We've been having a little trouble with a U-23-B tripping the low water pressure alarm. It's a fairly late model U-23 where the LWP tripping reduces excitation, but does not shut down the engine. At one point a couple weeks back the unit got up over 210 degrees. We replaced "the pill" and put on a new radiator cap which seems to have corrected the temperature issue, but we're still getting that pesky LWP trip.

So to the question. I see in Mr. GE's service manual that a clogged fuel strainer or fuel filter will cause a LWP trip? I don't for a minute think that's our problem since the fuel pressure is a solid 40 PSI at idle or wide open. But, I also do not see how in the world this could cause a LWP trip??? Can somebody explain this? It sure doesn't seem to make a lot of sense to me.

Also, it looks like the water fill neck is corroded and may not be letting the radiator cap seal correctly, so I'm changing that next. Anybody had this problem before and know of any other things to look at? The strange thing is that when you open the pressure release on the engine with it running but cold you get a fair amount of pressure released, but once it's hot you get very little?

Try looking at the gaskets that are connected to the cooling system(or water lines. Whichever term is correct) and connect the prime mover to the cooling system. Those could have been damaged by the heat. The sensor could also have been damaged, which could explain why it's still going off.

If my memory serves me correctly, the LWP sensor is in the governor and uses steel tubing to bring the pressure up to the governor. The steel tubing has a tendency to clog up with scale, causing false indications. Also, the rubber diaphram in the governor has a tendency to collect dirt in the reservoir and or may just be too old to give good pressure indication, or there may be some corrosion under the diaphram cap not allowing the sensor to reset. I would remove the diaphram cap at the governor and give it a visual inspection first(might want to have a replacement diaphram on hand before opening) EDIT:I seem to recall that there are at least two different LWP systems in use on the U series locomotives, I believe what i have described above as being for the modulating governor type system

The low fuel pressure trip would have to cause low engine rpm, which in turn will cause low water pressure to be sensed. Please PM me if you need further assistance and I can dig out my notebooks and refresh my memory.

Mike Tillger

Thanks for the suggestions guys. I did a little more checking today, and it's definitely loosing pressure as it warms up. I still want to check the diaphragm and plumbing going to it, but I kind of suspect a crack somewhere that's opening up as it warms. The unit has been on ethylene glycol for most of the last ten years so I wouldn't think corrosion would be too bad. I don't care for antifreeze in a locomotive, but it was what we inherited.

One of the things you may want to consider is what we did, put inline ball valves on both lines going to the governor. This way anytime you need to remove or work on the governor you can close the valves and not have to drain the cooling system below the governor's level.

Just inspect the whole cooling system for leaks, the fact you loose pressure at the cap when it gets hot indicates this. Check the oil too, you may have an internal leak in the prime mover or lube oil cooler. Go through the components and piping methodically and you should be able to find it. Don't rule out a bad cap either, just because it's new doesn't mean it's good. I thought they sold dyes that can be added to help locate leaks?

Mike, it looks like you may win the prize! We finally got it hot enough to put water in the radiators which was a bit of a challenge in colder weather. Turns out a hole in the right place in a GE radiator will shoot water 15' in the air in #8 throttle when the water finally gets to it! Fortunately we have a few spare cores we dropped by the radiator shop this morning to get checked out. We'll see if this fixes it, but I think its a safe bet.

Where the heck ARE you? Around here, it's been feeling hot enough to trip these alarms when the loco is shut down!

I'm here in the sunny South, and perhaps that wasn't the best explanation or it could use a bit more. Once the LWP trips the unit reduces excitation/engine speed. So left to its own resources its going to be slower to heat up. Even with it 97 degrees outside we had to run it in #5 throttle for nearly a half day to get it to do it. The leaking core was the engineer's side rear (short hood front locomotive) so it is the last core to see water as the unit heats up. I don't think we'd have gotten there on a 70 dgree day.

What is the heritage of this unit?
Thanks,
Mike Tillger