mcs51

Hi there,
To me, relays X (supply) and XII (fresh air blower) on a ’90 drawing can’t be energized !?!?

First page of attached document:
Terminal 85 of both relays should go to ground to be energized.
BUT
In this drawing they goes to terminal 50 of the defogger relay I-II and further to terminal 87 of the starter relay, second page of attached document.

That terminal 87 of the starter relay gets 12V when the car is started, key to ignition position.

Any idea where the 85 terminals of relays X & XII are really connected too?
Thanks for a reaction!

Regards

Alan

In exactly the way you describe. Terminal 50 of the starter is the solenoid, this is a very low DC impedance to ground (inductor) driven by the starter relay. All the time that the starter relay is INACTIVE that 50 connection is a pretty good ground for a relay coil that only requires <200mA to operate. When the starter is activated it is not - so the Blower fan and the X relay will be deactivated - which reduces the system load while starting (that is the whole point of using this as the ground).

Alan

mcs51

Thank you Alan for your answer!
The idea of reducing load while starting the engine, that I knew of
but… …
I didn’t know about the very low impedance of the starter motor.
I saw two coils in series and thought “That cannot work ?!!?”

davek9

Yes, thank you Alan!
I always imagined it was a function of the ignition switch, Do all MY's utilize the Starter solenoid as a ground for the same relays?

Dave K

mcs51

Nope, and that is also what confused me!!

Up to ’89 both relays X and fresh air blower are connected directly to ground, only the defogger is shut down when starting the car.
’90: relays X, Fresh air blower and defogger via starter coil
’91 onward: relays X and defogger only

I think they didn’t know what to do @ Porsche those days, so they changed things from time 2 time to complicate our lives today

Alan

No - the configurations changed by year, there are typically other methods to reduce load though.

On later models the X-relay was supplemented with a "Supply" relay with a similar load reduction purpose and configuration.

The ignition switch itself provides a drop out for the X-Bus ("X" in the diagrams) during starting - this was used directly on earlier models but was supplemented with relays as the accessory loads were increased over the years.

During starting the brake lights, headlights, fog lights, cooling fans, blower fan, window defoggers, heated seats are typically disabled to preserve power for starting.

Alan