What is a "speed switch?"
 

I have installed all the old wiring on my 1979 project. Because the PO, more specifically her mechanic, cut and taped wires everywhere I thought I should trace them all to make sure everything is back the way it should be. The factory wiring diagram has several wires going to the "speed switch." There is a wire from speedometer, wires from the fuel pump relay, and several others. Does anyone have any idea where or what this thing is? Just chasing the wires showed nothing under the bonnet.

Thanks,

Please correct me if I am wrong, (thank you), but wasn't there a time in US politics when many automobiles were being limited to top speed through limiters on the engines? I have seen some threads on this board about speedometers from that era. I suspect the "speed switch" would have been a device designed to cut or restrict fuel flow at a pre-determined speed. How un-Porsche!
Les

I found a non-Porsche book list the same thing as a rev limiter. My guess is that you are right about limiting top speed. I would still like to know where it is located on the car. I would also like to know how to bypass it since my engine management system will be controling all fuel and spark decisions.

Not in the U.S. Maybe in Canada or somewhere else. The speedometer law you speak of had two aspects - one was the 85 mph limit on the speedometer itself. The other was an accuracy standard because factory speedos had become so inaccurate, it was getting out of hand. Yeah, it was a drag having to estimate speed via tach, but top speed was not regulated. The Z28 of that era had an interesting speedo in that as a needle swept a mph scale, the needle was extended below the pivot to sweep a kph scale, as there was a push to "metrify". Once the 85 mph side was reached, the "bottom" kph side of the needle would sweep the mph side so you could tell how fast you were going by adding 85 to whatever the kph needle indicated. So if the kph half of the needle showed 35 mph, you were going 120 mph.

I don't think the US had any top speed limiters in the 70's or 80's, especially in Porsches. My vote is the rev limiter. They exist now, even in Europe. I don't think they could have cheaply and reliably restricted top speed in the '70's, no computer control makes it more difficult. The only thing that I know of that was limited that pertained to speed was the speedometer.

PS Your '79, assuming stock power should have a top speed between 140 and 145. Maybe upper 130's on a bad day, hot, headwind, bad gas...

:D

The factor wiring diagram shows it schematically between the tach and the fuel pump relay. I would bet that the speed limiter is behind the guages. Have no clue what it looks like. But would guess it looks like a normal black-and-boxy relay.

Jeez, you wouldn't want to run the thing out of gas at high RPM in the name of speed control! Must work some other way.

Regarding the speed switch function. This is what I figured out while trying to solve my non-working tach on my 72T. This little box (located on the relay / ignition module panel) controls when voltage is applied to the fuel cut off solenoid based on engine speed and throttle position; it cuts fuel during decelleration, which reduces hydrocarbon emissions.

How it works: The circuit is timed to close an internal relay contact when the micro switch on the throttle linkage is closed, AND the tach pulses are above 80 Hz or so (~1600 RPM?). The 12V output was verified at the relay contact when ~80 Hz was delivered to the unit by an external square wave pulse generator.

This is all just fine, but my tach still does not work. :mad: Ideas anyone??

Alex