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.