Greetings,
I've been troubleshooting (unsuccessfully so far) a problem with my A ignition. Near idle, both A and B seem fine, but at 3500 where I do my runup (brakes won't hold 4000), BOTH seems fine, until you go to the B ign. B is smooth, and A is rough. It's quite noticeable, but not like you're missing a cylinder or anything. When you go back to BOTH you can tell that BOTH isn't as smooth as B. I feel like this has been slowly getting worse over time, and it's only recently started to concern me. The RPM drop during the ignition test is only about 130-150, with the lower drop being the rough A ignition which makes no sense.
1- Replaced the plugs, no change
2- Swapped ignition modules, no change. Still A that's rough.
3- Ohmed across wires and coils (1T-2T, 1B-2B, etc). The plug connectors were dirty enough that it wasn't a great test, but 1T-2T seemed higher.
4- Replaced all plug resistor caps, cutting off 1/4" of cable on each. This made the resistance test reliable, 15k on all but 1T-2T, which was 17.5k. I removed the caps for 1T and 2T, and verified that the extra resistance is not the new plug caps, so it must be the wires or coil. No change in roughness, and now I have some radio noise as well. Awesome...
I did check the pickup coils to make sure they weren't loose, but I haven't checked the resistance yet, or the gap. I'd be surprised if it was either of those.
At this point, my primary suspect is the coil that runs 1T-3T. I need to verify that it's got a solid ground, though that wouldn't cause the resistance across the outputs to go up. Getting to the wires on the coil is of course a pain. Is it possible to measure each output coil separately? In other words, instead of measuring from 1T to 2T, would you get a reading from 1T to ground, and 3T to ground? If so, I'd expect that to be half the normal coil resistance for each side.
For the next test, I'm thinking I can swap the input connectors for the coils between modules, of course staying with the same cylinder (1T-2T with 1B-2B, and 3T-4T with 3B-4B) I'm thinking that would be relatively easy, and safe. If A continues to be rough, then the problem is the trigger coils or wires before the module, and if it moves to B then it's down to coils or wires. Is this a known good troubleshooting test? I can't see any reason why it wouldn't work, but I figured it might be safer to ask.
How often do the coils go bad? Anyone in the US got old style coils laying around? I have another ancient engine on a test stand that I can probably borrow parts from if needed, though I hate to break them both at the same time :-)
Thanks,
Rusty
