radio interference without reducing your spark.

Plug condition is critical to spark intensity. Correct PN plug is also important. 2243's go in the L134 and 2245 goes in the F134. I was raised in a shop environment where we periodically cleaned, gapped and tested spark plugs. Granted the equipment for doing this now approaches $1,000 dollars but many quality shops have this equipment and can do this cleaning and testing for you. Don't overlook local small airport FBO's where we aircraft mechanics have all been cleaning, gapping and spark testing plugs all our lives!

JeepdaddyRC · Post by **JeepdaddyRC** » Sun Feb 28, 2021 7:34 pm

Please confirm for this newbie. If I DO NOT get a fat blue spark from coil secondary terminal to block (from A to B in test shown below), it would be impossible to have a good strong spark at plugs. Is that correct?

RonD2 · Post by **RonD2** » Sun Feb 28, 2021 9:29 pm

Yes, that's true (provided everything else is working correctly) ---- which seems to be the problem --- nobody is sure everything else is working correctly.

It's difficult, if not impossible, to explain basic electricity to a "newbie", especially over the interweb. It's also very difficult to troubleshoot something you can't see that's also hundreds of miles away. Throw a malfunctioning or intermittent malfunctioning part and maybe some bad wires into the mix? Your guess is as good as mine or anybody else's. Can't even get the terminology straight. Folks here want to help. I'm following your posts, but I can't make heads nor tails of what's really going on under your hood. If you post some photos that might be helpful.

This vintage ignition system design has worked on millions of cars over the decades, with plenty of mechanics and electricians repairing the same in short order. Pretty simple really.

Have you been studying the books?

The Army Technical Manual TM9-8000, Principles of Automotive Vehicles (January 1956) is a great place to start. More than 500 pages of the best money I ever spent.
Free download here: https://radionerds.com/index.php/File:T ... HICLES.pdf

Post by **wesk** » Mon Mar 01, 2021 12:15 am

Please confirm for this newbie. If I DO NOT get a fat blue spark from coil secondary terminal to block (from A to B in test shown below), it would be impossible to have a good strong spark at plugs. Is that correct?

Well if you think a bit on this.

1 - Gap this system is designed to produce a blue spark at is .030. So by all rights if you cause the coil to discharge properly across a gap of 0.030 then and it is a week yellow spark then you are correct.

2 - Were you holding the tip of that test wire 0.030" from the ground source?

JeepdaddyRC · Post by **JeepdaddyRC** » Mon Mar 01, 2021 4:01 pm

Wes,
Tested again to confirm findings.
The color of the small spark using a civilian spark plug from the coil secondary terminal to the block is white/yellow.
It is not blue and definitely not fat.
This spark test was repeated numerous times by opening the points with wire at different distances from the block, with a goal to duplicate the .030 spark gap.
Each small spark was white/yellow.

RonD2 · Post by **RonD2** » Mon Mar 01, 2021 6:09 pm

This spark test was repeated numerous times by opening the points with wire at different distances from the block, with a goal to duplicate the .030 spark gap.

What does "opening the points with wire at different distances" mean? It's hard to tell from your one-sentence description. Are you opening and closing the ignition breaker points (properly set gap of .020) by hand to fire the coil --- or by cranking the engine over with the starter? Doing it by hand could be rough on the coil.

And I'd be surprised if most people could eye-ball a .030 spark-plug gap by hand. I know I can't even come close. A correct hot-blue spark will jump a lot more than .030.

If I may make a suggestion, it might be time to consider exchanging phone numbers and talk this over in real time?

JeepdaddyRC · Post by **JeepdaddyRC** » Mon Mar 01, 2021 8:14 pm

Clarification: Opened the ignition breaker points by hand (with plastic stick) to fire the coil per Wes's spark test. White/yellow spark seen with plug wire close to block (.030).

I have neglected to mention this M38A1 Jeep has a military radio (RT-70/GRC & AM65/GRC Amplifier). The power is off. Could that be a factor?