Alternator D- (Part 2)

patkeefe · 12-14-2014, 09:33 AM

Question pertains to an SC, which was upgraded to a higher output alternator years ago. The external regulator was removed, as this is a later style alternator.

Should D- be grounded with the system off?

If I take a VOM, and test continuity to ground, I get a closed circuit. I have a generator ground at one of the bolts through the diode pack, and another ground at the generator body through one of the 6mm studs.

If I should have continuity in D-. then what is the purpose of grounding the WUR and AAR through D-?

TIA,
Pat

timmy2 · timmy2's Garage

As D- is basically just a ground point it should be at ground potential when the system is off.

As far as the WUR and AAR and other devices using that as a common ground point, it makes sense as a convenient place for connecting them to ground.

patkeefe · 11:24 AM

WUR and AAR are special, time dependent devices. They are powered from 30 on the fuel pump relay. So, D- has to hold off the connection until the engine starts and the alternator is making current. A CIS WUR should not start heating until the car is running.

edit...I forgot how 30 works...30 (fuel pump) makes when the airflow plate rises, during cranking. But, the WUR should still not heat until the engine is running. So, during cranking, 30 is powered to the WUR, so D- should still hold the circuit open.

timmy2 · timmy2's Garage

During cranking and whenever the fuel pump runs normally, or is bypassed with a jumper for testing, the WUR will be active.

This is why for cold start pressure readings the WUR connector is disconnected.

The ground path is always there.

patkeefe · 12-14-2014, 02:38 PM

What about when it's not jumped 87 to 30 and the key is on? Is the WUR heating?

timmy2 · timmy2's Garage

If the ignition is in the run position before starting the car, and the air flow switch is closed, The fuel pump relay coil picks up and cuts power to the fuel pump and the WUR, AAR etc. by moving the contact over to 87

It is when you turn the key to crank that the terminal 87 gets power and energizes the pump and the WUR circuit along with the starter and CSI etc.
Then when you release the key to run the fuel pump relay drops out and the pump etc keep running through 87a because the coil on the fuel pump relay cannot operate until the air flow switch closes. This allows the power to flow from 87a to 30.

You want to jumper 87a to 30 for running the fuel pump. Not 87.

Hope this helps a little...

patkeefe · 12-14-2014, 05:59 PM

Yes Dennis, that is correct. The fuel pump isn't running because of the airflow switch being closed. But, one side of the airflow switch goes to FP relay 30, and the other side is D-.

I'm really thinking this is a diode bias issue. I need to get my old schoolbooks out and read up on diode biasing.

timmy2 · timmy2's Garage

AFS (G19) goes to 85, one side of the coil, does not go to 30. See below. Power flow is from top to bottom. (Conventional flow theory)

I could walk you through the entire start sequence if you need me to, but trust me, the Fuel pump relay coil gets energized when the flow switch is closed and the ignition is on, thus breaking the circuit from 87a to 30, de-energizing the WUR and fuel pump.
Terminal 87a does not conduct current flow to terminal 30 again until the flow switch opens, cutting power flow through the coil, and the contacts drop back to the de-energized state and there is a connection from 87a to 30.
The relays in the prints are shown in the de-energized state.

timmy2 · timmy2's Garage

The diode is there to prevent the collapsing field of the coil when de-energized from inducing a current that would cause chattering and possibly keeping the relay in an oscillating state. Smooth transition from on to off is why it is there.

Many years ago I had a call to fix an electric siren horn in a coal plant that wouldn't shut off because the collapsing field kept it operating, put a diode across the coil and it shut right up!

patkeefe · 12-15-2014, 07:00 AM

Dennis, sent you a PM. You are correct on the AFS to 30. Oops.

boyt911sc · boyt911sc's Garage

Pat,

You are over thinking this simple concept. Dennis is right on his explanations about how the FP relay and the AF switch work in tandem. There is a detail discussion of the FP relay in one of DKlever48's posts. D- is nothing but a known good source for ground contact.

Tony