It doesn't Hummm or squeal.....

[fred cook]

I drove my 80 911SC today (as usual) and it ran fine. After I got
home, my wife asked me to run to town to pick something up. When I started the Pcar, it started fine but only idled at about 400 rpm and was really down on power. While I was out I bought some gas and put some STP water remover in the tank thinking I had gotten some rotten gas. On the way home it ran slightly better but was still really down on power. After I got back home, I checked a couple of things (looked for arcing plug wires, etc.) including listening for the CD "squeal". Silence, no squeal, no hummmmm, no nothing! I pulled the plug on the bottom of the CD unit, checked for burned or corroded connectors (all ok) and replaced the plug. Still no squeal or hummm. Is there any other test that I can do to make sure the fault is in the CD unit before dropping the big bucks for a new unit? The only other test that I can think of is to make sure the unit is getting 12V. Any and all suggestions will be appreciated!

Fred Cook
'80 911SC coupe (no squeal, no hummmm)

[fred cook]

Just checked, am getting power to term 15 on the CD unit. Unless someone knows a chant or has some magic smoke (all electrical entities contain magic smoke. If the magic smoke ever gets out of the wires, it will no longer function!) it looks like it time for a new CD unit. Which is really NOT the way I wanted to spend $350!

Anyone have a good spare they would like to part with?

Fred Cook
'80 911SC coupe (no squeal, no hummmm)

[Early_S_Man]

Fred,

It is a Bosch CDI unit, and not a Permatune, right?

Failure modes for the Bosch unit were discussed in the following thread:

What goes wrong with OEM CDI units?

If your Bosch CDI just quit whining suddenly, then it is probably a relatively easy fix ... replacement of the big 2N3055 power transistor in the DC-to-DC inverter circuit.

There are several places that repair Bosch units, or sell replacements on an exchange basis.

[fred cook]

Warren,

Thanks for the reply. Yep, pure ole Bosch CD unit! And near as I can tell, it just "quit" after driving home last night. I believe the CD unit and the coil are the originals so they have had a long life (nearly 24 years) but have gone only 106,000 miles.

Do you know what resistance values the coil should show? If I had that information, I would be able to test the coil for a shorted situation. While I don't want to put a new box on an old coil, I don't like replacing parts that have not been diagnosed as having failed.

Fred Cook
'80 911SC coupe (no squeal, no hummm)

[Early_S_Man]

Fred,

The primary should be 0.4 - 0.6 Ohms, and the secondary 650 -790 Ohms at room temperatue. Don't worry about the primary reading so much, because many inexpensive DMMs don't measure resistance values under 1.0 Ohm reliably ... the secondary is what fails, and it causes the coil to overheat and leak oil/tar ...

If you are familiar with what a TO-3 power transistor is ... and don't mind taking a few pics, and keeping good notes ... and unsoldering about 20 wires to get to the power transistor ... replacing the $1.59 2N3055 transistor is a simple fix!

[fred cook]

Warren,

Just checked the coil. The primary resistance came in at .7 ohms and the secondary at 698 ohms. No sign of leakage or heat stress on the coil (probably the original Bosch unit). So....now it looks like time to make a decision on the CD unit. "To Bosch or Not to Bosch, that is the question!" Decisions, decisions!

Fred Cook
'80 911SC coupe (no squeal, no hummm)