Help w/ 35 Merc alarm problem

[Stizo]

A friend of mine has a mid 80's smokercraft with an oil injected 35 hp Mercury 2 cylinder. He took it out on the river and told me after running 4-5 miles at 3/4 throttle the audible alarm started going off intermittingly. When he got the boat home he asked for my help. We put the muffs on it and it fired up no problem and ran fine ( exhaust smoke looked normal too)with NO alarm sounding after running more than 15 minutes on the hose at various RPM ranges. It pees water fine so it appears his water pump is okay and there were no signs of blockage on the intakes. We then looked at the oil pump / resevoir assembly. His manual says to untie the hold down straps and turn the entire assembly upside down to test the alarm. We did this numerous times and could not get the alarm to sound. Looked for loose elctrical connections but all looked OK. Something is wrong...any thoughts anybody? Thanks in advance.

Stizo

[Fishin4Life]

If it is oil injected it could have been low on oil. Did that all check out ok?

[Stizo]

Yup,

3/4 full

Stizo

[Chris Haley]

Might need a new impeller, since you ran it with the muffs. The pressure of your hose would push the water through fine with out it heating up at all.

[elevatorguy]

I had this happen to me on a yamaha. It was fine wide open and at an idle but half throttle the alarm would go off. It ended being the water pump. And while it was running it seemed to pump water fine but it was a bad pump. So I would definately take a look at the pump. Maybe just try the impeller first it is worth a look.