Spark from "coil" but not to plugs...

[Pat S]

74 911, 2.7, Bosch Distributer, CIS, Pertronix

I had a mechanical overrev (missed shift) last summer and did a top end rebuild. Reassembled and installed the engine but it won't fire.
Using a timing light I found that there is no spark to any of the spark plugs. However, when I hook it up to the wire leaving the "coil" it does read that it is firing.
So I'm a bit lost here...
I pulled the pertronix and it bench tests fine. Replaced cap and rotor with new ones. I also thought that maybe I accidently mis-aligned the rotor to be TDC on 4 instead of 1, so I rotated the distributer to the other side.

Am I right in assuming that if the "coil" is firing that everything in the system seems to be working right? The only thing left after that is the rotor right?

So what else should I try/what am I missing?

Thanks,
Pat

[Mo_Gearhead]

The coil has 12V to it?

When you place the timing light on ANY plug wire end ...you get no spark?

You are putting the timing light on the coil wire between the distributor cap and coil? Hmmm???

Have you tested for spark at each distributor cap terminal?

Have you ohm-ed the NEW rotor and CAP to be certain they have continuity?

If you have spark at the distributor cap terminals, that only leaves plug wires/plugs?

[Pat S]

Originally Posted by Mo_Gearhead

The coil has 12V to it?

Do you mean to the wire that comes from the CDI box? If so there is about 330 VDC there and it drops from when the key is turned on, according to the haynes manual this should be 350-400 it is a bit low but the "coil" is firing. I know it ins't really a coil but I don't remember what it is really called.

When you place the timing light on ANY plug wire end ...you get no spark?

Correct for any spark plug wire.

You are putting the timing light on the coil wire between the distributor cap and coil? Hmmm???

Yes, that is where I get a reading from the light so I know it is firing.

Have you tested for spark at each distributor cap terminal?

No, I haven't checked that.

Have you ohm-ed the NEW rotor and CAP to be certain they have continuity?

I just checked that an neither the old or new one have continuity... Both are rev limiting style. Something must be frying them....

Thanks for the help, any idea where to go from here?
Pat

[BK911]

I had this exact problem with a 2.4E. I replaced the cap and rotor and still had the same problem. It turned out to be the coil. It gave enough juice to trigger the timing light at the center terminal, but not enough at the individual leads. Check the resistance across the coil. The acceptable values are poseted somewhere.

Good luck!
BK

[wowzer911]

If you end up buying a new coil, (I have the same setup as you) the Pertronix Flamethrower has worked well for me.

[911pcars]

With an ohmmeter, confirm the rotor has continuity between the center and tip.

Sherwood

[Pat S]

I ohmed the coil and I'm getting almost good readings. .6 across the primary and 621 on the secondary. Spec is .4-.6 on primary and 650 - 790 on secondary, so it is just a bit below. I'll try to pick up a new one to swap it so I can rule it out.

However both the old and new rotors have no continuity. Both were rev limiting style and it seems there is a resistor built into them. My guess is that something else is wrong and is causing that to blow.

Any ideas on what might cause that symptom?

[Pat S]

I just purchased a brand new rotor and the new one does not have continuity. So this seems to be a red herring... I have a new coil on order also and will see how that works out.

[Bill in OKC]

That is odd about the rotor not having continuity. It is normally two pieces of metal with a rivet holding the pieces together on top. I agree with the coil though. It takes more juice to fire the plug under operating conditions. I had a ...Pontiac... once and the coil tested almost good and fired plugs I was holding to the block - I could see the spark. But it took a new coil to fix the problem.

[911pcars]

"However both the old and new rotors have no continuity. Both were rev limiting style and it seems there is a resistor built into them. My guess is that something else is wrong and is causing that to blow."

The term "continuity" means there's a path for current flow. In the case of a resistor in a circuit, the continuity meter may indicate no continuity in that the resistance reading won't be "0" or there won't be an audible signal from the meter. If the multimeter has a resistance range selector, it must be set to a resistance range that includes the circuit resistance. For example, if a manual ohmmeter is preselected to measure 0-100Ω (ohms) and the circuit happens to have 200 ohms resistance, the meter will read off the scale (infinite resistance or more accurately, beyond the preset range). Since both old and new rotors are displaying the same "non-continuity" results, I might guess that's your situation.

If the meter has "autoranging" capability, the meter automatically switches to a measurement range that includes the measured resistance. Thus, continuity may be 3000Ω. For secondary voltage resistance, that would be typical. If you're using an autoranging ohmmeter and the rotor continuity/resistance is infinite, than there is a problem. Although unlikely, that would be improbable for a new rotor, especially a "straight-through" rotor without centrifugal cutout capabilities or built-in resistance.

Sherwood

[Pat S]

New coil, same results, spark to dist, but not out.

I'm using a digital meter with infinite range. I get no reading from the center to the contact of the rotor.



I even tried measuring the ohms on the scale up to 20M with nothing. It's an open circuit on the brand new, just out of the box, never installed one. (NOTE that these are all rev limiting style) I wonder if there is a gap inside the epoxy that must arc?
Actually, since I have a few, I'll try to dremel one apart.

I also picked up a set of points. I'll try removing the pertronix and installing those. After that, the only thing left will be to swap the CDI box and the distributer itself.

Pat

[rick-l]

Did you hold the coil wire 1/4" away from the block to see if you have a spark and not just some discharge that triggers the timing light?

Be careful

[rrichar]

Pat,

Just by chance I did the same test a few days ago on two rotors. Both had continuity across the points you show in your diagram. Just make all you are touching is the metal on the part the touches the cap. I found it was easy to put the lead on the plastic part without noticing. I can't remember the scale but the reading was approx. 5 ohms on digital meter.

[real550A]

For about 8 bucks you can replace the rev limiting rotor with a
standard VW model. Do a search here and you'll find the part #
listed. It's a direct replacement w/no resistor. I've found them
at NAPA.