Humming noise, and won't start
 

Just drove my 1978 911 SC today around town, got home and parked it, went back out to start it and--no go!! Just a humming noise, seems to be coming from right in the middle of the car. Starter motor not turning. This happened once before, about a year ago. I let it sit for about 6 hours, went back and it started right up! What's going on here? Thanks for any advice!

My guess is that your starter is suffering from heat soak. The hot engine without moving air will overheat the starter. In a few hours, when it cools off, it works again.

The humming is probably the fuel pump. Can you bump start it? Will the motor run well after a bump start? If yes, it is no problem with the fuel pump.

The starter may also have a bad spot on the commutator. If you can roll the car in gear slightly, it may move the commutator just enough to get the starter to work.

Hope this helps.

It also could be the solonoid, but to replace that, it is easiest to just replace the starter assembly.

When this happens, try smacking it w/ a hammer. It sometimes temporarilly helps, and it usually makes me feel better! These old things "freeze up" from the heat as was said. You can change the grease, rebuild, etc, but when mine finally gave out I just went for a new high torque model.

I see you are new. If you use the "Search this Forum" button you will find out more. You can also check the parts catalog.

And don't forget to post pictures of your car.

...

Well, the hammer was tempting, but I opted for waiting overnight and it started this morning. Rough at first, maybe it was flooded, but warmed up okay. Still wondering about the cause and the fix for it. Does this mean I need a new solenoid for the starter? Or carry a rubber mallet in the back seat?:D

A very simple clean/oil of the Solenoid plunger will take care of the problem.
You'll need to take the starter off to do this and that's when the fun starts.
Do a search on how to hug your transmission to get to the 10mm Allen barrel nut on top of the starter.
If you have high mileage, consider a rebuild, highly recommended.

Solenoid is most likely shot. It will work cold, not hot. Classic SC problem right up there with the windows switch.

Before you do any of that, a very easy and cheap thing to do is to check + clean off any corrosion on your ground connections. I had a variety of mechanics and helpers on this forum offering me suggestions on almost exactly the same problem on my 1987 911 Carrera, and it never fixed the problem.

It got worse and worse until eventually I got stranded at home. At that point I started from the very beginning and spent a good few hours with ground connections (especially the transmission-to-ground strap), baking powder solution, and a wire brush, and I've never had any problems since then.

Dug up this link to the relevant part of my story:

http://forums.pelicanparts.com/porsche-911-technical-forum/268314-87-3-2-wont-start-long.html

Perhaps that will help?

Good point about the corrosion issue and ground straps.
I keep the connections clean and take it for granted that people do the same.
Most often the neg connection from the battery to the body gets neglected.
Use dielectric grease on all connections/pins to prevent corrosion.