EMD GP9 Excitation

Problem of the week. My GP9 went down with a sticky reverser, and no loading trouble. After rebuilding the air reverser motor and getting it squared away, I still cannot get it to load. The Series contactors pick up, the GF, BF, and SF contactors pick up but still nothing. ER, PC, GR, etc. are all in proper position.

When I put the VOM across the SF contacts, I don't get any voltage. When the BF is open, the main contacts show voltage. When the relay kicks in that voltage disappears. I am puzzled.

When I started working for the railroad, the GP9 & GP7 units were about gone so I don't have much experience on them. I have worked on a lot of Battery Field Excitation units over the years. Just asking, could the Load Regulator be stuck in Minimum Field and not releasing and traveling toward Maximum Field. Just a thought.

Its turning and doing its thing. It goes all the way to max field, seeing no load on the gen.

WP Mechanic wrote:When the BF is open, the main contacts show voltage. When the relay kicks in that voltage disappears. I am puzzled.

Are you measuring this across the two contacts on the BF? If so, that makes perfect sense. Open, there's a voltage differential. Closed, the voltage is the same and you will not read anything, as the circuit is closed.

Unless you mean, measured between one of the BF terminals and the battery switch.

I measured across the BF terminals... oops.

WP Mechanic, I believe that CN Sparky has gotten you straighten out on the Voltage reading at the Battery Field Contactor "BF". Like I told you earlier in a post, I basically have never done much troubleshooting on a GP9. Most of my troubleshooting with Battery Field Excitation has been on SW1500 units with a D32 Main Generator. My ohm reading on the Battery Field Winding on the D32 is about 1.1 ohms and the Shunt Field Winding is around 77 ohms. On the SW1500 in Notch 1, your Battery Field Voltage should be around 12.6 VDC with 8.8 Amps of Current Flow. In Notch 2, the Battery Field Voltage should be around 25 VDC with 20.7 Amps of Current Flow. These "BF" Voltage and Current Reading I have was taken with the unit hooked to a Loadbox. I know this unit being a GP9, the readings will not be the same, but maybe close. Does the GP9 have a D22 Main Generator in it? Let me know how it goes.

Since its in un-rebuilt state, I'm going to assume it still has a D12B generator.

Also I have no AC voltage, but blocked the NVR in place while testing the excitation circuits. Also, the batteries in the loco are shot, having to be jumped each time its started. I'm on vacation right now so I won't be back to work on it for another week.

No AC problem has been repaired, just some stuck brushes and dirty slip rings. Still chasing down excitation issues.

I am glad that you got the No AC problem fixed. Over the years I have had similar problems with locomotives that have been in storage for long periods of time when the brushes freeze up in the brush holders. In one of my previous post to you, I touched on the possible voltages and currents you should be seeing at the Battery Field Circuit. You might try cutting the tape off of the connections at the Battery Field Wires at the Generator and apply your meter there and verify what type of readings you have when trying to Load the unit. If you are pumping Voltage into the Battery Field Windings, you should be getting an output out of the Main Generator.