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by Ben
(Kitchener, ON, Canada)

Hello,

I have a Mastercraft 8G Air Compressor that does not kick in no matter what the pressure setting is.

Mastercraft 8 gallon air compressor
Mastercraft 8 gallon air compressor

Immediately when the power is turned on the fan starts but the compressor never seems to kick in. I have checked that there is 120V across the relay.

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Good that you’ve checked for power across the switch, Ben.

If the fan starts when you power up the compressor, that suggests that the electric motor itself is running, and if it is, then the compressor pump should be pumping.

It were me I’d remove the shroud and see how the linkage between the motor shaft and the pump is. Maybe the motor shaft is turning but it’s no longer connected to the pump drive?

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Checked piston and linkage
by: Ben

Hi Bill,
I finally had a chance to take the air compressor apart and here are my results.
1. When the piston and linkage are removed and the crank attached to the motor is visible, I can see that it turns when the motor turns.
2. When I reassemble the linkage and piston, for the most part the piston doesn’t move. I saw it move slowly lower until it reached the bottom and stayed there.
3. Reassembling all components made no difference.
Does the pressure switch determine whether the piston engages or not? At one point, when I would drop or move the air compressor it would kick in but then now that doesn’t seem to have any affect on its operation.

Thanks for your help.
Ben

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Checked piston and linkage
by: Bill

“Does the pressure switch determine whether the piston engages or not?” No, the pressure switch has nothing to do with the piston engaging. The pressure switch only turns power on and off to the motor circuit, based on the tank air pressure.

Your compressor is starting. What seems to the be issue is when you add load to the motor – reconnecting the pump for example – the motor cannot handle the load.

This is often either a power supply issue or a failing motor.

Make sure you are not using an extension cord. Try to find a plug that doesn’t have a lot of other things pulling power from that same circuit and one that has a 20 AMP breaker. Plug the compressor in there and try it.

If it still doesn’t work, and you don’t have a mechanical issue with the pump – then I would suspect the start / run capacitor next. See the page on this site about how to check them.

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I Checked Power Source & Capacitor
by: Ben

Hi Bill,
I plugged the air compressor directly into the outlet, skipping a power strip, and it made no difference. I was also able to run my neighbors air compressor (looked to be the same model) from the same outlet.

I removed the shroud and tried to follow the “Checking Capacitors” troubleshooting page. Without power, the capacitor measured 0 Ohms between the leads. When I turned the power on, the fan started and the resistance rose to 1000 – 1200 (may have been negative) Ohms where it seemed to stay within that range. I swapped leads and it seemed to stay in the same range which I believe was always negative.
It looks like the capacitor is a CBB65 if that helps at all.

Otherwise, perhaps I misunderstood something earlier. Is the fan driven by something other than the motor? It looked like the fan was connected to the shaft coming from the motor. Which I would assume would be the same shaft that extends out the other end and couples with the piston linkage. Last time I disassembled it, it seemed like the linkage was intact and it had no where to go. If that’s the case, if the fan is turning, wouldn’t the other end connected to the piston? Or are there two motors. One driving the fan and one driving the piston?
Anyway, hopefully this helps shed some light on the issue. If not, I am open to additional troubleshooting suggestions.
Thanks in advance,
Ben

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Checking cap
by: Bill

I don’t believe you check the cap with the power on, Ben. Maybe a more electrically informed individual  can confirm this?

I have uploaded the manual for this compressor on the Mastercraft air compressor page.  Check out page 28 about compressors not starting.

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Capacitor Troubleshooting
by: Ben

Hi Bill,
The power was being supplied to the capacitor but perhaps my Mastercraft multimeter was not functioning properly. I have a newer, likely more reliable one that I can use to measure the resistance across the capacitor leads. But it behaved differently between when the power was on (~1200 Ohms) and when the power was off (0 Ohms). I would have expected to see a constant voltage across the leads since it is a common use (perhaps the main use) of a capacitor to ensure a constant voltage at least with electronics. Perhaps I did not have the voltmeter set to a high enough voltage since the reading was jumping from 0 to something very sporadically.

Anyway, I will give it a shot today. I pulled the majority of the components apart while troubleshooting (except for the motor itself). Also, based on the manual, I would expect the problem to lie with one of the following items:


6. The air tank pressure exceeds
the preset pressure switch limit ****(not the case but perhaps a faulty switch).

7. The safety valve is stuck open. *** I assume this is the blow off valve that has the ring that can be manually pulled — this is working

8. Electrical connections are loose. *** this could be, and perhaps would explain the strange capacitor behavior. An electrical schematic would be helpful.

9. The motor capacitor or safety valve is defective *** I assume this is the case if the wiring / connections are fine. If I remove the leads from the capacitor, should I see 120VAC between the leads?

Thanks in advance,
Ben

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Re-checked Resistance & Voltage
by: Ben

Hi Bill,

I had a chance to re-measure the resistance and voltage using a better multimeter.

The resistance across the capacitor leads was 0 Ohms when the power switch was off and jumped to infinity immediately as soon as the switch was turned on. I repeated this a couple of times with the same result.

The voltage measured across the capacitor leads was 215 VAC with the power switch on.

Is this the expected behavior? If so, what is the next step? If not, does this confirm I need a new capacitor with the same specs?

Thanks,
Ben

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Mastercraft not starting
by: Bill

Typically the meter should rise slowly to infinity, Ben. Not seeing your meter I cannot be sure the speed of the rise.

Although, from the sounds of it, the cap is good which means that the problem is likely the motor itself.

The symptoms suggest increasing load stops the compressor.

The load is from the pump running and building pressure, but also just be reattaching the linkage to the pump to the motor shaft. That adds load to the motor.

If the pump is sound, and attaching the linkage stops the motor, and the capacitor is good, then to my mind it’s got to be the compressor motor that is suspect.


It’s a mechanical Issue – Crank is slipping
by: Ben

Hi Bill,
As a next step I thought it would make sense to check the mechanical components of the air compressor since the motor was turning.
I drained the oil and removed the front piece with the oil level sight. Once inside I manually turned the fan and noticed that the linkage between the motor shaft and crank was slipping but the Allen bolt holding them together was turning. I tried to tighten the Allen bolt but it seemed to be as tight as it would go. For whatever reason it seems like it is not tight enough (and there is not keying between the crank and shaft) to ensure that the crank turns with the shaft.
Have you seen this before? Any thoughts on how I can fix this?
Thanks,
Ben

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Compressor crank slipping
by: Bill

Thanks for the update.

Getting the nut out and seeing if you can get it to lock on the shaft is the obvious answer.

If you can do so safely, heat might help break the bond on the threads. Or, some liquid thread loosening.

When you get the nut out see if it is possible to lightly grind a small flat on the shaft and get the nut to lock on that.

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