Rev Limiter 83 sc

[hughc]

Where do I locate the rev limiter on my '83 sc? and how do I go about checking it?
thx

Hugh

[Flat6pac]

Limiter is part of the tach...
Bruce

[Bob Kontak]

Originally Posted by hughc

Where do I locate the rev limiter on my '83 sc? and how do I go about checking it?

Once or twice I exceeded the rev limit on my 81. It's not pretty. The inertia function on your seat belts kick in. It's like a face plant into the steering wheel without belts.

However, it is a way to test the functionality.

[Mark Salvetti]

I'll defer to Bruce, but I can tell you that on the earlier SCs it's a black rectangular relay mounted to the body behind the fuel gauge. It works by shutting off the fuel pump.

I know because mine failed, and the engine kept cutting out randomly until I disconnected the wires before I finally replaced it.

Mark

[boyt911sc]

Originally Posted by Flat6pac

Limiter is part of the tach...
Bruce

Bruce,

Hate to disagree with you my friend. The rev limiter in an SC is controlled by the tachometer. It would not function correctly with out a good working tachometer. However, a tachometer does not need a rev limiter to operate properly. In short, a rev limiter needs a tach. But a tach does not need a rev limiter. Try and test them to understand how they work.

Tony

[Jonny H]

Depends on the country.

On this side of the pond the rev limiter is part of the rotor arm in the distributor. It cuts the ignition. Purely mechanical.

In the US, the fuel pump is switched off by an ECU.

[Reiver]

Originally Posted by Jonny H

Depends on the country.

On this side of the pond the rev limiter is part of the rotor arm in the distributor. It cuts the ignition. Purely mechanical.

In the US, the fuel pump is switched off by an ECU.

Or...if you have a transplanted 930-10 in a US car you take the replace the Euro rotor and allow the little box behind the tach to do the job....my ecu is also programmable.

[boyt911sc]

Originally Posted by Jonny H

Depends on the country.

On this side of the pond the rev limiter is part of the rotor arm in the distributor. It cuts the ignition. Purely mechanical.

In the US, the fuel pump is switched off by an ECU.

Jonny,

The ‘78~’79 SC’s exported to USA/Can do not have ECU. The ECU’s were first introduced with the introduction of the Lambda system (OXS) in ‘80 CIS up to ‘83. The rev limiter is triggered by the tachometer for these models.

Tony

[Reiver]

Originally Posted by boyt911sc

Jonny,

The ‘78~’79 SC’s exported to USA/Can do not have ECU. The ECU’s were first introduced with the introduction of the Lambda system (OXS) in ‘80 CIS up to ‘83. The rev limiter is triggered by the tachometer for these models.

Tony

Tony...nice to know but the OP has an '83 so I suspect our responses were directed there.

[tirwin]

This is how it works on my ‘83.

The RPM limiter switch is located behind the dash/at the back of the trunk. On a left hand drive it’s on the driver’s side. You can reach in with your hand and find it. I believe it is a 6-pin connector. Only 4 wires used. Bentley will have the wiring color codes in the electrical diagrams in the back.

I can’t think of an easy way to test it. When the rev limit is hit, the rev limiter puts a ground on pin 85 of the fuel pump relay. When the ground is present on 85, the fuel pump doesn’t have power (circuit path 87->30 is picked instead of 87a->30).

I don’t know what the input signal to the RPM limiter looks like. It is a red/white wire going from the tach to the RPM limiter. Could it be as simple as if 12v is present on the red/white wire, the RPM limiter kicks in? Or is the signal on some voltage scale that equates to engine RPM. Never thought about it before to be honest. Just spitballing. Maybe others know for sure.

[uwanna]

Originally Posted by tirwin

This is how it works on my ‘83.

The RPM limiter switch is located behind the dash/at the back of the trunk. On a left hand drive it’s on the driver’s side. You can reach in with your hand and find it. I believe it is a 6-pin connector. Only 4 wires used. Bentley will have the wiring color codes in the electrical diagrams in the back.

I can’t think of an easy way to test it. When the rev limit is hit, the rev limiter puts a ground on pin 85 of the fuel pump relay. When the ground is present on 85, the fuel pump doesn’t have power (circuit path 87->30 is picked instead of 87a->30).

I don’t know what the input signal to the RPM limiter looks like. It is a red/white wire going from the tach to the RPM limiter. Could it be as simple as if 12v is present on the red/white wire, the RPM limiter kicks in? Or is the signal on some voltage scale that equates to engine RPM. Never thought about it before to be honest. Just spitballing. Maybe others know for sure.

Yea Tim, you da man! Right on explanation. We ran all around that
circuitry when we troubleshot your weird fuel pump problem and figured out how it works.

[dicklague]

My 1973 had a rotor with a weight that centrifugal force would ground the secondary voltage at 7200 RPM. When it hit it was like turning off the ignition.....BOOM up against the belts you go.

I put in a high end aftermarket CDI [NOT MSD!!!] with a soft rev limiter. I set it for 6600 RPM and it just starts missing when you get to that RPM......much better. I LOVE the soft rev limiter!!!

I had to find a solid rotor with no advance and no resistor installed.

[hughc]

Thank you for all of your input. I'll try to clarify.

This is my problem. Anytime while driving the vehicle, be it around town or at highway speeds the engine cuts out. It just stops. I can restart the engine by turning the key to ACC and back to RUN and the engine restarts and carries on it's merry way.


After corresponding with Tony privately he suggested that my problem could originate from either a faulty AFS or faulty rev limiter, either of which might cause the fuel pump to stop.

With my limited knowledge of the circuitry this explanation makes sense to me and is the troubleshooting direction I want to pursue.

My problem now is how do I go about troubleshooting each of these components, therefore my question about the rev limiter.


I have to find some way to verify that one or both of these components is working (or not working) as it should


That, gentlemen, is my dilemma.


Hope you can help.

thx
Hugh


So now what I want to do is somehow

[boyt911sc]

Hugh,

Something very simple skipped my mind about your problem. Do you have the red FP relay? Pull it out and inspect the pins. Use a razor blade to spread apart the pins and insert it back. If you are really interested in learning how to troubleshoot your CIS, you could do it. There is really nothing difficult or sophisticated that a typical DIYer like you or me could not perform.

How do you get started and feel comfortable working with CIS? I will be back home in a couple of days and will send some pictures of things you will need like reference reading materials, basic tools, etc. With some practice and experience, you will able to do all your CIS troubleshooting with ease and confidence. Next, if you are ready is engine teardown! Maybe later.......

Tony

[Bob Kontak]

Originally Posted by boyt911sc

Do you have the red FP relay? Pull it out and inspect the pins. Use a razor blade to spread apart the pins and insert it back.

+1

It's the closest relay to the steering wheel on the row of relays on the drivers side near the fuel filler neck. Spread the pins a wee bit with a single edged razor or even a fresh box cutter blade. Don't go nuts.

Push the relay in and pull it out a few times to scuff the mating surfaces.

[Pazuzu]

I'm pretty sure you can disable the rev limiter completely, since it's just a relay that runs +12V to the fuel pump relay ground to shut it off. Pull it out, and it's dead. Quick way to remove that from your troubleshooting.

To be clear, it's not really a relay apparently, it's drawn as a transistor on the electrical drawings. It has 4 inputs:
+12V connected to the hot side of the fuel pump, from fuse #16
Trigger output to the ground side of the fuel pump relay
Ground wire
Trigger in from tachometer

If trigger in from tach goes high, then the transistor changes trigger out from ground to +12v. If you remove the rev limiter, then nothing changes in how the fuel pump works with the air metering plate, fuse, ignition, etc. The only thing to watch for is the +12V source in one of the sockets for the rev limiter.

[tirwin]

Yea, if it's just troubleshooting for this problem I would just disconnect the rev limiter. The downside is, of course, you have no rev limiter but as long as you are the only one driving the car I wouldn't worry about it just for testing purposes to see if that eliminates your problem.

I left my rev limiter disconnected because I have a soft rev limiter in my MSD ignition and I didn't want to have competing rev limiters.

[Mark Salvetti]

Yes, just disconnect. We had this discussion over on Rennlist a couple of years ago. Here is the thread, includes a photo of the rev limiter box.

https://rennlist.com/forums/911-forum/963522-weird-cutting-out-help-with-advice-2.html

Mark

[hughc]

Originally Posted by boyt911sc

Hugh,

Something very simple skipped my mind about your problem. Do you have the red FP relay? Pull it out and inspect the pins. Use a razor blade to spread apart the pins and insert it back. If you are really interested in learning how to troubleshoot your CIS, you could do it. There is really nothing difficult or sophisticated that a typical DIYer like you or me could not perform.

How do you get started and feel comfortable working with CIS? I will be back home in a couple of days and will send some pictures of things you will need like reference reading materials, basic tools, etc. With some practice and experience, you will able to do all your CIS troubleshooting with ease and confidence. Next, if you are ready is engine teardown! Maybe later.......

Tony

Thanks Tony,
As I said in my pm to you, spreading the pins on the FP relay was done at some point last summer, but I'm sure it won't hurt to re-visit that possibility.

And thanks also for the offer to educate me on CIS troubleshooting.
Not sure if this old gray matter bit is up to it but we can certainly have a go at it.

Hugh