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1975 Dodge Truck no spark

Tiny
OLDDODGETRUCK
  • MEMBER
  • 1975 DODGE TRUCK
  • 5.2L
  • V8
  • 4WD
  • AUTOMATIC
  • 133,000 MILES
I have a 1975 Dodge W100 with a '68 318 industrial and it was running perfectly until a few days ago. My dad was driving when it suddenly started running very roughly and eventually he pulled over. Nothing appeared to be wrong, but he couldn't get it to start again. We dragged it home behind our tractor and I started messing with it. It's getting fuel, I can see it through the carb. I hooked up the timing light and the timing is way off. A few of the other wires didn't give a reading at all. I checked the starter coil and it was bad so I replaced that, but it didn't help. The spark plugs look good and I just replaced the points, rotor, cap, condensor. I also gapped the plugs and points a month ago, but I didn't have time to gap them tonight. Any ideas on why it would suddenly lose it's spark?
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Tuesday, September 2nd, 2014 AT 7:15 PM

13 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,874 POSTS
I remember this truck. So the problem is you have no spark at all? First, I would confirm the points are opening properly and that isn't an issue. Also, when you try to start it, does it fire for a split second and then shut off?
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Tuesday, September 2nd, 2014 AT 7:29 PM
Tiny
OLDDODGETRUCK
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  • 66 POSTS
Yes, I can sometimes get it to fire for a second, but it always dies.
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Tuesday, September 2nd, 2014 AT 8:12 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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If the engine will fire in the start position and then die in the run position, the ballast resistor is most likely the problem. Here is how it works. When you turn the key to the start position, power to the ignition bypasses the ballast resistor so you get a hotter spark to the ignition to start the engine. When the key returns to run, power is supplied through the resistor to cut power and protect the ignition system. So, if it fires in the start position and dies in the run position, power is most likely not getting through the resistor.
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Wednesday, September 3rd, 2014 AT 4:38 AM
Tiny
OLDDODGETRUCK
  • MEMBER
  • 66 POSTS
It turned out the coil was shot, the resistor was bad (it fell apart when I took it off), and finally I re-checked the points. The points looked good but when I tried adjust them a little more, a little plastic piece fell off, so they are no longer opening. So, I think it was a combination of the 3 that made it stop working. Thanks for the info!
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Wednesday, September 3rd, 2014 AT 6:31 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Glad to help. Just FYI, the best way to adjust points is by degrees of dwell. Using a feeler gauge will get you close, but not exactly where it needs to be.

It has been YEARS since I worked on this type of system, but if I remember correctly, with a V8 you want 45 degrees dwell. It really does make a difference.
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Wednesday, September 3rd, 2014 AT 7:58 PM
Tiny
OLDDODGETRUCK
  • MEMBER
  • 66 POSTS
Sorry to bother you again, but the truck still refuses to fire, the new points didn't help. I checked to see if their is power going in the coil and it's about 7 volts. The rotor spins, but when I put the timing light on there is no spark in the plug wires. I have no idea why the spark is disappearing. The rotor, cap, plugs, and wires are all pretty new. The cap and rotor don't have any scorch marks on them. What is going on?
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Thursday, September 4th, 2014 AT 3:20 PM
Tiny
JACOBANDNICKOLAS
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Did you replace the resister?
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Thursday, September 4th, 2014 AT 6:09 PM
Tiny
OLDDODGETRUCK
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  • 66 POSTS
Yes, the old 1 shattered into 3 or 4 pieces when I took it off
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Thursday, September 4th, 2014 AT 6:30 PM
Tiny
JACOBANDNICKOLAS
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Check power side of resister to see if you have 12v. If you don't, I think we have an ignition switch issue.
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Thursday, September 4th, 2014 AT 6:44 PM
Tiny
OLDDODGETRUCK
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  • 66 POSTS
11 volts going in and 7 volts coming out
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Thursday, September 4th, 2014 AT 7:02 PM
Tiny
JACOBANDNICKOLAS
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That sounds a little weak, but the resister is doing what is should. Tell me this (sorry I have so many questions, but it's been a while with this system) do you have power to the coil in the start position but not the run position?
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Friday, September 5th, 2014 AT 8:20 PM
Tiny
OLDDODGETRUCK
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I have been doing this mostly myself so I haven't been able to check if I am getting power on the start position, but I am getting power on the run position. I am pretty sure I am getting power on the start position though. Tomorrow I am going to replace the rotor and cap again to see if something is defective there.
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Friday, September 5th, 2014 AT 10:20 PM
Tiny
JACOBANDNICKOLAS
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If you can get help, check on the start position too. Power doesn't go through the resister in the start position. Also, make sure the rotor is turning when you have the cap off.
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Sunday, September 7th, 2014 AT 8:29 PM

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