no spark on 79 911

i dont have any spark i thought it might be the cdi box but it makes a wineing noise i got power going in when i turn the engine over but nothing coming out should i replace it or not ??????? i think they are big bucks

To test to see if it is indeed the CD unit, you can bypass it to test if it is the dead link in your engine. Usually, CD units are "in between" the coil and distributor in the electrical circuit. You can (as I did when my CD unit died) bypass the circuit by shorting out the leads so the CD unit is not used - this will lead to rough running in most cases, but it will run. This will tell you if your CD unit is bad. I am sorry this is so vague, as I don't know what your wires to the coil look like - otherwise I'd be more exact.

In general, you will have a power input cord to the positive low tension lead on the coil, & you'll have a wire running from the distributor to the coil's low tension negative lead. That is all one needs to run an engine, and the CD unit is in between - the positive runs to the CD then to the (+) low tension lead, and on the distributor the wire goes to the CD unit, then to the (-) low tension lead on the coil. Can you see how it's "in between"? Anyhow, a quick bypass will tell you what's up! My CD unit went bad even though I had that high pitched whine - just one transistor went out while the other one still worked. I hope this helps you figure out your problem.

If the unit is making the whining noise, chances are that it is good. It is not a guarantee, but the odds are greatly in your favor.

One thing that if VERY OFTEN overlooked is the connection between the distributor and the CD unit. Look at the wire that runs between the two. About halfway between the two, sitting kind of hidden by the cam oiling line, is a 2 prong connector. Disconnect the connector and clean up the contacts as best you can with contact cleaner, WD40, etc. You can even get a small piece of 300 or so grit sandpaper in there.

Try this out and let us know what happens!

Brian

I just took a cd unit apart. In this box, the SCR trigger diode had shorted and actually smoked part of a trace. The oscillator circuit was still intact and operational, but without the SCR trigger, it could not work. I'll let you know how I make out when I replace the diode.