Power, but no spark (1 Viewer)

[JustTodd]

12/72 fj40, died while sitting in a drive thru, just stopped running no hesitation no acting like it was outta fuel just died, thought it was some sort of carb issue, so let her sit for a couple hours went back and she started right up, so drove her home, next morning I'm taking my daughter to school and leave a stop light and it back fires (never done that before) and dies going thru the intersection, let her sit for a couple hours go back and she just cranks wont start, so start going thru the fuel system, filter, checking lines, etc. Get to the carb and crack the inlet line and get air out, then she fires right up, get her home again and decided to just go ahead and rebuild the carb, replace filter, just basic stuff, get it all done nothing looked out of sorts, get her fired back up and she runs for about 20 minutes then just shut off again no hesitation, no chugging just dead. But now after a couple of hours she doesn't restart just cranks and cranks, so I check for spark no spark, immediately think that coil is bad it's not old but whatever no big deal, so I do the whole thing plugs, coil, cap, rotor, condensers (at distributor and at coil), and ballast resistor, and points put everybody back where they came from, but she don't start. Got voltage on both sides of coil, voltage at cap, points, everywhere I'm supposed to, but can get any spark. If I momentarily ground the - side of coil I get a spark there other no nothing. I'm dying here guys and need help bad.

 

[Krondor]

My gut tells me that it is a faulty ignition switch. Sounds like you have traced everything and made sure each of the individual pieces work. Everything working then sudden shut off is kind of strange.

 

[JohnnyC]

Test your ignition system ... run a wire from Bat + to coil +

See if it starts ... if does

Jump to other side of the ballast and see if it starts running thru ballast ... if does you have an issue between ign switch and it’s wiring

Most likely ign switch ... it does that to mine when bad

 

[JohnnyC]

Remember this is temporary... you need to remove that wire and not leave power to coil when engine is not running

I keep a spare wire to do just this if my switch is bad I can still drive... if ballast is bad I can still drive

 

[Steamer]

And if the carb has a sight glass, check the fuel level when it dies.

 

[JustTodd]

My gut tells me that it is a faulty ignition switch. Sounds like you have traced everything and made sure each of the individual pieces work. Everything working then sudden shut off is kind of strange.

Even though I have voltage at the switch it could still be bad?

 

[Krondor]

From what your explaining with the sudden cut offs, something could jossel the ignition, and a faulty component could intermittently disconnect. Other times you would get voltage just fine like when you measure it at home. Just a thought. Something pretty inexpensive to change out.

 

[JohnnyC]

Kurt has aftermarket as well as oem ignition switches @cruiseroutfit

 

[Steamer]

Another easy test is to put a temporary pilot light on the + terminal of the coil. The next time it dies, don’t touch anything but check the light. If it stays lit and doesn’t start, it’s not the ignition switch or the associated wiring. I kept an led pilot lamp permanently wired and mounted under the hood. It helps speed up diagnosing ignition problems. With longer wires you could run it through a window temporary and tape it to the dash for an instant check.

 

[Kayaker]

My 72 did exactly the same thing a year ago. I used my timing light in much the same technique described by Steamer above, leaving it hooked up and running it inside the cab so I could see if the plugs were firing when it cut out. I ended up replacing the electrical wire from the coil to the distributor and using eye terminals to ensure a good connection. She hasn’t cut out on me since. Easiest and cheapest fix ever. Diagnosis took about two months!