Electrical Cranks OK, but No Start 92 Mustang 5.0 Coil Fat Blue Spark per jrichker

Apr 30, 2019
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Midwest, USA
jrichker posted a terrific checklist that expands on the Chilton's manual stuff. The starter solenoid is next to the coil, btw, and "Jumper the screw to the big bolt" is, for example, to jam a big bolt between the two solenoid lugs. The key phrase that helped me diagnose my problem was "a nice fat blue spark ". Here's what I did:
1992 5.0 no-start Mustang: Had spark, but thought it was weak. Flicker of test light at -12V coil terminal eased concerns about TFI & PIP problems. Could smell fuel at exhaust pipe indicating ok fuel pump & injectors but checked pressure at rail anyway: 42 psig w/pump running to 37-38 off. Fine. Cleaned slag off electrodes in distributor cap, scraped rotor edge to bright metal, resistance of coil wire to distributor was fine (~5000 ohms), spark plugs fine, set gap to specs. Could smell fuel when removing plugs, also indicating ok fuel delivery. Resistance on primary side of coil ok at ~0.4 ohms, but secondary resistance was 18,500 on digital meter and 20,500 on an analog. Cobbled up an air gap tester (1 inch notch in a board with 1/2" 'ears', tapped 1/4-20 and screwed in 2 pieces of 1/4" all-thread.) No spark until about 1/8" gap, and when it finally sparked steadily the gap turned out to be exactly the spark plug gap. That was direct from the coil terminal to negative post of battery in humid air with pointy electrodes. By the time that weak spark went through the coil wire, across the distributor, and down the plug wire into the compressed fuel-air mixture, there wasn't enough energy left to ignite the fire. Checked references and estimate the coil was putting out less than 5000 volts. New Blue Streak coil had same resistance for primary, 8650 on secondary, and threw a spark about an inch long. Previously doggy Pony now starts quickly and gets up and goes like she's supposed to; what a difference!
 
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Apr 30, 2019
3
1
13
66
Midwest, USA
It's about a hundred mile round trip to the big stores for me and forty for the closer stores that had all closed up for the night. So I opted to spend maybe 15 minutes to make the air gap tester out of stuff lying around. All I needed was a way to hold a constant gap between a couple of points to quantitatively measure how long a spark I could get. I had gotten so accustomed to rolling the engine over without it firing a lick that when it unexpectedly started with the new coil I jumped and lost contact with the coil post. The coil arced from the bare post down to the laminated steel core a few times as the rpm's quit. When I explained the science behind it all to my 16 yr old son, he said "Dad, you're way too proud of that thing." :)