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The reason this is important to me is that it makes any tachometer that is connected to the engine read high at idle, but then over 2000 RPM the tach reads normal.

Has anyone else out there seen this problem?

Does anyone have any idea what could cause this? Is this some kind of problem with my distributor pickup? Is this considered normal or abnormal behavour?

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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.

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Greg,

Is the secondary ignition ok, plug wires routed correctly and away from the pick up wires?

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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???

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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.

sounds like a good plan, I never thought of that, good way to eliminate other possibilities.


the extra pulses on the a/c waveform that line up with the extra pulse on the primary ign. waveform on Gregs scope are weird, where could that extra pulse come from? thats why I said possible wire routing, like maybe some outside source is triggering it , I don't know, sure is an interesting thread!, hope Greg gets the problem fixed.

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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.

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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???

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I hate to be a pessimist but I'd like to see some test results about this. More importantly, I'd like to see some results from an unbiased person (i.e., someone who is not trying to sell their own ignition system over the orange box)

In any event, I run the chrome box and when I first put it in, I had the distributor wires crossed as well and it ran like crap. I think the issue of firing on the leading edge is the ticket.

good diagnosis!!

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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

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Greg,
Just to confirm for any doubters out there, I fired up my Cuda today which had this tach problem for 15 years and it is fixed! I swapped the 2 pick up coil wires inadvertently when changing something (probably from a 5 pin to 4 pin ignition box)years ago. I reversed them and now it works like it did when new. Thankyou Thankyou Thankyou.
Mr. Bill

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And another round of Thankyou Thankyou Thankyou from me, too! Spent the morning in the salvage yard picking up some bits and pieces, including another distributor plug, and this afternoon after getting my new gages installed, I hooked up a pair of plugs to reverse the wires to the control box (orange). At first, timing was WAY off, so I had to tweak the timing just to make her run. Once she was running, the tach was reading accurately, both in the car, and with my hand-held tach. Reset the timing to 35 @ 3K, about 16 @ 800 idle, and all is good with the world again. Man, that just frustrated the heck out of me forever! I still don't know WHY it was backwards, all I can remember (from nearly 10 years ago) was that something was funny when I was putting my harness together. I almost think I found something strange in the factory harness, too, but I've slept too many times since then. No matter, it's fixed now.

Thanks again for this fix!

Clair