Quote:
Originally Posted by Lorenfb
"As everyone already knows the CDI is highly subject to alternator voltage spikes, anything on the "12 volt" supply above ~15 volts and the SCR begins to "self-fire" (tach jumping, etc.). This looks like it might be an input voltage "snubber" to "sink" any incoming voltages above...12 volt zener plus 2 diode drops....~13.2 volts"
Actually, both statements are incorrect.
1. Neither the early (3 pin) nor the late (6 pin) are "highly subject" the alternator
spikes. The early unit can operate with spikes less that about 22 volts. The later
unit has a regulator circuit which regulates from about 7-8 volts to over 20 volts.
As a result, neither the early nor the late CDI will "self-fire" based on the triggering
circuitry used on both. CDI failure usually occurs when an alt continuously over-charges.
The tach jumping as occurs on the 911SC results from the tach itself being sensitive
to over-voltage, as the CDI does not cause mis-fires nor poor running when over-voltage
occurs.
2. The Mickey Mouse components attached were most likely used to replace the
failed zener diode which protects the power transistor, i.e. As previously posted
one is a 1N1373A (82 volts). That was NOT used to snub the possible input spikes.
Just review the early CDI component layout and this will be apparent.
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Wow...Loren...??!!
You've been working, repairing Porsche CDI boxes all these years(***) and still have little idea as to their common failure mode/cause...?
*** My 78 SC Targa about 2 years ago.
Alternator proved to be faulty, stator windings intermittently shorting, resulting in high voltage spiking upon "un-short" of a stator winding. Installed new alternator but the tach still "jumped" after the engine was run for long enough for things to heat up. Sprayed CDI housing with cooling spray and tach would stop jumping until the CDI (SCR) heated up again.
CDI's SCR failed due to being run too long in over-voltage state.
During the drive from Seattle to ID the tach would start jumping and shortly thereafter the engine would quit. Limped the last 100 miles by alternately running with the voltage regulator connector unplugged, until battery got too low, plug in the regulator until the battery voltage got too high, etc, etc.