CDI igntion - proper C8 capacitor replacement part?

[HFR_Racer]

My CDI finally did go bad. I installed another CDI to confirm and the car started with no issues.

The symptoms of my bad CDI are as follows:

I drove my 930 on the highway approximately 1 hour at 70 mph with no issues. Outside temperature was 80 degrees. At the end of my trip the engine started to sputter very slightly and my engine rpm slowed. I was able to continue driving at low rpm for several minutes as the engine continued to sputter and bit. When I pressed the accelerator pedal the car felt like it was going to die as if the ignition was falling on its face ( not a powerful enough spark to allow the engine to accelerate ). Finally the CDI died and I could not get any spark.

IMPORTANT NOTE: The CDI still whines, but no spark.

I've completed the testing of the C8 capacitor and it is bad.

I am wondering if someone can recommend a replacement part # and supplier for the C8 ccapacitor. Should the new capacitor be electrolytic, mylar, etc?

Based on the failure mode that I stated above is it likely that only the C8 failed or other components as well?

[Joe Bob]

Email Ingo, 3.6@cox.net

[wwest]

Testing of capacitors is NOT an exact science.

Assuming C8 is THE spark power storage capacitor a ~1 MFD 400 volt rated mylar film would be best.

But in the meantime I'll be betting you have a failed/failing alternator/regulator. Otherwise a high resistance ("High" in this case meaning resistance of a few hundred milliohms) somewhere in the battery charging circuit. If there is a next time, engine sputter, turn the headlights on and see if the engine smooths out.

[RoninLB]

weak CDI ground can cause all kinds of issues especially if it's not directly to battery imho

seems like many guys just assume that if the CDI + is good the existing - is good enough

[HawgRyder]

When I went through mine to check it out, I replaced all the capacitors with top of the line gold seal units.
All capacitors will dry out or leak after a time...so replace them while you are in there.
Also...get a bigger SCR to replace the 6 AMP unit ...I used a 40 AMP one that was the same physical size but higher Amps and higher voltage rated.
Bob

[HFR_Racer]

Thanks for the quick response. Ground to my CDI is solid with virtually no resistance. My voltage regulator is relatively new and is putting out correct voltage to the CDI box. Ingo stated that a bad voltage regulator ( high voltage to a CDI ) will fry it

Your correct.. Capacitors are only good for a finite amount of time. I agree that replacing all of the capacitors is a great idea.

Why the need for a bigger SCR? Why not replace it with the same size as the OEM paart?

[HawgRyder]

I have to confess....I'm one of types that only ever wants to fix things once.
I hate assembly lines of the same thing day after day...LOL
So...bigger SCR...yup...one so big and robust that the world can melt before the SCR breaks down.
And...I had one...was surprised it fit in the same location...but it did...and the heat and current capability was fantastic.
I also used better quality resistors when replacing...and the smaller transistors inside got replaced for the same reason...but these were fitted with heat sinks to give longer life and run cooler.
I always over-build...use better bolts...stronger parts...etc.
I feel , if I drive down the road in my baby...the last thing I need to consider is her letting me down.
Remember....all machinery is female....treat them like a fine woman and they will work for you forever...but...disrespect them just once....and all bets are off...LOL.
Bob

[Lorenfb]

"But in the meantime I'll be betting you have a failed/failing alternator/regulator."

Generally not the cause of the mentioned failure, i.e. If one reviews how the Bosch CDI is designed.

[wwest]

Originally Posted by HawgRyder

I have to confess....I'm one of types that only ever wants to fix things once.
I hate assembly lines of the same thing day after day...LOL
So...bigger SCR...yup...one so big and robust that the world can melt before the SCR breaks down.
And...I had one...was surprised it fit in the same location...but it did...and the heat and current capability was fantastic.
I also used better quality resistors when replacing...and the smaller transistors inside got replaced for the same reason...but these were fitted with heat sinks to give longer life and run cooler.
I always over-build...use better bolts...stronger parts...etc.
I feel , if I drive down the road in my baby...the last thing I need to consider is her letting me down.
Remember....all machinery is female....treat them like a fine woman and they will work for you forever...but...disrespect them just once....and all bets are off...LOL.
Bob

Does size really matter..?

For SCRs, NO.

The MAIN problem with the OEM CDI is due to the SCR's high voltage "self-firing" susceptibility. Like most solid state devices of this type that self-firing voltage will get lower and lower as the device temperature rises.

A second susceptibility of SCRs is due to the applied voltage risetime. A really sharp, FAST, voltage rising, leading edge, and the SCR's internal capacitance will couple enough power over to the "gate" and thereby self-fire the SCR.

But back to the main problem. Your CDI uses a DC-DC inverter to raise the 12 volt input to something in the range of 300-400 volts. "The" capacitor is rated to a high enough voltage and energy storage capability for the job at hand, as is the SCR.

Here's the "rub".

The OEM CDI's internal DC-DC inverter is unregulated, the higher the 12 volt input rises the higher the capacitor charge will become. The CDI MUST work with the lowest possible, but reasonable system voltage. That might mean the CDI must remain fully functional during engine cranking, <10 volts. Minimum plug firing voltage is a 250 volt charge to "the" capacitor. That would mean a "normal" running/charging rate with ~14 volts would result in ~310 volt charge on "the" capacitor and the SCR anode.

Raise the engine area system voltage above 16 volts due to a poor F/R electrical connection or a failed/failing regulator and the SCR anode voltage rises to 400 volts and the 400 volt rated SCR begins to self-fire.

Add a POWER zener diode rated at say 14.5 volts and a low resistance, 0.5 ohms, high current resistor in line with the 12 volt "feed" to the CDI box and the OEM CDI should not again fail.

[wwest]

Originally Posted by HFR_Racer

Thanks for the quick response. Ground to my CDI is solid with virtually no resistance. My voltage regulator is relatively new and is putting out correct voltage to the CDI box. Ingo stated that a bad voltage regulator ( high voltage to a CDI ) will fry it

Your correct.. Capacitors are only good for a finite amount of time. I agree that replacing all of the capacitors is a great idea.

Why the need for a bigger SCR? Why not replace it with the same size as the OEM paart?

The only advantage to a BIGGER SCR is the extra heat handling capability. The more important SCR parameters are maximum anode voltage and DV/DT rating, tolerance of the SCR to fast rising anode voltages.

"..My voltage regulator is relatively new.."

High voltages are not always due to failed or failing regulators.

Poor circuit resistance (in the milliohms range) from the regulator to the battery, can result in HIGH voltages in the engine compartment with a perfectly normal battery terminal voltage.

Your alternator has 3-phase stator windings. A short, even an intermittent short, of a single winding phase, diode say, and the regulator/alternator system will still appear to be fully functional. But look at the ripple content of the system voltage, 13.5 volts average but P-P ripple of ~6 volts.

".. replacing..capacitors..good idea.."

Electrolytic capacitors, yes, YES.

Otherwise, more likely not.

[Lorenfb]

"The MAIN problem with the OEM CDI is due to the SCR's high voltage "self-firing" susceptibility."

Not really! Only when the CDI unit is not designed properly or incorrect components
are used which is not the case for the original Porsche Bosch CDI unit.

"The only advantage to a BIGGER SCR is the extra heat handling capability."

Again, not really! A bigger SCR can handle greater peak currents the result of
the greater energy, i.e. 1/2 x C x V (squared), i.e. an additional marginal of safety.

"The OEM CDI's internal DC-DC inverter is unregulated, the higher the 12 volt input rises the higher the capacitor charge will become."

Again, not totally correct! The Bosch CDI uses a flyback type of inverter which has
an output voltage NOT totally a function of the battery voltage and is limited by
a collector zener diode.

Bottom line: One needs to properly analyze the diagrams posted many times here on Pelican!

[HFR_Racer]

wwest:

You stated above to use a Mylar capacitor for C8. Are you saying to use Electrolytic for C1 ~C7?

If so, can you please elaborate on this subject.

My R6 resistor is toast as well. It tests open.

What are the parts that most commonly fail in the CDI box? Should I be looking at other components since my R6 and C8 have failed?

[wwest]

Do NOT use electrolytics except for capacitance vs space considerations.

Post schematics for any other questions.

[HFR_Racer]

wwest:

Ok... Your saying to use Mylar for all capacitors in the CDI. Use Electrolytic only if I run into a space problem. Correct?

Thanks a bunch... I'm awaiting a reply from Ingo as well. I may not be able to fully diagnose and get this thing running myself.

[wwest]

No, I'm ONLY saying don't use electrolytics except as necessary.

Paper, mylar, silver mica, etc, etc, okay.

[Lorenfb]

"except as necessary"

Like where? Have you not reviewed any of the schematics posted on Pelican?
It appears not by your posts in this thread.

Bottom line: There's no need for any component changes to the Bosch CDI!

[Trackrash]

Your alternator has 3-phase stator windings. A short, even an intermittent short, of a single winding phase, diode say, and the regulator/alternator system will still appear to be fully functional. But look at the ripple content of the system voltage, 13.5 volts average but P-P ripple of ~6 volts.

Will this only show up using a 'scope? What are the symptoms of this?

[wwest]

The alternator housing will get extemely hot due to the current flow in the shorted stator windings and the ability to maintain the battery at a decent SOC will be lacking at night with all the lights on.

Amazing as it may seem the VM might show a normal or even an elevated system voltage under light electrical loads as most VM's only display the peak DC voltage.

[RoninLB]

Originally Posted by Trackrash

Will this only show up using a 'scope? What are the symptoms of this?

i have an auto VM that measures ripple as VAC

meter says anything less than .3 VAC alternator is ok

that's at a high idle with headlights on

[wwest]

Originally Posted by RoninLB

i have an auto VM that measures ripple as VAC

meter says anything less than .3 VAC alternator is ok

that's at a high idle with headlights on

For troubleshooting purposes ripple content reading should be done ONLY in the engine compartment.

Analog VM generally cannot make low AC, ripple readings due to the need for rectification.

DMM should be used.