CDI Eval - Capacitor Discharge Ignition
 

How fragile are the CDI units from 914-6 / 911T engines? Is there a way for the DIY mechanic to evaluate them out of the car?

We have a 914-6 engine and don't know the history of it. I do know that it hasn't been in a car for over 10 years. We're going to start tearing down the engine for a complete rebuild in a couple weeks, and now would be the time to do something with the CDI (among other things) if warranted.

A 914World forum member mentioned Ashlocktech does CDI restorations. I don't mind paying people for good work, but I don't want to spend money in places it's not needed.

I guess I'm asking if this is a IF it will die or a WHEN it will die component.

Thanks for any experience you can share.

P.S. I'm cross-posting this topic on some different forums, given the 914-6 / 911T cross-over and that this seems to be a bit of an obscure topic. I hope this doesn't offend anyone.

Hi, the Bosch CDI's are, in some cases, are more than 50 years old and typically will die at some point. But I get exactly what you are saying about not spending $ unless necessary. The majority of the units I service are dead, but I do also get the occasional one that is still alive and kicking. The problem with the earlier units, ('69 thru '71) is that they had a power component poorly mounted and so that device (voltage regulator diode) blows out from overheating and then the unit runs at about 100V over it's intended operating voltage of ~400V. This stresses other components and eventually wipes those out. That being said, I've seen plenty of them operating at 500V and not dead yet, so you never know when that is going to happen. When I rebuild these early units, I modify the mounting of the power components to eliminate this problem and I always upgrade the discharge capacitor as the original one will eventually fail. (accounts for a good 75% of the failures I see). For now, I can provide some simple troubleshooting/bench test instructions if you simply want to see if the unit is operating and then decide from there if you want to move forward with having it rebuilt. Just email me, bob@ashlocktech.com --Bob

Bob, you very generously offered free testing for me on my SC CDI box. Is that a service you still offer?

Actually, the early Bosch CDIs are very reliable and rarely if ever fail, unless the battery is reverse-jumped or a high voltage charger is used to start the engine.
CDI rebuilding sources are available providing quality units for less than $300 with a 5 year warranty, if the CDI ever fails. Avoid wasting money on rebuilding a functional CDI.

Avoid rebuilt CDIs with non-Bosch parts;

http://forums.pelicanparts.com/uploa...1661011432.jpg

Dave - one vendor advertises rebuilding the CD with better parts, maybe replacing the SCR or the like. Bosch electronics seem to me to be built like a tank, generally - look at a O2 sensor CIS box, for instance - or the original you show. But 50 years of the progress of electronics and solid state stuff seems like some parts are, indeed, better if there should be a failure.

How would you know if your CDI is dying?
Are there symptoms apparent which would be a clue that it is time to replace the unit?
Is there apparent symptoms while driving the car that would lead one to believe the CDI was dying? ( I am assuming that if one is driving and the CDI unit is faulty or dies? )

The reason I ask is, I have observed that the "whine" is slowly getting lower and lower with one of my cars. In other words, there was a time when I would be in the driver's seat with windows up and I could hear the sound of the CDI when the key was turned to the "ON" position (ignition switch).
But, as of late, for me to hear anything I have to turn the key to "ON" position and then walk to the back of the car to hear the high-pitched sound of the CDI.

Appreciate your thoughts on this "dumb" question.