Thinking we should get some of these stickers for our pumps 

If you are switching the fan via a relay you do not need a diode. PS. If you have your indication lamp/light/led directly connected to fan, then yes you will need a high current Shockey diode- fit the tell-tale to relay coil to mitigate this. 20+ amp diodes are not cheap!  

Thank you, Alan!  I thought that some how the capacitor provided charge that avoided the need for a very high current on start up, so I'm glad be further educated. John

I think I may need advise on the one way diode. 

Hamish. Sounds like you are getting there and I would agree with the addition of the relay. I have a thermo switch and a manual override on my GT6, with the feed through a subsidiary fuse. I added an LED warning light, but had to add an inline diode in the fan feed to stop the motion generated current causing the warning light to glow. Ian

John You need the capacitor on a single phase AC motor to spoof a phase shift to get the armature to turn otherwise it can just sit there and vibrate if there is no capacitor or be very hard to start if the capacitor is weak. Not needed on a 3 phase motor as it has a natural phase shift but if you get the phases wired wrong the motor will spin backwards. I believe DC motors use commutators to achieve the same continuous magnetic changes in the armature windings that cause the motor to spin, but I am not very  familiar with DC motors Cheers Alan  

Well there you go!  My ignorance for all to see.  The AC/DC difference I was aware of, but not that it would affect the way a motor starts ( or how!) Thank you, Spit6! John  

John. Your equipment is AC current. Car fan is DC. Not the same.

The electric motor in my workshop is on the compressor, and that has an enormous capacitor on it.  Size of a can of beans.  Do others have them, drills and so on? In ignorance, I looked up how it might be wired to the motor, and found that there would be a centrifugal switch - to disconnect it once the motor is up to speed?    But if a cooling fan can draw 50A (!!) why don't they build a capacitor into the fan? Surely the motor speed could be detected electronically, without a mechanical switch?  Or am I just showing my ignorance? John