911 sc no spark solved
 

Finally got the old girl to fire up so I thought I should put together a quick ignition system troubleshooting sequence for beginners like me.

1. Check for spark by inserting a known good plug in the end of one of your spark plug leads attach jumper wire with alligator ends to spark plug threads and a good engine compartment ground. Get a helper to crank over the engine, bright blue spark all good. If no spark go to step 2
2. Disconnect the coil lead from the distributor and using a spark tester look for a spark when the engine is cranked. If you haven’t got a spark tester you could use a spark and jumper wire with alligator clips. Holding the lead with insulated pliers close to an earth point would be risky as the voltages here can be very high and if the gap is too wide the charge building up in the coil will be too great and may damage the coil or CDI box.
If you have a spark from the coil the problem could be in your distributor, distributor cap, rotor points (or pick up if you have breakerless ignition) or leads. If you don't have a spark the problem could be in one of 4 places: the coil, the green wire that goes from the distributor to the CDI box, the CDI box or the pulse generator in your distributor .
3. Check the coi for obvious damage and with your multimeter look for resistance across the two small connections ( marked 1 and A ), this is the primary coil and resistance should be very low, less than 1 ohm, 0.6 or so ( remember to zero your multimeter or deduct the resistance across your test wires) check for resistance across the high tension center connection and terminal 1 this should be much higher 600 - 800. If test is out replace the coil if test ok go to step 4
4. Turn the ignition on and if the CDI box makes a squealing noise you are getting power to the box. If there is no noise pull the plug at the bottom of the CDI box and check for power at terminal 15 which is the middle terminal on the outboard side of the six pin plug ( see a manual if you have got one) . If there is no power check the supply wire or your ignition switch, if there is power and your CDI box is silent the CDI box is most likely caput.
My CDI box was making a noise however this alone does not prove that it is working so I went to steP 5
5. With the engine cranking check for a 1 to 1.5 volts pulse across terminal 31D and 7 in the plug (these are the two end terminals side by side at the fwd end of the six pin plug.) If you have a pulse then the problem is in your CDI box as it was in mine so replace the Box. if you have no pulse go to 6.
6. Check for continuity in the wires inside the green shielded twin core wire that goes from the six pin plug (terminal 31D and 7) to the distributor, these are probably the most troublesome part of the system as they deteriorate and chafe inside the shield. The pulse generator very rarely fails . If green wire test faulty replace it.
There you have it a more or less complete ignition test for non geniuses.
ANY COMMENTS OR IMPROVEMENTS PLEASE.

PS. Some earlier cars have a 3 pin plug instead of a six pin plug so consult your manual for testing at these terminals.

Nice write-up. Have you submitted this as a DIY article? (see sticky)

Did I miss it or what was your problem/solution?

I would add for steps 1 & 2 that if you have an inductive timing light that is an easier and safer way to check for spark at the plugs.

I ran into a problem recently with a bad coil. It was making weak spark. Timing light on the coil to dizzy wire produced a light. Moved the inductive lead to the wire for cylinder #1 and no light. Pulled the coil wire and then did the ground test and found weak orange spark. Spark was strong enough at the coil wire but too weak at the plug wire to trigger the light. Changed coil and problem solved.

It may be personal preference but I find the timing light is quicker as a first check. No spark at all is obvious there is a problem. If there is spark, then pulling the plug to inspect spark quality is a good next step.

With regard to the CDI squeal... that's assuming the car still has an original Bosch. Lots of cars have other CDIs that people have used (Permatune, MSD, Classic Retrofit, ...) when their original Bosch failed. I think the only one that whines is the original Bosch.

I believe his CDI box was bad. (See step 5)

Newbie question here. When checkibg for distributor pulse following the instructions above do I probe 31D to 7 with my multimeter (VAC) or do I attach my multimeter negative to ground and then probe 31D and 7 separately?

When attaching probes to 31D and 7 I get no pulse which makes sense right, because no power is being delivered to the distributor? Is that correct?

When I ground the multimeter and probe 31D I get pulse. I then did the same to 7 and have pulse.

Am I doing this correctly? Thanks everyone!! Appreciate any advice you can give.

1981 911sc I also have no squealing from the CDI, but I have the 12vdc voltage on 15 and no return ground on 31-1. The schematic indicates, 31-1 goes to ground and B connection on the coil. Can someone double check my troubleshooting. Thanks for looking.

The 31-1 pin is normally connected to the case of the CDI, and is also connected to the coil pin that's grounded.
If the case of the CDI touches the fuse panel, the internal connection between the 31-1 pin and the case is damaged,
and the CDI won't function until it's grounded to the chassis. Make sure that the coil pin where the 31-1 wire goes
to is grounded to the coil mounting bracket.