I am befuddled.

I've been warming up my '73 periodically (2wd cab on '74 4x4 frame with a 390FE/NP435 combo) and went to start it on Friday to keep everything lubed... Cranks but does not fire.

Here is my trouble shooting procedure. It needs air, fuel, and spark. I am missing the latter....

1. Quick visual inspection... no disconnected wiring, no chafed wires, no rodent damage.... just like I left it two weeks ago when it did successfully crank and start....

2. Verified fuel pressure and fuel is being primed in the carb. Two nice jets when the throttle is cycled... fuel pressure is good. Loosened a hose clamp after the filter and it sprays out consistently.

3. Need spark. Pulled the coil wire from the dizzy and placed it near the cylinder head. Turned ignition on, remotely cranked it (jumping S terminal and battery hot at solenoid) and a white/yellowish spark jumps to ground.

4. Pulled #1 spark plug, reconnected its wire, and grounded its metal body against the cylinder head. Ignition on. Cranked with no spark.

5. Swapped the ignition control module with a known good OEM Motorcraft unit (my spare), plus a spare cap and spare rotor. Cranked again (ignition on) using the spark plug body grounded against the cylinder head. No spark. Tried the same procedure using #5 spark plug to eliminate the possibility of a bad plug wire. Still no spark.

6. Bad magnetic pick up? 'Disconnected the three-wire plug of the DSI distributor. Attached a Voltmeter (on AC setting or alternating current) to the orange and purple terminals. Ignition off. Cranked and got cycling up to 1 VAC. That is supposed to indicate a good pick up.

..... so, I thinking either it has bad magnetic pick up or it has a wiring problem such that the ICM is not receiving the signal from the dizzy's magnetic pick up.

But here is the oddity.... the coil is firing but the current is not being transferred from the cap to the plug wires and finally to ground. That would indicate a bad dizzy but that doesn't make sense because the current from the coil would have to be grounded out in its entirety.... And the coil signal is electrically insulated from ground by the dizzy rotor, cap, and plug wires.

Your input is appreciated. What say ye?

 

Sounds like the issue is cap/rotor related. If the coil is firing the breaker and box appear to be functioning

 

In #5 I swapped the cap and rotor with known good units... even Ohmed out both caps and both rotors and all checked good (zero Ohms or electrically shorted).

 

I had that exact same issue then I found the rotor on the fender

Bad wire to the distributor cap is my only thought unless it is the cap and rotor try bending up the tab on the rotor

 

Sounds like an issue with either the coil to cap wire or possibly a weak coil? You say that you get a weak spark with coil wire disconnected and grounded, but nothing out of the cap cylinder wires. Have another coil wire and coil you can test with, or ohm out the coil wire?

 

To the top...

 

Make a jumper and bypass the NSS.

 

I missed that. How strong was the spark at the coil wire? Did you ohm the coil itself? IIRC primary side of the coil should be a few ohms and the secondary side should be around 7k-10k.

 

Sounds like possibly a weak coil.
Have you checked to see if there is 12v at the coil while cranking? As an initial check, pull the small wire off the "S" terminal of the starter solenoid and measure for voltage at the terminal while someone is cranking the engine over.

 

x2 or x3 on the coil, check the power leads to it. Easiest way to do that is to just hotwire it, if it fires, there's your problem.

I would swap it with a known good unit if that doesn't do anything, maybe its shorting internally/externally.

Sam

 

in addition to the above, was the spark you were seeing when testing the coil wire consistently sparking, or was it only a couple of times when you started/stopped cranking?

 

Thanks Mike. I didn't check 12 VDC at coil while cranking. 'Figured (assumed!) it was acceptable since the spark was white with a yellow tinge.

Thanks. It's an NP435 and the OEM jumper is installed. I will verify by Ohming in and out of gear.

Thanks... Spark was white with a tinge of yellow. I did swap the coil and both behaved the same. However, I did not Ohm either of them. Shall do as a matter of thoroughness. A fellow gearhead at the office stated the coil may not have enough "ooomph" to bridge across plug gap.

The coil was sparking consistently while cranking.
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Alrightee... ya'll have given me enough to go on. Unfortunately, it looks like long hours at the office dealing with architects, engineers, and arborists... It's darn near 9 PM and I just got home. I will dutifully execute the recommendations and report back.

Additional comments and suggestions are of course welcome....

 

In your ignition circuit the largest resistance should be the spark plug gap because the air/fuel is compressed and harder to bridge. Sparking at the coil wire would mean the pick up is working, but the spark color should be more white/blue than white/yellow. If you have access to an oscilloscope you could look at the spark waveform and see the firing voltage as a large spike. Since you grounded a plug out of the engine and had no spark I am thinking your coil is either weak enough that it cannot bridge the gap in the distributor and the sparkplug together, or you have some bad plug wires, possibly a combination of the two. A vehicle will run on a piece of rubber hose used as a coil wire if you make it, but if a plug wire fails internally they won't spark. Makes no sense, huh?

 

good post hydro

 

a couple more things. There was no guarantee the spark plug was grounded when you tested possibly. you could eliminate the plug and just see if the plug wire(s) arcs to ground when cranking (you may need to stick in a bolt or screwdriver instead of the plug). My 460 flooded real bad one winter when I was trying to start, and the plugs wouldn't fire even after pulling them and cleaning them...had to replace them.