Capacitor In CDI Box

[sbrinz]

Hi Everyone,
I have a 1984 manual trans 928 that I am trying to get up and running. I have not been getting any spark nor voltage to the fuel pump. I took the CDI box out of the engine bay to examine inside and found a bad capacitor. I took that bad capacitor out of the box in hopes of finding a replacement. Since that time I have stupidly misplaced the capacitor and was hoping to get some help. I remember the numbers .01 and 400 on the original capacitor. My questions are: 1) does anyone have any information about the capacitor I am missing? 2) Where is the best place to find a replacement capacitor for my box?

[JK McDonald]

Hey Sbrinz, Just a suggestion - You should unsolder and test the 2 silver capped Power Transistors and the large SCR to the left of your missing capacitor. Often times you will find that the initial failure of the smaller components within an electronic ignition circuit can be traced back to one of the output control amps. The physical size of a component typically indicates how much work (current) it handles and heat it must dissipate.

One of the problems you may also run into is that the replacement Part Numbers and Spec's for some parts will have been updated and require a good cross reference catalog to locate. It might be a lot simpler to swap your CDI out. I sold a good spare CDI unit about a year ago to another member for $45. Perhaps there is another floating around somewhere....

Good Luck, Michael

[sbrinz]

Thanks for the reply Michael. I've noticed the out of date serial numbers being difficult to find with this CDI box. Good suggestion about a possible swap.

Spencer

[Outbackgeorgia]

Well,
If your numbers are correct it is .01 microfarads and 400 volts. Unfortunately, there are several types of capacitors, most likely yours is a Mylar capacitor. Does it physically look like any of the others? Why did it fail is the real question. Most likely, as Michael said, one of the active components probably has failed, and the capacitor is a resultant failure. My CDI failed with a bad SCR, but it should not be difficult to find a replacement. I just used a similar sized one with the highest voltage rating available, has been working for 20+ years.

[harborman]

You can get components at Digikey, Mouser, Radio Shack and other places. Capacitors usually short out while resistors normally open as a general rule. The dielectric in capacitors can deteriorate with age causing a failure. Another device in the circuit may have caused a failure as well.