Path of the Yellow AC Relay Wire?

[piscator]

Yesterday, I had my new variable speed controller running just fine, feeding it 12v from the AC relay in the smugglers box. Today the relay stopped working. Tested it on the bench, relay's good.

Turns out, the yellow wire that connects to terminal #86 on the AC relay has an intermittent short. I can't see where that yellow wire runs to without some major dis-assembly. I tried continuity testing between the wire at the AC relay and whatever other yellow wires I can see, but no success, yet. Does anyone know what path this yellow wire takes and where it terminates?

The Bentley schematic diagram shows this yellow wire terminating at "starter." That's really confusing me. I only have one wire to the starter motor, it's big, it's not yellow, and it carries +12 volts!

Any help will be greatly appreciated!

[Ayles]

Check right behind the mc. The yellow from the ac relay joins here then off to the starter. I no longer have ac so that yellow lead has been removed.

[timmy2]

Yup, behind the dash panel on the trunk side where the 6 pin plugs push through.

[piscator]

Ayles, thanks so much for the photo! That's a very big help. Thank-you too, Dennis.

I carefully dismounted the fuse panel yesterday and I was playing in that area around the master cylinder. But even after that, the relay functioned normally. So the problem must be intermittent. I'll see what I can find in that area.

The strange phenomenon is that, at times, I get a voltage on the yellow wire of about 3v. Then it goes away. Then it comes back. I'm not sure how that could happen, since I'm only working in the smugglers box and am not (and have not) been moving the wires around the master cylinder.

And if the yellow wire is somehow shorting out on a positive terminal, why am I seeing 3v and not 12v?

I might be able to supply the AC relay with a separate ground to terminal #86 and leave the yellow wire disconnected. But then I'd probably worry about the 3v short I hadn't tracked down.

More testing tomorrow. Thanks again for the guidance and please add more if you have any!

Happy Father's Day to the Dad's (like me)!!

[timmy2]

That is the starter solenoid circuit, likely used to kill A/C when cranking. (I haven’t studied the circuit for A/C in quite a while)
Phantom voltages impressed on it could be from a bad ignition switch or possibly a car alarm? Depending on the year there can be a lot of wires stacked up on that yellow wire at the firewall.
What year is your car?

[piscator]

Dennis, thank-you! Very interesting stuff!

First off, my 911 SC Targa is a 1979 and has no alarm system.

I’m sure you’re correct that it’s part of the starter solenoid circuit. It would be nice if the Bentley diagram made that more clear. Two questions:

1. Does it connect to anything or anywhere else? Or is it a solo path between the solenoid and the A/C relay?

2. If the yellow wire is simply a DC negative ground, how would connecting it to the solenoid’s DC negative terminal cut-out the A/C on starting? That’s confusing.

Those are the important ?? ‘s But my morning’s study does prompt this query:

Dennis, am I right that what you mean by “phantom voltage” is an induced current created from a capacitance between wires running in close proximity? This is not my strong area, but I learned a bit about capacitance when I got involved in HAM radio.

If that’s correct, I think this is also called “ghost voltage” as in this Fluke Meter description:

“For instance, a little-known voltmeter reading is called "ghost voltage." Digital voltmeters are so sensitive that even the earth's magnetic field will induce a very small voltage in its leads (as when you turn it on). You get the same result — a rapidly fluctuating, low voltage reading (ghost voltage) — when the leads are touching two wires that are not connected, as when there's a broken or disconnected wire stopping current flow.”

A link to the full PDF text is here:

https://www.newark.com/wcsstore/ExtendedSitesCatalogAssetStore/cms/asset/pdf/americas/common/fluke/DIDM.pdf

What I don’t understand (yet) is whether ‘ghost voltage’ represents a ‘real’ current that can cause a problem, or a ‘false’ (non-important) current that is detected only because of the very high sensitivity of modern digital multi-meters.

The Fluke article introduces a DMM that has a low impedance setting that ‘dumbs down’ the meter so it doesn’t read these currents. Other articles suggested that older, analog meters were better for testing current carrying circuits because their low impedance can’t ‘see’ the ‘false’ voltages.

So, are these ‘phantom’ voltages/currents are ‘real’ or ‘imagined’ ? If real, are they large enough to require a remedy? Or do we just leave them alone? Any insights will be much appreciated!

[timmy2]

Color factory diagrams may help.

[pete3799]

If i remember correctly the Bentley manual has the a/c relay in the smugglers box wired wrong (or number wrong).

[piscator]

Pete, thanks. I'll recheck the Bentley diagram to see if it accords with what I've learned.

Dennis, thanks for the diagram and for your assistance. I think I've figured it out and my biggest help was a comment you made on starter motor wiring in 2015, you said:

"My thoughts are to correct the wiring back to original so it functions the way it is supposed to.
But, that is a lot of work because there has been some unique modification done to make the yellow wire that is supposed to go hot in start go to ground...." Dennis

So the yellow wire IS supposed to carry +12v and the black wire is ground! For some reason, my wiring is backwards. The black wire is hot and the yellow wire is supplying ground. What you suggested might be a phantom voltage is more likely a short. And may well be a short against the black wire.

This afternoon I dropped the blower and pulled out the left side air valve to get a better view. I spent the day tracing wire runs and learning 'what goes where.' I'll say one thing, I now have to agree with the guys who say that electrics is not Porsche's strong suit -- what a bird's nest!!!

Some of it was quite well done, but then I also found wires wrapped around hoses, wires that are too short, grounds that should have been moved 1/2" so can get to them, etc. Porsche, at times, did put out substandard work, but our passion tends to varnish those faults. My car, for example, has some pretty ugly welds and rather shoddy paintwork in the trunk and on interior panels.

The surprise today was finding the intermittent wiper relay flopping around loose behind the blower! And -- finding the signal light timing relay half out of it's socket! The socket was manufactured incorrectly. I had to file the female socket terminals to accept the full length of the pins on the relay. I also had to file the relay pins to shorten them about 3mm. After that, the relay seated properly.

The thing is, this car has been in my family it's entire life and I know everything that's ever been done with it. Before today, no one has pulled out the blower. Which means that the floppy wiper relay and the signal light relay with 3/8" of it's terminal lengths exposed -- were almost certainly left that way by the Porsche techs that built the car!

O.K. It's not the biggest deal in the world, but it did surprise me.

Tomorrow, I'll start investigating the location of my short. Thanks again for the help!

[rnln]

Originally Posted by timmy2

That is the starter solenoid circuit, likely used to kill A/C when cranking. (I haven’t studied the circuit for A/C in quite a while)
Phantom voltages impressed on it could be from a bad ignition switch or possibly a car alarm? Depending on the year there can be a lot of wires stacked up on that yellow wire at the firewall.
What year is your car?

piscator,
this is correct, base on my experience. I used to have same problem yeas ago. It happened when it's hot. Sit and wait until it's cool off a bit, it would start. I can't remember how I fixed the problem, tried many things, but for temporary/emergency, you can run a separate wire from the ignition to the big hardness on the left of the engine bay, on the fuse box area.

[piscator]

Rnln,

Thanks! It's good to know I'm not alone. It may be heat and an overcooked wire that's caused this problem, but if you mean engine heat, my engine is out of the car. It would have to get pretty d-mn hot here in New England to reach engine temps! :-)

Another symptom I discovered while tracing wire runs, is the electric window relay. It feels hot, no, it feels warm. I can grab and hold it without flinching, but it is definitely warm. The windows are working fine and always have. I don't know if this is a new problem or something that's always been there. I've never spent much time fondling my relays! ;-)

[dhanl82]

Below I have attached a link to a post from this forum in 2011 that does a good job of explaining your issue - check post # 7.

Yellow WIre, Terminal 50, Starter, and A/C Relay Issue

Hope this helps.

Dave