Cranks but no start...
 

I drive my 83 SC nearly every day and it is well sorted (CIS working perfectly) and has been 100% reliable. Today I got in to run an errand, it cranked up and just started when I heard an audible click from the engine compartment - not a back fire - and it stalled. Now it just cranks over. I checked the fuel pump and fuel pump fuse and those are fine (replaced fuel pump a few years ago). I used an OTC 6589 spark tester and do not see a spark. I popped the distributor cap and all looks good there.

So now I need to diagnose why. This is my first non-trivial electrical issue so I could use some pointers/help. My Bentley went awol.

Shoot some kwik start in the intake. If it coughs it's a fuel issue. No cough it's electrical.

Early_S_Man (may he RIP) helped a Pelicanite back in 2004: http://forums.pelicanparts.com/porsche-911-technical-forum/177814-sc-wont-start-no-spark.html#post1469123

I tested the resistance on the CDI at #7 and #31b and get infinite resistance. He doesn't say what to expect if the CDI is bad. I'm 99.99% sure it is not a fuel issue Joe Bob.

I'm assuming he meant test the CDI itself and not the terminals on the wiring harness?
EDIT: he meant check the wiring harness to the distributor!

To check for spark, I usually hook up an inductive timing light and, where available, have an assistant crank the engine. This can be done with the fuel pump relay removed, for safety. If the light flashes, you are getting spark. If not, check electrical connections first.

to check the spark..simple remove the no 4 cyl(if no air) or no 1 cyl, with a screwdriver or if you have another spark plug ,have somebody crank(fuel relay out) and ground the spark plugs.. with the screwdriver go just near the intake and see..check you engine harness multi plug connection as well..

very simple test

Ivan

As I said in the first post, I've already checked for spark and there was none. My diagnosis followed this order:

1) check fuel pump running with relay bypass - good
2) check fuel pump relay - good
3) check for spark with spark tester (with assistant) - no spark
4) measure resistance at CDI terminals 7 & 31b - should be ~800Ω, I see infinite (open circuit)

I've now also verified 3 as Superman recommends using my timing light and friendly assistant - no spark.

Doesn't test 4 verify that the CD is malfunctioning? Are there any other verification/tests I can do on the CD to further diagnose it?

I should also say that I do not hear the high freq whine from the engine compartment when the ignition is on as I believe I normally hear it.

Well, through a minor miracle I found my Bentley! In reading 280-9 it says the CDI can not be tested directly. If so, what did Early_S_Man mean in the link I posted about - measuring 600Ω across terminal 7 and 31b?

You can do a "telegraph test" to isolate just the CDI.

http://forums.pelicanparts.com/porsche-911-technical-forum/995942-911-sc-no-spark-2.html

Thanks! Telegraph Test: http://forums.pelicanparts.com/porsche-911-technical-forum/449527-how-do-i-check-coil-timing-light.html#post4395758 seems a bit dangerous - closing points with a screwdriver?

But Early_S_Man's version isn't clear to me though: http://forums.pelicanparts.com/porsche-911-technical-forum/341881-do-i-need-replace-my-cdi-box.html#post3224792

I have a stock Bosch CDI.

Correction, my Bosch CDI is whining. I'm working to understand and conduct the other test procedures including the telegraph test. Being very cautious around high voltage...

Bentley 280-9 tests

ignition On, test for battery voltage at terminal 15: I see 11.48V

engine cranking, test for VAC between terminals 7 and 31d: I see no AC voltage. I removed the Fluke leads and swapped them to make sure I was getting good contact. Meter set to AC, no voltage.

So, Bentley says "if meter shows no pulse, distributor pulse generator is most likely faulty."

I need to search to understand what the distributor pulse generator is and what would make it fail.

Check the “green wire” as it exits the distributor for cracks in the insulation.

The green wire is black! The insulation is hard but I do not see cracks in it. It is bent at a sharp 90° angle as it comes out of the connector at the distributor.

Is there a way to test the wire vs the magnetic pulse generator in situ? Or is it distributor removal time?

I have a replacement Bosch 1 234 431 290 that I intended to install at some point. I did email you a few hours ago Timmy2 about getting the mating connector.

FWIW spark should be tested at compression chamber pressures. I once pulled a plug, layed it against the engine, attached to the spark plug wire...there was a spark. But when I put the spark plug into the head and tested using a timing light...no spark. True story.

If you have not made the green wire upgrade, then I suggest you do this. Perhaps you already have the part? BTW, read up on the install method. It makes a difference which wire you connect where. If this brings no joy, then ask a Pelicanhead over for beers and use his/her Bosch CDI unit. They are internally grounded, so you don't have to affix the unit to your fuse panel.

All plugs were installed, I used a spark tester and saw no spark. Also tested with timing light.

I do have the green wire part as mentioned above. I don't have the connector to splice into the existing green wire yet. Hopefully timmy2 will get back to my email with cost for the part so I can get the order out tomorrow.

I've got feelers out with my local crew, nothing has turned up yet. But the test for AC signal on pin 15 and 31b on the CDI harness is pretty conclusive the CDI is not the issue though, correct?

EDIT

Pelican email is now going to my junk mail....

I replied to you Michael.

So, Bentley says "if meter shows no pulse, distributor pulse generator is most likely faulty."

This also true if green wire faulty, but Bentley doesn't seem to consider this. So, before you condemn the distributor as I nearly did take it out and bench test it by connector a meter to the 2 contacts where the green wire goes in (not easily done), rotate the distributor drive by hand and look for a pulse. This worked for me and narrowed it down to the green wire.

Should also have said follow the Bentley manual for removing the distributor; crankshaft pulley needs to be a TDC with #1 firing, otherwise you will lose ignition timing but no big deal to reset it.