Used Mantra ebike 36 volt 10 am Brushless motor.Using 3 x 12 volt batteries in series(meter shows 40 volts).Bike has never run since I bought it. (bought as for repair). When connecting,battery flashes at terminal. Assumed duff controller,so removed controller from bike & bench tested -same result. Purchased brand new controller-same result. When I test Red/Black wires(battery connectors to controlller), meter shows live circuit??? see photo. Any ideas???

When you say flashes, what does that mean? A spark, welding what, there will likely be an initial surge from the battery pack to charge the caps. The real test is, are the wires getting hot or, is the fuse blowing (or do you have a fuse)? What is the amperage rating of your controller?? Let's say its 25A, hook a 25A fuse in line with the controller power, hook it up and see what happens. If the fuse blows immediately, you have a short somewhere. Chances are, its not in the controller but elsewhere. Providing us with a diagram of exactly how you have everything hooked up will greatly aid us in helping you troubleshoot. The tester you are using is very basic and simply looks for continuity. Well, a direct short has continuity so in essence, the tester doesn't help you locate the short.

What exactly are you trying to test? It looks like you have the black lead in the common jack and the red lead in what appears to be a jack for testing continuity. So if your probes are connected to the battery terminals, all you're testing is how quickly your meter is going to go up in smoke.

I am only testing the controller(completely disconnected from bike) Controller is for brushless motor 36v,max current 14 amp,rated current 7 amp. Battery input is 3 X 12 volt 10 amp in series. Red positive input wire has 15 amp fuse( which has no blown). When I connect, there is a spark at the battery positive terminal .I remove wire immeditely as I assume short circuit,so it is not connected long enough to get hot. Then I test red/black input wires on controller & it shows continuity(which seems wrong). This is same for the original controller and also a new unused one. If I connect the positive ,then the negative,it sparks at negative terminal.

One generally does get a spark when you connect a battery to a controller, as the capacitors in the controller fill up quickly and suck juice out of the battery. Is this what you are worried about? If so, it is normal, but you may need to replace your connectors now and then, or you can find various solutions on the net for arresting the spark, using a resistor which is hooked up first, then the main connector.

If you have a fuse inline, try putting a small one, like 5A or 2.5A, in it's place. Then leave it hooked up...... if it doesn't blow, it's OK. Then, hook it up normally.

I was testing red/black battery input to Controller .It seems wrong as I get continuiity. when these wires are connected to battery,it is like connecting positive to negative which will give flash at terminal! pic enclosed of battery(reads 40volts)

pancho,
Circuits (like a controller) will have continuity. It will not be a direct short. What does your meter read when you go from the red to the black on the motor controller? Certainly not a direct short but a value. Also don't forget that some components show impedance instead of resistance (ie a coil). If both the brand new controller and the used controller show the same resistance/impedance, they are both likely good. You really need to hook up the fuse as stated before or hook up a load resistor like chvid said. The load resistor should be about 10k ohms and 1 watt.

Bingo! The fuse did not blow & when I ignored the initial spark on connecting battery ,the motor works fine! Now I have to re-assemble.Thanks a lot for all your valuable advice. regards Pancho