No pressure motor runs but no pressure

Your expansion tank is no good.Try replacing the air tank, but if this doesn't help, it sounds like you may have burned out the pump.
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When first use the motor will run for a second until the pressure build up stops it from then on if it doesnt run when the rigger is pulled but you get mains water out of the nozzle I would be checking for a partial blockage in the wand nozzle or hose
If it is a capacitor , the motor will hum but not move
if it is any switch the motor will not run for the second when you turn it on

It isnt very clear your explanation of the problem but what type washer do you have first? It sounds like you have a bad unloader or need a new variable nozzle tip. Can you explain in more clear detail. Thanks

When water is running through system, the motor has to run to keep the pressure built up. So, if the water is leaking out, the motor think it needs to keep running to keep the pressure built up. The leak needs to be fixed to the motor will not run constantly.

If while running the air pressure in the tank comes up (watch the tank pressure guage) but then leaks out after the pressure switch turns the motor off, you need to check/replace the tank check valve. If the motor runs and runs and the tank pressure never comes up or gets over 30 to 40 psi you need to check: cold start valve if it has one; piston/ring/cylinder condition; valve plate reeds. Any of these will contribut to you not getting enough pressure in the tank for the pressure switch to turn the motor off.

hi there, if there is enough air pressure present in the compressor the motor will stop,the pressure switch is working.if there is a good pressure at start then suddenly lost the pressure,there is no maintaining pressure to use but on the pressure gauge appear to be have a good or enough air pressure. check if the pressure gauge have enough air pressure the motor is stop,meaning the cut out limit switch stop the motor,if you loosen the air pressure from the valve until the cut in pressure attain or meet the motor will run again there will be a cycle process of differential pressure of cut in and cut out set up on pressure switch. checking the compressor,be sure there is a high pressure on compressor and the compressor motor is stop,shut off service valve and disconnect unit device / tools from the service valve. if the compressor have enough air pressure crakc down the service valve and observe the pressure until it run again the compressor motor,if the pressure maintain the pressure you need to clean the bench, the problem is the unit device/tool you use is defective, but if the maintaining air pressure in the compressor is lost due to malfuction of motor and the compressor engine. check the pressure switch, check the motor function by loosening the belt and run the motor w/o the compressor engine if it is tripping brought it to service center, check also the lubricant oil,if neccessary change oil.have a nice day and fix my rate thanks a lot.

when i turn it on motor runs but it will not build up air pressure

This sounds like a problem with the pressure sensor which is located on the water inlet side of the pump. This switches the power to the motor when there is sufficient inlet pressure and water flow. This pressure switch can get partially siezed up, and not operate correctly.

You will need to dismatle the unit and identify the pressure switch, dismantle it and lubricate it with silicone grease.

either the motor start capacitor is weak on the motor case(cheap fix) or the motor windings are defective(expensive fix) new motor time