Pride Goes Before the Fall, and/or I should have added a fuse:

The car has been running fine since the diag/repair described above. I figured that if a) jumping the Rev Limiter made the car run properly AND there was no further issue, then the throttle switch mentioned by ProPorsche and Tony must be OK.

This morning I was just having a gander in the frunk and remembered that the interior/dome lights had stopped working; I saw fuse #18 was blown (5A). I went to install a new one and POOF, it sparks and my left index finger gets a small burn. There wasn’t any smoke, but I look in the area and see some burnt wires. Did they melt just now or before? Dunno.

So I pull the gas tank to gain access. More burnt wires now visible, all on the fuel pump/relay circuit. Part of the frunk harness.

I should have, at a minimum, put a fuse in the jumper wire I used at the rev limiter connector.

First I need to ascertain the root cause. I loosened up the fuse panel and it looks like the damage is limited to wires on the fuel circuit. I may be able to get my hands on a scrap harness for repair sections.

I’m thinking I’m lucky the car didn’t go up in smoke.

http://forums.pelicanparts.com/uploa...1559417707.jpg

http://forums.pelicanparts.com/uploa...1559417870.jpg

http://forums.pelicanparts.com/uploa...1559417981.jpg

Very lucky indeed. The clumpy morphology of the verdigris on the wire and burnt insulation is indication that this likely happened some time ago (likely months, not weeks).

I'll get my hands on a clamp meter to look at draw on the pump itself.

Tony had asked that I check the Fuel Flow Sensor; am I correct that since the car worked properly after I bypassed the Rev Limiter, that FFS is by definition OK?

I will also add a fuse on the circuit for the dash lights/ashtray lights....had been meaning to get to that. (There was a radio show on NPR today where the subject was how we (humans) often pay far too little time/attention/energy to PREVENTING problems, versus what we're prepared to endure to SOLVE problems once they've happened. Guilty, m'Lord!)

CIS troubleshooting..........
 

John,

If your tests showed that terminal #85 was good (?) why disconnectin the Rev limiter or AFS make a difference. Both AFS and Rev limiter are sources of ground contacts. My point is that you did NOT test terminal #85 correctly because if you did it right, either the AFS or RL would point as the culprit/s.

BTW, how did you test terminal #85 to eliminate AFS and RL as a culprit of your problem? Keep us posted.

Tony

Tony, I tested the relay properly but the socket I did (only) as per Bentley. At this point it looks like I'll be testing the socket after I've repaired the harness that supplies the socket (because they are now in a dreadful state) using the steps in the link you provided. I think I tested everything in the circuit except the AFS.

The question I'm asking about the AFS isn't intended to be lazy or step-skipping. When I was first faced with this, I worked my way up to the RL (via the wiring diagram and mention in MarkS's thread) and found that when I removed it and jumped the relevant terminals in the female connector (the RL has male spade fittings, a la relay), my problem seemed solved. So I'm asking if that in and of itself suggests the AFS is fine. Another way to ask: in my jumping the RL connector, was I bypassing the AFS's switch? Would it be possible for the AFS to be bad but not show up due to my jumper wire?

Respectfully, John

Learn from your experience.........
 

John,

I did not realize you did some alteration in your wiring. All the while I thought you were thinking of just eliminating the AFS plug and disconnecting the rev limiter to overcome your mysterious “ground”. So how did you do the rev limiter by pass set up? Did you use a wire jumper to connect something? Where did you get this idea from?

Anyway, you could help many by sharing with us about this experience. Do you know at this point what caused the wires to melt and burned? Thanks.

Tony

Tony, I’d looked at the wiring diagram and read of a couple of other guys who’d had the same problem and solved it jumping the the two appropriate wires at the connector for the RL. I picked up a pre-owned fuel system wiring sub harness just now and will be looking for whatever caused the short or large resistance. Thanks much.

Please share the information.......
 

John,

Do you mind sharing with us where the jumper wire were connected to? Please identify the wire color or terminal number from the wiring schematic. Thanks.

Tony

Tony, I jumped the Red and Brown/Red wires (see post 20 for a visual). Brown/Red is now my problem child.

This morning I was able to pick-up a used fuel relay > pump sub-harness from the always excellent Tom's Rennshop (local to me, thankfully). With the gas tank out and the car on my scissor lift, I started to remove the remains of the old one.

As has been said by others elsewhere, the Porsche wiring, at least of this era, is not fun to work with; perhaps it just seems that way because I'd rather not be in this situation in the first place. Put it this way: the Japanese do it much nicer; maybe later model P-cars do too.

What did I find? The wires within the sub-harness were toast to the point that the external sheathing had either split off and /or melted itself to surrounding sub-harnesses (as in, bundled wires in a sheath). The primary culprit, at least from what I can see so far, is the Brown/Red, which seems to travel:
AFS Switch > F/P Relay > RPM Limiter. It is contained in another sub-harness, which also includes Red, Brown, Red/White and Black/Red.

I am now studying Bentley (page 970-8 and 970-9 for anyone who wants to read along. Does anyone know if there's a resource that might show the colors of the wires inside bundles/sub-harnesses? PET shows P/N's for the sub-harnesses, but not a color code as to what's inside them. The wiring diagram in in Bentley just shows flow, not what I'm looking for. The reason I ask: I will likely need to replace the sub-harness that contains the (now melted) Brown/Red rearwards in the car. But I should be able to get there eventually with the Bentley diagrams.

This repair might take a while. Feedback and good karma much appreciated.
John

OMG!!!
Not surprised you burnt it up. Connecting red to brown at the speed switch connector effectively connects power to both sides of the relay coil and the protective grounding device circuitry downstream (air flow switch G19) with ignition on!
Not sure where that jumper idea came from, but it should be stricken from the forums if you found it here...

Really all that was needed was to unplug the speed switch to isolate the circuit. It won’t ground out the relay coil to kill the pump when disconnected. (No path to ground)

The colored ‘78 schematics are the same for this circuit on your car.
As can be seen below, looking at the speed switch J35 wire destination on the fuel pump relay, (middle of pic) you connected 85 to 86 with the jumper, effectively killing the ability of the coil and safety function to work and impressing 12 vdc on it when ignition is in run position.
Your air flow switch was now a dead short to ground when closed (not running) and ignition on! (12 vdc on that wire) I really hope you didn’t melt any wires in the through body harness or engine harness....
Replied to your PM with my phone number. Give me a call.

http://forums.pelicanparts.com/uploa...1559496810.jpg

Clarification..........
 

Dennis,

Glad you responded to this post. I was wondering what John did by “jumpering” the brown/red wire to WHERE(?). He was not very specific or I might have missed it. So I sent him a PM inquiring about the procedure. I even requested for sketch of his jumpered wiring to fully comprehend it. I was going over the wiring diagrams in the Bentley Manual and PSM since yesterday and I could only guess that the ground wire was inadvertently connected to a power source(?).

Since you spoke to John, you have more detail knowledge of what was done. Where did he get the idea of jumpering it? What was the purpose of using a jumper wire? Thanks.

Tony

I'll call Dennis shortly. I blame no one but myself, may well have misinterpreted what I'd read. When I was first working on the Start/Stall issue, and learned of the RL, I had tried running the car with the RL disconnected but that didn't work. In my ignorance (of wiring diagrams and circuitry in general, it appears), I was thinking that I was jumping the two legs that the relay would. Silly me and WTF.

Now it is getting clearer.........
 

John,

So you innocently connected the brown/red wire of the rev limiter to a RED wire at the RL plug? If this is the case, then we could prevent future incident like this one. I was trying to figure out where you connected the “jumpered wire” and I could not get a clear answer from you. Thanks.

Tony

Just got off the phone with John.
All is good, the alarm circuit took all of the damage. Could have been much worse.
John was lucky, he is now removing all alarm wiring from any power sources.
And installing the replacement fuel pump harness.

I’m hoping he will contact me when he is ready to turn the ignition switch again....
Just to check a few things with a meter first.
Let’s make sure everything is functioning correctly.

You do have a volt/ohm meter right John? :)

Sorry to hear about the mess :eek:

But does this mean the car was running fine with the short in place ? When I looked at your pic, I thought you were bypassing the tach signal to gnd not the red to brn, but on closer look, yeah, it's the latter.

I did 300 or so miles that way....

Ah, I guess the short burnt out the alarm circuit which made it go open and hence disabled the alarm/rev limiter circuit. I wonder if the fuse 18 blowing out was caused by the same.

Yes, will do and very much appreciate your help.

Post by DkLever48..........
 

John,

On May 12 (post #19), I made a suggestion to you to read DKLever48 post regarding FP relay socket tests. Had you done this, you would had discovered the wiring anomaly you had created by connecting the ground wire to a power source.

And I also missed your post describing jumpering the RED wire with the brown/red wire (ground). Your responses to some questions were vague and not clear to the readers. It took me several posts and PM before I got a confirmation of your jumpering procedure.

If we had a better feedback and communication, you could had averted this problem. You are very fortunate the damage caused by the wiring anomaly is minimal. We all mistake and learn from this experience.

Tony

Tony....please....lighten-up. I already confessed my sins AND omissions. Your remarks are a) not entirely accurate and b) kicking a man when he’s already down. Not what a good teacher does.
John