Starter Troubleshooting
 

Troubleshooting a starter on my 83 911 SC. Here's what i have so far (in rough order of troubleshooting steps):
1) Starter will not engage via the key/starter switch. Turning the key to the "start" position results in a "hum". No "click" from the starter solenoid.
2) Next i went to check the voltage at the Starter. I have 14.3 V at the starter. Drops to ~12.5 V when turning the ignition to start. However the starter does not turn nor do i get a click from the solenoid. New battery
3) Starter bench checks fine. Starter solenoid appears ok, Pinion gear pushes out, starter spins
4) Reinstalled starter... i jumpered terminals 30 (power/red) and 50 (yellow) with the starter in the car. Yellow wire disconnected.When I did this, the starter spins (pinion gear did not push out to engage the ring gear). Removed jumper and the starter spins continuously whether the 30 and 50 terminals are jumpered or not. Even with the ignition in the off position.
5) disconnected battery ground to get starter to stop turning. Reinstalled yellow wire to solenoid. Reattached ground. Starter spins regardless of ignition key position. Spins with key removed.

I'm thinking bad ignition switch?

Starting to sound like it ( sorry for the pund )

But before ordering a new ignition, have you figured out how to hot wire under the dash? That should confirm it, no?

The only thing that is strange is your voltage, 14.3v seems very high. is this after sitting an hour?

The ratio of voltage drop seems right when ignition is turned to start.

Sounds like the solenoid is stuck closed if just hooking up the ground regardless of key position causes it to spin.
Does it do the same thing with yellow wires disconnected?

above.
stuck solenoid.
perhaps from messing with it on the bench.

you can "rebuild" the solenoid if you can solder.
basically take the end off and clean the contacts as best you can and spin the contacts so new "meat" is making contact.

I agree with timmy2 that it appears to be a solenoid issue. Note the bold in your OP, above, as those are the areas that caught my interest after putting it all in context.

From 1, the hum you heard is likely the coil inside the solenoid failing to move the pinion and switch power to the starter motor.

From 2, another confirmation that the solenoid is not doing its two-part job--moving the pinion gear out to the flywheel and switching power from 30 to the starter motor.

From 3, when removed from the car, solenoid appears to be functioning properly--pinion moves out and starter motor spins.

From 4, jumping terminal 30 to 50 should have, as you know, engaged the pinion to the flywheel and powered the starter motor, but it did not, it only caused the starter to spin. Removing power, the motor continued to spin. That means, the switch inside the solenoid that shunts power from the lug of terminal 30 to the starter motor was stuck closed, as timmy2 suggests. Turning off the key would have no effect as 30 is a direct connection to the battery.

My guess is there is a mechanical failure inside the solenoid that, first, prevented the pinion from moving and the solenoid just hummed. Maybe something binding or on the verge of breaking. On the bench, any resistance to the movement of the solenoid mechanics was gone so everything worked as it should. Once reinstalled, the resistance returned. Jumping 30 to 50 gave the solenoid coil enough power to overcome the resistance, but whatever was weak, broke, causing the pinion to fail to move into the flywheel teeth but resulting in the solenoid shaft, which is connected to the switch that shunts power to the motor, to move and stick in the closed switch position.

Replace the solenoid/starter.

A new Low weight, high torque starter perhaps?

+1

In the past I have disassembled the solenoid/starter on several vehicles and cleaned and rotated solenoid contacts. This will certainly add life to the unit. Or you can just buy a new or rebuilt unit.

What you decide to do will depend on many factors, time, funds, curiosity, your mechanical/electrical capability and the most important part, a desire to hang out in the garage tinkering with Der Porsche.

Thanks. Great input. I was leaning towards a failed relay, but agree the solenoid sounds like the culprit. Will try to pull this weekend, clean and replace. Depending on what I find, rebuild / temp fix or replace. Just searched Pelican and a rebuilt Bosch is $175 (or $215 need to confirm). Cheap, relatively speaking anyway. Not worth the risk and hassel of a failure on the road trying to eak a few more miles out of the old starter. Stand by....