ignition switch problem

modelguy · 07-19-2006, 05:03 PM

When I try to start my 77 911 it'll start up and run as long as I hold the key in the start position. As soon as I let go of the key, the engine dies. I replaced the electrical part of the switch, but that did nothing to fix the problem. What next?

Walter_Middie · Walter_Middie's Garage

modelguy,

The CDI unit gets power from 2 places. One is the starter motor. This sounds like it is still working. The starter motor wire begins at the starter with a yellow wire, and goes to the CDI unit - it changes from a yellow wire to a black/red wire, and finally a solid red wire at the CDI unit.

The other place the CDI unit gets power from is during the "run" position of the ignition switch. This sounds like your problem. This wire goes to the CDI unit as a solid red wire.

Turn your ignition on to the run position, and check the red wire going to the CDI unit. It probably doesn't have +12v going to it like it should. Follow this solid red wire until you find where it has either broken or a connector is no longer making connection.

modelguy · 07-20-2006, 10:02 AM

On the three pin connector, the center wire (red) has +12V even with the key out of the switch. That's the only red wire I see. Is the other one the wire that plugs into the spade? Is there a way to get the car running by bypassing something?

modelguy · 07-21-2006, 01:29 PM

Ok, I retried a few things...the center (red) wire has power with the key in run position, none with key out. I checked all wiring and it all looks clean. Any thoughts? I really want to get this figured out. I have a permatune CDI unit if that matters.

Walter_Middie · Walter_Middie's Garage

OK - the other possibility is no fuel. When the key is in the start position, the cold start valve (CSV) is spraying fuel. When you let go of the key (and it returns to the run position) the CSV stops spraying fuel. You may not be getting any fuel to the cylinders - or not enough. Pull an injector, and see if it is spraying fuel when the key is in the run position, and you lift the air flow plate.

Walter_Middie · Walter_Middie's Garage

After thinking about this last night, it's more likely that the WUR is not getting power. That would act like it wants to start while cranking, but dies as soon as you let go of the key, and it returns to the run position. The WUR only has 2 wires going to it - 1 ground and 1 power. Check to make sure the WUR has power, and the connector is not loose. The WUR gets power from that big 14 pin connector on the back left of the engine compartment. The 14 pin connector gets power from the same source as the CDI box. Power goes into the 14 pin connector (from the CDI box) on the lower right most pin (#14), and goes out to the WUR on the top row, 3rd pin from the left (#3).

modelguy · 07-24-2006, 10:06 AM

At the 14 pin connector, the upper rightmost (#14) has power. The second lower pin (#3) and the upper second pin (#4) have about 9V each. The third upper pin from the left (#6) has no power. I tested at the WUR and no power in the plug. Is it possible to trace those wires without dropping the engine? My big hands don't let me get into tight spaces. Thanks for your help so far.

Walter_Middie · Walter_Middie's Garage

All the wires you need are easily accessible from the top of the engine.

While the key is in the start position, power will be sent to the cold start valve if the Thermo-Time Switch allows it. On the left side chain cover is the Thermo-Time Switch which is the basic control for the enrichment solenoid during the starting phase. If the car is warm from running recently, or enough time has elapsed since it was started, the Thermo-Time Switch will interrupt the power going to the cold start valve.

The cold start valve itself has a gray and white wire and a red and black wire from the thermo-switch: Note that the gray and white wire and the yellow wire are connected to the same terminal on the thermo-switch. The WUR (warm up regulator) mounted on an intake runner base on the driver's side of the engine should have a single red wire with white stripes connected to it.

Cold start valve circuit: There should be a wire from the ignition switch to terminal 50 (this is a small blade type lug that receives a female type connector) on the starter, from terminal 50 of the starter a wire (yellow) goes to the throttle valve switch. From the other side of the throttle valve switch a wire (also yellow) goes to the thermo-switch on the driver's side chain case, this wire from the throttle switch attaches to one terminal (this is the non-ground side terminal) of the thermo-switch, a second wire is also attached to this same terminal of the thermo-switch and goes to the cold start valve. From the other side (ground side) of the thermo-switch a second wire goes to the cold start valve.

I hope this isn't too confusing - I stole from previous posts originally done by Jim Sims and Warren Hall (you can't get any better than that). If you get lost, I can help by tracing wires or showing pictures from my car.

modelguy · 07-24-2006, 01:18 PM

I think pictures are worth 10,000 words at this point, especially if the wires are supposed to be easily accessible. Locations of parts you're describing would help the most, I've never dealt with this engine before.

Walter_Middie · Walter_Middie's Garage

I'll see if I can get some pictures - but I am also mixing up the logic for the cold start valve and the WUR. Your cold start valve sounds like it is working OK - it's the power to the WUR that you need to restore - so ignore most of my last post (that is the cold start valve logic). As a test, you can pull the electrical plug off the WUR, and attach separate wires to supply ground and +12v to the WUR, then try to start the car. If the car starts, this will show that the lack of power to the WUR is definitely your problem.

modelguy · 07-24-2006, 07:32 PM

The power wire to the WUR has almost no resistance to ground, so I have to start tracing I think. The problem is that the harness goes immediately under the airbox and I can't see where it comes out...or does it split apart under there to various connectors?

Walter_Middie · Walter_Middie's Garage

The ground wire just goes to the front of the engine and grounds on one of the bolts. You can run a new ground wire to anywhere in back that you can easily get to - from the WUR to your new ground.