CDI whining, new coil, but no spark from coil

[michaelm101]

Frustrating!
Thought I had it solved when I heard the CDI whining, but had no spark from the coil...installed new coil and still no spark.
Even had a Porsche mechanic come over who could not diagnose the problem.
This is a 1979 911SC I've owned free of problems for over twenty years. Started her up 3 mos ago no problem... Please help!!!

[rennworks]

Ck for corrosion at the tigger wheel connector in the distributor.
Warren

[michaelm101]

Thanx Warren, but I forgot to mention that with the new coil, I'm not getting a spark from the coil (when holding cord end to ground), before reaching the distributor...

[T77911S]

the CD can still be bad. best to find a good one and swap it out.

[Superman]

Yes, the CDI box can be bad and still whine.

The high voltage signal from the coil to the distributor cap is triggered by the hall sender in the distributor body. That wimpy signal is passed to the CDI box via a green wire that comes out of the distributor body. This green wire commonly fails. There is a factory repair 'pigtail' wire available.

Surely there is a way to test the CDI box. A way to manually 'trigger' the firing of the coil so you can verify that the CDI box and the new coil are working properly. I don't know how to do that, but I've watched it. Manual triggering is possible. If that causes a nice, snappy blue spark then I think you can conclude your green wire has failed.

[fred cook]

Are you getting +12v at the coil? Time to break out the old volt meter!

[ischmitz]

You might either have a bad trigger signal from your distributor or a broken trigger stage in the CDI box. Sometimes the signal transistors stop working rendering the box dead even if it whines. The whine is the DC/DC converter producing the 400V charging the capacitor.

A voltmeter doesn't really help here unfortunately.

Cheers,
Ingo

[Superman]

Yup. Either the CDI bos is broken inside but still whining.......or it's not getting the right signal from the Hall Sender. If someone could instruct Michael how to manually "trigger" the CDI box, then we would know whether the CDI box is broken or not. Switching out the CDI box is perhaps another way, except that if that does not result in a spark, then we would still need to know whether the second CDI box is functioning properly. and the coil, even though it is new. Again, manually triggering the existing system would be best. That would pretty much isolate the problem to the Hall Sender itself, or the green wire.

[holtjv]

My $' s on the green wire. A simple $60 replacement (or whatever astronomical price it is).

Had a similar problem on my 79 last year. It was not intuitive to me that, with the cdi working and a new coil that I wasn't getting spark out of the coil. But there you go--the dizzy needs to trigger it.

Good luck and make sure to tell us the resolution for future searchers.

[ischmitz]

To trigger a 6-pin box all you need is a AA battery, the positive of the battery goes to GND of the car and the negative of the battery goes to the trigger input. You would need to fabricate little extensions for the 6 pins between the harness connector and the box to get access. Or you go in at the dizzy to produce the trigger signal.

[T77911S]

Originally Posted by fred cook

Are you getting +12v at the coil? Time to break out the old volt meter!

these cars dont use the conventional 12V's to the coil. the CD sends 450v to the coil which is then stepped up about 100 times.

do you have the 6 pin or the 3 pin CD?

what usually goes bad is the "big" capacitor. they are getting old and they tend to dry up. if you have a mulitimeter, it is very easy to check.

[T77911S]

i will assume you understand what an "open" and "short" are and what the multimeter displays for both. 0.L = open and 0.0 = short.

here is the capacitor in a 6 pin. it is the big silver "can" at the bottom with the 2 green wires.



take the cd out and remove the cover.
lay it down with the black connector to the left.
about 1/3 the way up on both sides, you will see "C8" on the circuit board.
take a wire and connect these 2 terminals then remove. this puts the CAP at "zero". when you connect the multimeter it puts a small charge on the CAP, so "zero" the CAP each time you check it.



put the leads on the 2 "C8" pins. the meter display may jump around but then it should start counting up. what this is indicating is the resistance is increasing as the multimeter charges the CAP. if it shows zero (0.0) then the CAP is shorted and bad. if it does nothing (0.L) then it is open and it is bad.

the 3 pins are not that much different. mainly just the triggering circuit. the boards look older along with the components.

[michaelm101]

Thanx all you Porsche gurus! I consider myself technical, but not that technical... As a teenager I worked on my Yamaha YZs...I also helped my pops work on 60s-70s, family Station-Wagon V8s. My dad showed me how to use an ohm-meter several times, but I don't remember (very loong time ago). I'm assuming that picture is the inside of the CDI unit...Really don't have the time right now to dabble in this endeavor. If anyone is ever out in So Cal, please call upon me for a complimentary tennis lesson and a beer, maybe you can show me whazzup!

Email

[ischmitz]

where in SoCal?

[michaelm101]

San Gabriel Valley, Arcadia area...

[jtfreels]

I'm trying to test my 6 pin CDI box as I'm getting no spark. Changed out the "green wire" but found this thread.
My question is where exactly are the C8 pins I'm supposed to test?

On the left hand side it seems obvious there is a pin coming out of a circle labeled C8. I've got the arrow pointing to what I think is C8.

But on the right side which one is C8, printed on the board inside the yellow circle is C8 but which pin?

So far trying them all gives me no response so I guess the cap could be bad but I want to be sure.

Thanks

[HarryD]

This thread should help you get started and lead you other good ones. The posts by Early_S_Man (RIP) should get close attention.

Do I need to replace my CDI box?

[ischmitz]

The green wire on the left comes from the capacitor and so does the green wire on the right in your circle.

[jtfreels]

Thanks guys, Harry I've spent the last couple of weeks reading Early_S_Man posts, just wish I understood more than a 10th of it.

ischmitz, thanks, thats what I thought regarding the 2 green wires. So it appears that the cap is bad. I'm getting nothing on my meter when I connect these two points.

So tell me about rebuilding this thing.

Thanks

Jay

[ischmitz]

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