Help- 911T 2.7 Runs, then dies instantly CDI Good

[2276spyder550]

Hello

Seeking the infinite knowledge out there, to solve this running problem. Recently, my 2.7 Carb'd 911T is dying. I start the car cold, and during warm up it instantly dies. Then just cranks with no spark. It's clearly an electrical problem. Thinking it was the CDI box, I sent it off to the great Bob Ashlock. He confirmed the box is working as it should. Checked the powerlead to CDI and voltage is hovering around 6.7, when it should be 12V.

Batteries are reading around 11.6 at the time of the incident. This has happened once before, and the car fired back up and ran great for an evening blast.

Appreciate any ideas. This weekend is my birthday and I sure would love to drive the car.

Thank you
Jeff

[rwest]

Jeff,

A good battery should be reading in the mid 12s without the car on. I would charge or replace the battery as a first step. Could be that after all the draw from the starter, there just isn’t enough to run the CDI.

Rutager

[2276spyder550]

Originally Posted by rwest

Jeff,

A good battery should be reading in the mid 12s without the car on. I would charge or replace the battery as a first step. Could be that after all the draw from the starter, there just isn’t enough to run the CDI.

Rutager

Thank you Rutager. The batteries are new...I've got them on charge to ensure that isn't the issue. Will give it another shot when the CDI comes back this week.

I installed a modern internally regulated alternator and am concerned it's not being properly triggered due to the inadequate resistance in the gauge light bulb. I've got to wire up the resistor. Am concerned that the system isn't charging as it should.

[PeteKz]

Jeff, you may already know this: The Gen light must be an incandescent bulb, NOT an LED. It initiates the alternator field by sending current back through the bulb to the alternator. Too little resistance would not be a problem, but rather too much would be a problem.

That's easy to check: When you turn the key to ON, does the Gen light illuminate? Then when you get it started, does the Gen light go out? If so, the light works and is feeding current back to the alternator. Then check the running voltage. That should be around 14V.

If the voltage to the CDI is around 6.7, instead of 12.6, then you probably have a poor connection in the wiring harness. Pull apart the connectors and clean the pins and spring the pins so that they make better contact.

[mysocal911]

Originally Posted by PeteKz

Jeff, you may already know this: The Gen light must be an incandescent bulb, NOT an LED. It initiates the alternator field by sending current back through the bulb to the alternator. Too little resistance would not be a problem, but rather too much would be a problem.

That's easy to check: When you turn the key to ON, does the Gen light illuminate? Then when you get it started, does the Gen light go out? If so, the light works and is feeding current back to the alternator. Then check the running voltage. That should be around 14V.

If the voltage to the CDI is around 6.7, instead of 12.6, then you probably have a poor connection in the wiring harness. Pull apart the connectors and clean the pins and spring the pins so that they make better contact.

Actually, too little resistance is common when converting from an external regulated alt to an internal regulated alternator.
Typically, a 100 ohm resistor is added in parallel with the alt bulb.

[al lkosmal]

Not sure this addresses your problem, but there is a tach bulletin for the regulator/resistor requirement.

[rwest]

Jeff,

Sounds like you have the batteries under control. Have you checked grounds? Like from transmission to body?

You might need to start where you’re getting the low voltage and work your way back until you find proper voltage to find the issue.

Rutager

[2276spyder550]

Originally Posted by PeteKz

Jeff, you may already know this: The Gen light must be an incandescent bulb, NOT an LED. It initiates the alternator field by sending current back through the bulb to the alternator. Too little resistance would not be a problem, but rather too much would be a problem.

That's easy to check: When you turn the key to ON, does the Gen light illuminate? Then when you get it started, does the Gen light go out? If so, the light works and is feeding current back to the alternator. Then check the running voltage. That should be around 14V.

If the voltage to the CDI is around 6.7, instead of 12.6, then you probably have a poor connection in the wiring harness. Pull apart the connectors and clean the pins and spring the pins so that they make better contact.

Thanks so much for your reply, Pete. I have a dual battery set up, as original using Odyssey Batteries. They are only a year old and for some reason don't seem to be charging. Or there is a parasitic draw on the battery, as they lose charge over a day or two. I am really struggling to identify what is going on, very mysterious. The alternator light is green and does glow upon start up. With 200-300 rpms, the light goes out. I am going to go through all connections to ensure good contact. Good news is, I have a beautifully restored CDI box from Bob Ashlock arriving Friday and looking forward to having that piece of mind. Cheers

[2276spyder550]

[QUOTE=al lkosmal;12268134]Not sure this addresses your problem, but there is a tach bulletin for the regulator/resistor requirement.


Thank you for your reply, Al. I have seen that bulletin, as Allen provided it to me during my alternator replacement. I have ordered the resistors and electrical connectors. It seems it would be worthwhile installing that to rule out the charging issue. My laundry list of to dos continues to grow. But that is why we love these old cars...right?

Will keep you posted as I work to resolve this.

[mysocal911]

Originally Posted by 2276spyder550

Thanks so much for your reply, Pete. I have a dual battery set up, as original using Odyssey Batteries. They are only a year old and for some reason don't seem to be charging. Or there is a parasitic draw on the battery, as they lose charge over a day or two. I am really struggling to identify what is going on, very mysterious. The alternator light is green and does glow upon start up. With 200-300 rpms, the light goes out. I am going to go through all connections to ensure good contact. Good news is, I have a beautifully restored CDI box from Bob Ashlock arriving Friday and looking forward to having that piece of mind. Cheers

Hopefully, you are aware that this indicates the alternator has entered the charging mode, right?
At this point, your voltmeter at the battery should indicate an increase in voltage, e.g. 1-2 volts.

By the way, the Bosch 3-pin CDI unit rarely if ever fails, not like the 6-pin ones. Why waste your money?
The 3-pin CDIs typically fail when the battery is jumped in correctly, or a tech shorts the ignition coil to ground while troubleshooting.

[colingreene]

I advocated for him to have it rebuilt since the unit is 50 years old and with the batteries dying frequently it could have been damaged by jumping it off.
The car does see some distance trips and its worth the piece of mind.

[mysocal911]

Originally Posted by colingreene

I advocated for him to have it rebuilt since the unit is 50 years old and with the batteries dying frequently it could have been damaged by jumping it off.
The car does see some distance trips and its worth the piece of mind.

Then there's what called the infant mortality phase related to electronics;

1. mechanically system incur increased failures with time
2. electronic systems incur more failures in the early time phase and decrease with time

[colingreene]

You cant seriously be arguing that it is a waste to update a 50 year old component.
That would just be stupid.

[PeteKz]

Jeff, following up this thread, I've been away for a few days:

If the charge indicator light goes out when the engine is running, then you do not need the parallel resistor. Now focus your troubleshooting on connections and grounds.

[mysocal911]

Originally Posted by colingreene

You cant seriously be arguing that it is a waste to update a 50 year old component.
That would just be stupid.

I guess you really haven't seen what actually gets done and the quality of workmanship on some "rebuilt" CDIs that get recommended!

1. Same original components:



2. No resoldering of the OEM circuit board for increased reliability:



3. Poor quality installation of so-called "higher quality parts" update:



4. And a paint job with a Torx screw to replace the rivet and just cover the hole (doesn't hold the capacitor like the OEM).

[2276spyder550]

Originally Posted by PeteKz

Jeff, following up this thread, I've been away for a few days:

If the charge indicator light goes out when the engine is running, then you do not need the parallel resistor. Now focus your troubleshooting on connections and grounds.

Thanks Pete,
And yet the car sits as the short Summer moves along.

Now, the low voltage I found at the CDI power wire (red on 3 Prong plug) is showing no voltage.
Checked Points Dwell at top dead center and there was no gap. I adjusted to spec
Next is to check if accessories are working with ignition on...thinking perhaps I have a bad ignition switch. Discouraged...

[colingreene]

Originally Posted by mysocal911

I guess you really haven't seen what actually gets done and the quality of workmanship on some "rebuilt" CDIs that get recommended!

I would argue with you if I thought it was worth my time but I know it is not.

having had Bob at Ashlock repair a bunch of these i am confident in my recommendation.
I also repair vintage cars professionally, do you?

[mysocal911]

Originally Posted by colingreene

I would argue with you if I thought it was worth my time but I know it is not.

having had Bob at Ashlock repair a bunch of these i am confident in my recommendation.
I also repair vintage cars professionally, do you?

You do carry a spare, right?

[PeteKz]

Jeff, try jumping 12V from another power source in the engine bay directly to the power wire to the CDI. See if it runs correctly like that. Then work your way back to why you are not getting 12V there.