I have a small portable (electric) air compressor. If I start it up with no pressure in the tank, it runs fine and fills the tank with air like its supposed to. But if there's any air pressure already in the tank, the motor will not start, and I can't refill the air tank. If I release all of the pressure out of the tank manually and restart it, the motor will run fine and refills the tank. Any ideas?

First, welcome to the Pool. You will find that it is a great place to hang out.

While I can not address your particular Craftsman product, I can give you a couple of items to check.

Most compressors can not start with a full head of pressure. They should have a start unloader of some sort and a check valve between the compressor and the holding tank. The unloader relieves the pressure off the pistons and the check valve only allows air to flow in one direction, towards the tank.

When the compressor stops, the unloader will open allowing the air pressure to drop on the pistons and the check valve will close keeping the tank air from escaping through the unloader.

If the check valve is stuck open, then the compressor is trying to start with a full head of air. You may also note that the air pressure in the tank may be going down faster then normal when not in use.

Follow the lines from the compressor to the tank and you should be able to locate these items. Be careful as parts under pressure can and will blow off with great force even though you think the pressure is low. If you hear any air escaping while removing parts, wait until the sound stops and then remove slowly.

If this is of no help, you may have a bad motor that will not develop enough starting torque to over come starting pressure but runs fine once it gets going.

If you have a Sears/Craftsman model number, that may help if I can find a service manual on line. Check your manual if you have it. Sears/Craftsman are good about having all the information packed with the unit including a parts list.

Again, this is general and may not apply to you unit.

PS. I am adding this after checking my Craftsman compressor. The compressor may have a bad discharge reed valve.

There are 2 types of reed valves for each cylinder. As the piston goes down, the intake reed valve opens to let air into the cylinder. As the pisoton goes up and compresses the air, the discharge reed valve opens when the pressure is equal to the the tank pressure.

If this reed valve is damaged, it will allow tank pressure back into the cylinder. This would act the same as a bad check valve.

This is a simple repair on my unit.

This message has been edited. Last edited by: Walks On Water,

I think I solved the problem. All of the valves were functioning and the bleed off valve released the head pressure with no problem. Believe it or not, the problem was that I was using an extension cord. I re-read the instructions and they recommend use of no extension cord, or a 12 gage or larger cord if one must be used. I was using a 16 gage 50' cord. The loss of voltage was enough to not give the compressor enouigh juice to power itself up. I retried it without the cord, and it works perfectly! Thanks for the advice.

It's funny.

We always overlook the obvious.

I have a Craftsman air compressor that just went out this afternoon. We heard a little click at about 80 psi and then the pressure started dropping. The reeds that you mentioned seem to just float in the region of the valves. Is that normal? It seems strange that they are not attached in some what to the surface. I put it back together and the pressure will come up to 30lbs and then the compressor just runs, but no more pressure. Any thoughts?

The reed valves do "Just Float" over their openings on the intake side. In most compressors, the discharge valves have spring helpers.

Not knowing your unit, I would suggest you check Sears on-line parts center and look at the diagrams there for your unit.

Sears Parts

One thing I have found is the discharge tube from the compressor is just held in place with a rubber gasket on some units. This becomes dry and lets air excape and not letting it build up pressure.

Hope this helps and welcome to the pool.

Thanks for the info. I went a bit further in taking the unit apart and found that there is a gasket on the intake to compressor face of the system. The center part of the gasket that splits the intake from the compressor side had blown out. Since I was working on a car, I took part of one of the old gaskets from the car and put it in the bridged region for a short-term fix and it worked.

I still need to get a new gasket, but it has been interesting learning a bit about how the unit works. Thanks again, Bill