1970 911 no spark

[paid]

the troubleshooting test in Haynes manual leads me to believe the rpm sensor (in Haynes) or ballast resistor from pelicanparts schematic is at fault. Local mechanic thinks very unlikely. will car run if this is unplugged? before car died it was running rough from about 3800rpm on up, tach would not go above 4000 even though the engine would.

[304065]

WELCOME to the forum. Most guys post their location and their particular car in their signature line. Also, at the upper right there's a "search" feature that will reveal hundreds of threads on the topic of "speed switch" and "intermediate unit."

But to answer the question, a '70 911 doesn't use a ballast resistor, even though that's what some of the parts manuals call it-- that silver can that says, in German, "Install with contacts on bottom" is an adapter to produce a signal to drive the tach from the CDI's 460V output. That device has probably failed, leading to the symptoms you describe.

The RPM sensor could only cause this problem if it were somehow shorted out, which we've never heard of happening. The car will run with it unplugged.

Here's a thread to explain all this stuff, let us know what questions you have after reviewing it. And once again WELCOME.

Tach - oh - woe (pre '74)

[paid]

Thanks though still a bit confused, I think what my Haynes is calling rpm sensor is what parts schematics call ballast resistor and is really the voltage adapter you describe. Bosch part number 227 990 001, when I get home today I will check. and if says "...this side down" I will try and start car with it removed

kurt in mass
70 911T
82 Targa

[paid]

The thread you sent describes with a photo part 227 990 001 thanks!

[Zeke]

Hey, John, doesn't a bad coil exhibit those same symptoms?

[GotaT]

When you turn your ignition switch can you hear your CD box "whining" ? Located in the Left of the Engine compartment

[paid]

box make noise and passes test in haynes, part of my problem has been haynes mislabeling the transformer that converts the signal to something the tach can use. All tell me the rpm sensor never fail but the part I mean is not the rpm sensor, I am misinformed.

[paid]

checked the points - once properly gapped seems to run fine