1996 Mercury 90Hp - Fished today and motor worked fine until we started up to move the boat. Warning horn beeps intermitten - I checked the oil - it was a little low but not that low. Put more oil in reservoir and horn still honks. When you just turn the key on without the motor running the horn sounds - beep, beep, beep and will not go off.

Anybody have any ideas?

If your water pump is shooting water out the pee hole, I would check the float in the oil tank- they seem to stick or just go out- Mine did and I put a new tank in, no more beeps..but mine was not intermitten, is was a steady screaming...I followed the wires from the tank back to the plug on the side of motor, unplug it and see if the beep stops- I think its a brown and a black wire at the plug..

Solid beep - water impellar might need replacing/engine getting hot.

Intermittant beeping - check the oil reserve and do as kingdad101 says and replace the oil cap. Possible float getting drenched and sticking. You can unplug the wiring (blue wire on mine) from the oil cap and see if it stops to test if it is a faulty oil float/cap alarm. Might take it to a mechanic if it doesn't stop. Don't run it anymore if the beeping doesnt stop just in case.

Ok - I took the oil reservoir off and drained the oil. I then hooked the sensor back up and turned the key on. If I reach in with a hanger I can pull the float up and the alarm will stop. As soon as I let it go it will start beeping again.

Can I replace just the float part/sensor? If so how do I get the part number? It has a screw at the bottom of the tank and looks like it will come off.

Here's a pic of the tank.

Nah your gonna need to buy a new tank, the float is made with the tank,theres no part for that.about $70 bucks.

Check here for the part number:

http://www.mercurypartsexpress.com/PartsExpress/ProductSelection/ServiceParts/sphome.asp?CID=

YOu will need the S/N from the motor and then select oil injection parts from the list.

I bought a new cap for my oil tank and I think the it came with the float.

Good luck.

Thanks guys.

I found part# 87-12886A 6 but I'm not sure if it's correct. I am going to Rockwall Marine next week.

Check http://www.marineengine.com or http://www.iboats.com to see if they carry the part. May be cheaper to buy online and have it shipped given the price of gas these days.