1972 911S cold start circuit

[Lash LaRowe]

My cold start solenoid located on top of the fuel filter is not energizing when cold. I removed the wire from the cold start solenoid and jumped 12v to solenoid, it energized and pumped gas into velocity stacks. So I know that the solenoid works but not getting the 12v when engine is cold.

I cannot find a wiring diagram to see how the cold start circuit works. I did find the cold start relay located on electrical panel and it checks good.

I'm going to assume that when the engine is cold the crankcase thermostat will pass 12v DC or ground to the cold start relay coil. The voltage is then sent to the cold start solenoid.

Is the cold start solenoid a 2-3 second timed solenoid?


A circuit diagram would really help with this problem

My problem started when I removed a permatune CDI and installed an MSD. Never had a problem starting the engine before, now it takes some time to catch and then it wants to pop and run rough for 8-10 minuets then its almost as if a switch goes on and the car runs great. So I may also have a problem with the warm-up enrichment system, but I'm trying to take one step at a time.

I pulled the crankcase thermostat out and using an ohm meter i do not have continuity, (its cold in my detached garage 45F) I heated it up with a heat gun and never had continuity. So i'm going to assume its bad, i shorted wires together and still nothing

I'm about ready to put the Permatune back in but I keep thinking it should not have anything to do with the difficulty of starting and warming up

Hope someone can help

Lash

[edmayo]

Did you put the ground wire back on that would have been bolted to the relay board at the CD box location?

[Lash LaRowe]

Did you put the ground wire back on that would have been bolted to the relay board at the CD box location?

I checked and I do have the braided ground cable connected to the electrical panel back towards the fuel filter.

I felt around the back of the fuel filter and I found another wire that has a round metal connector that looks like it should be a ground connection, it has a dark orange or maybe a brown color but it is a solid color just not sure what color you would call it. This cable looks like it is going straight down back through the floor of the engine compartment.

The metal connector looks like its big enough for the bolt that connects the braided cable.

Let me know what you think

[edmayo]

In order for the solenoid to squirt fuel it must be grounded, since the fuel filter housing is mounted on rubber it is not grounded, therefore it has a brown ground wire that goes from one of the 10mm filter housing bolts to a ground stud that is welded to the inner fender panel in front of the filter housing. Just use an ohm meter to see if your filter housing has a circuit to ground, if not the squirt solenoid won't work. If that unattached wire you have has a 10mm eyelet, that is the end that goes to one of the filter housing bolts, if its an 8mm eyelet it goes to the grounding stud. All the solenoid does is get the engine started, mixture control during warm-up is a result of the thermostat action.

[Lash LaRowe]

I mentioned in my first post that I was able to jump 12 vdc to the cold start solenoid and with the ignition switch turned to where you can here the fuel pump I was able to squirt fuel into the velocity stacks. So that tells me I already had a good ground.

I was able to find info online showing a wiring diagram. On the cold start relay located on the electrical panel I have 12 vdc on pin 30 which should pass onto pin 87 which goes to the cold start solenoid when the coil is energized. It looks like the coil receives 12 vdc from the starter motor when it is engaged. I will move the Porsche over to the lift tomorrow and do some more troubleshooting.

Maybe I disturbed a wire on starter motor when I was following MSD installation instructions. MSD instructs you to provide 12 vdc from starter motor using a heavy gage wire (14 gauge). This would possibly cause my cold start problem but not the warm-up thermostat process.

I just might reinstall the Permatune tomorrow just to see if my problem goes away.

Lash

[edmayo]

Ahhh, yes you did,,,good point, yes, you may have bumped off the small yellow wire at the starter that feeds the thermo-switch. Speaking of that MSD hot wire attachment at the starter I have seen several home installs of the wire that simply run a wire directly from the starter to the MSD box,,,,,totally unprotected and running past the engine sheet metal just waiting to be rubbed through and create an instant arc welder. When I've been forced to rewire some ones installation I prefer to insert the wire in a vinyl sleeve then route it with the original harness that goes up along the shock cross member and then over to the relay board area.

[Richy]

The thermo time switch provides continuity to ground for the cold start solenoid relay coil . The wire from the starter heats the thermo switch to break continuity . There is never continuity between the terminals on the switch .

[gled49]

The continuity exists between the body of the thermo time switch and one terminal. The other terminal is where the yellow wire from the starter circuit connects. And then like Richy says above. Gled

[michaelaiellosr]

...
I cannot find a wiring diagram to see how the cold start circuit works. I did find the cold start relay located on electrical panel and it checks good.

A circuit diagram would really help with this problem "

Lash

For future reference, the Haynes manual has schematics that have come in very handy for me

[Lash LaRowe]

Great Information

Thanks Richy and Gled for explaining how that thermal time switch works.

When you engage the starter motor does the 12vdc that is applied to the thermal switch cause the switch to heat rapidly and then open the switch?
I read somewhere that the cold start solenoid only pumps gas for 2-3 seconds.

I found a Rentech wiring diagram online and was able to expand the diagram making it legible.
I do have a Chiltons and Haynes manuals but the wiring diagrams are very hard to follow.


Edmayo, when I installed the MSD I used a good quality plastic wire loom and made sure I routed the wire away from any potential pinch points

I will keep you informed as I make progress,

Lash