Taking CIS wires out of engine harness -alternator ground

[75 911s]

Paging Timmy2 or anyone who may have done this:

I have a bunch of extra plugs and CIS components that I wanted to delete from my harness to clean up the area between my Weber carbs.

I decided to delete those wires carefully without cutting anything, I just like to make it reversible and it's cleaner that way.

My car is a 1975 911s and the thermo-time switch harness was easy to delete. I just removed the yellow from the starter, and the red/black from the sensor at the other end. I'm looking for a plug to plug off the sensor hole in the chain box now.

Then it was onto a triple group of brown/blue/black wires going to the heater blower motor plug, I think the WUR, and the voltage regulator that runs over to the rear fuse panel.

The engine harness 12 pin plug was easy to pop the top off and remove the pins and offending wires:



Then came the the voltage regulator (now mounted on the alternator), heater blower motor, and one other CIS component that are tied into one harness.



These two brown Earths start at the alternator, and then one goes to the heat blower motor plug where it is tied into a blue which leads back to the 14 pin connector.

I want to remove this section of harness without cutting, so I need to remove all of it. Do I need to run a new brown wire from the alternator ground down to the harness over to the 14 pin connector? As if I was just bypassing this group of harnesses?



Will something be looking for the ground in the rear fuse panel at that peg?

[PeteKz]

Those brown wires are not necessary, however, it's good electrical practice to provide ground wires back to a certain common ground point. The alternator is directly grounded to the engine through a braided ground strap under the engine shroud. The engine in turn is grounded to the body by a braided ground strap at the nose of the transmission. Therefore, if you run your brown wires (or replacement brown wires) to a ground point on either the body or engine, you will complete the circuit. You do not need a brown wire through the 14 pin connector (in fact, my 1973 T with an SC engine in it does not have a brown wire pin in the engine harness side of the 14 pin connector)

[75 911s]

Thanks Pete! Huge help. I made this basic schematic before I saw your answer in order to clarify.



It could possibly help clarify for anyone who's thinking of doing the same in the future.

I'll also take a big pic of the actual 3 part harness when I get back to the shop. The three tied in components are the WUR, Heat blower motor, and external voltage regulator, and I completely removed this connected group without cutting anything.

This mod requires updated onboard voltage regulator which most new alternators have *and a full CIS delete / conversion to EFI or Carbs where you don't need a Warm Up Regulator. Typically one would just tie off the black wire at the alternator and not connect it to anything when upgrading to the onboard voltage regulator on early cars where it used to be mounted in the rear fuse panel.

[PeteKz]

Duane, I'd change one thing in your diagram: The alternator ground is to the "engine" rather than the "Chassis". Or at least label it "Alt Grnd to Eng/Chassis."



I've also been thinking about opening up the wiring loom and removing the old wires and plug to the old regulator. But, too many other things on the project list. Maybe I'll get around to sanitizing it later.

BTW, I also went to an internally regulated 175-amp alternator. I had to think about how much more amperage the red wires can carry if/when I put an all-electric AC in my car. As long as you're in there, you should beef up the wire from the alternator to the starter terminal. I went with a fine strand 6 ga wire. I'm concerned that even that is not enough for the additional current this alternator can produce. So I am leaving the other red wires that go back to the relay/fuse panel in place, to provide an additional path for current (like the ground wires, the red wires to other connectors can be eliminated, because the red wire from the alternator to the starter terminal then connects back to the battery cable, which connects to the rest of the electrical system)

[75 911s]

Originally Posted by PeteKz

Duane, I'd change one thing in your diagram: The alternator ground is to the "engine" rather than the "Chassis". Or at least label it "Alt Grnd to Eng/Chassis."

Pete, I updated the graphic, thanks for that.

I only have blue and red going to the alternator. I also have the ground strap to the engine. Nothing is connected to the grnd (D-) on the alternator. As I completely removed the voltage regulator harness, the WUR harness, and the heater blower motor which were all connected.

So you are saying this is how early cars with the upgraded alternator (on board voltage regulator) would be wired?

Side note: I have read about the heat soaked on board regulator, but so far no problems running it on 6 hour drives.

[Gordo2]

Not a direct correlation since I went to Xdi, but some info you may find useful:

Engine Harness / Multi-pin Connector for Carb Conversion '83 SC

Good luck,

[75 911s]

Originally Posted by Gordo2

Not a direct correlation since I went to Xdi, but some info you may find useful:

Engine Harness / Multi-pin Connector for Carb Conversion '83 SC

Good luck,

Gordo,

Thanks for that. BTW your weber tuning thread is the stuff of legends.

The only other issue I found in my research with this setup was in another thread the OP had an issue with the alternator not charging. The problem ended up being the ground cable to the engine/chassis was grounded to a powder coated fan housing, with the powder coat stopping the circuit.

I've grounded directly to the alternator housing so I think I should be good.

As always, anyone trouble shooting this should check the ground cable from the alternator, the ground strap from the transaxle to the chassis, and the battery ground cable. They should be in great condition, not oxidized or loose.