Ground Relay Circuit Trouble GP40

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  • Member since
    December, 2016
  • 9 posts
Ground Relay Circuit Trouble GP40
Posted by ONRelectrician on Tuesday, March 21, 2017 2:27 PM

Hello everyone, Im back here with another probelm we cant solve, and before we decide to take out a new rebuilt generator I thought I would ask on here to see if anyone has any ideas.

We have a GP40-2 locomotive that we are overhauling. We are down to the testing and cant get the ground relay to trip on test mode. In the manual it says if its not working, to remove wieres 011 and 021 from terminal board 39R4 and L4 then apply 74 volts to the terminals. If the GR circuit fails to pick up the you have a faulty componet.  

The GR DOES pick up when we do this test. it dosnt tell us what the problem is if it does pick up.

we qualified the wiring and compents and we are stuck. so before we assume its the gen maybe someone has an idea? Thanks in advance for any input.


i know this is a pretty simple circit but its driving us up the wall!

  • Member since
    June, 2004
  • From: roundhouse
  • 2,657 posts
Posted by Randy Stahl on Tuesday, March 21, 2017 5:32 PM

The 011 wire goes to the neutral on the right side of the AR10. The 022 wire goes to the neutral on the left side of the AR10. I am fairly certain there is nothing wrong with the AR10.

Check the ground relay cutout switch, make sure its actually working.

Tell us how you are testing the operation of the ground relay.



  • Member since
    December, 2016
  • 9 posts
Posted by ONRelectrician on Thursday, March 30, 2017 1:09 PM

Hello, thanks for the reply. I didnt get an email notification that someone had. But here I am..

The unit loads and runs fine.. but when we go to load test on the test panel, set up for power, hold down the TH modual test switch then go to throttle 1, it dosnt pick up like it should. There is no increase of voltage at the GN and GP tests points. We have tested and checked everything.


I believe in my last post I wrote when we apply 74V to terminals 39R and L5, the GR picks up like it should. but nothing when we go to test mode. Maybe we could put our own ground on the unit and see if it picks up that ways.


Thanks for the reply.

  • Member since
    June, 2004
  • From: roundhouse
  • 2,657 posts
Posted by Randy Stahl on Friday, March 31, 2017 4:54 AM

Use a sturdy piece of 4/0 wire like a battery jumper cable and connect one end to the independant air brake valve and the other end to one of the reversers. Notch it up in power and see what happens.

Alternatively you can ground one of the leads on the LTT contactors and run it in self load test.


Weve been testing the GR this way for 30 years.

  • Member since
    December, 2016
  • 9 posts
Posted by ONRelectrician on Thursday, April 20, 2017 10:28 AM

We have been away from this locomotive due to other repairs, but now that we are back on it... here is a little more info.


We did ground out a traction motor and still the GR did not trip.

We had another SD40 in the shop yesterday and we ran jumpers from loco2200 to loco 2201 (2201 is the loco with the GR problem) for the ground relay. We then grounded out the 2200 and the GR then picked up in the 2201.

We then did the opposite and it would not work.

To me the only thing left is the gen. But the unit loads and it seems to run fine. anyways we are grasping at straws now and hopeing maybe someone has another idea..

the gen was rebuilt from another loco that derailed.


so i thought id give an update to anyone with any other ideas . thanks!

  • Member since
    May, 2003
  • From: US
  • 13,522 posts
Posted by BaltACD on Thursday, April 20, 2017 10:57 AM

Electricity has to be the most frustrating element to work with - you can't see it 'moving' and can only detect its operation with various meters and detection tools.

On my race car I had a Ground problem I could not diagnos for 2 competitive years.  Sometimes the car would run the entitre on track session, sometimes it would only run 5 or 10 seconds - but it ALWAYS restarted without incident.

Recently lost power in my bedroom.  With the assistance of my son-in-law after going through every connection box in the house, we found issues with the 2 boxes that were furtherest from the bedroom.  Go Figure.


Never too old to have a happy childhood!

  • Member since
    March, 2016
  • 2,073 posts
Posted by RME on Sunday, April 23, 2017 11:29 AM

ONR:  I'm assuming you are using a megger for appropriate parts of the diagnosis, not a VOM, and that you have switched out appropriate components of the ground-relay circuit between comparable SD40s.

The thing here is that the ground relay is NOT dropping in the presence of an overload, not the other way around, with what I understand is a known-good relay.  Weird and untraceable ground faults, those have been well-known since the early days of production diesel-electrics.  But there, the "logic" is working like a GFCI so that any fault, even a relatively small one, produces a trip. 

I would submit that there have to be a limited number of causes, some artifactual to the way EMD has the ground relay actuation function, that keep the relay from tripping.  And I do wonder whether this is an issue that manifests only with high fault current.

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