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Posted by: Bleyseng Mar 23 2007, 09:49 AM

Here is a pic of the speed limiter relay for the 75-76 FI harness. It plugs into a connector halfway between the ECU plug and the MPS plug.
I think it mounts to the battery tray.


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Posted by: Ferg Mar 23 2007, 10:51 AM

Yep, that's where mine is on my 75.
Ferg

Posted by: Porsche Rescue Mar 23 2007, 11:53 AM

What does it do? New info. for me.

Posted by: Bleyseng Mar 23 2007, 06:21 PM

My guess is it replaced the revlimiter rotor so ya can't overrev the engine.

Posted by: ClayPerrine Mar 23 2007, 07:48 PM

Past a certain RPM it cuts off the fuel. The old rev limiting rotor allowed excessive unburned fuel because it cut the spark. This was done to meet emissions requirements in 75. It's amazing what I retained from the Bosch training classes from years ago.

Posted by: Porsche Rescue Mar 23 2007, 07:51 PM

Wonder if it is a part that could fail, leading one to believe there was a problem with the fuel pump/relay or some other fuel system component? I would never have thought to check for a fuel cut-off device!

Posted by: jk76.914 Mar 24 2007, 07:46 AM

And I thought the CU16X ECU did this by itself. So did the ECU energize the relay to turn off the injectors? My '76 has its original CU16X, but hasn't has the relay since I've owned it ('83).

Posted by: Bleyseng Mar 24 2007, 09:01 AM

I thought the late style ECU 052 had the fuel over run cutoff inside. So when you let off the gas at 60mph and coasted the fuel shutoff worked until 1500rpms when it came back on.

So this relay shuts off the fuel at what rpms?

Sh*t, I'll just have to hook it all up and test it out.

Posted by: jk76.914 Mar 24 2007, 09:22 AM

Oh crap. I got us off track. Clay is saying it cuts off fuel at high RPM, and I steered us over to overrun cutoff... So the overrun must still be handled by the ECU directly, and the RPM cutoff by the ECU but through the relay?

Posted by: ClayPerrine Mar 27 2007, 12:18 PM

It shuts of the injectors at high RPM. That does the same thing as shutting off the ignition, but without the emissions hit.

It does NOT shut off the fuel pump at all.

Posted by: Bleyseng Mar 27 2007, 07:55 PM

What rpm? or rather how high it high rpm?

Posted by: ClayPerrine Mar 27 2007, 09:40 PM

I honestly don't remember. I would have to dig out my training materials from years ago and look it up. I know it is in the "computer", and that it varied per model and sold location. Cali cars got a lower RPM that the rest of the 49 states, because of the cat.

Posted by: Bleyseng Mar 27 2007, 11:12 PM

it will be fun to do a test then of it.

Posted by: blitZ May 1 2007, 02:07 PM

So, it won't hurt anything to disconnect it?

Posted by: Bleyseng May 1 2007, 07:37 PM

not that I can tell...I have yet to test it other than the 052 ECU is in my car now. Trying to save on gas, hehe

Posted by: scotty b May 1 2007, 07:55 PM

If it shuts off the fuel at a specified RPM, then it has to kill the voltage temporarily to the injectors right? If that is the case then removing the limiter would result in NO voltage getting to the injectors at ANY time unless the hot wire was jumped no?

Posted by: Pat Garvey May 1 2007, 08:31 PM

So, why the change from the original spring-loaded distributor rotor that cut off the juice when you exceeded safe zone? Was the rotor too expensive (i still have 2 new ones of that configuration)? Was "the Box" less expensive (hard to believe)?

Or...was it an emissions thing?
Pat

Posted by: Bleyseng May 2 2007, 12:53 PM

On the rev limiter rotors it cuts off the Spark...
The speed limiter cuts off the injector signal. I haven't even looked at its wiring yet.

Posted by: Ferg May 2 2007, 01:47 PM

Pic of how mine is mounted on the 75.

Ferg


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Posted by: blitZ May 2 2007, 08:36 PM

I disconnected mine and drove around a bit, couldn't tell any difference. I usually shift at 5500 during spirited driving, so really never could tell if it was cutting out or not. Who knows maybe it had failed years ago. Anyways, I nice weight savings.

Actually tached it out for the first time since I removed the limiter. It used get to 5200 or so then creep up. I thought that was normal, now it shoots up to 6k quickly.