Another CIS Start then Dies Question (fixed: bad Rev Limiter)
 

(Note: if you come upon this thread as you try to repair YOUR SC, be advised that the repair I first affected proved to be both WRONG and DAMAGED my car's wiring. This occured due to my incomplete understanding of the fuel pump relay circuits operation. Responses on Page 2 from Timmy2 and boyt911sc will explain where I went wrong.)

Good Evening,

This is the first time I've had a CIS issue with my 82 SC. 190K miles, I've had since 2014, bought with engine apart, I rebuilt it. I left the CIS alone and it's been perfection in the 10K miles I've put on it. Was always pro-maintained by original owner. I'm one of those people who enthuse about CIS, but clearly I'm having a hard time grasping the electrical side.

Issue: I started car this morning, moved it out of garage. It did sound odd; I wondered if my a/c belt was noisy (has been before), so at idle I spritzed a tiny diagnostic burst of WD on the belt, didn't change the noise, so I turned off the engine to have a think; this noise may or may not be relevant. Shortly thereafter, I went to restart it and it would crank, catch beautifully, then die within a second. I have since read my Bentley manual, read lots of posts here and generally tried to figure it out.

I believe the main observation is this:
With the air-filter cover off and ignition on, lifting the air-meter arm does NOT cause the fuel pump to come on.
But jumping 87a and 30 DOES switch the pump on, and it sounds fine.
- I switched relays with horn and another spare German red relay; no change.
- I DO have 12V present on the relay socket at 87a and 86 with ignition ON.
- the ground path on 85 seems good.

I also have checked fuse cleanliness, terminal tightness of fuse panel hardware etc. The female connectors in the FP relay socket look good. Car does have fuel in the tank. Wiring at the starter looks good. The pop-off valve is sound and the big boot connecting CIS to the throttle is tight. BTW, I run the car without O2 sensor being plugged-in....When I first set it up after the engine rebuild, it was happier that way. The control unit and relay are still under the right seat and are dry.

I am not a high-level electrical guy but it seems that something in the circuitry for the fuel pump relay is causing a lack of 12V to the fuel pump in what should be the RUN position.

I see that "RPM Limiter" device in the wiring diagram. Could that be interrupting the electrical flow?

Throughout all this, with the FP Relay in, I can try starting the car and it will catch, hit 1200 RPM then die within a second or two. I am not holding the key in the Start position when I do this. The car was perfectly OK until this. Recent work included suspension refresh and installation of SSI's but I've put three hundred perfect miles on since those were completed.

Is this the "RPM Limiter?" I found it near the left hood hinge:

http://forums.pelicanparts.com/uploa...1557613744.jpg

http://forums.pelicanparts.com/uploa...1557613773.jpg

Any thoughts as to what I'm missing or should be doing will be appreciated.

Thanks, John/Connecticut

John, does it stay running when the relay is jump red or bypassed?


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The fact the pump does not run when the intake plunger is lifted makes me think the ground circuit is not being broken and thus the relay does not switch back to connect 87a with 30.

Remove the relay and connect one lead of a test light to a known hot, and the other lead to the socket #85. The light should illuminate indicating that #85 is grounded. Lift the plunger in the intake. Does the light go out? If not, there is a short to ground somewhere along the wire or the switch in the intake is bad. If it does go out, the circuit is good and you have eliminated one possible trouble area.

If it does go out, replace the relay. With ignition ON, slowly lift and let fall the intake plunger. Does the relay click each time it is lifted and again when it comes to rest? If so, normal. If not, and relay is known to be good, you have an issue with the 12v circuit which may be the rev limiter.

Thanks very much Carl and Ossi. I’ll get back on it in the morning and report back. John

Carl....YES.....that makes it run and stay running.
Will proceed as per Ossi next and report back.
Progress! Thanks much

85 circuit tests good

Ok... If after starting the airflow plate returns to the rest position that would tell the relay to shut off power to the fuel pump. As it should. This could be indicative of a large air leak.

http://forums.pelicanparts.com/porsche-911-technical-forum/828834-cis-rebuild-wont-stay-running.html

Did your car recently experienced a back fire?


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One thing you could try, it would require someone to be in the drivers seat and try to start the car while you are in the back and manually raise the airflow sensor plate just a little. If it now stays running I would say that the possibility of an air leak is pretty good. If not... then back to an electrical problem.


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Runs perfect with relay removed and 30 > 87a jumped. No backfire history. Starts (as normal): immediately, proper fast idle etc. I think it’s electrical. (Feeling rather silly for not checking this yesterday.)
Remembered that I’d bought Rob Siegel’s “European Automotive Electrical Systems” book, so just did the complete tests he describes for relays on both my (German) Wherle relays...both test to spec.
Is the cubic black relay I posted above the Rev Limiter? I’ll go do some searching for images of it. I see another one back near the gauges, but oblong (like the size of two square relays joined end to end) but looking at the colors of the wires going to its connector, I’m thinking that’s the seatbelt buzzer.
Thanks all for help thus far.

BTW just found great thread started by HughC and answered by MSalvetti, showing a photo of the Rev Limiter. Will head in there now to hopefully pull the connector and re-test.
In this thread, there was also the reminder from Tony to spread the relay pins with a razor blade. Just did that - had cleaned the posts early on) and it did not fix it.

jj..have you ever tried to unplug the bosh connector behind the sensor plate.It is a safety device ,it will cut the fuel pump off if it is kaput -broken grounded..simulating car is on the roof....
this one
Ivan
http://img.pccreation.net/photos/20190512175413551.JPG

Mark Salvetti and I now share SC’s with unplugged and bypassed Rev Limiter units. That fixed it. Jumped Red and Red/Brown (unit and connector are shown in Bentley). I may look for a replacement or just take the unit out for inspection. Worth noting my car had an electric fire in the OO’s ownership with the main fuse panel and some/all of the frunk wiring harness replaced...there’s still some yellow extinguisher dust/nastiness back in there.... a corrosive, right? Maybe that took its toll on the unit. But for now I have a running SC; yesterday was a glorious Spring day but today it’s pouring. No complaints. Major thanks gentlemen. John

so did you or did you not checked the plug??
Ivan

Ivan....no didn’t check that. Would it make the Rev Limiter look bad, even if it wasn’t? Thanks

for the future if have a problem and do all the checking on relays ..check this plug first;-)

THIS....
I believe the main observation is this:
With the air-filter cover off and ignition on, lifting the air-meter arm does NOT cause the fuel pump to come on."

If you would have disconnect the plug the fuel pump will run with the key on...

Ivan

Please share your test.........
 

John,

How did you perform the test for terminal #85 (FP relay socket) with the ignition switch @ON position? Thanks.

Tony

Hi Tony!
Today I tested that the way Ossi suggested using a test lamp. Yesterday I did the same but using an ohmmeter, as per Bentley, with one lead attached to a ground and the other to socket 85, and lifting the air flow arm opens the circuit. Thanks, John

I'll look at the wiring diagram for how that switch is tied into the circuit, which I now understand (a bit) better. Thanks.

Fuel Pump Relay and socket..........
 

John,

When you get chance, search for DKLever48’s post about FP relay and socket tests. The thread discussed how the 5 terminals are tested and identified for operation. Everything you need to know is well covered in the thread.

BTW, terminal #85 (ground) has several sources for ground contact:

a). AFS (air flow sensor) switch. This is normally closed and opens up once the engine starts to run and produce vacuum/suction causing the ground contact to open wide.

b). Rev limiter. This is normally open and have no ground contact until the tachometer sends signal to the rev limiter to close or make ground contact. Thus causing the FP stop running.

c). Car immobilizer (alarm). If the alarm system is triggered, the brown/red (terminal #85) will be grounded.

Tony

Tony, will do, but it'll be a week or so.

If of interest, here are pics of the Rev Limiter connector and page from Bentley:

http://forums.pelicanparts.com/uploa...1557698337.jpg
http://forums.pelicanparts.com/uploa...1557698433.jpg
http://forums.pelicanparts.com/uploa...1557698460.jpg

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