Difficult case of no spark. Experts wanted!
 

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,
Jeremyhttp://forums.pelicanparts.com/uploa...1645918042.jpg

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?

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, 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.

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.

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

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.

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?

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!

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

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

http://forums.pelicanparts.com/uploa...1645977727.jpg

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

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

Yes

First, do the above suggested tests.

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.

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

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

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

That's good.

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

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.

For Daves test ,the top fuse should be constant 12 vdc. The lower one is fed by the same circuit as the CDI power supply is on.
If a different 12 v supply fixes the problem, then go up front and move the red wire from terminal 8 to terminal 7 and see if the problem goes away. (Alternatively jumper terminal 7 to terminal 8)
If it does, it is the fuse block internal jumper. If it doesn’t it may be the ignition switch.

You are the man!!!!! I jumped the B pin from the fuse and it started right up. I noticed that the tach wasn't working so that must be tied into that B terminal somehow. The car wouldn't stop until I disconnected the jump wire. So what are some possibilities on the poor power source to the CDI?
Thanks for all of your help!

Read my post above yours to troubleshoot the feed to the CDI power.
Could also be as simple as a poor connection in the 14 pin connector below the CDI.
Pull it apart and see if there is any corrosion or even just loose.

Thank you! I will check these

Looks like you are on it. Listen to Timmy2

Also, check the ignition 6 pin plug under the dash is fully inserted. Farthest one to the right side .

Hi Dennis. The 14 pin is clean, the 6 pin under the dash is fully inserted and the jumper from 7 to 8 didn't fix the problem. So it looks like it is the ignition switch. Anything else I should test before I tackle the electric portion of the ignition switch? Thanks again for all of your help?
Jeremy

With the key in run, do you have 12 vdc that will light a test lamp style voltage tester on the same fuse terminals 7&8?
If no, it is likely the ignition switch.

Hi Dennis,
Thanks again for all of your help. I think I have almost narrowed it down but need just a little more help to get to the finish line. I decided to give all my connections a through cleaning as I noticed a little corrosion. I started under the bonnet and even though there was conductivity in all of the fuse connections I cleaned with a combo of a wire brush, files, CRC electronic cleaner and then place dielectric gel. I noted on the fused side of 7 & 8 a fair amount of corrosion within the brass fittings. I then moved to the back and did the same thing. I opened up the two 14 pin connectors and cleaned out the male and female ends, all the connections on the speed switch, rear defogger and the other relay. I had already done the CDI connections. I then checked the ignition switch, pulled off the plug to inspect. All looked good, but I placed some dielectric gel for good measure. I hooked everything back up and to my surprise, the CDI was singing like a choir boy. I go to start up the car and.....it won't turn over. I was about to rip my hair out but I'm already bald. I had previously had a difficult starting problem, but cleaned and placed dielectric on the transmission ground strap, the 2 battery straps and all of the wires coming off the 2 batteries and that solved that problem. I woke up this morning and went though everything again, checking all my connections, removing and reinstalling the 14 pin connectors. After doing this I turned the key to the car and.....it started. I was very happy but...I turned it off and tried to restart it and nothing. I was then thinking an intermittent starter switch problem. I again pulled off the started switch plug. It was getting 12 volts at terminal 30. Plugged it back in.

Then I checked the voltage at the (2) 14 pin connectors in the engine compartment. I don't know the correct names but I think the more forward connector is the Chassis 14 pin and the more rearward the engine 14 pin.

The chassis (forward) 14 pin had no volts at any of the connections with the car key off. With the key in the on position, there was 12 V at 5,12, and 13. When I attempted to "crank" the engine the 1 (yellow solenoid wire) went to 12 V.

The engine (rearward) 14 pin had a constant 12V at 14. The number 1 (yellow solenoid wire) didn't change when I attempted to "crank" the engine but stayed near 0 volts.

I then go to hook everything up and now it is starting again. I started and stopped it 10 times and now it is starting every time.

Questions:
1. What does the 14 pin voltage data tell you?
2. The fact that the chassis (forward) 14 pin connector #1 connection reliably went to 12 V when I attempted to start the car tell us that the ignition switch is ok? It would do this reliably even when the car would not turn over.
3. Is it safe to say the spark problem was caused by resistance due to corrosion. The CDI has been singing loud since I cleaned all the connections.
4. Why do you think the car all of a sudden wouldn't crank? This hadn't been a problem until I cleaned up all the connections. Could it be due to manipulation of the wires on the rear engine harnesses or something to do with the speed switch or another switch on that panel of connectors in the back?
5. The one thing I haven't done is clean the connections to the starter and the solenoid. Could this have something to do with it?
6. What are the 3 relays on the panel. I have no idea what the most forward one is, but I think the middle is the defroster, and the most rear is the speed switch. Could something in the speed switch cause the intermittent starting problem or the findings of the solenoid wire readings above?
7. Is it normal for the engine (rearward) #1 yellow solenoid connection to stay near 0 when attempting to crank the engine.

I am certainly glad it is starting, but I worry about reliability. I think Hagerty's is going to balk at my next tow request.

I have included a picture of the relays and connections in the engine compartment. I have now yet secured them back up but have grounded the plate.

Thanks for any input,

Jeremyhttp://forums.pelicanparts.com/uploa...1646160572.jpg

Rear 14 pin to engine is notorious for the yellow wire connection needing attention.
Sounds like you have addressed all of the connections and should be good to go until/if any corrosion builds up again. All your measurements sound correct.
At a minimum, you know where to look for issues should it act up again.

A small Dremel wire wheel bit spun lightly in the female contacts does a good job to clean.
Try some De-Oxit D5 as a cleaner and final treatment as well. (Good stuff for contacts, cleans and lubes and preserves)

Great job going through everything that is in the circuits.
Happy motoring.:)

Most forward unit is the remote voltage regulator for the alternator.

Just an update. I got my baby back on the road! Being new at the air-cooled 911 fraternity, I certainly learned a lot of things. Specifically:

1. Old cars can get corrosion of the wires and connectors. Mine was restored in the late 90's, has less than 2000 miles since restoration, was always stored in a climate controlled garage, and never saw the rain. I figured the wiring would be free of corrosion. I was wrong! The two issues I had with the car, wouldn't turn over and no spark, were a direct result of corrosion. Upon close inspection, there was corrosion of the 7 and 8 connections of the top bonnet fuse panel that caused conductivity issues with power getting to the CDI. After I cleaned these up, the CDI sang loud and clear and produced spark. I also traced the dreaded yellow wire to the coil. There was corrosion on the engine (rear) 14 pin connector. I cleaned this up paying special attention to the 1 pin and it cranked over like new. I subsequently cleaned up every wire and connection visible in the bonnet, engine compartment, starter, and ground straps. I found corrosion on all of those areas. Reading through the forum posts, there is a tendency to chase after the things that are less likely the cause of issues. I was convinced I had a ignition switch and starter problems. I had those ordered and could have spent a lot of time, money and frustration changing out those parts. The sage people on the forum echo this throughout.

2: What a wonderful resource this forum is! Timmy2 (Dennis) and HarryD held my hand through this process and seemed genuinely happy to help a novice that asked too many irrelevant questions. A huge thank you to them and Pelican for supporting the site!

Great news!!!!


Glad to help. Many here have helped me over the years and I am just paying it forward.

It's unfortunate that a lot of time and effort was wasted on your part the result of misinformation in this thread, not withstanding your having a good at the time CDI unit rebuilt.
It wasn't until post 20 where the suggestion was made to jumper 12V to pin B of the CDI unit that the engine started and you posted (post 22);

"You are the man!!!!! I jumped the B pin from the fuse and it started right up."

Glad our efforts were productive.

Thanks Dave, Sorry I didn't mention you specifically. That was definitely the turning point! Let me know when you make it down to Laguna Beach. Lunch is on me.

Thanks for the offer. I'm often in southern Orange Country helping Porsche repair shops troubleshooting.
If you need any personal help in the future, just email, I might be near you working.