How to test rev limiter on SC (CIS)
 

There is more questions here than just the headline of the thread:

The CIS rev limit is at 7000 rpm. Is this correct?
The CIS rev limit is by fuel shut off, correct?
How does the fuel shut of rev limiter work, e.g. where does it get the RPM information?
How do I safely test if my rev limiter works properly?
Can I add (belt and suspenders!) an earlier distributor rotor that has the electro-mechanical switch? If so, PN would be great!

THANKS!

George

Your rev limiter is built in to the distributor and is an ignition cut off (at least my CIS 2.7 was that way). It uses a spring and mass concept enabling centrifugal force to short out the rotor and cut the ignition. I think the cut-off is quite a bit below 7000rpm. Maybe 6700 at best. I added a 7300rpm rotor from an early S to allow me to rev the engine a little higher.

Hope this helps...

Mike

Actually, the SC does not use the Dist. Rotor type of cut-off. The SCs and Carreras use a Rev Limiter called a Speed Relay (part no. 911.617.136.00). It is a small black box located behind your Fuel/Oil level gauge. My tachometer was a few hundred RPM off and I was hitting my Rev Limiter seemingly early. You'll know when you the limiter, the power will cut abruptly.

RoW SC's use a rev-limiting ignition rotor.

Oops, sorry. I don't know what I'm talking about. :>)

Mike

Ok, so the box knows the RPM, how does it get the fuel to shut off?

Thanks,

George

The speed relay 'interrupts' the fuel supply at the fuel pump relay. Your fuel pump momentarily stops pumping fuel.

Okay, that was worded funny. Of course there is no fuel running through the fuel pump relay. Otherwise, I hope this helps.

Resurrecting an old thread. Yes, SC rev limiters cut off the fuel pump, using a relay. My question:

I have revved this engine to 7000 rpm. I am afraid to go further. Is there another way I can test this rev limiter?

'78 SC

Uh, that's probably prudent.

Signal generator. Feed the rev limiter a 350Hz ~4V square wave and it should switch off the pump.

According to this thread http://forums.pelicanparts.com/porsche-911-technical-forum/280714-tachometer-compatibility-chart.html, the SC tach expects the same input as the Motronic puts out. Like this http://forums.pelicanparts.com/porsche-911-technical-forum/202003-tachometer-pulses-per-rev.html

3*7000/60 = 350Hz.

Or just don't rev over 7000 RPM. Little point taking an SC that high anyway - and the rev limiter won't help at all on a missed shift.

Just to add... The rev limiter box works by putting a ground on pin #85 of the fuel pump relay. When a ground is present and the ignition is run, the relay switches from 30-87a to 30-86 if my memory serves. That shuts off the fuel pump.

So if you do the test spuggy suggested then you should see the ground appear on pin #85 when the rev limit is reached.

There are 3 things that can put a ground on pin #85. 1) factory alarm 2) rev limiter 3) air pressure plate (safety switch)

Typo........
 

Tim,

Maybe you meant 30-87a to 30-87? The switch pivots @30 to 87a or 87 not 86.

Tony

Here is the wiring diagram.

https://photos.smugmug.com/Cars/Pors...Limiter-X3.jpg

Mark

Didn't work on my ROW SC. the rev counter was as far around as the needle could go for quite some time.

Ì dont mean to high jack this thread but the above diagram is very interesting in that my engine will occasionally quit at speed.
It was suggested that either the speed limiter OR the air flow sensor contact could be the culprit.
I've disconnected the speed limiter and I am still having the quitting issue.
When I had my engine out this past winter, I could see nothing wrong with the air flow sensor contact. It's either on or off, right?
So I am at a lost.
I suppose there is a remote possibility that there is a bad, broken wire somewhere but I wouldn't expect it to be intermittent.
Looks like I'll be living with that for a while.
Hugh

Thanks to tracking down a thread from Mr. Salvetti, a week ago I diagnosed this rev limiter relay as the cause of a start-then-die condition afflicting my US 1982 SC. I jumped terminals 1 and 4, making the car operable again... will be careful about not over-revving it. John

Thanks, Tony. I was typing on my phone while on the road and was going from memory. Pin #85 is burned into my brain because of all the trouble that mystery ground gave me.

Hugh,

There is a 3rd thing that can put a ground on pin #85 — a factory alarm, if the car was ever equipped with one. Some of these cars had non-factory alarms too and the wiring may have been butchered at some point so keep that in mind.

Last year I had a strange problem affect me. Something was putting a ground on pin #85 out of the blue. It just magically showed up overnight. Car would start and then immediately die. The fuel pump relay was working as designed. I never found the source of the mystery ground but I found there was junction going to pin #85 on the factory wiring diagram. I traced out the wire and the junction was near the fuel tank and brake master cylinder area. It’s basically a Y. I disconnected one leg of the Y that was the source of the ground and the problem went away. The rev limiter and the pressure plate were unaffected so assume that wire was part of the alarm wiring harness. I don’t think my car ever had a factory alarm though. No idea where that came from. Suspect there is or was a ground loop in a circuit.

I have been meaning to trace that wire out. I suspect it goes back behind the fresh air blower motor area so I’d have to pull all that crap out and I’ve been too busy.

Thank you Tim. You may have something there.
My sc had an after market alarm when I got it, which I removed (too quickly perhaps) as I suspected it was putting a drain on the battery.
I dont recall having the quitting problem until the second season of riding the vehicle but it may be somehow related

Not sure how I'll get my head around tracing that wiring as there was no schematics included with the vehicle but as I have time I'll try to compare wiring under dash with the schematics in the Bentley.
I'll also try to see if there is any other wiring around pin 85 that shouldn't be there.
Thx again

Hugh, if you've disconnected the rev limiter, then temporarily disconnect the air pressure plate sensor. If the problem still persists it would point to an intermittent ground showing up on pin 85. Find and disconnect that Y connection. If the problem still persists then it has nothing to do with the fuel pump relay. Or a quick test would be to just put a jumper wire across 87a and 30 on the fuel pump relay and go for a drive. That way you're bypassing the pin 85 logic. That would at least tell you if the problem has anything to do with the fuel pump relay circuit.

OK, I believe the rev limiter for the Carrera is built into the DME, and not done with that relay. I was curious and went to the Pelican Parts catalog and entered that part number. I was stunned when the relay came up as costing more that $500! Wow.

I know this is off topic from the original question, but Pelican's catalog said it could not confirm that relay fits the Carrera.

I do find it interesting that the SC had gotten away from the old rotor rev limiter like the early cars and the 914 use. Back when I had my 914 my brother went out and swapped in a VW bug 4,000 RPM limiter into my 914. I took of to go to work and the car would not rev above 4K. It was a no fun drive and I figure out what he did right away.

I do now for a fact that is a Carrera owner wants to change the RPM the rev limit, the DME program has to be changed. I don't believe the relay does it, but I could be wrong. Does anyone know for sure?

That’s correct, the Carrera’s use the DME and not the relay.

I think when I replaced that relay on my SC it was about $150, but that was also over 20 years ago.

Mark

Mark, on your Rennlist post, someone said they're now >$500 new; that said, I'd imagine there are used ones out there. I've never hit my rev limiter under acceleration so am not too worried running without it. I don't think in bypassing mine I'm compromising the rest of this circuit? Cheers, John.

The off the shelf Steve Wong map raises the limit from 6520 to 6760rpm. Done with the chip/DME.

Tim,
I just recently put about 350 mi, mostly highway, on the sc and I haven't had that quitting problem that I had last summer. So until it rears it's ugly head again I'll not give it too much worry. Your suggestion to disconnect the air pressure plate sensor is certainly valid, although getting back there to disconnect it won't be that easy.
I am having another problem where the engine idle will go to zero at some point between being stone cold and fully warmed up. Once fully warmed the car seems to run fine. Obviously that scenario is indicative of some other problem which will have to be dealt with. My first guess is that there might be unmetered air in the equation.
Tony, boyt911sc, supplied new WUR last season which I will have confidence in until I really suspect it.

The good people at Advanced Distributors rebuilt my distributor last fall and I give credit to how nice the engine is running to that rebuild. I was told that the rebuild was LONG overdue. I don't think the timing is perfect yet but it's close and certainly better than it has been since I've owned the vehicle.

I'll start another thread on timing in the hopes of getting some good advice.

Thanks for your help.
Hughc

Tim, that quitting problem has reared its ugly head again.
I've resorted to removing the FP relay and replacing with the jumper wire but that hasn't prevented the car from quitting.
I had intensions yesterday of doing a 200 mi drive (with the jumper wire in pace) but the car quit 3 times within 10 mi so I turned back to get another vehicle.
So it appears the quitting is not related to the fuel pump relay.
Can you suggest what I can check next?
Thx
Hugh

Intermittent CDI failure will cause the engine to quit. Have you checked for spark when it stops?
Also a coil going bad can do the same. You don’t have a silver Bosch coil do you?

I never actually stop the vehicle, I just keep rolling, turn the key to "acc", then turn the key to "run" and carry on my way. Smooth as silk.
Really odd.
The CDI was recently rebuilt plus I have a newly rebuilt CDI as a spare which I will try.
While I'm at it I'll have a look at crimping the CDI connectors a bit to assure they are making good contact
I've thought of the coil even though I have a black coil, but doesn't the coil want to fail when it gets hot then require a cool down period before itll be good again?
I think I'll also crawl under the vehicle to have a look at the connections at the FP.
I'll be away for four or five days so won't be able to get much h done till next W/E.
Thanks for your suggestions and I'll keep you informed.
Hugh

A lot of those speed relays go south and cause a weird jerkyness when cruising or at idle. Unplugging it fixes that. You could add a cutoff rotor in the distributor from a 930 if you feel the need for over-rev protection.

If by speed relay you are referring to the rev limiter then in my case it is disconnected and completely out of the picture.
Currently the FP relay is removed and replaced with a jumper wire and still the engine quits unexpectedly.

Unless your fuel pump is cutting out intermittently, I think the way you have it wired up eliminates the fuel pump circuit as the cause. Disconnecting the air flow switch is all that is left.
Did you check that all grounds and battery connectors are clean and tight yet?
Especially the one from transmission to body.
Check and tighten all fuse terminals as well as ensure the fuse terminals are clean.
Green wire at distributor is good?

I've just crawled out from underneath the car to check both terminals on the fuel pump. I was able to turn both nuts about 1 to 1 1/2 flats.
I dont know if that's enough to make a difference but I won't force them any more.
The green wire was replaced last year and I would like to think that all is good in that area.
Not sure how to confirm though.
When I replaced my engine this past winter I was ever mindful that the ground strap was well secured, even to the point of thoroughly cleaning all surfaces.
I have in the last year or so tightened all fuse terminals and lightly sanded all fuse ends. I suppose the brass fuse terminals could also be cleaned. I'll put that on my list.
I'm at this moment trying to remove all six terminals from the wire going to the CDI box with the intention of squeasing the lips just a bit to give a tighter fit to the spade terminals.
I seem to recall checking all ground points within the last two years but that could also be revisited . I'll also put that on my list of things to do.
As far as disconnecting the air flow sensor switch I thought by removing the FP relay and replacing with a jumper wire that that switch would no longer come into play, but I could be wrong and I still dont know how to disconnect it 'cause I certainly can't reach behind the engine to physically remove it.
Ill finish the job on the CDI connection and drive the vehicle for a bit and see if the problem has hopefully disappeared.

I must say, other than the annoying quitting problem, my vehicle is running nicer than it ever has (under my ownership)
I'll keep you informed.
Thx

Interesting. I think my rev limiter system is not working, but I also have some 'bucking' at creep-through-parking-lot speeds.

Basically, it's an off/on thing, not a surge.

Thank you John. Again. :)

Just re-read some of your posts. The never stopping the vehicle and just turn to accessory then run thing is possibly a big clue.
Could be as simple as needing to push in the ignition switch 6 pin plug at the firewall, or maybe the switch itself is acting up.
Losing ignition power in run from the ignition switch would cause your symptoms.

Thx Tim
Those are two more possibilities that my friend and I discussed earlier today. Rather than concentrate on the problem being a fuel pump problem maybe it's time to consider an ignition problem.
I managed to tighten , just slightly, the six connectors to the CDI, but I wont get any more done tonight as I'm leaving tomorrow for a few days.
I'll check the six pin plug and the ignition switch for anything obvious once I get back.
I did go for a short run after I finished the CDI thing ( and the fuel pump connections) and the car ran fine but that doesn't necessarily mean anything.
The car might run fine for several days OR it could quit a half dozen times in a 30 mi. run.
So I'll get back on it next weekend.
Thanks for sticking with me.

Troubleshooting.........
 

Hugh,

You are correct about the air flow switch. Once you remove the FP relay and use a jumper between 87a & 30 to run the FP, the air flow switch does not come into the picture anymore. The FP solely depends now directly on the ignition switch for power. Are you using a jumper wire with in-line fuse?

There is a good chance that the electrical problem you are having with the “jumper” method is caused by intermittent power interruption due to poor electrical contact. To test this assumption, I would install a low wattage pilot lamp to monitor the power supply to the FP during operation. Extend the monitor light (lamp) into the cabin and do a test drive. Next time the FP stopped running, the monitor lamp should be off too.

For all you know, the problem with the FP is caused by the “jumper” wire or worse by terminal #15 (ignition switch). Keep us posted.

Tony

Hugh,

By bypassing the FP relay you’ve eliminated anything putting a ground on pin 85 - rev limiter, faulty alarm, air plate. So that’s a good test in the sense that it eliminates several possibilities at once. If you have another CDI that would be an easy test to eliminate as another candidate. If you methodically eliminate the possibilities then whatever you’re left with is the culprit. You’ve already ruled out some of the others. If you swap the CDI then you’re probably down to coil and Dennis’ (timmy2) idea of the ignition switch or loose connection.

I chased some electrical gremlins. One was that one of the three original fuse panel sections had a crack. As a result, sometimes the fuses would get a little loose and momentarily lose connection. Maddening. Persistence is sometimes the best thing in your toolbox.

Hi all. As ive entioned earlier I'll be away for four or five days so nothing will be done to the vehicle.
Tony, I've thought of installing a LED across the fuel pump to see if it stops running when the car quits but instead what I think I might do, upon my return, is just keep driving the car to see if it quits again.
If it does quit I will just allow the car to roll to a stop and not touch the key. With the jumper wire still in place and the ignition key still in the "on" position the fuel pump should continue to run. Right??
If the fuel pump is still running, as it should, then we have confirmed that the problem is probably ignition related and I can continue to troubleshoot in that direction.
On the other hand, if the fuel pump has stopped then I have to rethink the problem to try to determine why the pump has shut off.
Make sense ??