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Hi guys, I need help. I had my motor apart this winter to rebuild the center section ( steering, shock mounts, etc). It's a 1994 Merc, 90 HP. I had the power head off and the lower unit out. I have it all back together properly, or so I thought. I even took pictures of everything before disassembly to make sure everything went where it's supposed to and it all looks right. The problem is when I turn on the key and the test horn sounds, it will not go off. It's a steady beep not an intermittent beep as with a low oil warning. The motor starts and runs fine but the horn stays on. It's pumping plenty of water through the tube and out the hole. I took the large connector plug apart, made sure all the prongs were clean. I even spread them a little with a knife blade to make sure there's good contact then used silicone grease and plugged it back in. No change. I did notice that there is no juice at the terminal on the starboard side of the motor where the temp sender wire connects to the warning module. Is that normal. Can anyone tell me what to check next?

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If your 94 Merc 90 HP is anything like my old 91 Merc 115 HP, the little electronic control box for the low oil warning and temp alarm is known to go bad easily. With the temp sensor disconnected and the low oil sensor disconnected, there should be no continuity between their respective input leads coming from the warning control module.

Double-check all the connections there to make sure you didn't make an assembly mistake.

If your setup is like mine, the temp sensor is actually a snap switch located behind a rubber plug in the head. It should be a normally open circuit (at normal operating temps and below) between the two leads. I'd check that.

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OK here's what I have. There's the module with five wires from it aft to forward there's purple which is the power feed from the key, then two blues that go to the oil tank sensor, then black that grounds to the block, then tan with a blue tracer that goes to a single post junction block connecting to the temp sensor wire and another tan/blue wire that goes back into the main cable to the key. I removed the temp sensor and wire and tested it. In the (cold) state it is an open circuit. If you power up the purple to the module there is no juice to the two oil tank wires or the tan wire to the temp sender. If you connect the tan/blue from the module to the tan/blue that goes back to the main cable to the key the alarm sounds and stays on. Everything appears to have been reassembled exactly the way it was originally but I didn't have this problem before. Actually the only wires I disconnected when I took it apart were the two oil tank wires. The rest related to the trim motor which works fine

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one more thing. I just went out and did a continuity test on the wires as you suggested and I do have continuity between the two blue wires that go to the oil tank sensor. Does that mean that the module went bad? There's none between the blue oil tank wires and the tan temp sensor wire.

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OMC used all Tan wires for their warning system. Mercury looks like it is slightly different with the blue wires from the Oil tank.

See this article on the Warning Horn system on OMC just to compare to yours.
http://www.whalercentral.com/articles...ticle_id=9

Also see this article on how the other OMC/BRP wires the newer warning horn system.
http://www.whalercentral.com/articles...icle_id=72

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Sounds like the module might be bad. The continuous warning should not be going off unless the temp sensor wire goes to ground. If you disconnect the temp sensor, there shouldn't be any way to get a continuous alarm unless the module is bad as far as I understand.

Normal operation of the temp sensor is, when it reaches a overheated state, the snap switch activates and goes to ground.

The tan/blue wire that goes into the harness I believe runs to the horn under the console.

In the 19 years I owned my Merc 115, I had to replace that module at least four times. It failed exactly as you describe every time.

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Is it possible you may have pinched a wire during your motor reassembly? I've seen this happen before.

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I checked all the wires for damage and there is none. I am borderline fanatical about protecting parts from damage during assembly. I also mark wires with colored tape, tags with numbers, whatever it takes to make sure things go back together properly. I just did a continuity test on the oil tank sender and it is also an open circuit with oil in the tank. The continuity between the two oil tank wires from the module leads me to believe that the module is shorted inside and can not get past the initial test cycle. I just did one more test. With both the oil tank sender and the temp sender wires disconnected. only the power feed to the module connected, the tan/blue wire from the module connected to the tan/blue wire back to the horn and the black module ground connected, the horn sounds continuously. If you disconnect the black ground wire from the block the horn stops. This also leads me to believe there is a short in the module. Does anyone disagree????

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I have the module off the motor and on the bench. There is definate continuity between the black ground wire and both of the blue oil tank sensor wires.

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Just to double-check, did you try it with everything connected EXCEPT the oil tank sending wires?

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Yes, I did that early on. Everything connected except the oil sender wires. Still had a continuous horn

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Sounds kaput to me.

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On a Merc, a continuous horn when the engine is at normal temp and getting plenty of oil, is almost always a bad alarm module.

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I think it's toast. I ordered a new module this morning. There goes another hundred bucks I could have spent on Mahogany. Thanks for the input people.

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The new module came the next day. Can you believe it? I installed it and everything is working as it should. Thanks again for all the help

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Glad that fixed the problem.
If it lasted 16 years, that isn't too bad at all.