CDI ignition troubleshooting

[SCCA-racer03]

Picked up a 1973.5 911T. It ran when I got it, now it cranks but no start. Its getting plenty of fuel. No spark at output of coil. Getting correct voltage at CDI unit, but I'm not sure if its working. Also, it has the regular, smaller size coil installed, not the bosch super blue that it about .5" longer. I heard somewhere you are not supposed to use regular coils with this system? Is the CDI box gone?

[SCCA-racer03]

Ohm reading on coil is fine, I even swapped to a super blue coil, no difference

[jluetjen]

Misery loves company. I've been working on a similar issue for some time. You might find some ideas there.

[SCCA-racer03]

ok, well I removed my CDI and I'm gonna switch it with a friends and see if there's any signs of life.

[304065]

Does the CDI whine when you power it up? There's a noticeable high frequency (2Khz) vibration from the inverter section, sounds like a camera's flash unit (same principle too).

What's the coil part number? If you try to use an old-school Kettering coil you will fry stuff. The CDI coil IS smaller than the blue, so you might be in good shape there. Keep the blue on for now, though.

[SCCA-racer03]

I can't hear it whine when the ignition is on, but for about 5 to 7 seconds after the switch is turned off, I do hear the "camera flash" hi pitched noise.

[SCCA-racer03]

coil p/n is 0 221 121 001 then 912 under it

There is no voltage at the coil terminals (1, 15) with the ignition on.

[304065]

0.221.121.001 is the correct coil for an MFI car. Is yours black or silver? If Black, OK.

As far as connecting your voltmeter to the coil terminals, if you want to destroy the CDI unit it is far easier to hit it with a sledgehammer. Although a sledgehammer can't give you a 460 volt shock like the CDI will.



Seriously, don't connect anything to the coil terminals.

So it doesn't whine when you put power to it but it does whine when you shut it off? What color wires are going to it? That's the reverse of what's supposed to happen, makes me think the wires are not correctly connected.

[Early_S_Man]

Please discontinue your 'shotgun' attempts at troubleshooting until you have reviewed and understand the basic principles of CDI operation!!!

Have you ever heard why the probability of a kill ratio for a US Navy ASROC was 2.0 ... as in 1.0 for target, and 1.0 for launching submarine??? Thus far, you are the person on the launch button of the ASROC, a rocket-boosted, nuclear warhead-equipped torpedo ...

The theory of operation and basic testing are described in a Bosch document posted in the following thread:

History of Bosch CDI ... toubleshooting info, parts list changes, and schematics ...

[SCCA-racer03]

Umm....oops

[HarryD]

Do you have the stock bosch unit or an aftermarket unit (permatune, msd etc)?