Is my CDI box bad?

[David E. Clark]

The car is a 1973 911E just put back together after a two year rebuild. It ran fine before I started the rebuild - no ignition problems. I did not replace the CDI box but I did clean it up. Now I get no spark. I'm looking for confirmation before I spend a pile of money on a rebuilt CDI or a move to MSD.

The facts:

Battery (Odyssey 1200) shows 12.6 volts.

12.5 volts at Pin B (center pin) on the CDI plug.

Engine turns over strong.

No whining from CDI box.

No spark using "telegraph test."

Coil tests good. Primary resistance is 0.6 ohms. It should be 0.65 +/- 10%



Secondary resistance is 711 ohms. It should be 676 +/- 10%



Pin A (left) to pin D (ground) produces a reading of "OL" - the same as if I do not connect the leads to anything at all. The reading should be ≈ 3,300 ohms.

]

Pin B (center) to pin C (right) produces 34.4 ohms. The reading should be ≈ 100 ohms.



Pin D (ground) to mounting hole produces 0.0 ohms. This is correct.



So do I need a new CDI box? Have I done all the correct tests? What else should I do? Any help appreciated.

David

[TonyGO]

If you happen to need MSD i Have A complete system for good price digitial MSD & msd coil & tach adapter MSD &spark plugs & even msd Coil wire,was going to install in my 79 sc decided to keep stock..thanks TonyG

[David E. Clark]

[E Sully]

I bench tested the coil and CDI of my 1973.5T in the basement. I connected it using 12 awg wire with stacon crimps, a large spring clip to ground the threads of the spark plug, and used a standard wall switch to simulate the points opening and closing.

[Bob Ashlock]

The way described above is a good bench test, similar to what I use. There are a couple of ways these boxes can fail, sometimes spontaneously like you may have seen. First, if it doesn't squeel, something has gone sideways with the dc-dc converter inside. Usually this is the power transistor. Or, if it is squeeling, but triggering it does not produce spark, then the SCR or something in the trigger circuit has failed. I have fixed these for people before, but if you're handy and know a bit of electronics, you can probably do it yourself.

[304065]

David
The box has an inverter circuit that creates an AC, which runs through a transformer to step up the voltage, which then charges the capacitor. The vibration of the windings in this circuit is what causes the whine. No whine= bad circuit. I would send yours to the usual suspects for repair.