Hello Guys,
I have a story of no spark with my 1978. I had taken the car on a 2000 mile trip last fall along the California coast and it ran well. On the return trip there was some clutch chatter that I was concerned about. Then my power steering pump began leaking so it was time to do some work on the car. Over the winter with other projects and corona I finally was able to serviced the two identified problems and tried to start the car but no dice. I further diagnosed that there was no spark from the coil. I went back and cleaned the connections at the starter and alternator and they all seem fine. Tested the voltage, with the key on, at the positive coil terminal and it reads 4.12, With the engine cranking the test lite flickers on the positive coil terminal and is solid lite on the negative post. I tested the ballast resisters and they are with in spec. So I must have messed something up but I am not sure what. Thoughts

Hope you are being safe.
Andy Anderson
Silver City NM

 

did you replace the green wire?

 

Green wire first thing to check as Stan says below. Also, is your distributor installed in the proper orientation - rotor pointing to index mark when the motor is set to TDC and timing gear marks both aligned?

 

So yes Green Wire, I have one and it looks a bit fried. Some of the outside insulation is frayed with wire exposed. I have a spare and if it checks out I will give it a try. If not I will order one up. They are not cheap and I wonder why a wire would be so expensive. Would a shorted out wire cause a fuse to blow. I have not checked them or the relays yet. Not sure why this would cause the no spark problem as I did not work around the distributor with the previous repairs.

 

Yep.
And pull the distributor cap off and check around. Had a similar situation and found that the rotor had cracked and was loose.
-Jason

 

You are probably on the right track....go back to whatever you touched last.

There's two small wires at the starter, which go to different posts. One operates the solenoid and the other bypasses one of the ballast resistors to provide more voltage to the coil when the car is cranking.

Yellow to 50 on the solenoid and black/yellow to 15A on the solenoid. Confirm that you have 12 volts on 15A when the engine is cranking. Then confirm that you have higher voltage at the coil, when cranking.

 

So Green wire connectors came apart in my hands when removing. The back up also is falling apart so ordered a new one. I might need a bit more direction from Greg, I will look at those wires again and see if I understand how to confirm the voltage.

thanks
Andy Anderson

 

well first put in a new green wire it fixes about 97 % of not running issues

 

The voltage to the coil drops too low (under cranking) if it has to go through both ballast resistors. Because of this, Porsche added an extra wire (which originates at the starter) that supplies full voltage to only one of the ballast resistors, in order to provide more voltage under cranking conditions. This wire is black/yellow and goes to a separate post (from the yellow wire) on the solenoid.

Just make sure of the wiring at the solenoid is correct. Hook up a voltmeter to the black/yellow wire and make sure it is near 12 volts when you crank the car.
At the coil, you should see higher voltage (under cranking conditions), than the voltage you see with just the key on.

If all of that works properly, the starter is correctly wired and functional.

 

So the voltage at the yellow striped wire is 10.12 and at the Positive terminal at the coil is 4. Still a mystery.

 

Is this during cranking?
Good luck,
Dave

 

yes during cranking.

 

That sounds like a voltage drop from bad wiring. Check your harness to see if you have any crazy corrosion.

 

The yellow/black wire should be battery voltage. I assume the voltage at the jump post is 12+ during cranking. If so, I agree you have too much of a voltage drop. Suggest bad wiring or connections, or measured wrong (I've done it.). You could run a temporary line to the 0.4ohm resistor and connect during cranking, to check starting.
Good luck,
Dave

 

A very common failure point is the ground strap at the back.

IIRC, early cars have a somewhat different setup than the 'strap & butterfly nut' that later cars do.

The easiest way to check this is to take a pair of jumper cables, use one of them from the ground post of the battery to a good ground on the car (rear coilover mount in the back works well).

You are NOT trying to jump start the car, you are simply making sure there is a good ground connection between the battery and the chassis of the car.