Difficult case of no spark. Experts wanted!

[Jeremy911T]

Yesterday I created a post about an ignition problem I am having. Thank you for all those who commented. I have now run all the tests I know of and still cannot find the culprit. I have a 71 911T with Zenith Carbs and a Marelli distributor. It underwent a complete restoration in the late 90’s and recently came out of a 3.5-year hibernation. After some initial work rebuilding the carbs and getting the some the brake work done, it ran great for a few days. Then the CDI unit went bad, had it rebuilt, and again ran great until 2 days ago. After a 10 mile trip I went to restart it after lunch and it wouldn’t fire up. The engine would turn over fine. Waited 20 minutes same thing. Had it towed home and pulled off the fuel line and the pump was pushing plenty of fuel. Checked for spark and nothing.
I pulled off the distributor cap and when I toggle the advance the points are opening and closing. I also ran some fine grit sandpaper gently over them just to make sure they were clean.
I read 12 V at the center plug on the CDI with the ignition on.
I performed the telegraph test-> no spark.
I put a spark tester in the coil high tension lead and cranked the engine-> no spark
I pulled a plug wire and put the spark tester in the plug wire and cranked the engine-> no spark.
I initially thought it was the coil or the CDI unit. I recently had the CDI unit rebuilt by Bob Ashlock. He bench tested the CDI unit (including heating it up and running it hard) and the coil and they were both good. I even borrowed another good CDI unit just to make sure and still no spark.
I then checked the continuity of the wire between the C pin on the plug and the distributer and it was good. Likewise the continuity from the A pin to the coil was good. Bob also said to check the voltage at the C pin wire at its connection to the distributor. He said with the key on there should be 12 V going from the C terminal wire on the CDI to the distributor. I pulled the C wire from the distributor and put the key in the on position and no voltage at the end going to the distributor. So 12 V at the B terminal of the CDI plug but nothing at the end of the C wire.
In addition, I have cleaned, sanded, and placed dielectric gel on all the grounds and checked every connection I can see. I have included a shot of my wiring near the CDI. There are 3 loose female ends (2 coming out the same clump of wires) but I believe these have always been unconnected. The ground to the back of the CDI (bare ground) is connected to the ground in back of the plate that the CDI and other relays are mounted onto.
I know that is a lot of information but I am hoping someone can lead me down the right path.
Thanks in advance,
Jeremy

[timmy2]

I see the 2 pole fuel pump wiring connector, the MFI fuel enrichment wire, the rear defrost and the license plate wiring in your photo.
Looks like the CDI wires are tucked under the CDI unit?
Have you checked the fuse for the CDI wire in the front?

And why start a new thread?

[mysocal911]

Originally Posted by Jeremy911T

Yesterday I created a post about an ignition problem I am having. Thank you for all those who commented. I have now run all the tests I know of and still cannot find the culprit. I have a 71 911T with Zenith Carbs and a Marelli distributor. It underwent a complete restoration in the late 90’s and recently came out of a 3.5-year hibernation. After some initial work rebuilding the carbs and getting the some the brake work done, it ran great for a few days. Then the CDI unit went bad, had it rebuilt, and again ran great until 2 days ago. After a 10 mile trip I went to restart it after lunch and it wouldn’t fire up. The engine would turn over fine. Waited 20 minutes same thing. Had it towed home and pulled off the fuel line and the pump was pushing plenty of fuel. Checked for spark and nothing.
I pulled off the distributor cap and when I toggle the advance the points are opening and closing. I also ran some fine grit sandpaper gently over them just to make sure they were clean.
I read 12 V at the center plug on the CDI with the ignition on.
I performed the telegraph test-> no spark.
I put a spark tester in the coil high tension lead and cranked the engine-> no spark
I pulled a plug wire and put the spark tester in the plug wire and cranked the engine-> no spark.
I initially thought it was the coil or the CDI unit. I recently had the CDI unit rebuilt by Bob Ashlock. He bench tested the CDI unit (including heating it up and running it hard) and the coil and they were both good. I even borrowed another good CDI unit just to make sure and still no spark.
I then checked the continuity of the wire between the C pin on the plug and the distributer and it was good. Likewise the continuity from the A pin to the coil was good. Bob also said to check the voltage at the C pin wire at its connection to the distributor. He said with the key on there should be 12 V going from the C terminal wire on the CDI to the distributor. I pulled the C wire from the distributor and put the key in the on position and no voltage at the end going to the distributor. So 12 V at the B terminal of the CDI plug but nothing at the end of the C wire.
In addition, I have cleaned, sanded, and placed dielectric gel on all the grounds and checked every connection I can see. I have included a shot of my wiring near the CDI. There are 3 loose female ends (2 coming out the same clump of wires) but I believe these have always been unconnected. The ground to the back of the CDI (bare ground) is connected to the ground in back of the plate that the CDI and other relays are mounted onto.
I know that is a lot of information but I am hoping someone can lead me down the right path.
Thanks in advance,
Jeremy

Sounds like your original CDI was probably OK, since your running problems haven't changed. The three pin CDIs rarely if ever fail,
i.e. unless the battery is jumped incorrectly. Furthermore, why hasn't your CDI "rebuilder" continued to help you to solve your problem?
Since you spent money, potentially unnecessarily, he should spend the time to help you resolve your problem, e.g. most who rebuild CDIs will do that.

Once the "C" wire is disconnected from the distributor and the tach wire is removed , it should measure about the battery voltage (12V).

[Jeremy911T]

Originally Posted by mysocal911

Sounds like your original CDI was probably OK, since your running problems haven't changed. The three pin CDIs rarely if ever fail,
i.e. unless the battery is jumped incorrectly. Furthermore, why hasn't your CDI "rebuilder" helped you solve your problem?
Since you spent money, potentially unnecessarily, he should spend the time to help you resolve your problem, e.g. most who rebuild CDIs will do that.

Hi Dave, Actually he has. The car ran great and started right up after the CDI was replaced. He benched tested it prior to rebuild and it was bad. He has been very helpful in trying to help me find the current problem.

[Jeremy911T]

Originally Posted by timmy2

I see the 2 pole fuel pump wiring connector, the MFI fuel enrichment wire, the rear defrost and the license plate wiring in your photo.
Looks like the CDI wires are tucked under the CDI unit?
Have you checked the fuse for the CDI wire in the front?

And why start a new thread?

Thanks Dennis. New to the forum thing. Probably just should have added to the original thread. I will check the fuse in the front. Thanks for identifying the extra wires. The car is carbed so I imagine those were just part of the extra wires when the new wiring harness was placed during the restoration.

[Jeremy911T]

Originally Posted by timmy2

I see the 2 pole fuel pump wiring connector, the MFI fuel enrichment wire, the rear defrost and the license plate wiring in your photo.
Looks like the CDI wires are tucked under the CDI unit?
Have you checked the fuse for the CDI wire in the front?

And why start a new thread?

Don't see anything in the 2 vertically stacked fuse panels on the drivers side of the bonnet that indicates it is for the CDI

[timmy2]

Take a look at the wire going into fuse 8 top stack headlamp side. It is unfused, but only sees power from a jumper on the back of the fuse block from fuse 7.

[Jeremy911T]

Originally Posted by timmy2

Take a look at the wire going into fuse 8 top stack headlamp side. It is unfused, but only sees power from a jumper on the back of the fuse block from fuse 7.

Thanks Dennis. I will check it out.

[Jeremy911T]

Originally Posted by mysocal911

Sounds like your original CDI was probably OK, since your running problems haven't changed. The three pin CDIs rarely if ever fail,
i.e. unless the battery is jumped incorrectly. Furthermore, why hasn't your CDI "rebuilder" continued to help you to solve your problem?
Since you spent money, potentially unnecessarily, he should spend the time to help you resolve your problem, e.g. most who rebuild CDIs will do that.

Once the "C" wire is disconnected from the distributor and the tach wire is removed , it should measure about the battery voltage (12V).

Hi Dave. That is what is so confusing about this. The B terminal on the CDI plug measures 12 volts when the ignition is on but the C wire removed from the distributor and separated from the tach wire measures zero volts with the ignition on. The C wire also shows to be in continuity. It seems like the power from the CDI is not exiting the C terminal down to the distributor. The CDI checks out on the bench and I tried another known working CDI unit and the same result. Can't figure it out. Any thoughts?

[HarryD]

Originally Posted by Jeremy911T

Yesterday I created a post about an ignition problem I am having. Thank you for all those who commented. I have now run all the tests I know of and still cannot find the culprit. I have a 71 911T with Zenith Carbs and a Marelli distributor. It underwent a complete restoration in the late 90’s and recently came out of a 3.5-year hibernation. After some initial work rebuilding the carbs and getting the some the brake work done, it ran great for a few days. Then the CDI unit went bad, had it rebuilt, and again ran great until 2 days ago. After a 10 mile trip I went to restart it after lunch and it wouldn’t fire up. The engine would turn over fine. Waited 20 minutes same thing. Had it towed home and pulled off the fuel line and the pump was pushing plenty of fuel. Checked for spark and nothing.
I pulled off the distributor cap and when I toggle the advance the points are opening and closing. I also ran some fine grit sandpaper gently over them just to make sure they were clean.

If you put an ohm meter across the point and crank the car, does the meter go from 0 to infinite ohms? Report findings.

I read 12 V at the center plug on the CDI with the ignition on.
I performed the telegraph test-> no spark.

If the points indeed opening and closing (see above), then you have a bad signal wire from the points to the CDI, a failed CDI, a bad signal from the CDI to the Coil or a bad coil.

Coil: Measure the resistance/continuity between the two outer terminals on the coil and between each of the outer terminals and the high tension tower in the middle of the coil. Report findings. Per the Porsche Factory manual the CDI puts out about 400v. The coil Primary resistance should be 0.4 to 0.6 ohms, and the secondary is 650 to 790 ohms.

Check continuity of the wire between the points and the plug that connects to the CDI. Center is power. On one side is the wire to the points and on the other is the wire to the coil. Report findings.

... until this is addressed the rest of the tests are meaningless.

[Jeremy911T]

Originally Posted by HarryD

If you put an ohm meter across the point and crank the car, does the meter go from 0 to infinite ohms? Report findings.



If the points indeed opening and closing (see above), then you have a bad signal wire from the points to the CDI, a failed CDI, a bad signal from the CDI to the Coil or a bad coil.

Coil: Measure the resistance/continuity between the two outer terminals on the coil and between each of the outer terminals and the high tension tower in the middle of the coil. Report findings. Per the Porsche Factory manual the CDI puts out about 400v. The coil Primary resistance should be 0.4 to 0.6 ohms, and the secondary is 650 to 790 ohms.

Check continuity of the wire between the points and the plug that connects to the CDI. Center is power. On one side is the wire to the points and on the other is the wire to the coil. Report findings.

... until this is addressed the rest of the tests are meaningless.

Thanks Harry, I really appreciate your response. I will try all of these tomorrow and report back. What is the process of "putting an ohm meter across the point and crank the car"? I want to make sure I do it the correct way. When I was opening and closing the points I was doing this by turning the distributor advance clockwise and counter clockwise. Is this the correct way to open and close the points when performing the telegraph test? Thanks again!

[Jeremy911T]

Originally Posted by HarryD

If you put an ohm meter across the point and crank the car, does the meter go from 0 to infinite ohms? Report findings.



If the points indeed opening and closing (see above), then you have a bad signal wire from the points to the CDI, a failed CDI, a bad signal from the CDI to the Coil or a bad coil.

Coil: Measure the resistance/continuity between the two outer terminals on the coil and between each of the outer terminals and the high tension tower in the middle of the coil. Report findings. Per the Porsche Factory manual the CDI puts out about 400v. The coil Primary resistance should be 0.4 to 0.6 ohms, and the secondary is 650 to 790 ohms.

Check continuity of the wire between the points and the plug that connects to the CDI. Center is power. On one side is the wire to the points and on the other is the wire to the coil. Report findings.

... until this is addressed the rest of the tests are meaningless.

One more question, When you say, "check continuity of the wire between the points and the plug that connects to the CDI" do you mean the wire that goes from CDI plug terminal and connects to the outside of the distributor (along with the tach wire) or from the CDI plug terminal to inside the distributor on the points themselves. i.e do I need to pull off the cap and rotor and touch the points somewhere in the distributor? If so where on the points do I put the multimeter probe? I am learning to use a multimeter but am definitely not an expert. Thx

[mysocal911]

Originally Posted by Jeremy911T

Hi Dave. That is what is so confusing about this. The B terminal on the CDI plug measures 12 volts when the ignition is on but the C wire removed from the distributor and separated from the tach wire measures zero volts with the ignition on.

There're only two possibilities;
1. The "C" wire is still shorted to ground.
2. There's an internal problem in the CDI.

You need to power-up the CDI out of the car and measure the "C" pin of the CDI.

Originally Posted by Jeremy911T

The C wire also shows to be in continuity. It seems like the power from the CDI is not exiting the C terminal down to the distributor. The CDI checks out on the bench and I tried another known working CDI unit and the same result. Can't figure it out. Any thoughts?

You checked the CDI on the bench?

You need to disconnect the CDI from its connector and measure terminal "C" with a meter to ground (CDI case) for continuity, key-off.
Then measure the "C" pin (ohms), with the CDI disconnected, to ground (CDI case).

Did your rebuilder ever indicate the failure mode of your CDI?

By the way, where are you located?

[Jeremy911T]

Originally Posted by mysocal911

There're only two possibilities;
1. The "C" wire is still shorted to ground.
2. There's an internal problem in the CDI.

You need to power-up the CDI out of the car and measure the "C" pin of the CDI.



You checked the CDI on the bench?

You need to disconnect the CDI from its connector and measure terminal "C" with a meter to ground (CDI case) for continuity, key-off.
Then measure the "C" pin (ohms), with the CDI disconnected, to ground (CDI case).

Did your rebuilder ever indicate the failure mode of your CDI?

By the way, where are you located?

Thanks Dave. I really appreciate your input. I will attach a photo of the CDI report and the parts replaced. It appears that the discharge capacitor was to blame.

Just so I am straight on the terminology, when you say "terminal C" do you mean the plug side that connects into "pin C" on the CDI unit itself? Is that correct?

So the first test you suggested, "You need to power-up the CDI out of the car and measure the "C" pin of the CDI." Can I place an alligator clip from the B terminal on the plug to the B pin on the CDI unit to give it power and then measure the volts at the C pin on the CDI unit? I am assuming that the C pin should measure 12 volts.

For the second test, "You need to disconnect the CDI from its connector and measure terminal "C" with a meter to ground (CDI case) for continuity, key-off". After pulling the plug off the CDI and with the key off, I would measure for continuity from the C terminal (plug side) to the CDI case.

Then with the key still off, measure the resistance (ohms) from the C pin on the CDI unit to ground, in this case the CDI case.

When I measure to ground on the CDI case, are you referring to the ground pin on the CDI unit in front of the A, B, and C pins or anywhere on the case itself? I am assuming the case acts as a ground. Sorry about all the questions. I am a little new with all the terminology and I want to make sure I am testing correctly.

Could I check the individual wires from the CDI to the coil and distributor by bypassing them with alligator clips. For example, I connect a clip from the B plug (power source) to the B pin (CDI unit). Then I place alligator clip from the C pin (CDI unit) to the connection on the distributor (on the common connection with the tach wire). Another alligator clip from the A pin (CDI unit) to the coil. Another clip could ground the CDI unit. If the car runs I know I have a bad C or A wire and the CDI is ok. Then it is just a process of putting one of the wires in question back in the circuit to find the bad one. If it still doesn't spark after bypassing the B and C wires it would tell us that the CDI unit is bad. Is my reasoning sound or am I missing something?

I live just a mile down the road in Laguna Beach, do you make house calls?

Thanks,
Jeremy

[Jeremy911T]

Originally Posted by mysocal911

There're only two possibilities;
1. The "C" wire is still shorted to ground.
2. There's an internal problem in the CDI.

You need to power-up the CDI out of the car and measure the "C" pin of the CDI.



You checked the CDI on the bench?

You need to disconnect the CDI from its connector and measure terminal "C" with a meter to ground (CDI case) for continuity, key-off.
Then measure the "C" pin (ohms), with the CDI disconnected, to ground (CDI case).

Did your rebuilder ever indicate the failure mode of your CDI?

By the way, where are you located?

[mysocal911]

Originally Posted by Jeremy911T

Just so I am straight on the terminology, when you say "terminal C" do you mean the plug side that connects into "pin C" on the CDI unit itself? Is that correct?

With the key-off, disconnect the CDI plug and measure the resistance at terminal C to the ground pin (the one for the body of the CDI).
of the

Originally Posted by Jeremy911T

So the first test you suggested, "You need to power-up the CDI out of the car and measure the "C" pin of the CDI." Can I place an alligator clip from the B terminal on the plug to the B pin on the CDI unit to give it power and then measure the volts at the C pin on the CDI unit? I am assuming that the C pin should measure 12 volts.

Correct. The CDI for this test is still mounted with just one connection, 12V to the B pin.

Originally Posted by Jeremy911T

For the second test, "You need to disconnect the CDI from its connector and measure terminal "C" with a meter to ground (CDI case) for continuity, key-off". After pulling the plug off the CDI and with the key off, I would measure for continuity from the C terminal (plug side) to the CDI case.

Then with the key still off, measure the resistance (ohms) from the C pin on the CDI unit to ground, in this case the CDI case.

Correct. You should measure either 100 ohms or 30 ohms.

Originally Posted by Jeremy911T

When I measure to ground on the CDI case, are you referring to the ground pin on the CDI unit in front of the A, B, and C pins or anywhere on the case itself? I am assuming the case acts as a ground. Sorry about all the questions. I am a little new with all the terminology and I want to make sure I am testing correctly.

Yes

Originally Posted by Jeremy911T

Could I check the individual wires from the CDI to the coil and distributor by bypassing them with alligator clips. For example, I connect a clip from the B plug (power source) to the B pin (CDI unit). Then I place alligator clip from the C pin (CDI unit) to the connection on the distributor (on the common connection with the tach wire). Another alligator clip from the A pin (CDI unit) to the coil. Another clip could ground the CDI unit. If the car runs I know I have a bad C or A wire and the CDI is ok. Then it is just a process of putting one of the wires in question back in the circuit to find the bad one. If it still doesn't spark after bypassing the B and C wires it would tell us that the CDI unit is bad. Is my reasoning sound or am I missing something?

First, do the above suggested tests.

[mysocal911]

Originally Posted by Jeremy911T

So based on this report, your CDI unit was functional when it arrived at the rebuilder, and it failed during testing by the rebuilder. That's very unfortunate for you!
Given that, your original problem still exists.

[Jeremy911T]

Originally Posted by mysocal911

There're only two possibilities;
1. The "C" wire is still shorted to ground.
2. There's an internal problem in the CDI.

You need to power-up the CDI out of the car and measure the "C" pin of the CDI.



You checked the CDI on the bench?

You need to disconnect the CDI from its connector and measure terminal "C" with a meter to ground (CDI case) for continuity, key-off.
Then measure the "C" pin (ohms), with the CDI disconnected, to ground (CDI case).

Did your rebuilder ever indicate the failure mode of your CDI?

By the way, where are you located?

Hi Dave, First thank you for responding and helping. I appreciate it. I ran all the tests and this is what I found:

1. The B terminal on the plug measures 12 V with the ignition on. However, when I connect the B terminal on the plug with the B pin on the CDI via an alligator clip, the voltage on the B pin goes to <1 V and the C pin reads < 1 V. If I connect the B pin on the CDI directly to the battery the C pin reads 12 V. I think this is where the problem may lie. When the CDI unit is plugged into its connector, the voltage drops to <1.

2. When I check the continuity between the C terminal on the plug to it's connection to the distributor, it reads <1 ohm. Looks like the C wire to the distributor is ok??

3. When I measure terminal C to ground on the CDI case, it measure 1-10 ohms depending where I ground on the case.

4. I also checked the continuity of the coil and everything is within spec.

Looks like we may be narrowing it down. What do you think could be going on?

Thanks,

Jeremy

[Jeremy911T]

Originally Posted by HarryD

If you put an ohm meter across the point and crank the car, does the meter go from 0 to infinite ohms? Report findings.



If the points indeed opening and closing (see above), then you have a bad signal wire from the points to the CDI, a failed CDI, a bad signal from the CDI to the Coil or a bad coil.

Coil: Measure the resistance/continuity between the two outer terminals on the coil and between each of the outer terminals and the high tension tower in the middle of the coil. Report findings. Per the Porsche Factory manual the CDI puts out about 400v. The coil Primary resistance should be 0.4 to 0.6 ohms, and the secondary is 650 to 790 ohms.

Check continuity of the wire between the points and the plug that connects to the CDI. Center is power. On one side is the wire to the points and on the other is the wire to the coil. Report findings.

... until this is addressed the rest of the tests are meaningless.

Hi Harry,
I just posted the results from the tests I ran. Any thoughts?

[mysocal911]

Originally Posted by Jeremy911T

Hi Dave, First thank you for responding and helping. I appreciate it. I ran all the tests and this is what I found:

1. The B terminal on the plug measures 12 V with the ignition on. However, when I connect the B terminal on the plug with the B pin on the CDI via an alligator clip, the voltage on the B pin goes to <1 V and the C pin reads < 1 V. If I connect the B pin on the CDI directly to the battery the C pin reads 12 V. I think this is where the problem may lie. When the CDI unit is plugged into its connector, the voltage drops to <1.

That's the source of your problem. That power source normally comes from your ignition switch.

Originally Posted by Jeremy911T

2. When I check the continuity between the C terminal on the plug to it's connection to the distributor, it reads <1 ohm. Looks like the C wire to the distributor is ok??

That's good.

Originally Posted by Jeremy911T

3. When I measure terminal C to ground on the CDI case, it measure 1-10 ohms depending where I ground on the case.

Actually, that measurement should be between the B pin and the C pin with the CDI disconnected.

Originally Posted by Jeremy911T

4. I also checked the continuity of the coil and everything is within spec.

So it looks like you've had an intermittent/poor power source to your CDI pin B.
Before troubleshooting the power problem, use a jumper wire to power the B pin by
connecting the wire to one of the fuses on the left side near the big engine harness
connector. Hopefully, your engine should start.