Dave & Dennis,

Gentlemen, thank you for your patience in helping me to understand this! The function of this yellow wire has been a real brain teaser for me, but I think I’ve learned quite a bit from working on it.

Rick 1’s comments from the other thread are excellent! When something complicated is restated in several ways, it helps to break through my foggy comprehension. For me, that’s what makes everyone’s contributions to these threads so important!

My, hopefully, last query for you is:

What will happen if I connect pin 1 on the 14 wire connector (located at the backside of the engine compartment fuse block) to ground?

With the engine in the car, a yellow wire from this pin 1 connects to the starter solenoid. Am I correct that this pin 1 should be +12v hot when the ignition is in start position? But (because it’s connected to the starter) becomes the neutral ground for the a/c relay in any other ignition position?

If I run a test lead from the battery negative post to this pin 1 will the a/c relay function as intended? Or would this cause the fire 76FJ warned against?

If it’s safe to test the circuit in this way (maybe with an in-line fuse) I’d be able to see how this works and put the whole subject aside until I get the engine back in the car.

Again, thanks for your help!

yellow wire
 

If I am correctly understanding your question, you want to ground the side of the 14 pin connector that comes from term 86 at the ac relay. I may be incorrect but it seems this would disable the function that cuts out the ac circuit while the engine is cranking, thus taking voltage away from the starter during cranking. I do not remember which side - male or female - of the 14 pin connector comes from the ac relay. If you accidentally ground the side from the starter you will have a dead short when the starter is cranking causing a fire.

I may be missing something but what is the reason driving your intent to modify the factory electrical system. My 82 sc is wired as factory and functions as it should.

As I said, I may have missed something earlier in your post.

Hope this helps.

Dave

Don’t do it (ground the yellow at the 14 pin), your car won’t start. You would have a dead short to ground in the crank position every time you try to start the car!!!
A new through body harness is around $900, you would need one after trying to start the car with the yellow wire grounded.... maybe even a new dash harness that is more$$$

Just leave it as designed. It works.

Dave & Dennis,

There is NO engine, starter, starter solenoid, yellow solenoid wire, or transmission in the car. So, no worries. The car's not going to start no matter what I do! I’ll be putting it back together, as it was, with no changes to any wiring.

Although I did mention the possibility of grounding the a/c relay inside the smuggler's box, I now understand that this would disable the a/c cut-out during cranking and is NOT a good idea.

What I want to do, is a quick experiment to see the circuit ‘in action.’ I want to (just for a minute or two) run an alligator clipped test lead from pin 1 on the 14 wire connector on the back of the engine compartment fuse-block to ground.

If the ignition switch remains in the run or off position, when I do this, it should be a safe test -- I think. Grounding engine compartment pin 1 (temporarily) should provide ground to pin 86 on the a/c relay, through the yellow wire running through the firewall. This should trip the relay. I just want to test this to see if my understanding is correct.

With the (temporary) test lead removed from ground and the ignition in the ‘start/crank’ position, there should be +12v at pin 1 in the engine compartment. In this condition, a/c relay pin 86 will also be +12v hot. This ensures that the relay can’t trip and prevents the a/c from using energy when the engine is starting. I'd like to see this, on my meter, as well.

This is what you guys have taught me and I very much appreciate it! I wanted to see this work, in the car, even with the engine out. That’s why I wanted to set up a test lead to temporarily ground pin 1.

If this is still a bad idea or if my understanding is incorrect, I hope you’ll let me know!

Thanks, as always!

I hesitate to post this because I don't know the '79/CIS electrics in this area.
And I think others may be reluctant to give a thumbs up lest something goes bad.

Some of us initially assumed that the car was intact, so there's that unknown. You would need to verify that any wires that are "always hot" were completely isolated / not touching anything. So personally I don't feel confident in giving you a yes. You'd have to be the judge there. I have another minor concern below.

It would be safer in my mind to just put an ohmmeter from the relay hot side to pin 1 - you should measure 50-200 ohms (the AC relay winding) and whatever that part of the harness wiring resistance is. If you take the powered leap, still do this first because you want to make sure you have the right pins.

Barring any objection from others and in consideration of doing the above, if you choose to proceed, you must ground pin 1 and verifying it with an ohmmeter that it was now grounded at the relay itself ... before putting the key in run.

This would be done to ensure that there was no voltage on the Ylw wires (because it is essentially grounded to 0V) which reduces the risk of something else happening wherever the other yellow wires are if they were to be at +12vdc. This is due of course to many of the yellow wires being common to each other.

In cars with a DME for example (motronic), if the engine were removed and you did this test by applying juice before you simulated a ground on pin 1, the DME (which would be on) would see the +12 on the ylw wire as starting voltage which would trigger the it to turn on the DME relay and thus the Fuel Pump.

Not good (if there's gas in the tank and the lines are disconnected).

I don't know the equivalent FP ckt in the 79 CIS or if the yellow wire is involved.

But that problem should be eliminated if you ground pin 1 first (and pop the FP relay and fuse).

These are the risks as I see them anyway - a loose hot and an un-grounded or hot yellow wire.

I don't know what to tell you, I understand your motivation, but I am not 100% on this, and I figured you deserved some response.

I can assure you the yellow wire on the male pin one of the 14 pin connector on the rear fuse panel runs directly to the yellow wire connection area at the dash harness 6 pin through body connector.

I have built numerous through body harnesses for multiple years of 911’s and they are all the same.
I advise not to do that test just because it is easy to go to crank....

I defer to you on this Dennis, but I am curious about what you mean by 'go to crank'.

Dan & Dennis,

Thanks for getting back to me, it’s very much appreciated!

On your advice, I WILL NOT ground pin 1 on the 14 wire harness in the engine compartment.

I do think it would be safe, particularly after the testing Dan suggests, but it's not necessary. My goal has been to resolve my mystery and learn how this works. That goal has been met and for that I thank you! I’m not so impetuous that I need to take this any further when there’s a chance that doing so is potentially hazardous.

BTW: I think what Dennis means is that it would be too easy to inadvertently turn the key to the 'start/crank' position and cause damage. I suspect an inline fuse would mitigate the threat, but again, the test is not essential and is easily skipped.

Dan I’m especially indebted to you for your careful writing, which has helped me understand more than just the immediate issue. I plan to make the ohmmeter tests you suggest and we’ll see what that tells us.

I won’t get to that for a couple of weeks because I’m off to my family’s annual get-together in the grand state of Maine! I had hoped to have the car done by now, but you all know how that goes!

When I get the a/c up and running and the a/c relay functioning as it was designed, I’ll post here to let you know.

Thanks again gentlemen, I could not have gotten to this point without your help!

For Dan,
As Robert clarified above, I meant turn the key to the crank/start position on the keyed switch with the grounding jumper in place on the yellow wire at pin 1 of the 14 pin connector.
Thereby impressing the entire unfused 12vdc current available from the battery on the grounded yellow wire, creating a dead short through the circuit, “Letting the smoke out”....
The ignition switch has unfused power directly from the battery that is in play when in the crank/start position.

Robert - You're welcome. I waited a day before sending that response just in case you got a better answer, and then I let 'er rip. Good luck and yeah, let us know - Enjoy your vacation!
Dennis - Aha! (duh) thanks, yes, one 'key' risk too many.