Two Speed Pump Woes, Capacitors Etc.

I have an AO Smith two speed pool pump that is switched manually from back of motor. Pump will run fine in slow. When I swtich to fast in runs fine for a while then cuts out . If I switch to slow it will run again.
Obvious questions. If it is a thermal protection scenario would the pump still work in slow but not in Fast? ie are there two different thermal protection circuits
Secondly if I start pump in high it hums and does not run if I toggle to slow it will run in slow and if I then toggle to high it will run. Again just for a while then cuts out
Capacitors?? Is the run capacitor a likely culprit (easy cheap) Does the slow speed circuit not use this capacitor just the high speed circuit
Can I dismiss the start capacitor as it seems to start fine in slow , but then again not in high??
I dont have electric motor wiring knowledge but a fair comprehension about moving things !!
Ant comments on how these two speeds work , capacitors etc appreciated.
I can just swap out both for $20.00 but would actually like to understand ita bit

Regards Mike

I'm going to see if Mark, who knows pool motors better than I, has time to answer.

Mean while, if you have the AO Smith model #, you can possibly look up specs on their website:
http://www.centuryelectricmotor.com/...logSearch.aspx

A motor will be under loaded in low speed so yes, it could possibly trip the thermal limiter only on high speed while not on low speed.

But a couple question to narrow down the problem.

Motor model#?

Is there a centrifugal switch in the motor?

Is there a single capacitor OR are there two capacitors, one on motor body (i.e. hump) and one in the end cap?

Have you tried to spin the impeller by hand and does it feel rough?

Motor is an AO Smith UQS 1202R rated at 2 hp, 230 V . It has two capacitors, assuming run and start.

One is rated 35MFD 370V (run Cap) other is 63-77 MFD 270 V (inside end cap)
If by centrifugal switch you mean the V shaped spring mounted unit at the end of the main shaft in the end cap that then yes. It closes when pump is turned on right.

had the pump head off and replaced seals yesterday. spins freely by itself , more resistance when pump backing plate / impeller is installed but still easy enough to turn by hand. Bearings sounded /felt fine.

When I removed end cap to check wiring one lead was discolored (as if it got hot) and plastic sleeves of the disconnect terminals was hard / and cracked off when I removed terminal. Re soldered one terminal that looked a little questionable. End cap area was clean , no obvious cat hair,dust ,grime or earwigs .Both Capacitors look Ok not swollen etc.

On another note last time i was running on high I had to get it to run from switching to lo first the flip to high. If I try to start in high it hums and doesn;t run. When i did get it running in high (lo/ high trick) it ran fine then made some strange noises, not worn bearing noises but more like it wasn;t getting enough juice then was fine then would do it again.

If switch to lo it just runs fine.

Thats about all I can can add.

Not true . It was working fine for a long time then slowly and more regularly it would trip when on high. More recently it has got wore and worse and now will probably only run 5 minutes or so (depending on ambient Temp also)

Let's try to eliminate things one at a time.

First clean the centrifugal switch contacts and see if that helps. A fine grit metal sand paper works well. Sometimes the contacts can corrode and cause intermittent problems. Because the motor doesn't start in high speed, this could be part of the problem.

Next, if you have a multi-meter you can check the capacitors:

1 Fully discharge capacitors with a screw driver across the capacitor terminals
2 Remove capacitor(s)
3 Set ohm meter on lowest resistance setting
4 Measure resistance across terminals - Should first read close to 0 ohms and then steadily climb to infinity. If it is shorted or always open, then the capacitor is bad.

If both capacitors check out and the centrifugal switch has been cleaned, then it could be a short in the main winding. The low speed winding is separate from the main winding so you can get thermal overload one and not the other. Unfortunately, that would not be something that could be easily fixed.