We have a problem with our '52 SW9 that I have hit a brick wall in diagnosing. Early style control stand, air throttle.

The issue: When you try to crank the engine, the P1 and P2 contactors engage and disengage causing quite the spark show, and the wheel slip light comes on. The engine does crank, slower than normal, and this only occurs when the cylinder cocks are closed. Cranks fine if they are open. I have checked the negative to the start contactor coil and the other relays that share a common negative and find no voltage drop that would draw through other relays. If I cut out a truck using the switch, everything works fine. Cut the truck back in after it starts and the locomotive operates normal.

Any early EMD gurus see this before? Any help or suggestions would be appreciated.
Eric

I had a similar issue on a CF7 about 15 years ago.... Traction motor was turning while cranking the prime mover.... If I remember correctly, it was something like a stuck contactor in the reverser... CF7's use an older style air reverser, so if the control air pressures down, it shouldn't go into "gear" at all...and this one wanted to move in reverse while cranking, it acted like the battery was low as well...due to the drag of trying to run at least one traction motor....

Interlocks on starting relays ST- & ST+ are failing to open or may have been jumpered around.
Mike Tillger

See if the parallel relay is closing when you start the engine, if so, than you know that the problem is before that. I have a feeling it will close because of what you said about the motor cut out switch, when you cut out a truck, it keeps the locomotive from going into transition by cutting out the parallel relay. You should be able to accomplish the same thing by leaving the transition forestalling switch ON.

When you press the start button, 5 things should change in the control electrical system: the 2 starting contactors, their 2 interlocks and the start button. The contactors themselves probably are not causing the problem, but their interlocks might. Remember that starting contactors are opposite of their interlocks, when one is open, the other is closed.

My guess is that the parallel relay is somehow getting tied into the starting system, starting contactor interlocks are a starting point but most likely not the problem, if those are not the culprit, than they being bypassed somewhere else in the control system meaning that the positive side of the parallel relay coil is being connected to the positive side of the control system.

These are by no means the only ways your problem can occur, and I am also no EMD electrician but maybe we can as least establish if the parallel relay is being energized. Hope this helps.

Whoops! Don't listen to all of what I said. Just read this instead.

We KNOW it is somehow also associated with the starting system.

If the starting interlocks are staying closed than that would also require that the line that they normally break when starting be energized. In other words, the throttle would be open and the reverser be in forward or reverse. But I highly doubt that you missed something like that, so maybe all of that was being bypassed somehow like what Mike said. And remember that it would only be bypassed WHILE starting the engine, very curious problem!

My original post was for an SW8, very similar electrical system, but not identical. Upon studying the SW9 electrical schematic, it would appear that the the entire transition system is bypassed when you cut out a truck meaning that when a truck is cut out, the locomotive is always in parallel, it just skips the parallel relay and energizes the P1 and P2 contactors.

That helps in troubleshooting because it points us in the direction of the transition system (FTR, PR). The problem could be in/around the parallel relay IF it is energizing when you press start. FTR is a momentary relay, should only energize to initiate transition, then drop back out. The parallel relay (PR) is a latching relay, once in it stays in until you use the forestalling switch or cut the throttle to idle. If it is being energized, find where the power is coming from, that is your smoke trail...

So I think your problem lies somewhere between the starting system and part of the transition system.

Sorry for the misunderstanding with the first post, shows how little I know! Happy troubleshooting,

- Walker

Is the low voltage system free of grounds on both positive and negative side?