Distributor Waveforms - Bench Testing

[Captain Obvious]

I was giving my distributor a once over while I had it out of the car for my oil pump replacement, and while it was laying on the bench, I figured I would hook it up to a scope while I give it a spin with a screwdriver bit in my drill. Just to see what would happen. No real conclusions, but thought someone might like to see the pics.

This is a manual trans 77 USA market distributor, but I suspect the waveforms for the pickups would basically be the same for all of the non-points distributors starting in 74.

Here's my fancy test set-up:

And here is the corresponding waveform. Important to note that the steep falling edge of the waveform is where the point on the reluctor is passing the metal tab on the electronic pickup, and that's the point at which the ignition module would fire the plug:

[ZCurves]

Very interesting. I rebuilt my Distributor a couple of years ago and it has always had a little 'hop' on the tach. The car runs OK, it just isn't as smooth as it should be, I am going to replace it with a "professional" rebuilt unit in the coming days. I wonder what the waveform of my own bench job distributor would look like? Hmmmm.

[Captain Obvious]

That waveform is pretty much a function only of the pickup and the reluctor wheel. As long as the pickup is working, I bet all of the stock distributors look the same. I guess you could get some timing jitter if the bearings are shot, and you might get some amplitude variation or asymmetry if there is some kind of problem with the pickup, but other than that, I don't know what could go wrong.

Haha. Send it to me. I'd be happy to give it whirl (pun intended) and take some pics. Heck, I'll even include RPM and voltage measurements. I didn't bother to measure anything on mine. I was just looking at the pretty pictures.

[GreenZZZ]

Amazing fun you can have with an oscilloscope. $10,000 scopes I used in college can be found on eBay for $120 now. Thanks for posting! I scoped a rough running 84 Vanagon (which uses the same injection as a a 280ZX) and when I clipped to the injectors it was a square wave 85% on 15% off which was obviously a flooding condition. Tracked the problem to a grounded O2 sensor.

[Captain Obvious]

$10,000 scopes I used in college can be found on eBay for $120 now.

Heh. Sounds like I should take a look at ebay for a better scope. The hardest part about taking that pic was figuring out how to get the camera and the scope to play well together.

It's a ten-fifteen year old analog scope while the camera is a two year old decent quality one. In other words, with a quick decent quality camera and a non-storage scope with very little natural persistence, the camera was too fast for the scope. All I would get was a single dot where the trace was at the instant the "shutter" opened.

I had to get into the cameras settings and manually slow it down to a longer shutter opening to get a pic of the trace. I wish I had a digital storage scope...

[GreenZZZ]

Mine is a Tektronix 2246. Looks similar to yours.

[emptech]

Speaking of using a bench scope ( I have several, all tek) for automotive use, I would loosely

couple to the hot wire right off the coil. Loosely means a few turns of hookup wire around the

high tension wire, the end to the scope probe, just creating a small capacitor. The trigger of

the scope couples to plug #1. Given the proper sweep rate for the rpm range, you can see

a representation of the waveforms for all plugs, the distributor being your multiplexer. The

order would be the firing order of course.

I suppose if you wanted to get fancy, you could put a second channel on the trigger from your

points or pickup.

Jim

[siteunseen]

Captain Obvious are you thinking of upgrading to the early ZX distributor? Advance has the Cardone 31-619 for $133.99. Reatailmenot.com has a coupon code $30 off $100+ so that be around $115 for a rebuilt one. I've found some Zs & ZXs at a local junk yard and may look at there's as I'll have to get that diffrent mount bracket. I'll do mine after I get this motor back in.

[Captain Obvious]

Captain Obvious are you thinking of upgrading to the early ZX distributor?

Thinking about it? Sure.

Am I going to go through with it? Probably not until I have a real reason to. My stock ignition seems to be working as intended, and I've got so many other things that need attention that I probably won't poke that hornets nest until I have a good reason to.

That's a good price you've got there though....