High voltage with LED indicator.

I work on a lot of drip irrigation controllers and have been experiencing a large problem with the controllers frying due to high voltage. In the fields we have 480v 3 phase going to a transformer stepping down to 120v. Well we have high voltage which in turn is making our 120v stay up around 140v. I have "surge protectors" that are good for voltage spikes but cannot protect from the constant 140v input longterm. I am not smart enough to make a regulator that will keep my voltage from exceeding 120v so I want to make a simple indicator that I can mount in the face panel of these timers that will light up when voltage exceeds say 125v or so. It will not stop the problem but will at least let the customer know when his controller is acting funny its not the controllers fault but yet the incoming power that is causing the problems. Keep in mind I don't know it all, I do plenty of wiring and some small sized soldering but I am by far not a professional. Hence why I would like to build an indicator rather then a regulator.
Any information would be greatly appreciated.

 

Thanks for the input. Now let me inform you I've worked in the agricultural irrigation areas most of my life and have been wiring things up for a good long while but I have no education on it other then my own hands on experiance. Would a UPS be able to keep the outgoing voltage at 120 if I had a constant incoming voltage of say 140v or so. And if so I could mount something like this in a small enclosure after my power and before my timer and supposedly be ok? http://www.apc.com/resource/include/techspec_index.cfm?base_sku=BE350G

Now the ferroresonant transformer. This sounds like the way I most likely should go but I have no idea what it is. Sorry for the ignorance. I am quite limited on things such as an oscilloscope or a logger. I've used loggers in the past (borrowed) for pivots that were cooking motors overnight and they were quite effective. A solid state regulator would be great, but where to start looking for one is beyond me. I will be googling it in just a second.
Thank both of yall for the input, I learn a little everyday.

 

I just need a stable 120v. Most commonly my readings are around 135v to 145v. The controllers don't pull much at all. I can run one off a 200w inverter with an extension cord out of my pickup just to program one if I don't have power. The controllers are usually under a carport or some kind of overhead tin covering somewhat exposed to the elements. Our voltage goes up and down throughout the day and night because we are out in the middle of farms with multiple wells running all around. I would say from the hundreds of fields I've been to in this area the 480v three phase power ranges from 460 to 520 volts. That variation of voltage makes the 120v vary since its going through a transformer that steps down the 480 to 120. Then inside the controllers theres a small transformer that runs the 120v down to 15 volts 24 volts and 5 volts. When I have say 520 on the 3 phase I usually end up with around 138 to 140 which makes my 24v be around 28 or so and then I'm cooking solenoids that the controller runs. The 5v turns to around 6.5v and kills things like the CPU in the controller. the 15v turns to around 17 - 18 volts and kills out things like my relay boards and so fourth on my controller. Then there is also another controller that runs my wireless valve control and the high voltage makes it just flip out all over until it cooks something. Not sure if this information I'm giving you is very good since I'm not real smart on electricity but thats about as good as I know how to explain it.

 

I have a 50 dollar APC brand UPS coming to try on one setup. If this works then everything will be good and I believe that the particular one I ordered might even fit inside the controllers I'm using. I really appreciate the input and the help. I learned a few things too. Thanks again, and I will let you know if this solves the problem. So glad I didn't have to bust out my old soldering iron.