CDI issue and how to test

[speedo]

Needed a jump today for my 74 911 as it was down on battery voltage. I have a small drain in the system I need to diagnose. Jumped it and limped it back into town. It started running erratically then lost power and died. Charging the battery now...but am wondering if coincidentally my CDI might have died. Don't remember the "whine" in the engine compartment while trying to jump it. I realize the "whine" is a good way to know if it is happy. I have a box of 3 pin CDIs. If the "whine" for my existing CDI has gone away, is there a way to bench test my other CDIs or is the best way to test...to just install in the car?

Also, on the battery drain issue...if I remember correctly when the car is running, measuring the voltage across the battery terminals, should be 14 volts. Less than 14 the alternator has issues. More than 14 volts and the voltage regulator has issues? Am I remembering correctly?

[speedo]

Charged the battery for an hour (not even to full capacity) and keyed the ignition to listen for the "whine" and really couldn't hear it. I here the fuel pump pressurizing the FI, but do determined "whine" from the CDI. I then turned it over and it fired right up. Left it idling and listened for the "whine" while running and couldn't hear it.
As CDI's fail...do they start to cycle? Works then doesn't and repeat?

Left the battery to fully charge overnight. I'd like to put an extra CDI (known to be good) in my tool bag as a backup. Unless there is an easy CDI test...I guess I will swap them into the car to see if they work. Car starts, they work, car doesn't start, they don't.

Wondering also if my coil could be the culprit...?

[garment]

Can you ground a spark plug to the engine case and look for a spark? When my CDI died I used this test to diagnose.

[speedo]

Yeah, I read that you pull the center wire off the dizzy and attach it to a spark plug and then ground it to the engine case and have someone turn the engine over to see if you have spark? Does that sound correct?

[speedo]

Originally Posted by garment

Can you ground a spark plug to the engine case and look for a spark? When my CDI died I used this test to diagnose.

When they die, do they die slowly, or cycle?

[Jonny H]

Originally Posted by speedo

When they die, do they die slowly, or cycle?

Common failure modes:

1)No whine. Internal power supply is dead. Hard fault.

2) Input circuit failure / 'drift'. The distributor signal is not 'seen' by the box so no spark is produced. Can be a hard fault or can also be heat dependent.

3) Fractured pin connection (6 pin units). The fragile pin connections inside the box have cracked. Hard fault or intermittent fault.

[Jonny H]

Originally Posted by speedo

Yeah, I read that you pull the center wire off the dizzy and attach it to a spark plug and then ground it to the engine case and have someone turn the engine over to see if you have spark? Does that sound correct?

Kind of. The spark is produced when the wire is removed from ground. If you lightly 'brush' the wire on a ground, you will generate a train of pulses so should get a few sparks.

To eliminate the distributor from your 'enquiries', remove the king lead from the coil end and directly push a spare spark plug into the coil, then use a jump lead to ground the spark plug body to the engine case. This is a bit easier/safer than having spark plugs 'loose' on leads.

[WMontald]

To create a spark, The CDI is equipped with a Condenser to store the energy before it is released to the coil. Most of the time it is the condenser that fails after years of use or Moisture.
When it fails, it is directly, No spark anymore.
You can replace it yourself, but demands some soldering to get access to it and resoldering to connect everything again. My Father did this for me as he uis used to repair electronic stuff, with diagrams found on the pelican forums. I believe I have a manual about the CDI unit on a HD somewhere. I can dig it up if needed.

[kenzinger]

My CDI failure was intermittent. I think it was related to engine compartment temperatures, as it would only happen after idling for a bit or sitting in traffic.

[mysocal911]

Originally Posted by kenzinger

My CDI failure was intermittent. I think it was related to engine compartment temperatures, as it would only happen after idling for a bit or sitting in traffic.

Typically the .01/630V dv/dt capacitor is bad.

[speedo]

Originally Posted by mysocal911

Typically the .01/630V dv/dt capacitor is bad.

Is this difficult to source or replace? Why would it be temperature sensitive?

[garment]

Bob Ashlock will repair it to “better than new” status. Look up Ashlock tech. No affiliation, he sold me a banger replacement with a great guarantee.

[mysocal911]

Originally Posted by speedo

Is this difficult to source or replace? Why would it be temperature sensitive?

It fails and the SCR, under a hot temperature, doesn't turn-off preventing an output to the ignition coil.

[mysocal911]

Here's an extensive thread on Porsche CDIs; History of Bosch CDI ... toubleshooting info, parts list changes, and schematics ...

As one reads the many threads on CDIs, one learns that there're a few reliable sources for having Porsche CDIs rebuilt, with a 5 year warranty.
Do a Pelican Parts search!

[manbridge 74]

Mine failed in 2018. Started easily but started cutting out after fifteen minutes. Higher revs made it worse as it’s working harder. Easy to test your other ones by plugging one in and laying it on a towel by bank one cylinders. I run an additional jumper to ground case, quick and easiest way...

[Nditiz1]

Bob Ashlock repaired/refurb mine to better than new. It would cut out after 15 mins of idling in the garage. Issue went away after Bob sent it back. I thought I had another issue with it when my car started cutting out while driving on the highway, but it ended up being the green wire.