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Join Date: Nov 2007
Location: Fullerton, California
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And, here are the signal waveforms comparing "normal" polarity and "reverse" polarity showing why consistent triggering is only achieved on the falling-edge of the waveform.
Old 11-11-2016, 09:18 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #21 (permalink)
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This is really interesting. One more example of how great this forum is. I replaced my failed wire years ago on my 81 with a C.C. rotatating dizzy. I had no idea of any potential timing issues but,
my car ran fine after the swap. Are there different, model specific replacement wires?
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81 Pacific Blue 930 Euro coupe slicktop on a strict diet, Rarlyl8 headers, Blowzilla turbo, Tial waste gate, Full bay I/C, Home made center out exhaust, Leask WUR, MSD 6AL, PLX wideband
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Old 11-11-2016, 10:24 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #22 (permalink)
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From me- if you have the wrong wire on your 3.0 Turbo but it seems to run fine, is there an adapter connected? Is the timing setting "off" from what you would expect these vintage cars to be set at? With things like this, there aren't going to be many genuine original cars without replacement parts. The heat factor burns these up sooner than you might imagine. The answer is looking a clues. My brother's original experience and available information indicated these were a universal replacement. In his case, with a timing change it seemed to work fine. After enough heat cycles, exposure to moisture, general wear and tear... things start to show up and suddenly you are chasing your tail trying to trouble shoot why your car isn't running right.


I asked my brother if he was OK with me posting a bit more from him. yes. Here:
=================
What ticked me off was no info on manuals,W.S.M. SERVICE INFO...
My investigation was a nightmare.
The issue was somewhat mentioned in some talk of "reversed phasing" of the ignition.

I know it was a long time ago and I had to learn the hard way what was gong on.
I had a really badly over heated BLUE Signal line on my 3.0L turbo engine (with standard C.W. turning distributor) and got a new signal wire.
Only I got what they had: a green one.
I Put it on.
It ran Ok,................. I thought.....

Then some time later. Especially in warm desert dry air, it ran weird: had a strange lean type of miss fire in it? Like turning the CIS's C.O. screw real lean -it runs rough kind of like a 3.2 Carrera does with that rapidly hunting idle.-whereas a over -rich idle is a slow hunting around idle,... and rough,.... sluggish.
I checked the timing and adjusted it back to where it was supposed to be and tuned the Co and it was the same.

So, I finally looked at the inside of my distributor cap.
I saw arching and carbon tracks on one side of each of the 6 brass terminals??
Why only on one side?

So, I cut a piece of the distributor cap to be able to see the rotor as t was firing with a timing light.
AMAZING what I saw: the rotor was firing almost half way between the brass terminals!?
I modified the star wheel trigger wheel to re-position it to fire on center. I wrote about that as it worked fine.

But then I discovered why all of this:
The blue signal Line's two wired are wired different than the green signal line wires.

YOU CAN USE A GREEN SIGNAL LINE IN A C.W. ROTATING DISTRIBUTOR-ONLY REVERSE THE WIRES ON THE C.D.I. END.

I,....of course, did it the hard way.*:-w waiting

PS from 1978 the engine wiring harness is different as it has a 2-pin quick dis-connector going to the ignition distributor. This enables you to remove the distributor easier and also change the signal line easier.
New GREEN signal lines sometimes come with a new quick disconnect, pins, and new rubber boot to swap the pins over for a 3.0L turbo

======
That's all folks. I run into some of these weird type issues also. We have a new set of problems because we don't put enough miles on them. I keep an aircraft maintenance logbook to verify various changes, upgrades, and repairs. This helps. Things that would make themselves known after a few thousand miles in the old days can take years with limited miles. How much do we remember about what we did 7 years ago? Good luck and have fun.
Old 11-11-2016, 02:52 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #23 (permalink)
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The trigger circuit in the CDI is a negative zero crossing detector. As Bob's trace shows, the slope of the negative going edge is slow/soft if you get the polarity wrong.

Although you can time up the car and get it to run, the dynamic timing through the RPM range will be greatly affected. Why? This is because the amplitude of the dizzy waveform increases from a few volts to almost 50V at full RPM. This greatly effects the angle of the slope if the wrong 'soft' edge is used causing large errors in timing.

As a side note, even on the correct sharp edge, a timing error of around 3 or 4 degrees retard is still present at 7000RPM due to the analog nature of the trigger circuit. That error is at least linear with RPM.
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Old 11-14-2016, 02:39 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #24 (permalink)
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I went through this on my 78 3.0SC engine in my 74 Targa (a terrific combo). The green wire was baked over the years & was causing the car to intermittently quit when the coax shorted. I found out by moving it while the engine ran -- and it stalled and wouldn't restart until I could attain a good position (this got worse every time I moved the wire). I didn't want to buy the (expensive -- and short) green wire (with the connector) going to the CD box. I wanted a single wire to go from the dizzy straight to the CD without a connector in between. So I made my own coax wire because I couldn't find anything I could use off the shelf -- they all had a thin gauge core & a massive amount of shielding -- and the OEM wire appeared to have matching impedance for both. I used a standard 16 gauge wire for both the core and the shield. I matched the length of the core wire, then stripped the insulation from the shield wire and used the bare wire bundle to meticulously spiral-wrap it around the core. It was hard to spiral an equal length wire around the core --I had to untwist the strands & then spiral the flat collection of strands around the core so that I could completely shield it with the exact same length & impedance. Then I used heat shrink tubing to encase the new coax. I soldered the old dizzy connectors to the new coax so it would plug on like the old stator. On the CD end, I soldered a pair of spade connectors that perfectly fit the CD connector into the When I started it -- it backfired through intake so much (Thank you, pop-off!!!) that it barely ran -- timing was very retarded, so I twisted the dizzy but found I could not advance it enough. Then I pulled the dizzy and advanced the gear by 1 tooth -- now I had the opposite problem. The timing was extremely over advanced, the car could barley crank -- I had to twist the dizzy to full retard -- but it was still way advanced. This really got my head scratching because it seemed that timing was now a HALF tooth off on the dizzy. Once I thought it through, I realized that the polarity of the stator got reversed -- and that caused the CDI to read a shift in the pulse of about 30 deg. Those teeth on the star wheel (reluctor) create a small voltage pulse when they pass the stator (pick-up coil) -- and the width of the star elements were just about the right amount of degrees in error. So now the CD was either seeing the pulse as the tooth was passing the pick-up as BEFORE instead of AFTER (or vice-versa)-- causing a timing change of about 30 deg. Once I switched the coax connections to the CD box -- it fired right up and has run great for 2 years now. Someone mentioned using a 928 coax -- never thought of that -- may be a really good idea --it does look a lot longer than the 5" wire of the 911 part, but appears to still use the same connectors (although its even more expensive). I made my own because I feel that midway connector is just not needed and adds another failure pointe point.

Last edited by Rumble-Bee; 11-26-2016 at 10:07 AM.. Reason: Fact corrections
Old 11-26-2016, 09:21 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #25 (permalink)
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