How do I diagnose my ignition problem if it’s the CDI box
 

How do I diagnose my ignition problem if it’s the CD box? I have a 1974 911, with 95,445 miles. I was driving down the freeway when I felt the engine quit tried to accelerate it backfired and then quit so I had to pull over and park it on the side of the freeway. Luckily I was able to call A relative who is an auto mechanic. He removed the distributor cap and rotor and told me to crank the engine over and he used the screwdriver to see if he could get any sparks and he said there was no electricity going to the points so he said he thought it was the condenser. So would that be right or, do I need to check further and how would you check to see if it is the CD box? I have no understanding of electronics or electricity in a car system. I’m hoping how to figure this out before just buying parts in putting in the engine compartment and finding out that it wasn’t that. Any good information is welcomed. Thank you, Marion Fisk

To start with, does it emit a high pitch sound when turning on the ignition?

to clarify: the sound should com from the CDI ignition box.
if not, it's probably dead
a common failure (age, bad solder/condenser inside)

if it does make a sound, then move on to checking wiring and also check coil.

- First check the inductive coil within the distributor if its not flawed: Disconnect the green wire plugs from the CDI and use an ohm meter where on both terminals of the green wire it should read about 600 ohms
- After that (if you don't have an oscilloscope) and with still the green wire disconnected from the CDI, let someone just crank the engine by starting about 2-3 seconds. Here a DMM should read on the terminals of the green wire about 5 volts and more, depending on the rpm. Actually its not a flat voltage output but an zero crossing inductive signal from negative to positive voltage.

https://www.911-hkz.de/ZV%20500upm.bmp
(Image from www.911-hkz.de)

- Finally with re connected green wire to the CDI unit, check with a timing light for a given spark on the ignition wire.


This test only lets you know if the inductive coil is ok and the CDI is "working".
For a detailed CDI test you'll need an oscilloscope with a high voltage probe for testing the high voltage output of the CDI at all revs at the "A" terminal of your ignition coil. Caution: High voltage here!

https://www.911-hkz.de/HKZKlemmeA.bmp
(Image from www.911-hkz.de)

If Marion's 1974 911 is stock, he won't have the infamous "green wire". He should have a 3 pin CDI:

http://forums.pelicanparts.com/uploa...1604500198.jpg

You will not see any voltage at the coil like on a classic points and coil system, as the coil only gets a pulse from the CDI (shown in the AndrewCologne graphs above).

To get a spark to the spark plug in the engine:
First, you need power to the CDI.
Next you need to have a way to trigger the CDI

Then the CDI will send a pulse to the coil which will increase the voltage and send it through the high tension lead to the rotor and to the selected spark plug. You need to verify that each step in this chain is working. 



1) Verify that the CDI is getting power. Do you hear it whining? If so, you are getting power to it. 



2) Verify that the CDI is producing a spark. You can do this with a "telegraph" test. Take the high tension lead off of the center of the distributor and put a spark plug on it. Ground the threaded part of the spark plug. With your plug attached to the center lead, open the distributor, remove the rotor, place your key in the run position, and manually open and close the points. Each time you cycle, you should see a spark from the plug. If you do not, you may have a bad CDI.



3) Verify that the coil gets a signal when you crank. Replace the rotor and distributor cap. Leave the plug on the center high tension lead. Crank the engine. Do you see a spark? If so, you are getting spark to the distributor. If not, your point gap may be off or there is some other problem with the connections. 



4) Verify that the spark goes to the spark plug wires. Reattach the center lead to the distributor. Attach your spark plug to one of the leads going to the cylinder spark plugs and ground. Crank engine. If you see a spark you may have a timing problem. If no spark, you have a problem with your rotor or distributor cap or your wires are improperly seated. 


Do the points open and close? There is no condenser in a CD system.

Yes I hear the high pitch sound coming from the CDI box. my car is stock as far as I know it does not have the green wire wherever that is the three wires coming out of the bottom of the CDI box the wire on the left outside Is white the middle wire is red and the right outside wire is white. The coil has a white and brown wire coming from contacts labeled A and B.

I am not sure if the points are opening and closing but I think it’s time I put a new set of points in the distributor. Right now I don’t have anyone to assist me on cranking the engine over while I look at the distributor to check.

Usually no spark problems are dirt on the point. Drag a piece of papper between the point.

You can put the car into 4th gear and go back to the engine compartment to watch the distributor turn and the points open and close as you slowly push the car forward.

If the CD is dead, you can add a condenser and wire the coil like pre CD cars in a pinch.

Do these tests. Tell us what you find.

Don't do step #4 by pushing the car in gear - if it starts, it could get away from you. Obvious, but I thought I should say something.

Wow. There is some really smart and in-depth advise offered up here. I'm amazed at the depth of experience and knowledge of Pelican community.

I've just had a similar experience today on a 73T that's been asleep in my barn for the last 4 years. We had spark last week and could get it to fire on starting fluid. I rebuilt the fuel pump and blew out the lines and got fuel flowing but then had no spark.

We cleaned all of the connections for the CDI and ignition system, but the one that got us running was the big 20ish pin connector that's below the CDI box. We cleaned it with alcohol and Q-tip and it fired right up. I know this is really a simpleton approach but it's not the first bad connection issue I've had.

One thing to note was the orange alternator light, in the tach I think, was on when the connection was good and off when it wasn't.

Totally correct, I mixed that up.