Quick CDI Sound Question
 

I’m still trying to sort out a no spark condition on a 1976 911 and I checked terminal 15 for voltage and it looks good but I don’t hear a high pitched noise from the CDI. I’m wondering how hard it is to hear. My next question is whenever the ignition is turned on there is a humming/buzzing sound from some other sort of system that would drown out a lot of sound from the CDI in my opinion. I’m wondering if it’s fuel or oil or what and if that’s normal.

The CDI charging the capacitors is much like a camera flash gun charging. Its not all that loud.

Sounds like your fuel pump is running when the key is on. Some of them can make a bit of a racket. Doesn't mean they're necessarily bad or suspect.

Not entirely sure when the connector that provides ground to the fuel pump (and thus inhibited the pump when the motor wasn't cranking or running) got added to the CIS air meter. Pretty sure my '77 had it. Maybe the '76 doesn't - or it was unplugged or otherwise bypassed?

Try pulling the fuel pump relay or fuse.

I thought that too and replace the relay and no change. Whenever I push bar in the air box I can hear the injectors going. I really think my CDI is bad.

a working Bosch CDI makes a 3.6kHz whine noise. It's a clean sine wave tone. I would not consider this a buzz sound as the tune is much higher. More like the test tone from an old TV station.

If there is no sound coming from the box with the ignition switch ON and you listening with the rear lid up it either doesn't receive power or it needs repair

Ingo

The sound intensity of the power converter inside varies unit to unit. All in the 3.5 - 4KHz range. Sometimes it is pretty subtle, hard to hear if the fuel pump is running at the same time. If in doubt, you can remove, put it in a quiet environment, and apply +12V to the CENTER pin of the connector with the case grounded. If it still does not 'squeal', it definitely will not work in the car. If it does squeal, that's good, but that does not guarantee it will work because the converter is just one part of the device. It could have other problems, the most common being a dead discharge capacitor buried deep inside. These units are also susceptible to reverse-voltage events. Even a momentary reverse battery situation will take them right out.

If the noise you describe is the fuel pump running with the key on, this could easily be loud enough to make it hard to hear the CDI. I could never hear my CDI over the fuel pump.

Replacing the relay doesn't help. Pull the relay temporarily to kill the fuel pump and check if you can hear the CDI then.


Well, yes, that certainly can/does happen.

@GiantRider
Check out this vid of mine. You can clearly hear the CDI:

https://youtu.be/oES4XNNnWYU

This second vid is of the CDI sound (slightly different) after it was having a warm stall issue ~:30

https://youtu.be/_9kAjGGLTgk

Wow thanks for that video that is tremendously helpful. I’m contacting shops to see if they could drop it in another car to test it but I’m also really interested in doing the MSD install

More torque/HP and better long term reliability, right?

Before doing a MSD install check out Classic Retrofit's CDI+ Ignition which has dual spark at all RPMs.

Pelican Parts listing: Classic Retrofit CDI+ Digital CD Ignition Unit

Another big torque/HP gain here too, plus proven long life reliability, right?

That is my understanding but I have not used the product.

That was sarcasm from Loren. Ignore it.

A portable AM radio placed near the CDI will usually detect and amplify the inverter noise from stray radio interference not completely shielded by the case. In the case of my CDI, my hearing has degraded with age and I can no longer hear higher than 13kHz. The AM radio will pick up the higher frequencies and allow me to hear it plainly within my audible range. Fred

650+ customers would say so.

https://www.instagram.com/p/Bz_hPCklto9/?igshid=6vp1dk3xf83u

https://m.youtube.com/watch?feature=youtu.be&v=UtnrLj1pVF4&fbclid=IwAR1X 6oBp1wObnSTDj-WJq9o73ROe1MAPUvsLDa7zW6-mX78R6QKnhx0jE14

Jonny, Well done. I'm only at 151 units sold in 5 years and that being with a more varied market. You must be doing something right despite the criticism from one particular individual. From my testing with my own CDI, and experience, your design should produce more HP than the standard Bosch design whether it be the older 3 pin or the later 8 pin HKZ boxes. Good show! Fred
EDIT; For anyone that thinks it is total energy to the spark gap that makes power, consider this: My CDI on a 200HP hopped up VW engine made 6 more HP than a high energy magneto, with more stable AFR readings across the rpm range.

Just a follow up on the problem being solved. It turns out that the problem was a bad CDI. I sent the CDI box in to Pelican Parts for a rebuild and I am very happy with the service they did. I was told that if the CDI was good that I would be refunded which is nice. The unit came back looking brand new and with a very detailed description of what was done. I was dragging my feet on getting my Porsche running again but I stuck the rebuilt CDI in there and it fired right up!

That is good news. I have my Bosch CDI nicely stored in a box in a cabinet in my shop. My '73 MFI 2.7 really benefited from a switch to a modern CDI box. Much better starting, idle mid-range, upper RPM and a nice soft rev limiter I have set at 6600 RPM. Not sure if your '76 has points,but getting rid of them is a good idea too.

I use a Daytona- Sensors CD-1 with their coil and love it.

Remember the Bosch is a 50 year old design with 45 year old electronic components.