I did a check on the sears web site for you compressor, the part no. you need is No.43, it cost $53.99 and is called the pressure switch... it is suppose to turn the compressor off electricallly as the pressure goes up to its limints(in your case probably 150 lbs. of pressure)... it might just be the contacts sticking on your pressure switch?? you can take off the sheet metal case off the switch and see what happpens when you get the top pressure like 150 lbs???? make sure the little metal arm that is on side of the pressure switch box is not bendt into the on position as this might be what is stopping it from turning the electricity off???? hope this helps.
Your compressor should have a pressure switch on it (looks like a black box) where the the power wire and the motor wire run to. This is what controls the start & stop cycle of the compressor. It's also connected to the tank to sense the pressure and should also have a connection for a little hose that relieves the piston pressure once the motor shuts off.
It's a mechanical device and may just be "gunked up" based on your description of the *two* pressure related problems you described. Once your tank is completely depressureized you can try and "futz" around with it (unplug you machine please - that's real live 'lectricity running through them wires) and try and clean/oil it.
There should be a bulky set-screw at the top that controls the on-off pressure. You can try and turn it back and forth to loosen any stickyness - make sure you set it back to the original position.
It could also be that the electrical contacts are fryed and the switch is actually working - It's hard to hear when the machine is running, but there should be a definite "click" when the tank reaches the cutoff pressure
If you really have to, you can take it off and disconnect everything but this is a real PITA. you will have to reseal and reconnect everything after.
Failing that, you will need to replace it. a big new one (5HP 220v) cost me about $65 Canadian (40US)
Although I agree with the sentiment of this statement, PLEASE don't do that. Sandpaper leaves particles imbedded in the contacts which cause arcing and subsequent failure of the contacts - especially on high current applications like air compressor motors.
Instead, use a burnisher or riffling file to clean the contacts. Anything that doesn't shed abrasives or other particles when used, but removes debris and restores contact surfaces. Take it easy, because many better relay and switch contacts are silver or gold plated. Don't want to remove any more of the coating than absolutely necessary.
Hi, I guess many folks never worked on contacts. During my working days burnished zillion contacts. Well worked in phone switching plant once in the days of step-by-step, strogger stuffs. Any old timers remember this? Tony
I started working for the phone company (BT) just as the last of the Strowgers were disappearing. I still remember contact burninshing, the most tedious job in an exchange.
Now I restore Japanese swords. A usable material for the hilt wrapping on a WW2 military mounting is the flat cotton tubular tape used as the polishing medium for bank cleaning on a Strowger. Instead of slipping a length over a polishing hook, dye it green or brown and use it stretched flat.
i have used very fine emery cloth on power contacts. you can use just typing paper and a solvent like 90% Isopropal alcohol, force the contacts closed on paper and pull it thru. repeat until the paper remains white. --Loren
Back in the day of engines with contact points ignition seems most people knew not to use an abrasive coated paper to clean them.
Something that should be noted, on a set of coated contacts, once the coating has been breached performance drops, sometimes drastically. A set of contacts may last 10 years with the coating intact, may last 10 cycles once the base metals been exposed (I know it's an extreme example).
While sanding and burnishing might work for a while, once the contacts are worn, they'll tend to stick again. I've noticed that the quality of contacts on newer compressors has become worse than on old ones, or maybe I'm just getting cynical.
A solution that fixes the problem once and for all, and for less than the part cost of a new pressure switch is to use the old pressure switch (after you've pried the welded contacts appart) to drive a transformer/filter to drive the LED of a solid state relay. The relay drives the motor and will never arc; it should last forever. I've done that on one setup and it has worked great for years (5hp 220v compressor). I now have another compressor's switch going bad and ran across a surplus 240v 40A Furnas DPST contactor for $6, so I'll use that in place of the solid state relay. The contactor is a simpler (no transformer/filter and heatsink on the relay needed) and cheaper solution and this compressor doesn't get nearly the use of the other one. It won't last forever, but the 40A contacts should be good for far longer than a replacement pressure switch.
Another approach, after burnishing the contacts is to apply a de-oxidizing / anti-arc coating. Caig Laboratories makes an entire line.
Search the site for the application guide.
I can vouch for it's effectiveness. I used to maintain an old 16-track MCI tape deck. Each channel had 4 relays in addition to the relays in the transport controls. Once I started using Cramolin (now called ProGold, I think) I seldom had to do anything to that machine.
Are the contacts rough and pitted? Good, that's the way they should be. I used to repair forklift trucks and found that people not having experience with pure silver contacts handling high amperage would file them smooth, wherupon they would stick. Unless a very fine file was used, there would be tiny ridges left on the contact surface. This would result in a very small contact area having to conduct hundreds of amps which melts the metal and welds the contacts together. It is better to leave the contacts rough as they have burned themselves a larger contact area. This applys only to silver contacts handeling high currents, not combustion engine contact points. Engineman1