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 electrical interference 
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Joined: Sat Jul 25, 2015 2:31 pm
Posts: 75

question about the engine missing above 6000 rpm. After some thinking about it, the miss I am now having started after I used a larger wire from the alternator back to the battery in the trunk. I also had my head up my azz how I ran the wires back to the battery. My MSD box is under the dash and the engine is a 440 mopar, so I'm sure you know the distributor is next to the alternator on the right side of the engine. So, of course I ran the trigger wires, wires from the box to coil and the charging wire together and thru the same hole in the firewall. Coil is mounted on the fender well not far from the alternator, not sure where to move it to keep the coil wire as short as possible or even if that is an issue.

So, my question is, after I separate the wires from each other and use different holes thru the firewall, is there some type of thin wrap, or braided sleeve I can use around the wires to help eliminate, or isolate any chance of interference? I found some braided sleeve, 2 things, they are very proud of their product and you have to buy enough for 200 cars.

I read about another fellow mopar guy had the same problem, he traced his issue back to the trigger wires coming from the distributor. He shortened them up, and ran them under the intake away from any electrical source he could. I plan on running mine under the intake as well, just wanted to make sure and wrap it with something for a piece of mind.

Cant believe I did that.


Thu Jan 12, 2017 9:19 pm
Site Admin

Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1704
Notice the MSD extension cord is a twisted pair, the twist cancels EMI but it can only do so much.

I think if you move the purple and green trigger wires away from the orange and black coil wires and other high amperage carrying wires it should be OK. Motors, like fans and pumps, are offenders. Keep the purple and green away from the high-voltage coil wire also.

The alternator B+ back to the battery is DC (obviously) but it has a little bit of AC ripple. Measure the AC voltage at the B+ stud on the alternator at about 3000 RPM with all the load turned on you can, lights, radio, heater, fans, pumps, etc. Should be no more than about 0.1 (one tenth) V AC.

The problem comes when the coil orange and black wires are parallel with the trigger purple and green. Sometimes a system with the trigger and coil primary and/or secondary wires tie-wrapped together, or tie-wrapped together on a frame tube, will fire itself continually when power is switched on, engine not running. This can be a safety issue because the engine can fire and turn or start when you are not expecting it to from just turning on the ignition switch without using the starter.

Thu Jan 12, 2017 11:52 pm

Joined: Sat Jul 25, 2015 2:31 pm
Posts: 75
Thank you Tuner,

I will give that a try and measure the alternator AC voltage like you suggested.

Fri Jan 13, 2017 8:50 am
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Joined: Fri Feb 08, 2013 5:50 pm
Posts: 2305
Hydraulic cam? Could be bad springs or hydraulic lifters pumping up.

Fri Jan 13, 2017 10:40 am

Joined: Sat Jul 25, 2015 2:31 pm
Posts: 75
Solid roller, the springs were just put on last spring. Only ran the car about 4 weekends after that, so I doubt it has 10 runs on them total. I was standing there watching the fellow test them when he setting the spring pressures. ANother thing I was trying to learn.

When i first put the engine in the car and took it to the track, it would rev to 6800 with no problem, and I didn't take it any higher, got nervous, LOL it was overgeared. I changed the gears and fixed the starting issue with larger wires, better grounds, and a larger wire from the alt to the battery at the same time. Fixed that problem. So, when I ran the larger charging wire back, I put it thru the same hole and wrapped it with the existing wires to the dist and coil. They are the only wires on the right side of the car, as trying to be neat. I wasn't thinking about interference issues. That is when the problem started, I started blaming the carb. Another thing I shouldn't have done.

Reading on this site has helped me allot, but still learning. A couple of other sites you ask questions, which I don't anymore, you would get so whacked up answers, even if you don't know the problem, common sense was telling me it was wrong. That is why I ask you guys when I don't know something. I respect your opinions.

I'm pretty sure its the trigger wires being in too close to the other wires, I'm going to try that first, but it will another 6 to 8 weeks before its warm enough outside to play with it.

Fri Jan 13, 2017 12:13 pm
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