Gentlemen,

My '79 F250 w/351m stopped running while on the road today. Yesterday I did ~200 miles with it no problem. Today, while on a short trip at about 55mph, I first felt a small momentary loss of power. Then a few seconds later a larger and longer lasting loss of power. Then a minute later the engine outright stopped combusting and was just spinning 'cause I was at speed, but it re-started after a few seconds. Then a hlaf minute later, it stopped again and would not restart, so I pulled into a sideroad to park while I had some momentum left.

The engine turns over just fine.

I popped the distributor cap and verified that the rotor turns, so the timing chain's at least not broken.

I pressed the acceleration pump on the carb and it squirts. On turning the engine over a bit, you can smell gas. So I'm figuring the bowl is full which means I've got gas pumping.

Next, spark.

I felt that the low-voltage leads to the coil weren't making perfect contact with the coil contacts, so I cleaned them and tightened them down so that fit snugly on the posts. After this, I tried turning it over again, and it did start and idled for about thirty seconds before quitting again. So, thinking the coil contacts might be the thing, I re-checked them, but contact is still good and it will not start. Maybe a coincidence that it started after fiddling with it..

So I got it towed home by a friend and, now with tools, pulled a park plug, plugged it back into its boot, held the hex area up against an exhaust manifold bolt with the head scraped clean, and had my buddy turn it over - nothing. If there is a spark there, it's too faint to see even in the shade. So, I'm concluding, no spark.

I've searched a few "no spark" threads on this site, but found no comprehensive methodical approach to tracking down the source of "no spark".

My 79 351M has a module.

All contributions would be greatly appreciated - as would wording for dummies.

Thanks.

 

Electronic ignition control module! If you go aftermarket (non-Motorcraft) get two. They ain't that expensive. That way, the next time it happens, you'll have a spare in the glove box so you ain't gotta spend the bucks on the tow.

 

It doesn't help to blindly replace parts, test first. Test for spark at the coil wire. Test for power at the + terminal of coil with key on. Test for power on red wire of two wire connector at module, with key on. Should be power at both.

Most auto parts stores will test the module for free. The pickup coil in distributer can be tested with an ohm meter, but I'm not sure how.

Let us know what you find so far.

Marty

 

Coils eventually go dead.

You can check a coil for resistance to determine if a circuit is bad.

What colour was the spark when you saw it.

Yellow or Blue ?

The positive terminal on the coil is 12v with the key on >
Check that ?

The other thing is the device in the dizzy that sends a signal.They go bad as well.

Don't buy parts ...trouble-shoot first

 

OK - important lesson learned.

It was none of the major components.

I checked at the posts of the coils and got 12v. I checked for spark out of the coil, and nothing. I replaced the coil with a known good, and still nothing. Seemed impossible. Until I mentioned to my neighbour who was watching that I had heard some sparking when I wiggled the blue coil wire. It dawnede on me that this was still true even after having tightened the connector to fit snugly over the post. So, very close inspection - the copper strands of the wire have flexed/corroded between where the connector grabs the insulation and where the wire is clamped into the connector. So, it looks like it's connected, but it's not. Unless you fiddle with the wire and get a temporary connection, which of course vibrates apart in 30 seconds of idling.

Trouble-shoot first.

 

Its usally the simple things that get us in trouble. Something about the forests and the trees. Everybody says to change the module. I have been driving my 77 since almost new and have never had a problem with a module. I have replaced two coils and I always carry a spare.

 

There us a step by step diagnostic procedure in most service manuals. "Pliers crimped" connectors should be replaced and crimped with the proper tool. Use some silicone dielectric grease on the end of the wire before crimping to protect the crimped connection from oxidation.

 

Take a little advice from someone that has been in the mobile communications field for over forty years. Never use the pliers type crimp always use the crimp tool that is designed for un-insulated crimps. Even on the insulated crimps. This tool puts a little dimple on the back side of the crimp and make a very solid connection. This type of crimp will last for years without becoming intermittant.