I am using a Tektronix TDS3054B. It is a 500Mhz 5Gs/Sec 4 channel scope. I don't have the one with the nifty web server in it, unfortunately, so I have to take pictures of the screen with my camera.

Here is a picture showing two cylinders firing.
The yellow trace is connected to the minus side of the coil. The blue and purple traces are connected to the pickup coil in the distributor. In this picture you can clearly see the extra thin pulse right after the main pulse. You can also see (in the blue line) that the pickup coil commanded the Orange ECU to fire twice.



Here is the original picture in full 5MPixel resolution:

http://rt-eng.com/mopar/tachboard/2004-09-21_014.jpg

GregGarner
www.rt-eng.com
greg@rt-eng.com

I also looked at the three distributors I had handy, and all three had reluctors that are held on with a roll pin.

GregGarner
www.rt-eng.com
greg@rt-eng.com

cool pics, yea I see the extra events, I played with a tds 3014, loved it, but had to return it, the rent was getting expensive, can you transfer image to the floppy on the scope?

The TDS3054 won't save an image, but it will save a CVS (Commad Delimited) text file. That is how I created the first graph (at the top of this thread) in excel.

GregGarner
www.rt-eng.com
greg@rt-eng.com

Ok, here is the latest. I completely rebuilt my distributor. It now has no play in it at all.
Unfortunately, the system is still double firing.

I then noticed that the shaft that drives the distributor was very worn on top and allowed some play in the distributor, so I yanked that out and put in a new one.

Again, this made no difference, the system still double fires at idle.

I then got a silver ECU from a friends car, and put it on the car. No difference, it still double fires. I checked the silver ECU on the other car, and it does not double fire.

I am using all different parts inside the distributor, except for the pickup coil. I even changed the reluctor and tried a different reluctor.

I only had one good pickup coil around, so I plan on buying a rebuilt distributor tomorrow and trying that. I may also purchase a new MP distributor and try one of these as well.

Anyone else have any other ideas to try?

GregGarner
www.rt-eng.com
greg@rt-eng.com

It's got to be either the pickup coil or something in the engine or wiring at this point, as we have eliminated everything else it could be.

maybe the wireing from the dist is picking up a signal from another wire.Have you tried to separate the ignition wiring from other power sources?

I would turn the distributor in a lathe or drill press to eliminate a noise issue. I have done that in the past to test distributors. Turn the shaft out of the car and look at the waveforms.

I was just checking my MSD this week with my scope. Neat how it was firing 3 pulses at idle, then 2 and then one. I thought that the 7AL2 puts about 450 volts to the coil, but when I zeroed in on the pulse it was off scale with a X10 probe. Couldn't find a X100 probe, although didn't look too hard.

I think Don at 4 Sec Flat had some tests on the "orange box", that were not very flattering.. AIR, he found SERIOUS loss of plug voltage at higer rpms.
Maybe a good distributor and a good box, [I think he has one that shows significantly better performance] would really "step up to the plate" w/ some improved performance???

Can a mopar ECU misfire due to Voltage spikes or a faulty alternator emmiting a frequency.I heard that GM modules would go bad early as constant miss firing would wear them out.Are the mopar ones effected by these conditions

You guys are not going to believe what the problem turned out to be. Thanks for all the excellent suggestions, as they helped me track this problem down.

First of all, I got a replacement distributor from O-reilly, very nice looking, for $54 including tax. I popped that puppy in, expecting my problem to go away, and it still double fired.

So I thought, well, it must be either the engine or the wiring. I then hooked up the distributor to a drill and looked at the waveforms as suggested. It was a very nice waveform with no double pulses.

I made a nice little test harness out of two of those 4 pin trailer wiring harness jacks, so that I could plug in between the distributor and the wiring harness and look at the signals with the scope. I then looked at the distributor pickup signal, and this time there was no double firing. So I added the scope probe back to the minus side of the coil, and sure enough, no double firing. Somehow adding the test harness fixed the problem!

Possibilities that went through my head: Inductive pickup where my wires were picking up spark, extra capacitance of the scope probe, extra inductance of the 1 ft test harness. I move the harness around, thinking that if it were inductive pickup, then it should be easy to make it happen. Moving the wires did not make it double fire. Finally it hit me: I had wired the test harness such that it swapped the two pickup wires, changing the polarity of the pickup signal. What luck!

I then yanked out the handy dandy Moparts electronic distributor wiring, and looked closer at my harness. The harness I have has a brown wire and a black wire going from the 2 prong plug to the Orange ECU unit. The harness is wired exactly backwards from what the moparts diagram says it should be! The brown and black wires are not swapped, so I think it was wired this way from wherever the harness came from, which is no telling where.

I yanked the harness out of the car and peeled back the tape next to the two prong plug and swapped the wires, heatshrinking and soldering the connections. I then put the harness back in the car, and no double fire!

I then hooked up one of my tachs with a reproduction board in it, and the tach worked perfectly, reading the proper RPM everywhere.

Whew, what a relief!

Here is the URL of the wiring harness so you can check your harness and see if it is backwards. This diagram is definately correct, it shows the correct polarity.

http://www.moparts.com/Tech/Archive/elec/3.html

Here is my Half Baked Theory on what is going on:

The Orange box is probably supposed to fire on the rising edge as the reluctor approaches the pickup coil. Since I had my polarity backwards on the pickup coil, it was firing as the reluctor left the pickup coil. As the ignition coil fired, it generated a 260V positive spike on the minus pickup of the coil. This spike is inductively coupled to the two pickup wires, and since the reluctor is not near the pickup coil anymore to create a voltage source, it causes the Orange box to fire again. This may not be exactly what is going on, but it is something like this.

Does Don/FBO have a link on the web to the aftermarket ECU units you guys were mentioning above?

GregGarner
www.rt-eng.com
Greg@rt-eng.com

I tried this on my car this AM and it made it run rough and actually made the tach read even higher!! What's up?

Great troubleshooting!

Jon:

Make sure you retime the car after you swap the wiring. It changes the timing pretty drastically because now it will be firing when the reluctor approaches the pickup coil instead of firing when the reluctor leaves the pickup coil. This could explain why it seemed to run rougher.

As far as it reading higher, I can't explain that. Was your ECU wired backwards according to the wiring diagram? Do you have access to a scope so you view the signal on the minus side of the coil?

GregGarner
www.rt-eng.com
greg@rt-eng.com

I'll check it after work tonight, don't have time right now. I have a buddy that has a scope.

So your saying its the black molded plug going to the pick up.With the female and male terminals.Was the problem on the repo harness .Or the mopar replacment one.

When I converted my Dart from the 318 to the 383 I lenthened the wire harness for the dist up front and it had an aftermarket blue ECU on it that worked fine. About a year later I put 3 Orange ECU boxes on it and it would not start with any of them ?? Come to find out I had gotten the pickup wires crossed when I made the harnes for the 383 and the aftermarket ECU worked but the MP Orange ECU would not run until I fixed the harness. Ron