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Finally had some free time to work on the truck. A new distributor took care of the problem.
Good to hear. That's why I say with the age of these engines, when you have ignition issues that you think are in the distributor, just buy a reman unit and replace yours. Nothing last forever.
Two adjacent teeth were hitting firmly. The teeth on either side of them were just grazing. The other four on the opposite side were not making contact at all.
Still haven't determined the physical aspect of why the teeth were hitting the pickup coil. But in the meantime, I've been trying to understand why that even matters. Wouldn't you think the pickup coil would still properly sense the rotational position of the teeth, and fire the spark accordingly? I think I may have found the answer in this test of the Duraspark system:
Scroll down to Step 1, after all the illustrations. With the engine not running and the key in the run position, the test says to tap the distributor body with a screwdriver handle and the system should fire a spark. My hunch is when the rotor teeth were striking the pickup coil, it was the same as this test. The shock was causing the spark to occur too early for those two cylinders.
This would hopefully explain the vibration and why it felt like the engine was firing too early on at least a couple of cylinders. I've had dead cylinders before, and it wasn't even that noticeable, just a minor loss of power. This was more of a sensation of the engine fighting back. If my theory is correct, those two cylinders were firing much too early. Even if there was then a second spark occuring at the correct time, the fuel had already been ignited and there was nothing left to burn.
That's my story and I'm sticking to it. I will update when I figure out why the teeth were physically hitting.
I will update when I figure out why the teeth were physically hitting.
I found the problem, and it wasn't what I was expecting.
1) The rotor (called armature in the first picture) was not fully seated. It's a press fit on the end of the shaft, with nothing to retain it except the tension of the roll pin. The rotor was sitting slightly ****-eyed, which partially explained why only 4 of the 8 teeth were hitting.
2) The pickup coil sits on a plate that is rotated by the vacuum advance actuator. This plate has a bushing that rides on a stationary collar below. The two surfaces were worn, especially the rotating portion. Due to the strong magnet (between the pickup coil and plate), the whole assembly was tipping up about 0.050" and closing the gap.
Here's a view showing the bottom of the rotating plate and the stationary collar on which it rides. The stationary collar seems to be hardened material and wasn't worn too much. The bushing in the rotating plate seemed a smidge softer, and perhaps was designed as a sacrificial wear piece because it is easily replaced (part of the pickup coil assembly):
Here's another close-up showing the damage to the face of the pickup coil. Note how only half of the face shows damage from contact with the rotor. This is because the rotor was tipped slightly, and then the whole rotating plate/pickup coil was also tipping up:
I plan to keep this distributor as a spare and will replace the pickup coil. Hopefully the new assembly will fit tighter and not tip up.
I also played around with the screwdriver tap test, as mentioned in my previous post. I hooked up an oscilloscope to the two wires from the pickup coil. Sure enough, if you tapped the distributor body, a spike signal was generated without rotating the shaft. Since the rotor was obviously hitting the pickup coil, I'm pretty sure this was causing at least two cylinders to fire early.
If you ever need to replace the pickup coil, this thread has some good information. This was in a different forum for earlier models, but I believe it should also be applicable to any Duraspark system:
Thanks for the update as I am sure it will help someone at some time.
When I seen about the screw driver tapping of the dist. I was like "you for real on this?"
I have never heard of this test when checking for spark.
So if I under stand right use a spark plug in the end of the coil wire and tap the dist. body and if the pick up coil and wiring is good you should get a spark.
I guess this would also check the ICM (box) as it is what tells the coil to spark right?
Dave ----
I don't know if it's a good practice or not, but I always tapped that stator down to make sure it was seated. I figure since it takes pressure to lift it up.
It's not a bad idea to apply pressure to seat it.
I need to remember that tip about getting a spark out of it.
When I seen about the screw driver tapping of the dist. I was like "you for real on this?"
I was just as skeptical. I didn't run the full test of the ignition system. I only had the bare distributor on my workbench. No ignition coil, control module, etc. I connected my oscilloscope to read across the two wires on the pickup coil inside the distributor. No power supply, just a very simple hookup. And much to my surprise, when I tapped the distributor body with a screwdriver handle, I saw a spike of approximately 8 volts generated by the pickup coil. I didn't measure the current, but it was probably microamps. This voltage spike was merely a signal to the module, and if the rest of the system was connected (with the fender-mounted module powered), a spark would have fired.
How does the tap test produces a voltage spike? I couldn't make any sense of it. I was careful not to turn the distributor shaft. All I know is it did work.
For giggles, I compared the voltage spike when turning the distributor shaft, no tapping. Spinning it by hand, I saw a slightly stronger spike of approximately 12 volts. Reading between the lines, either voltage level would be sufficient to tell the module when to fire.
One other variable: This distributor was sitting on my workbench. I don't know how differently the tap test would respond if the distributor was installed and clamped down.
A new pickup coil is in hand (WVE #4P1189). I carefully miked it to compare dimensions with the old one. Everything was spot on, such the inside diameter of the bushing. When installed, it was still able to tip up at a slight angle, like the old one. This tipping action decreased the gap, but I couldn't see a reasonable means to reduce this play without causing binding.
The new coil still didn't hit, so I was perplexed. Then I took one last measurement that 'splained everything. The old coil was about .035" closer to the centerline of the bushing in the plate. This little bit of extra clearance kept the new one from hitting. Was the old one defective? Must have been. Was this a one-off problem, never to be seen again?
