alarm horn merc 150

[David Reardon]

I have a 1993 mercury 150 carberator model serial no od254023. All instuments and remote controls are from a 1987 mariner 150 serial no a906062. the power head has been rebuilt and the oil injuction system removed. I just rewired the hole boat and am having trouble with the alarm horn.<br /><br /> I replaced the old horn (a big one about 2" round) with a small one (about 1/2" round)from mercury and now the horn sounds as soon as the key is turned. the horm connects to a purple wire form the key switch and a brown one that goes to the engine At the engine it goes to a lug that has a wire going to the port head sensor and that sensor has a wire going to ground. The lug also has a wire going to the starboard head and that sensor has only that one wire(no ground wire).<br /><br /> Is this configuration correct and my trouble the horns or am I missing something.<br /><br />Thanks for your time

 

[WillyBWright]

Re: alarm horn merc 150

Welcome to iboats.<br /><br />The two-wire sensor is probably a temperature sender. Output is variable depending on temperature. The single lead one it the overheat sensor. If the motor gets too hot, it grounds the circuit and sounds the horn.<br /><br />Tan wires are for the thermal sensors. Just tan is for a temp gauge. Tan with a blue tracer is for the overheat alarm. Tan with a black tracer is also a temperature sensor but those usually go to a computer (those are the 2-lead sensors). I'm not quite sure what one would be doing on a carbed model.<br /><br />I should add that the oil injection system has a sensor in the oil pump drive that sends a tiny amount of current to a module that sounds the alarm if there is no voltage from it while running. Without an injection pump, no voltage is being produced. The alarm module thinks the oil pump has failed and is letting you know.<br /><br />So at this time you probably have two problems that aren't mechanical. Disconnecting the overheat alarm from the 2-lead sensor should stop the horn from going beeeeeeeeep as soon as you turn on the key (except for a few test beeps). But you'll probably get a beep beep beep once the motor is running. With the oil pump gone, getting that to go away is a lot more "interesting".<br /><br />Just out of curiosity, why did you eliminate the oil pump? You just drastically reduced the value of that motor. You didn't succomb to the old myth about "unreliable" oil pumps, did you?

 

[rodbolt]

Re: alarm horn merc 150

hello<br /> this is going to be both difficult and confusing. the Vmercs on most(not all) models used a temp SWITCH on the stbd head, most of the switches(not all) had a black wire that went to the junction block where it tied to a tan/blue. at the junction block,located on the stbd side just above the starter solinoid inspect that the tan or tan blue is not on the same lug as the purple. now with the key on and the alarm horn sounding dissconnect the tan wires at the terminal. if the horn quits sounding we have isolated it to the engine. make sure the screw beieng used to secure the plastic junction terminal to the tan wire is not so long that it can touch the engine block. the temp SENSOR,not switch, looks phsically the same as the temp SWITCH. its possible they got switched. using an ohm meter test the switchs/sensores. the switch will show an open circuit between the lead and engine block. the sensor will show resistance and the resistance will vary with temp. but sometimes when the alarm module is removed the wires get placed on the wrong screw at the junction terminal just above the starter solinoid..<br /> I know this sounds confusing but merc made many running changes with senders and switche's and have made some alterations in mounting. most senders have a ground lead,however most replacement switches do as well. about the only way to positivly test is with an ohm meter.<br /> it all comes down to understanding basic DC circuits. the purple wire is 12v positive to the horn(for this think of the horn as a bulb) the tan lead going back to the engine is a "ground" the temp switch is just that, a switch. switch closes and current flows and a bulb lights or a horn sounds. if anything allows current to flow to ground from the tan lead on the horn you have a circuit and a tone. <br />hope this helps .

 

[David Reardon]

Re: alarm horn merc 150

Dear rodbolt & WillyBWright<br /><br />I checked and the port sensor is a temp sensor (its reading about 800 ohm).It is now tied down to the same lug as the temp switch on the starboard side of the engine and when i disconnect it the horn stops. Am i correct to think that the temp sensor should only be connected to a gauge and not the horn. If this is correct it fixes my problem Please advise. <br /><br />AND THANKS

 

[rodbolt]

Re: alarm horn merc 150

you got it<br /> or you can buy and install another swicth in the port head.