Pressure switch contacts keep burning out.

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by JB Johnson
(Sidney, Ohio)

I have a 5HP Sanborn air compressor that keeps failing the pressure switch every month or so. I am on my fourth in the last year.

Newer Sanborn 5 HP air compressor
Newer Sanborn 5 HP air compressor

The switch will stick “ON” because the contacts will burn and weld together.


Compressor contacts keep burning out
by: Bill

What’s the supply amperage, distance from power source to compressor, and ratings for the pressure switch you are using, please?


Contacts burning
by: Doug in

Do you have a specific model number?

In addition to what Bill asks, you may want to check the motor capacitors – possibly one is failing, causing excess amperage.


Points sticking
by: Anonymous

How do you check the capacitor to see if it failing?


Testing capacitors
by: Bill

You go to the page on this website entitled testing capacitors!

It’s linked from the sitemap page and troubleshooting pages.

Let us know what you found if you would, please.

If you wish to learn how to adjust air compressor pressure switch control valve, visit our detailed page for more information!

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I have a 3 hp, 15 amp rating, 240 volt 60 gallon compressor. Its is wired to a 240v outlet that is pushing 245v. The compressor is connected to the outlet via 10 gauge wire 3 conductors. I turned the compressor on twice and both times the switch sparked and it ran the motor. i tried turning it on again and nothing happened, there is power to the 10 gauge cable, but no power going the motor lines and the pressure switch contacts slightly melted on one side.what could this be?

Pray tell where the three wires are connected. Sounds like some sort of mis-wire, as in ground where it should be hot. You’re certain the motor is 240v?

I changed the start and run capacitor as-well as the switch. The motor runs alright by it self. Now, if i attach the belt to the drive shaft of the pump, the motor wont start the pump and it will overload and trip the breaker. I tried manually spinning the pump but not its not enough to get the pump going. I got this compressor used so I dont know if the motor is capable of still spinning the pump. The output of the motor is 2.98kw and the pump is supposed to be 3 hp.


OK. Can you show/give your pulley calculation?

Pump specs


1 hot wire to L1(black)1 hot wire to L2(white) on switch and ground to green screw. T1 and T2 go to motor(black and white)

OK. That sounds right. One more thing – please measure across the contacts on the pressure switch. Just to be absolutely sure the power from the outlet is going where you think it’s going…

The wire from outlet to switch is about 10 feet and switch to motor is about 2-3ft.
Im sending photos in sequence from outlet to switch to inside motor contacts.


2nd part


3rd photo


Update. A start and run capacitor came with the unit used. Start capacitor specs 189-225 microfarads and 220 v rating 60hz Run capacitor specs 30 +5 microfarads 370v I replaced both capacitors, but I bought a higher microfarad start capacitor instead, but same rated run capacitor. Then the motor would spin but the shaft of the pump would not spin as stated previously. Now under my impression the two capacitors were switched or the start capacitor isnt capable of carrying enough of a charge to start the pump. Therefore I switched the capacitors, the start where the run was and… Read more »

That could indicate they were wrongly installed in the first place, but then the run capacitor is/was bad. Blowing the reset is typical of a bad run cap. You might check across the terminals where the run cap goes, should be pretty close to a short (motor winding). And you know how to check the cap, at least for gross failure, right? If not:

I think we’re making headway!

The pix look OK. I wanted to measure across the switch contacts for 240. Not that I question in the guts you did, but that the wiring in the outlet and your cord matches up.
So, across the two yellow spades 240(245?), and then spades to ground, around 120.
And finally, with the motor running, little to no change in voltage either across or to ground?
Pulleys / rpm calc?

Yes 240v is going to the motor no doubt.
Last photo is the reading to the motor from switch.

Now, I was wrong about the capacitors being switched, because when I switched them back to the original order now the compressor runs fine for minutes on end. Yet there is still an issue. Everytime I need to run the compressor, I have to run it without the belt/drive shaft for about 30 seconds; turn it off; attach the belt and then it will run. Seems like the capacitor needs to charge up in order to have enough of a load to actually get the pump/drive shaft spinning.


Something like that. So maybe it was just poor connections the first time.
Given your experience with “warming up” the motor,, sounds like a weak run cap. See these things are a little like batteries, in that they can go flat. The difference is they can reform and then work well enough with some volts across them. So, you might want to get a replacement run cap and see if that works.

Let me add that some of the info regarding the purpose of the start cap is incorrect. What it does is kick the motor in the right direction as the magnetic fields are created. The it disconnects via the centrifugal switch. The run cap helps keep it going once it’s running.
So, if it starts without a buzzy noise, then dies, it’s either a motor winding or the run cap at fault.