What I learned from not one but TWO CDI failures

[mysocal911]

Originally Posted by Sunroof

I have been advised that if and when the CDI (three pin) CDI unit get very hot, they will shut down so engine compartment temps play a roll. I have had several shut downs over the years with my 1973.5T/CIS. What was so amazing was that once the unit cooled down the car would run well again. Fact or fiction? I do not know what internal components in the CDI would fail in particular from heat and I would like to know. you have three components that work together; the coil, CDI unit and wire harness that connects the CDI to the distributor/coil.

Maybe someone can explain the heat issue better..............please.

Bob
73.5T

The temperature failure described above represents less than 5% of the Bosch CDI failures,
both for the 3 & 6 pins. That failure affects the anode voltage rise time of the SCR causing it
to conduct while the energy storage capacitor is being charged.

[mysocal911]

Originally Posted by Jonny H

A clonking suspension indicates worn bushes, Replacing the bushes shows up worn dampers etc and so it goes on. Same with the electronics.

Poor analogy!

Originally Posted by Jonny H

Unless you replace all the parts, something is gonna go pop again.

A naive statement! So you're suggesting that ANY component failure in ANY electronic circuit
board requires replacement of all the components, right? Please! Bosch now provides rebuilt
electronics for the pre-2000 Porsches, and they are NOT replacing all the components.

Originally Posted by Jonny H

When the Bosch CDI was new, it was designed to work with the other parts of the ignition when those parts were also new. There was no requirement to protect the box against short circuited coils, failed voltage regulators or over zealous battery boosters (they didn't even exist!). These represent new failure modes that did not exist 'in period'.

Another naive statement! Today the Bosch Porsche DME ECM, e.g. 2015, is NOT designed
to protect for all possible failures, e.g. the COP drivers for battery overvoltage or reverse polarity.
It's all a matter of cost and the probability of that failure. That's basically the same design strategy
used by Bosch when they designed their CDI units. Any system design is approached the same way.

[dicklague]

Jonny, you should know by now that the old Bosch CDI is just fine!!!!


I personally want to get in my car and DRIVE..... MY Bosch CDI is just fine sitting in a drawer under my workbench. I have Pertronix hall effect system for points and a modern reliable CDI with an "E" coil to drive the plugs and it all runs fine and has never stranded me in 10 years.

AND if I had your system it would cosmetically look like a Bosch.

[and no red box for me]

[upwardr]

So dumb question, how do you know when the CDI is not working correctly. My car ran fine when parked last fall but now refuses to run. It kind of fludders runs for 5 seconds backfires and will not rev up. The coil seems to spark when the ign wire is retracted.
Could this indicate a CDI failure, haven't had this touched in my 30 yrs of ownership.

[Jonny H]

http://youtu.be/BUl6PooveJE

[mysocal911]

Originally Posted by upwardr

So dumb question, how do you know when the CDI is not working correctly. My car ran fine when parked last fall but now refuses to run. It kind of fludders runs for 5 seconds backfires and will not rev up. The coil seems to spark when the ign wire is retracted.
Could this indicate a CDI failure, haven't had this touched in my 30 yrs of ownership.

1. Just because the engine backfires & fails to rev, it doesn't necessarily indicate a bad CDI.
2. First check that the ignition coil (hopefully not a Bosch silver) wire when placed
about 10mm from the engine sheet metal produces a strong blue spark.
3. If the distributor has points, check for their proper setting and clean with emery
cloth.
4. Make sure that a good 12 volts is powering the CDI.
5. Try and borrow another CDI form a friend/Pelicanite.
6. Next, use carb cleaner to determine if you have a very lean mixture (backfiring).
7. Bosch CDIs rarely if ever fail because of non-use.

[Bill Douglas]

We live dangerously. I had no idea these things are prone to failure. I'll carry a spare in frunk.

[zelrik911]

Originally Posted by Bill Douglas

We live dangerously. I had no idea these things are prone to failure. I'll carry a spare in frunk.

Here is a picture from a Forum post about 2 years ago that I saved;



It seems that the 911 with this module ran well ( perhaps not as good as with proper CDI) but good enough to get you out of a spot.

I was going to make a mini loom of 4 wires connecting to the Engine/CDI plug (a 6 pin) following to a common Ignition module like this. That would have got me going & then I would just keep it as a spare in the tool roll.

In the end I just got a MSD StreetFire & all is now good.