Weak spark / no spark from coil. Experts needed.

[HFR_Racer]

I've also posted this inquiry on the 930 forum. I'm hoping to get more repsonses here since this is driving my crazy.

A couple of weeks ago my 87 930 left me stranded on the side of the highway
Prior to that day I had been fighting to find what I though was a vacuum leak. I was experiencing engine miss at low rpms. I found a few leaks and sealed them up. While driving home one evening my 930 simply lost power. I immediately thought that it was fuel or ignition failure. I was simply driving on the highway. No boosting

Once I got it home I completed the full diagnosis of the ignition system since I found that I had no spark. My initial prognosis was that there was no high output signal coming from terminal "A" of the CDI box. I was only getting 0.1 VAC. This is approximately the same as the pulse from the pickup coil in the distributor so I figured my CDI box was bad. I purchased a good/used CDI box from a fellow Pelican. While I was waiting for the spare CDI box to ship I also found that I wasn't getting a VAC pulse from the distributor. I've since gone through my dizzy and replaced the coil pickup. I am getting good voltage now from my distributor pickup coil. I can spin it up by hand on the bench and get 0.1 ~ 3 VAC. Yes.. It was truly bad.

OUCH.. I just found the answer. The primary winding resistance should be 0.2~0.7 ohms and the secondary should be 450~900 ohms per the coil manufacturers website. Darn it.. I bet I fried my spare CDi box as well because of the darn coil being bad. Any thoughts? Is the CDi box smart enough to not damage itself if the ignition coil goes bad?

Since then I re-insatlled the distributor and tried the spare CDI box. I'm only getting 0.2 VAC. This is the same as my original CDI box. Is it possible that I fried the spare CDI as well. I took special care not to fire the CDI box with the coil wire out of the coil.. Although I did find that this wire wasn't fully seated into the coil.. I hope that this didn't fry my coil..

My coil tests as follows: ~ 700 ohms from coil terminal "4" to coil terminal "A". ~700 ohms from coil terminal "1" to coil terminal "4". Coil termina "1" has continuity to ground as it should. Terminal "A" has continuity direcly to terminal "A" on the CDi box. I know that your not supposed to test from terminal "A" to terminal "1". I've not done that

NOET: ABOVE IS NOT THE CORRECT METHOD FOR TESTING AN IGNITION COIL. YOU TEST FROM TERMINAL "A" OR "+" ( SMALL TERMINAL) TO TERMINAL "4" (OUTPUT TO THE DISTRIBUTOR) TO TEST THE PRIMARY SIDE OF THE COIL. RESISTANCE SHOULD BE VERY LOW 0.2~0.7 OHMS. YOU TEST FROM TERMINAL "A" OR "+" ( SMALL TERMINAL) TO TERMINAL "-" (SMALL TERMINAL)TO TEST THE SECONDARY SIDE OF THE COIL. RESISTANCE SHOULD BE VERY HIGH 4500~9000 OHMS OR MORE.

IMPORTANT NOTE: I wonder since I have the same resistance within the coil on the primary and secondary side if that means that my coil is bad?? Also, if the coil is bad could it then fry my CDI box? And thus have fried my spare CDI box?

Can someone please elaborate if this is a good coil based on these numbers?

APPARENTLY NOT MANY ON THIS SITE KNOW HOW TO TEST AN IGNITION COIL. NO RESPONSES.

I FOUND THAT MY COIL TESTING METHOD WAS FLAWED. ALSO, 712 OHMS RESISTANCE IS CLEARLY NO GOOD. I'M DISAPPOINTED THAT NOBODY ON THIS SITE SPOKE UP AND TOLD ME THAT MY COIL WAS CLEARLY BAD BASED ON THE RESISTANCE NUMBERS I WAS POSTING.




Is there anything else that can prevent the high voltage signal from coming from the CDi box?

I've got good ignition voltage to the CDi box.
My CDi is well grounded and 0.4 ohms resistance to chassis ground.
I'm getting a pulse from the dizzy from the GREEN wire.
I've even tried to unplug the tach although I truly don't see how the tach can interfere with the CDI box. The CDi box drives the tach correct. Not vica-versa.

All help is truly appreciated. All I want to do is drive my 930.

Help please

[kodioneill]

Does the CDI box wine when the ignition is on? Do you have power to the box when cranking? You may lose your power flow while cranking, while it may be normal when the key is in the on position.

[fastfredracing]

Just something to consider. Have you thought about just going to MSD? In probably the same amount of time, and about the same money you will spend diagnosing and repairing this, you could be up and running.
Of course , I understand if you want to keep the factory cd ignition, and diagnose and repair it.
I am at basically the same crossroad as you right now. My 8 pin cdi box is dead, and I cant decide If I want to repair it, or just go the tried and true route. You can probably be into the MSD system for $300-400, and have nice easy reliable ignition system which you can get parts for in one day anywhere in the country.

[HFR_Racer]

Yes. As stated in the original post. I've completed all of the recommended checks in the Porsche service manual for ignition troubleshooting.

The CDI box does have 12+ VDC when the ignition switch is in both the RUN and START positions.

I'm not considering using an MSD box at this time. I still have to diagnose this issue whether I have a CDI or an MSD ignition box. They are basically the same. The input and output wiring is the same as the Bosch CDI. I fully realize that the MSd product is easily available.

You can still get CDI boxes as well. Many come up for sale on this board.

I'm hoping that someone will fully read my post and answer the remaining question.

If your ignition coil goes bad (short circuit internally), will the CDI box get fried as well? I've ordered a new coil so I will be able to test the theory in a few days.

[47silver]

My MSD fried when I put on a plastic over on my coil, the one that goes on the end and covers the terminals. But when I washed the car, water got into the cover (had the drain hole pointing up) shorted the coil and fried my msd.
My coil however did not fail, but i think a bad coil would short out the cdi just like the water shorted out my msd.

[ischmitz]

First, get a new or known good coil and try again. The high resistance on the primary means that your coil is bad. Then try if the car fires up again. You won't be able to measure much on the output of the CDI box since there is a capacitor between the DC/DC converter and Terminal A. So unless you connect a load to Terminal A or ground it the capacitor won't charge.

Also be careful - a shorted capactor can deliver the full 440V to Terminal A and that hurts a lot. The warning sticker on the box is there for a reason (in case the capacitor failed)

BTW, it is hightly unlikely to damage a Bosch CDI with a bad coil as I explained in my email.

Ingo

[T77911S]

also, check grounding from CD to engine, actually back to the neg side of the coil, not just CD to the body.

[HFR_Racer]

Thanks Ingo and Ty..

I did check the ground from the engine to the CDI box during my troubleshooting and it's good.

A good coil should have 0.2~0.7 ohms resistance on the primary side.
A good coil should have 4,500 ~ 9,000 ohms resistance on the secondary side.

I installed a new coil and now she runs. Both of my CDI boxes are good as well. I never needed to spedn $250 to purchase the secondary CDI box