Behind the green wire

[Aurel]

I need help troubleshooting a no spark issue on my 911sc (1978)

I have an MSD street fire that gets power and produces sparks when triggered by grounding the white wire.

But I get no sparks at the plugs when cranking, so I assume something is wrong with the distributor.

There is one spark when I turn the key on and, but no more sparks while cranking.

The coil is good, I have another good spare one and I get the same issue with both coils.

The rotor is a no resistor type that MSD recommends, so I know I don’t have a burnt resistor issue in the rotor.

I recently replaced the green wire to the distributor, so I know that is not the issue either, and I just checked its continuity is good.

This leaves me wondering what is behind the green wire that generates the spark trigger signal to the CDI box, and could that have failed? I do not know the details of the distributor trigger mechanism, I am assuming it is magnetic but wondering if there are known parts that can fail and cause a no spark situation.

[john walker's workshop]

Ohm the distributor unit where the green wire plugs in. Recently had no spark in an 87 930 and the distributor showed open circuit. I think somewhere around 3K ohms is normal.

[Aurel]

Originally Posted by john walker's workshop

Ohm the distributor unit where the green wire plugs in. Recently had no spark in an 87 930 and the distributor showed open circuit. I think somewhere around 3K ohms is normal.

Thank you for the tip, John. I am measuring around 750 ohms, so it may be a bit low for normal…

[Alan L]

No, 650 is about right. Your circuit is OK. You are measuring the ohms in the pick up coil in the dizzy. Spec is 650 +/- 50 from memory.
Alan

[Aurel]

Doing some google research on the distributor, I found this very nice website that I am happy to share:

https://nineelevenheaven.wordpress.com/maintenance-of-the-distributor/

[wazzz]

Originally Posted by Aurel

Doing some google research on the distributor, I found this very nice website that I am happy to share:

https://nineelevenheaven.wordpress.com/maintenance-of-the-distributor/

website belongs to pelican fellow member AndrewCologne.

[Aurel]

I started taking appart the distributor, it won’t hurt to give a little overhaul. I found the fiber shim was broken and the shaft vertical play is about 1 mm, so new set of shims ordered.I will also clean and relubricate the advance mechanism…

[Alan L]

Originally Posted by john walker's workshop

Ohm the distributor unit where the green wire plugs in. Recently had no spark in an 87 930 and the distributor showed open circuit. I think somewhere around 3K ohms is normal.

Hi John. Had same issue in my 930 about a year ago - intermittent no spark, which finally failed to no spark. Took a while, but finally diagnosed no circuit in the dizzy coil. A well known engine builder here (can't remember who) mentioned he had never seen one fail. Glad to know I have company.
What was your solution?
I found a 80's merc dizzy with the same coil, and swapped it in.
Regards
Alan

[Aurel]

I am wondering if too much vertical play in the dizzy shaft may have been the issue. I will know when I fix that. My dizzy coil seems ok.

[john walker's workshop]

Originally Posted by Alan L

Hi John. Had same issue in my 930 about a year ago - intermittent no spark, which finally failed to no spark. Took a while, but finally diagnosed no circuit in the dizzy coil. A well known engine builder here (can't remember who) mentioned he had never seen one fail. Glad to know I have company.
What was your solution?
I found a 80's merc dizzy with the same coil, and swapped it in.
Regards
Alan

I used one from an SC distributor out of the core box.

[David Inc.]

I had a no-start a few years backwith a brand new green wire from Uro. The dizzy tested fine and the green wire tested fine. The problem was that one of the little clamps inside the plug on the green wire at the dizzy end was too loose and wasn't actually gripping the pin in the dizzy. I contacted Uro and they were aware that there had been an issue with a batch.

Maybe not your issue, but something to check. I had to reach in with a pin to bend the clamp so that it would actually grip the pin. After that it started right up.

When you test the wire with a probe, the probe is round and thick--the pins in the dizzy are flat, so the clamps need to be fully tight for them to grip.

Edit: If I remember right I realized something was up when I left the green wire plugged into the dizzy, and testing the green wire from the CDI end showed an open circuit.

[Bob Ashlock]

seldom but only once have heard of the reluctor pickup coil failing. Since you measured ~700 Ohms at the distributor, plug its "green wire" coax cable back in and then unplug the plug at the CDI box. Measure into the harness plug corresponding pins 31d and 7 and see if you are getting 700 there. That will check the integrity of the coax cable. These 'green wire' cables are known to crack internally, etc. so manipulate the cable in the areas near the distributor to verify your measurement is hot hopping around.

[Alan L]

Originally Posted by Bob Ashlock

seldom but only once have heard of the reluctor pickup coil failing. Since you measured ~700 Ohms at the distributor, plug its "green wire" coax cable back in and then unplug the plug at the CDI box. Measure into the harness plug corresponding pins 31d and 7 and see if you are getting 700 there. That will check the integrity of the coax cable. These 'green wire' cables are known to crack internally, etc. so manipulate the cable in the areas near the distributor to verify your measurement is hot hopping around.

Yes, this would be the proper process to check that system. He hasn't actually explianed what he did to test.
In my case I finally got the coil out of the dizzy and checked it on the bench - no circuit.
Alan

[Aurel]

I have a theory on what may have caused my distributor to no longer distribute sparks: the fiber shim, which is over 1 mm thick, was completely disintegrated. This caused the shaft to drop about 1 mm, maybe causing the graphite electrode in the cap to no longer make contact with the rotor. I just got the shims, so once I reassemble the distributor, I will know if my theory was correct.

Also, I measured the 750 ohm to the dizzy coil through the green wire plugged into it, so that eliminated those two components as source of the failure.

[Mahler9th]

Several of us had high displacement, single plug engines for racing here in Norcal.

These used some variations of 911 sc-style distributors employing the little green wire, and all used MSD ignitions and coils.

Several of us eventually had distributor failures. Mine happened entering turn 6 at Sears Point during a race.

In my case, I removed the distributor and took it to JWE, where Jerry confirmed a failure and rectified it.

It can happen.

In some cases, the symptoms were related to heat.

Used to carry a spare green wire or two. These were Porsche parts.

Never needed to use one.

[Alan L]

Yeah mine shut down intermittently on the track too.
Not good. I replaced the green wire and the problem persisted. Took a while to nail it.
Getting engine shut downs on the track is not good for you or your car's health.
Alan

[Aurel]

I tested my rebuilt distributor, and still get no spark at the plugs…very frustrating!

Then I wondered if the coil was weak, I swapped with another one and got the same.
Now I am starting to wonder if all my coils are bad, so I measured them all on the bench:

Primary secondary
My old Bosch 0.17 ohm 835 ohm
MSD blaster#1 0.12 ohm 1210 ohm
MSD blaster#2 0.12 ohm 1180 ohm

It seems like the secondary resistance is low on all these coils. I know it should be 4.5 kohms for the MSDs. So should I get a new coil?

[mysocal911]

You need to test the distributor output voltage; minimum .60 VAC while cranking.
The tach should be receiving a signal from the ignition box causing a needle movement.

[PeteKz]

CDI box failures at this age are common.

[AndrewCologne]

Originally Posted by Aurel

I tested my rebuilt distributor, and still get no spark at the plugs…very frustrating!

Then I wondered if the coil was weak, I swapped with another one and got the same.
Now I am starting to wonder if all my coils are bad, so I measured them all on the bench:

Primary secondary
My old Bosch 0.17 ohm 835 ohm
MSD blaster#1 0.12 ohm 1210 ohm
MSD blaster#2 0.12 ohm 1180 ohm

It seems like the secondary resistance is low on all these coils. I know it should be 4.5 kohms for the MSDs. So should I get a new coil?

Get a little cheap oscilloscope where you can measure the dizzys sensor output
https://hackaday.com/2017/11/09/review-jye-tech-dso150-oscilloscope-kit/

Thats how it should look (from another oscilloscope model)


If the signal is ok then you should check the cdi output, but here you need a high voltage probe and check in the www the correct approach for measuring.