SW1500 Aux Gen. Overcharging.

Our shop horse broke down a couple nights ago. I was call out to work on it for no battery charge.
I found the 150 amp aux gen fuse blown. Replaced it and started it up.
It worked fine for awhile, then started overcharging. I pulled the 15 amp aux gen field fuse till output went to 0 then put it back in. It worked fine for awhile then started overcharging again!
I installed a new voltage regulator panel but it still stops regulating after awhile. Especially after speeding up and slowing down the engine. Pulling the aux gen field fuse and putting it back in always stops the overcharging but it's not long before it starts it again. I found no low voltage grounds and no shorted wires. I cant believe that two volt reg panels would be bad but I guess it's possibe. I ran into this problem once before, also on an SW1500, and never did get that one figgured out. I've never had this kind of problem on newer locomotives with a VR10 card, just with the old style Volt Regulator Panel. Anyone have any ideas?

cb&q bob wrote:(Excerpt) ... Anyone have any ideas?

Sounds like a heat-related crack is opening a circuit or breaking-down a component. Try spraying with compressed air to cool areas, see if you can pinpoint the problem that way.

Check batteries while charging , it could be one or two cells short.

Does the SW1500 have a Battery Charging Rectifier (BCR) panel? If so, could the problem be caused by a shorted rectifier or diode in the panel?

No, It does'nt have a rectifier because it has a DC aux gen. It does have something called a CRBC. This mainly keeps the batteries from feeding back through the Aux gen and keeping the fuel pump and cab heater circuits de energized when the engine is shut down.
What I know is that the voltage regulator is shorting through and not regulating at all but it only does it some of the time. Usually, once a VR fails, it fails all the time. Thats whats stumping me.

This is a long shot but has the Aux Gen Flashed over?

Nelson Bay wrote:Does the SW1500 have a Battery Charging Rectifier (BCR) panel? If so, could the problem be caused by a shorted rectifier or diode in the panel?

You may be thinking of the MP15AC..

ExEMDLOCOTester wrote:

Nelson Bay wrote:Does the SW1500 have a Battery Charging Rectifier (BCR) panel? If so, could the problem be caused by a shorted rectifier or diode in the panel?

You may be thinking of the MP15AC..

Yeah I probably was.

The aux gen has'nt flashed over but you can stick your nose up close and tell by the smell that it has been hot. The com is a pretty dark color too. I've seen alot worse . Maybe it has some open windings that could be causing some kind of spiking that is causing the voltage regulator to stop regulating. I wish I had an oscilliscope to put on it.

cb&q bob wrote:The aux gen has'nt flashed over but you can stick your nose up close and tell by the smell that it has been hot. The com is a pretty dark color too. I've seen alot worse . Maybe it has some open windings that could be causing some kind of spiking that is causing the voltage regulator to stop regulating. I wish I had an oscilliscope to put on it.

Any Luck ?????

Nope, It's still broke.
I've been on vacation for eleven days and just got back to work on Tuesday. It's still doing the same thing. I heard from someone that one of the guys said the starting field in the main gen was grounded which made sense because I noticed that it would pick up the ground relay when cranking the engine and would have to be reset. I never did get the test light to light up when checking for low voltage grounds though and kind of forgot about it as being a cause for the aux gen problems but now I'm starting to have second thoughts since we dont seem to be finding any other problems.

90% of older EMD's trip ground relay when starting, the ground relay only detects High voltage grounds not low voltage.

cb&q bob wrote:Nope, It's still broke.
I've been on vacation for eleven days and just got back to work on Tuesday. It's still doing the same thing. I heard from someone that one of the guys said the starting field in the main gen was grounded which made sense because I noticed that it would pick up the ground relay when cranking the engine and would have to be reset. I never did get the test light to light up when checking for low voltage grounds though and kind of forgot about it as being a cause for the aux gen problems but now I'm starting to have second thoughts since we dont seem to be finding any other problems.

Disconnect both batteries at the terminals + & - and test (voltage) from each battery terminal to the carbody. I have seen a grounded battery do some strange things. Also check for a tie from high side to the control side. You may have to open a few circuits to make it happen correctly. I haven't seen a SW 1500 print in 18 years, so I need you to understand that I am doing this from memory.

cb&q

Do you have a hi-pot?

DutchRailnut wrote:90% of older EMD's trip ground relay when starting, the ground relay only detects High voltage grounds not low voltage.

Thanks for the info. I thought only 86.37% of them tripped the ground relay on start-up.