911SC EFI Conversion - CDI Switched Power Source Question

[drabels]

Hi guys,

Calling all electronics/wiring gurus familiar with the SC!

So, I'm in the middle of doing an EFI conversion for my SC (1981 Euro.)

I have done a lot of work so far, including many, many hours going over wiring diagrams. I'm setting this up first on a test bench and it's coming along nicely.

The question I have relates to switched power sources in the SC.

From what I can see in the engine bay, there is a very convenient switched (15) power output that goes to the CDI unit (two pin connector.) Since I'm going CoP ignition and the CDI is being deleted, I thought I could use this to control a relay that in turn powers the main ECU.

The only issue I have with using this, is that this power source appears to be unfused! In my mind, all loads should be be fused, no?

Now to be clear, I have access to the current flow diagrams for many model years of SC. From what I can determine, the switched power (that is, "15" from the ignition switch) provides fused power to all manner of devices in the SC.

But when it gets to the back of the car, it simply powers the CDI unit directly. Can this really be true, or am I misinterpreting something in the diagrams?

As a reference point, refer to the 1982 diagrams available here:

http://classickabelboomcompany.com/wiring-diagram-porsche/

Obviously for my build, I can simply add a fuse before I power the ECU, but I'm trying to figure out if I'm overlooking something. I'd rather not add it if I don't need it.

Regards,

Daniel

[scottrx7tt]

You already have 3 fused circuits back there. Chances are if you Are going EFI, you probably aren’t running the heater blower. Why not just use that circuit?

[drabels]

Originally Posted by scottrx7tt

You already have 3 fused circuits back there. Chances are if you Are going EFI, you probably aren’t running the heater blower. Why not just use that circuit?

Hi Scott,

Yes, it's true - I've elminated a number of things back there. However, I am also removing the stock bullet fuse holder and adding my own to support more circuits (for O2, injectors, ignition, etc.)

To re-add another fuse holder for switched power is obviously not an insurmountable task.

From what I can see in the 82' diagram, the same current track feeds fuse 15 (fuel pump), fuse 22 (heater ventilator relay), fuse 12 (rear window defogger) and fuse 11 for the turn signal relay and backup light switch.

So I'm just puzzled as to why then CDI would be fed power without a fuse!? A number of the instruments up front appear to have the same wiring. I would love to know the rational behind this... Or maybe it's just what it appears to be.

Perhaps I've answered my own question. Thanks.

Daniel

[Jonny H]

^ correct, CDI box is not fused on any factory 911. Baffles me too!

[scottrx7tt]

I actually have an aftermarket fuse panel in the back that holds 6 circuits. I use one of the switched unused circuits from the original 3 gang panel to a relay and ran a hot wire from the starter motor to power the panel.

[drabels]

Originally Posted by Jonny H

^ correct, CDI box is not fused on any factory 911. Baffles me too!

Alright! This is what I wanted to hear, as it confirms that a/ I know how to read the diagrams right and b/ it is indeed a "strange" design decision.

One gets the feeling that as the induction/ignition system changed over time, they struggled somewhat to modify the loom in a semi-sane manner.

See for the example the comments in the workshop manual under how to read a current flow diagrams. Increased complexity + component counts = hard to read diagrams. Also, I noticed that with the introduction of CIS in 1973, they made compromises in wiring design "for production reasons" (page 773.)

I just can't wrap my mind around the fact that something like a CDI box, with all these high voltages surrounding it (never mind the energy stored in the capacitor) does not even have a fuse behind it.

Oh well, never mind... Another fuse it is!

[drabels]

Originally Posted by scottrx7tt

I actually have an aftermarket fuse panel in the back that holds 6 circuits. I use one of the switched unused circuits from the original 3 gang panel to a relay and ran a hot wire from the starter motor to power the panel.

Okay, thanks for that. I was thinking of tapping into pin 14 on the 14 pin connector as a unswitched power source, but maybe what you've done there is a better approach. I guess you can run a thicker gauge wire?

[Fred Winterburn]

Actually it was very common on many old cars of most makes not to fuse the ignition or the headlights. Some things you want to work reliably without a shaky fuse failing, leaving you in the dark or the engine cutting out at the worst possible moment. It was a different time and a different philosophy but when you look at the those german fuses it makes a lot of sense to me! Fred

Originally Posted by Jonny H

^ correct, CDI box is not fused on any factory 911. Baffles me too!

[scottrx7tt]

I believe I have ether 10 or 12 gauge wire running over to the new 6 gang panel.

Originally Posted by drabels

Okay, thanks for that. I was thinking of tapping into pin 14 on the 14 pin connector as a unswitched power source, but maybe what you've done there is a better approach. I guess you can run a thicker gauge wire?

[jpnovak]

I always use one of the 2X switched segments on the 3 fuse holder to trigger a relay and support an external fuse/power distribution block for all my EFI installs. This removes all electrical load for the additional circuits from the factory wiring. Power for fuse distribution block is an 8G wire from starter lug.

Keep them all fused and separate. No need to cut into 14 pin connector or harness unless you are pulling that out.

[scottrx7tt]

I just checked mine. The wiring from the starter to the new panel is 10 gauge.

Originally Posted by scottrx7tt

I actually have an aftermarket fuse panel in the back that holds 6 circuits. I use one of the switched unused circuits from the original 3 gang panel to a relay and ran a hot wire from the starter motor to power the panel.

[drabels]

Cheers for all the feedback guys.

I recently dug into the engine harness and carefully removed the CIS parts no longer required.

Today I opened the harness that goes into the main 14-pin connector and some more secrets were revealed. You've all given me a few ideas to explore.

Thanks again,

Daniel