Dead tach

[Baxter2012]

I have a 1970 911 e
The tach is dead totally. I have located the bl/pu wire that runs from the tach to the intermediate unit but according to my schematic it ends there and picks up again at the speed switch then on to the distributor.
I hooked another tach to the distributor and got no signal either.
Can I run a wire from the coil direct to the tach to test for signal w/o messing up the tach. As I understand it this was the way it was before 70 and the intermediate unit.
The only thing I se between the intermediate unit and the speed switch is a black wire, is that how the bl/pu
Is bridged between them.
Any idea on how to diagnose this issue would be much appreciated.
Thanks

[Richardnew]

I just had my tach repaired by Seattle Speedometer. Buz did a great job.

Richard Newton
Winter Storage Ideas

[classic]

Bypassing the intermediate unit on a 69-70 Tach will result in no reading, as the Tach can't read the signal.

Was your spare Tach from a 69-70?

If its a later Tach, you could bypass the Intermediate unit to fault test the unit.

[Baxter2012]

So again what is the speed switch in the equation

[304065]

Baxter, I'll respond to your PM here.

You have a 1970 911E with an intermediate unit. And your tach doesn't work.

Do NOT run anything from the coil positive, that is a +350V pulse from the CDI that will destroy everything electronic it touches.

Here is an appropriate diagram:



As you can see from the diagram, the "intermediate unit" IS connected to coil positive, it receives the high-voltage pulse and transforms it into something the tach can actually accept. The black/violet wire comes off the intermediate unit and goes to the back of the tach to the /1 terminal.

The speed switch, on the other hand, receives its signal from the points. The signal from the points is an 11 volt square wave with about 420mA of current-- it is created by the pull-up resistor inside the CDI box. This square wave is read by the speed switch which then outputs a voltage to the MFI fuel shutoff solenoid in accordance with the speed switch logic.

If you were to run a wire from the points directly to the tach this would not work, as said above your tach with a production date prior to 10/70 does not run off the 11 volt square wave, it runs off a high-voltage pulse from the intermediate unit.

That's all there is to it-- no other diagrams or explanations are relevant- if you have a "new" wire harness that is the first place I would look-- and look to support from the vendor of the harness- diagnosis of electrical problems over the Internet is difficult enough with original factory parts but when aftermarket parts are introduced it becomes impossible-- and many aftermarket vendors don't provide support.

Good luck!

[Baxter2012]

So 2 is the intermediate
3 - is speed switch
5- is shut off selinoid

[304065]

No- 3 is the CDI, 2 is the intermediate unit, 5 is the speed switch.

[savaden]

What the speed switch does is shut off the fuel supply when the engine is not turning. In the case of an accident, you don't want to keep supplying fuel to a dead engine. This has been accomplished with electronic fuel pump relays and incorporated directly into engine management later on.

SV

[304065]

That is not the function of the speed switch.

The fuel supply is an electric pump that runs independent of whether the engine is turning or not.

The speed switch is an "overrun cutoff" that improves fuel economy and prevents fire from shooting out of the tailpipes when the throttle is snapped closed.

When the throttle is closed and the engine RPM is above 1500 RPM the speed switch commands the solenoid to move the main rack to shut off the fuel. When the engine RPM falls below 1300 RPM and the throttle is closed, the solenoid is released and the rack returns to the normal position.

This most often happens at the end of a long straight on the track-- when you take your foot off full throttle and put it on full brakes the throttle snaps closed but the engine is still turning-- raw fuel will dump into the cylinders, washing the walls down which is bad for lubrication, and also wastes gas. The fuel, having nowhere to go and no oxygen to burn, passes into the exhaust, which upon reaching the oxygen in the atmosphere will shoot a short flame out of the pipes.

I found this effective for keeping other cars off the tail in the braking zone. . . actually my speed switch was "disabled" because one of the transistors legs had rusted through. Shake it: if you hear a noise. . time to rebuild it



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