CDI Eval - Capacitor Discharge Ignition Options

[DennisV]

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

We don't know the history of the engine. I do know that it hasn't been in a car for over 10 years. We're going to start tearing 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 forum member mentioned Ashlocktech does CDI restorations, but I will spend the $350 elsewhere it's not needed for this.

Thanks for any experience you can share.

P.S. For these 914-6 / 911T engine topics, I never quite know whether to post them here or over on a 911 board like Rennlist. Is it bad form to cross-post? (IMG:style_emoticons/default/confused24.gif)

[targa72e]

Matter of risk aversion. Whole car is 50 years old. Any part could fail at any time. Electronic parts that leave you stranded on side or road probably more likely (IMG:style_emoticons/default/sad.gif)
Evaluate based on your comfort level.

That said, I have not had to tow home one of my old cars in the last 15 years. I always worry about taking them on long drives.

My daily drivers for the last 15 years have been new BMW's. Every one of them has been towed or limped to dealer. For some reason I don't hesitate to jump in them for a long drive (partially do to free road side assistance).
Might just be that I know every part I have touched on old cars that could possibly come back to bite me. New cars never touched by me are assumed to be perfect (IMG:style_emoticons/default/smile.gif)



PS: planning on driving 73 -6 conversion 1000 Miles round trip to red rocks classic. planing on carrying plenty of spares and travailing with a friend (they are planning on driving a car that's only 40 years old, not as much worry about (IMG:style_emoticons/default/lol-2.gif) We figure with two cars one of use can drive the other to get parts if needed (IMG:style_emoticons/default/smile.gif) )

john

[Superhawk996]

New cars never touched by me are assumed to be perfect (IMG:style_emoticons/default/smile.gif)

(IMG:style_emoticons/default/av-943.gif) Just saw a newish Jeep Gladiator going on the flatbed yesterday - blocking traffic of course.

As you've noted, new cars don't guarantee that they will never be on the back of a flatbed.

Quite the opposite. A failed crankshaft sensor will leave you walking without warning. They go from OK to Not OK in a fraction of a second. I use that as only one example of where modern electronics can fail and leave you stranded.

Old (pre-80's) cars on the other hand, I can usually jury rig a solution to get me home.

A few examples:

Broken throttle cable -- replaced by using shoe laces tied together to pull the throttle linkage by hand.

Broken clutch cable -- creative shifting to avoid stopping. Having to start the car in 1st gear at the few lights I couldn't time or get though on a slow roll. Not good for the starter or battery but got me there.

Distributor center cap with failed carbon button. This was the only instance I've ever had on a carburated car where it just quit running without warning. Solution: Jammed some copper wire into the button pocket just about long enough to touch the rotor. It was misfiring bad but got me there.

Don't get me wrong, modern cars have a lot of advantages but jury rigging a fix to get you home is a whole lot harder than it used to be.

Moral of the story: Go drive that 914 on long trips!

To answer OP's question. VIntage CDI not easily tested DIY. Due to components inside the box (vintage switching components SCR/thyristor/transistors, and aging capicitors) CDI is subject to instantaneous failure just like any electronics on modern vehicles. Even if it tests OK today, a vintage component inside the box may fail tomorrow. If it were me, I'd replace the 50 year old part, putting it aside for someone concerned about originality, and replace with a more modern CDI that doesn't have 50 years of age / use on it. Modern electronic components like SCR's/Thyristors/Transistors/MOSFET's are generally more robust & durable than vintage components that perform the same function.

Here is a simplified schematic of what a CDI is (this is a motorcycle application with the stator supplying excitation to charge the capacitor)


If this diagram has no meaning to you - move on. DIY testing is not for you.

If you want more info on actual Bosch CDI start at this link for more info.
http://forums.pelicanparts.com/porsche-911...schematics.html