Rev limiter not working

[Leland Pate]

Wow, I'm on a roll tonight...


Anyway, this past week I confirmed my suspicion that my rev limiter is not working on my car.
I'm not talking about a mechanical over-rev... I'm tlaking about punching it to redline and getting a little sloppy on my shifting to 2nd gear, causing the engine to momentarily reach 7K.

No harm done or anything, but I've always been reluctant to believe that the rev limiter was working, (and didn't want to find out) but this confirms my belief. I flat out watched the needle... and it wasn't a "bounce" like some people experience.

I've never "heard" a rev limiter kick in on my car, nor do I know how they are supposed to work.

On a friends 993, you DEFINATELY know when it hits the rev limiter... it does that "stuttering" noise which is more than audible.


So, what's going on? Is there anything i can do?

Is the rev limiter on a SC built into the distributor or in the CDS box?


Ideas?

[pwd72s]

Lee, for what it's worth...on an old 911S, it's built into the rotor...you just bend the tab the spring hooks to in order to bring the limit up or down. And it takes very little bending, with the newer replacement rotor. You might take a peek at your rotor...on the early cars, the replacement rotor is a simple weight/centrifiugal force/spring thing. Probably used in bombs all the time, but that's your area of expertise...

[rstoll]

Maybe your rev limiter was the mal-adjusted throttle cable.

[cary]

I can't remeber what the 3.0 has. My 3.2 it comes from the brain. I think the DME cuts the fuel supply, not the spark.

Here's my old car 75s. with the 3.0. Oregon region member that lives in Seaside .............I love that car.

[Early_S_Man]

Lee,

Your rev limiter is one of the infamous 'speed relays' that operates the fuel pump relay, which interrupts power to the fuel pump. The same signal that drives the tachometer is fed to the speed relay, which is labeled 'J-35 Speed Switch' in your part II of the '78 circuit diagram in th factory service manual.

[Leland Pate]

RObert, I can never tell if your serious or not ... but if you are- the two are unrelated. I hit 7K while i was fooling around a couple days ago. Just didn't have time and remember to post at the same time until now.

Cary, I also seem to recall the rev limiter on 3.0L cutting the fuel supply... I just can't see how this would work efficientally.
I mean, with a switch in the CD box or something on the dist. to instantly cut or retard the ignition makes sence... that is instant.
To me, it would seem that there would be fuel in the fuel lines and such that is still going to be burned even after the supply is cut somehow.

I'm going to do a search and see what I can come up with... probably should have done that first, huh.

[rstoll]

Lee, I am never serious if I can help it.

[Leland Pate]

Warren... I've been doing a little thinking... (I'm not the best at reading current flow diagrams) ... could this be in any way related to the fact that my "inertial fuel cutoff switch" has been disconnected??? The switch on the back of the injector boot on my CIS?
It was fualty and I had to unplug it or the fuel pump wouldn't run.
Now the downside is that whenever the key is in the "on" position, the pump starts running.
Could the fact that this harness is undone have any effect on the rev limiter not working?

I need to go pull out my factory manuals...


I wonder if there's any way to install a better rev limiter on my car.... ??? ??? ??? ??? ???

[Tony911Z]

The rev limiter relay module is located behind the instruments, I think closest behind the oil tem. When they added the cat to the emissions system they could no longer use the ignition type rotor rev limiter as found in the earlier cars.

The SC's rev limiter cuts fuel. A few years ago, I discovered that my car had no rev limiter either. When I found the module it was disconnected. When I reconnected it, the car would die immediately. The failure mode seems to be to always cut fuel. I'll bet yours was disconnected as well.

I priced a new relay at Stoddards for over three huindred bucks. My SC is sans cat so I've been thinking about installing the centrifugal-type rev limiting rotor.

[Superman]

I've been to seven grand too, so maybe my rev limiter really isn't working. Hmmmmm.

At any rate, Mr. Pate, I can explain some of the logic around this system. Cutting spark causes the unfortunate condition of having fuel going through your engine and without being burned. The fuel washes the cylinder walls and then burns in the exhaust, killing catalytic converters. So the better system is one that robs fuel. Pistons are not bothered by a spark with no fuel. And cutting power to the fuel pump of a CIS system shuts the engine down just as immediately as cutting spark. Seriously. Start your engine and then go and pull your FP relay. the effect is instant. And finally, see if there is some mechanism, if hte part is too expensive, that will stop your fuel pump in the case of an accident. That is the mechanism that you unplugged, I think. It would not be too tough to rig up a relay that will close when it quits seeing your ignition signal at the coil. Those are the ones I'm most used to. the ones that have a relay that shuts off the FP if the engine stops running. Your starting system already runs the FP during starting. Anyway, these cars are designed to stop pumping fuel in the event of an accident, and they should retain that feature. IMHO. I'll be checking mine out.

[island911]

Originally posted by Superman
I've been to seven grand too, so maybe my rev limiter really isn't working. Hmmmmm.

At any rate, Mr. Pate, I can explain some of the logic around this system. Cutting spark causes the unfortunate condition of having fuel going through your engine and without being burned. The fuel washes the cylinder walls and then burns in the exhaust, killing catalytic converters.

Esp. when when the spark comes back, and all that unburned fuel/air in the exhaust goes BOOM !

Guys, see if you can take it to 7200rpm.
Bentley says the SC's are limited "around 7000 RPM" I also remember reading the spec for mine was 6750±250. FWIW

[Leland Pate]

Ok, here is the latest.

First I want to thank all of you for your responses.

I went back out there and played with that switch.
Here's the scoop: About three years ago, my car died and wouldn't restart. My mechanic said that the air plate sensor and or harness was bad, making the engine believe that it wasn't running and cutting the fuel pump. So, he disconnected it, saying that it would be very time consuming to track down the problem.
The car started fine, the only draw back is that when ever the key was turned "on" the fuel pump started running. The switch is designed to only allow the fuel pump to operate when it senses air flow over the air plate on the injection system.
This is what stops the fuel pump in a crash...etc.

So, I pulled out my Factory manual like Warren suggested and looked at the diagram in question.
As aweful as I am at reading these current flow diagrams, I did finally figure out that it sure looks like this faulty switch on my car plays a part in the rev limiter...

As best I can tell (Warren, please feel free to correct me) from the diagram, this is how the system is supposed to work:

The CD unit tells the Tach that it has moved past the maximum rev limit.

The Tach tells the air flow sensor plate switch to shut off...

Then the sensor plate switch sends a signal to the relay and it cuts the power from the fuel pump?


Is this even close???

Superman, I see what you are saying about the unburnt gas. That makes sense. Plus what else occured to me is that as soon as the fuel pump is cut off, the instant drop in pressure in the lines will cause the injector lock balls to close the injectors, imediately preventing all fuel from getting to the combustion chamber.

Ok, now for Part II.


I went out there armed with this information and plugged in the air plate sensor harness again. I didn't expect it to start because this was supposedly a faulty circuit a couple years ago. "Vroom"... fired right up, I was shocked! I drove it around for about 20 minutes and didn't have a promlem!

The best part is, that I no longer have to worry about the key being in the on position for too long. Now it works like it should... the fuel pump only comes on when the engine is cranking or running.

But, the bad part... I warmed it up and found a deserted stretch of road and slowly increased the rpms until I was at almost 6700 rpm... nothing... still no limiter.

So, now it's back to the drawing board.

Tony, can you be a little more specific?
You mean it's located behind the oil temp guage?
I can't find a rev limiter "module" anywhere in my factory manuals.

My car is also "sans-cat, what would you have to do to add a centrifugual limiter to the rotor???


I'm going to throw some pants on and go take a look and see if I can't find this module that you are refering to...


Thanks for all of the tips so far!

[Erakad]

This just got my interest because I just installed a rev-limiting rotor on my 83 Euro....with cat! The Euros, I'm told, had no other form of limiting revs, hence this particular rotor. Question is, why (other than smog control) can you not have a cat and the rev limiting rotor on the same vehicle? Is this a US model unique problem, or one I should remedy soon by removing the cat?

Ya'll are a never ending supply of information, I'm continually amazed with what goes on here! I personally would never had made a connection between the rotor and the cat.....amazing

[Early_S_Man]

Lee,

Let me see if I can clarify the matter ... assuming everything is normal and working ... the way it left the factory:

1. When you turn the ignition switch on, initially, the Air Flow Switch is closed, and the fuel pump relay aperates, so the fuel pump has no power.

2. When you turn the ignition switch to the 'start' position, the signal to the starter solenoid provides power to the fuel pump through contact #87 of the fuel pump relay.

3. When the engine fires, air movement opens the Air Flow Switch contact and the fuel pump relay releases, providing normal power to the fuel pump.

4. During normal engine operation the CDI-unit sends pulses to the tachometer, which displays the current rpm level, and the same exact pulses go to the Rpm Switch ... which does nothing as long as the rpm level is below 6500 rpm +/- 200 rpm.

5. At some rpm level between 6300 rpm and 6700 rpm (if the Speed Switch is within normal tolerances) the relay inside the speed switch opeates, providing a ground signal at its' #4 terminal for terminal 85 of the Fuel Pump Relay ... and the fuel pump power is interrupted by the fuel pump relay!

[Leland Pate]

Warren, I was confused about the speed switch/air sensor deal.
It wasn't until about 15 minutes ago that I realized that they were two different units!

The wiring diagrams in the Bentley maual are much easier for me to follow as they are broken down a bit more.

Now I see that there is a speed switch and that it is totally different than the air sensor switch mounted on the injection system...


Ok, so now... where exactly is this speed switch located???

I can't seem to find a photo or doagram anywhere!



Thanks so much for helping me narrow this one down!

[Leland Pate]

I usually try not to "bump" my own topics, but I really need help here.

So far, I've looked though 101 Projects, my factory mauals, and my Bentley book all to no avail.
They all talk about the speed switch, but none of them show a photo, diagram or give it's location.

Please, if anyone out there knows where this doo hicky is located, please let me know.

[Early_S_Man]

It is in front of the instruments ... somewhere, and you probably need to attack from the front ... carpet loosened and folded down, in trunk!

If you find the 'TD' terminal on the tach. ... White/Red wire on later SC's though the '78 schematic isn't labeled ... you can probably trace it down. It will be a medium-small sized aluminum or black plastic box.

[Leland Pate]

Ok second question...
Warren, if I do find that this box is bad, and that a replacement is super expensive, can I backdate to a '77 rotor that has the inertial weight in it?

My car doesn't have a Cat.


Has anyone done this?
Can you do this on a US spec car?
Erikad has a Euro...

Hmmmm... this would seem to be the best solution for a malfunctioning speed sensor.

[Early_S_Man]

Lee,

I just checked Stoddard's ... no go on the SC part, but the Turbo unit is ... 93061812300 R.P.M. CONTROL Call for Availability $248.05

If it is bad, send it to me, and I will see if it can be fixed. It may just have a 'rusted-thru' transistor lead ... like the MFI speed switches (like mine had) almost always get! No big deal to repair.

The '77 rotor and cap are different, and rotation is opposite than your SC dizzy ...

Maybe one of the guys deployed in Europe can pick up a ROW SC rotor for you? The '81 930/10 ROW engine uses a 6800 rpm rotor.

[pwd72s]

Warren?