79 911 SC CDI Woes

[OsoMoore]

I just finished a fuse panel rebuild, and drove the car about 25 miles.
After work yesterday I took a short 1 mile drive to the post office, and then the car wouldn't start. It's been a warm day, but not too hot.

I did some diagnosis on-site. Here's the rundown:
1) Engine cranks strongly.
2) Fuel pump runs when I lift the air flow sensor plate.
3) Injectors do their soft squeal when I lift the plate.
4) Battery reads 12.5v at the terminal. 12.5v at the fuse panel.
5) With the key in Run, CDI center pin (see picture) reads 11.5 volts, measured from an engine mount bolt. Reads 11.5 measured from the fan strap.
6) With the key in Off, CDI center pin reads 0.32v.

I would appreciate any advice on next steps/testing. Thanks!

[pa911]

Do you have a Bosch CDI ? if so do you hear it whine with key on?

[OsoMoore]

Yes, I do have a Bosch CDI. Normally I expect it to make the whining noise. But it is not doing that anymore. It made the whine properly before the no – start.
http://imgur.com/a/RDpDgSU

[ischmitz]

If it's a stock Bosch 6-pin box it needs to whine it needs help (aka repair). If in doubt you can power it up on the bench (GND and +12V) and you need to hear it making its characteristic sound.

Even though 11.5V isn't great it's not root cause for your issue. The box would whine even around 8V - 10V.

[OsoMoore]

Is the voltage on that bottom center pin the only voltage I should expect to see?
Also have a friend with an SC I could ask to swing by and try his cdi.

[timmy2]

Yes, only one voltage reading to take.
It is the input to the CDI you are measuring.

[OsoMoore]

Found a basic hookup schematic. Feeling sad about this CDI.
I guess I could look inside and see if there's anything obviously dead.

[ischmitz]

One quick tip for bench-testing. The two pins where the brown wires are connected to are interchangeable. To test the box remove it from the car and test with an Ohm meter that there is 0 - 2 Ohm between the two pins and each pin to the chassis of the box. If any of that doesn't check out you found your issue.

Then get a couple of wires and connect +12V to where the red lead goes. And the GND lead can go to either of the pins where the brown wires go or even to the outside chassis of the box. It needs to make a noise.

A non-humming CDI is either a failed internal connection of on of the the GND leads or a damaged PCB due to overvoltage in 99% of the cases. If the PCB is damaged it often has taken out other components internally that need to be replaced.

[RDM]

I chased intermittent high heat no run / no start conditions for about 3 years. A Classic Retrofit CDI+ solved all that. Unfortunately, no CDI whine though.

[Nditiz1]

Send it out to Bob Ashlock. $250 later you will have a brand new looking refurbished better than factory cdi.

[ischmitz]

Just to help and clarify for the original poster:

A standard Bosch CDI box needs to make the sound. If it’s quiet it’s broken. Even with the sound there can be other issues. But the whine is a pre-requisite for a working box.

And it can be repaired. Intermittent heat-related issues or completely dead units can readily be brought back to where provide years of trouble free service. Unusually at a much lower cost than buying aftermarket.

The CDI+ mentioned here is one the the aftermarket choices with completely different internals. That’s why it isn’t making the sound.

[911MANN]

I had my factory Bosch CDI box repaired/rebuilt several years ago when it gave me problems. These folks did a nice job and returned it in less than one week, if I recall correctly. Specialized ECU Repair/ Oakland Park, Florida/ www.ecudoctors.com. Sorry, I can't recall the cost, but I don't think too bad.

[OsoMoore]

Wow, thanks for all the inputs from everyone. I'm going to pull my box and try some testing tonight.
Just to be sure - When putting 12v on the box, I can just use the car battery directly (with appropriate wires and polarization), right?

[ischmitz]

Yes - just keep in mind that you don’t short the wires from the battery. A car battery can deliver incredible currents melting standard wire...

[OsoMoore]

The mystery deepens! Here's a pin numbered pic for number references for my description.

I tested my CDI box with 12 volts on the bottom center pin (5) and ground to the chassis. The box did it's happy whine immediately. So that's awesome, CDI may yet be alive.
However when installed the car carefully plugged in, there is no whine. So pure 12v clearly isn't making it to the pin 5, or the ground isn't making it.
When I tested the plug, I found 12v was on the TOP center pin (2). No voltage on the other pins.
Also I found chassis/pin 2/pin 3 all had 0-2 ohms resistance between them.

So now I have no idea what is going on. The car was running, and then I shut it off, and then the voltage changed pins... somehow...

[ischmitz]

Forget the top center pin (2). You’re just seeing the pull-up resistor in the tachometer. That has no meaning.

Find out why there is no +12V on the bottom center pin of the CDI plug. Check the harness and fuses. I don’t have a schematic in front of me but that +12V comes from the ignition switch through the connector that plugs into the electric panel in the engine bay.

[OsoMoore]

Originally Posted by ischmitz

Forget the top center pin (2). You’re just seeing the pull-up resistor in the tachometer. That has no meaning.

Find out why there is no +12V on the bottom center pin of the CDI plug. Check the harness and fuses. I don’t have a schematic in front of me but that +12V comes from the ignition switch through the connector that plugs into the electric panel in the engine bay.

Thanks ischmitz, I'll track that. I did just rebuild the panel over the previous week. I have the schematic and I'll dig into it. Do you know what the CDI's label is in the schematic? Here's the schematic in case you don't have it on hand.

https://drive.google.com/file/d/10zLlpM7QzYCnEbQatDyevZSZjS-JG6nk/view?usp=sharing

[timmy2]

CDI is N-15 in the schematics.
Top of Fuse 11 feeds it. Wire runs direct from fuse to the “T” connector beside the CDI and then to the fuse panel lowest fuse.

Funny that others are recommending different CDI repair people when Ingo (who is helping you) does CDI repairs..

[OsoMoore]

Got! Fuses 11 and 12 are ganged at the head.
There was a small red wire on 11, in behind a large red wire. The small red wire had shifted free. It already barely reached, so I guess it hadn't had a firm contact.

I crimped an extension on to it (too hard to reach to solder) and now everything works.
Thank you so much everyone for helping me figure it out!

[OsoMoore]

And I spent 5 hours waiting for a AAA tow on Friday night...