Railway Preservation News

GP38-2 Problem
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Author:  Berks315 [ Wed Nov 06, 2019 9:14 am ]
Post subject:  GP38-2 Problem


Having a slight issue with a 38-2. when putting the reverser in either direction it will not come out of low idle. Notch one and two make power in low idle. Ammeter goes to 600 amps like a rocket. Notch 3 engine RPM finally increase.


Author:  E.B. Levin [ Wed Nov 06, 2019 7:56 pm ]
Post subject:  Re: GP38-2 Problem

There should be two relays NIR and LIT. NIR is Normal Idle Relay and LIT is Low Idle Relay. They may be mounted separately or on a TESCO Low Idle Panel. I would look at the feed to the NIR which should be energized when the reverser is in F or R position. LIT is energized when the reverser is centered. I would suspect your trouble is within these relays and/or their associated interlocks.

If equipped with electronic cooling controls there is typically a feed from this panel to the NIR that picks up engine speed until temp warms up.


Author:  dieselloco [ Wed Nov 06, 2019 11:20 pm ]
Post subject:  Re: GP38-2 Problem

the very first place I'd look is the control stand. The roller switches for the eight speeds of the throttle are used in various combinations to give you 8 speeds, low idle and shut down. And these switches activate 4 solenoids in the governor. Different systems on the locomotive use these signals from the control stand to do other things such as giving reference signal for how much the engine will load in each throttle notch. Take a test light and check each roller switch to make sure they're making contact. Sometimes a meter will give you a false reading and when under a load the contacts in the roller switch can't sustain the amperage needed to power the circuit.
Good luck!!

Author:  Overmod [ Thu Nov 07, 2019 12:50 pm ]
Post subject:  Re: GP38-2 Problem

If you have, or can build, a megger, use that after you use the light on the switches SUBJECT TO THE IMPORTANT CAVEAT MR. LEVIN MENTIONS BELOW. A wide range of components will appear to check out fine on a low-voltage light or typical multimeter, but not at all when full current has to be carried. You need to conduct testing with good controlled-current. This is precisely the advice dieselloco gives above, adding only the need for specialized equipment to accomplish it.

Author:  E.B. Levin [ Thu Nov 07, 2019 8:34 pm ]
Post subject:  Re: GP38-2 Problem

Be careful what you decide to merger on Dash 2 control systems. The throttle micro-switches along with picking up the governor solenoids also go back into the card rack to the throttle response circuit in TH module. Based on throttle position this card sends a reference voltage to Rate Control module which is the first step in main generator excitation control.

In low idle governor solenoids A-D are energized when the reverser is centered. If the unit won't come out of low idle you need to find out what is keeping the D solenoid energized when the reverser is thrown.

For reference the notch progression energize the governor solenoids in this order:

Stop - D
Low Idle A/D
Idle - No Solenoids Energized
Notch 2 - A
Notch 3 - C
Notch 4 - A/C
Notch 5 - B/C/D
Notch 6 - A/B/C/D
Notch 7 - B/C
Notch 8 - A/B/C

Author:  Berks315 [ Fri Nov 08, 2019 6:38 pm ]
Post subject:  Re: GP38-2 Problem

Thank you all,

I got some time to work on it a little.
Reverser in either direction- stays in low idle,notch 1 and 2 load. Engine remains in low idle.Notch 3 full RPM. 4-8 full RPM.

The manual I have has a test for TH test point 2 but module does not have a test point 2.
TH-TP 3 to 14 is supposed to read 9 VDC I have 68 VDC.

Hope to have more time nex week to keep looking at it

Author:  Flagg [ Fri Nov 08, 2019 8:21 pm ]
Post subject:  Re: GP38-2 Problem

When I worked on the last shortline we had several GP-38 engines and what you described would appear not often but regular enough that it always seemed to be the roller switches. The low idle would be one of the two that is in contact with the reverser handle and the reving up would be one of the eight ( i think, its been awhile) that are in a row above. (Looking at them from the back side)They wear internally and hang up losing contact. I remember that if it loses the even notches it was the bottom one. Id,d start with the bottom two. Watch as they are not all interchangeable. Some are normally open and some are normally closed. If you wiggle the throttle handle in the positions will it somtimes work? That is a good sign that its a roller switch.

Author:  Brasspounder [ Sat Nov 09, 2019 5:22 pm ]
Post subject:  Re: GP38-2 Problem

As mentioned earlier, check your roller switches in your control stand first. One or more may not be making full or any contact. Start with reverse and forward first. Beware there are multiple part numbers for these switches within the stand.

Author:  dieselloco [ Tue Nov 12, 2019 2:02 pm ]
Post subject:  Re: GP38-2 Problem

Wish I had some Dash2 blueprints laying around. If D solenoid stays energized or possibly stuck the unit will shut down when the reverser is moved out of neutral position. As Mr Levin provided the sequence of the speeds with which roller switches are closed. Notch 6 was the one we used all the time to verify if all control stand switches were working. Quite often a kind electrician would mark the roller switches A,B,C and D. I seem to remember that a switch is used to tell if the reverser is indeed in the neutral position. Earlier non dash2 locomotives had a LIP low idle panel that was an optional accessory.

Author:  E.B. Levin [ Tue Nov 12, 2019 7:49 pm ]
Post subject:  Re: GP38-2 Problem

I pulled wire running list 8493547 for your GP38-2. It shows revision H 3-27-79 as equipped with a Low Idle Panel which is located in zone 36. I believe I sent you this along with the schematic some time ago.

The reason it's not shutting down as a result of this issue is both A & D solenoids are energized by the LIP. If A was missing the unit wouldn't run at all. The LIP and the associated relays mounted to it is the first place to look.

Don't waste your time trying to troubleshoot the excitation issues as they are a result of improper throttle sequencing influencing the TH module. TH sends reference voltages for throttle notches through its resistors to the RC module.


Author:  Berks315 [ Thu Nov 14, 2019 11:51 am ]
Post subject:  Re: GP38-2 Problem

Thanks for all the help so far. Replaced the low idle board. Relay was not closing. Now it is working and on to next issue won't load.

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