Running down the lake the "Low Oil " light and buzzer came on. There was over 3/4 in the big tank but the tank on the motor was down to the bottom line. I filled the motor tank and engine ran fine until it again ran low. My question is how can I check the oil pump in the big tank? Is there any filter in the oil line that might be getting plugged up? I believe that the oil pump should run all the time the key is on, is that correct?

What does the oil level look like in the tank on the motor? There should be a filter in the tank that is on the engine. That could be plugged up and not letting enough oil into the tank on the motor. Hard to say without looking at each of the parts to see if one of the tanks is really low or if a sensor is causing a problem. If you replace the filter in the tank on the motor, you can pull the tank and clean it out as well to make sure there isn't any gunk in it. Once you replace it, there is a grey switch just to the left of it to pump oil from the big tank to it so that you can fill it before you start the motor again.

Change the filter at bottom level outside of the main oil tank. #23 http://www.boats.net/parts/search/Yamaha/Outboard/2002/LZ150TXRA/OIL%20TANK/parts.html

There is plenty of oil in the tank in the boat. When the alarm goes off there is only about 1/4 of the tank on the one on the motor. If I use the switch then it should pump oil to the tank? So if it does not then I have a pump problem?

Yes, the grey one-way toggle switch should pump oil to the motor tank from the big tank. If it were me, here are the troubleshooting steps I would go through. Empty the tank motor and check the filter in it. Clean or replace the filter, clean any gunk from the tank. Run the oil hose to a container and hit the switch to see if it pumps oil. If it does, then I would check sensors to see if they looked OK. If it doesn't, I would start looking for a big tank oil filter or in-line oil filter to see if it was clogged. If it still doesn't pump, then I would pull the oil pump and test it.

I tend to start from the easiest to more involved. Pulling the tank on the motor and checking it is no big deal. There should be a tube on the bottom to trap contaminants. Make sure to clean that out as well. The other stuff isn't hard, just potentially harder to get to.

Not necessarily it could be the on board sensor.

IF you can pump oil to the on-board tank from the toggle switch, then it is the sensor in that tank that is bad. That sensor should signal to turn on the pump to pump the oil. If you are manually having to do it then that sensor must be bad.

Which I believe is #2 in the link above.

I can not pump from the grey switch. I was under the impression that the pump in the big tank runs when ever the key is in the on position, is this correct? I measured the voltage on the brown wire going to the oil pump and there is 12 volts when the key is on. I have the tank out now. I will replace the filter on the outside of the tank. How can I check the pump and also how does the pump attach to the tank?

clean filter and your issue could be #26. We are assuming that you have power to the pump and that has been eliminated.

Here is what I have done: removed tank and cleaned it. Hooked external power to pump and it runs and pumps. I checked voltage at the plug and when I activate gray switch I have 11.6 volts at the plug. When I connect the plug to the tank and activate the switch the pump does not run.

You might want to grab a Seloc manual for the engine. It's about $25 and well worth it. It has much more detail (sometimes too much) than the Yamaha manual. I use it and the Yamaha service manual to run down things where I cannot find an obvious problem. Seloc has a troubleshooting flowchart for the oil pump. Was the engine trimmed up or flat when you tried the emergency switch? It will not work unless the engine is down.

http://www.amazon.com/gp/product/B0015GD1YK/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

The flowchart in section 5-8 walks through the troubleshooting steps in detail and will walk you down to the conclusion of a wiring problem, a pump problem, or a sensor problem. It will also walk you through testing each sensor as well. The pump will only run when the sensors tell it that the engine tank needs oil by closing the circuit. It could end up being an electrical problem, so having a manual to walk through the steps to check each component will keep you from throwing parts at it and drill down to the real problem.