Overheat Alarm

[TwoFish]

I?m having a problem with the overheat buzzer on my 1998 75 hp Force. It makes a weak buzzing sound intermittently. I?m still trying to track down when it happens. I have just replaced the main engine wiring loom because the plug at the back of the motor has corroded internally. I thought this had fixed the problem but last time out the buzzer sounded very weakly again.

I have earthed the wire on the engine to check the buzzer is working OK and it sounds quite loudly. It doesn?t seem to have relationship to engine temp as it still sounds after the motor has cooled for a couple of hours at sea. I tried it again in the driveway the next day and the buzzer was silent. Wether it is something to do with being in water or a loose connection that moves when the motor is tilted I don?t know.

I?m not sure of the best way of tackling the problem. As I understand it the buzzer has power to it all the time and the heat sensor on the engine grounds the circuit to make the alarm work. Is there a way of testing the heat sensor off of the motor using heat? I have already tested it to see if it is earthing out when it is cold but it seems OK.

Thanks

Dean

 

[mla2ofus]

Re: Overheat Alarm

Just a wild shot, but it sounds like the alarm wire and a ground(earth) wire inside the harness are bare or insulation is split and are getting enough water between them to make the alarm sound weakly. You said you earthed the wire to engine and it sounded loud, so the alarm going off and sounding weak says it's getting a high resistance ground possibly thru the water between the wires, especially if you're running in salt water.
Something to try first is using a blow drier on the sensor to see if the alarm sounds weak when the sensor closes the circuit.
JMHO,
Mike

 

[pnwboat]

Re: Overheat Alarm

If it still sounds after the motor has cooled for a few hours, I kind of doubt that you are experiencing an engine over heating problem. You should be able to touch the head without burning your hand under normal running conditions. May be a little hot but not too bad.

Yes you can test it off the motor. There is a small internal circular spring clip in the hole in the head that holds the rubber plug that in turn holds the spring that pushes the sensor against the head. Use a meter to check resistance. Check resistance between the metal body of the sensor and the end of the wire that is attached to it. Remove the clip and pull the sensor from the head.

When it is cold, it should read infinite resistance or "open". Heat it up with a hair dryer and at some point, the internal heat sensitive bi-metal element should expand and close the connection. At that point, the resistance should go to zero or almost a dead short. Any other readings would indicate a bad sensor. It should read either open or shorted. Nothing in between.

Here is a replacement that is relatively cheap, and they do ship down under.

http://www.ebay.com/itm/MERCURY-THERMOSWITCH-819022-/170525244876?item=170525244876&vxp=mtr

To determine if it is the sensor or the wiring, all you have to do is to remove the sensor from the head, thus eliminating the ground connection for the sensor. If the buzzer still goes off, the problem must be the wiring. If that temporarily fixes the problem, then I would strongly suspect the sensor itself.

 

[TwoFish]

Re: Overheat Alarm

Thanks for the replies.

I was hoping for a simple test to check. Now I have one I’ll give it a try this weekend and report back.

Cheers

Dean

 

[TwoFish]

Re: Overheat Alarm

A quick update.

I checked the over heat sensor and it is working OK.

The buzzer is silent now when the ignition is turned on so I will have to wait until it sounds again and disconnect the tan wire to see if it stops.

As far as I can tell it can only be two things.

1. The boat is getting hot and the sensor was not making good contact with the block get a good ground hence the weak buzzing sound.

2. I have a short some were in the system. I haven’t tried tilting or turning the motor to see if this causes it to short. I’ll give that a try next time I’m near the boat

I will post any further developments.

Thanks again for the previous suggestions.

Dean

 

[pnwboat]

Re: Overheat Alarm

If the motor is in fact overheating, and it's not a short in the wiring, then you might want to go ahead and check the thermostat. Easy to check. Drop it in a pan of hot water. It should start opening between 130? - 143? F (54.44? - 61.67? C) depending on what version thermostat you have. As the temp. increases, the wider the thermostat should open. Also might want to consider replacing the water pump impeller if it hasn't been recently changed. A bad impeller is a common cause for overheating.

 

[TwoFish]

Re: Overheat Alarm

Thanks pnwboat,

I have just finished servicing the motor before Christmas and checked both at the time. All suggestions are appreciated as it is difficult to think of all the possibilities myself.

Just tilted and turned the motor to see if it would trigger the alarm but no luck. I will need to wait until I have it on the water again.

 

[jerryjerry05]

Re: Overheat Alarm

You say you check the impeller?Anytime you get it down that far,change the impeller.
A soft buzzing could be a bad sending unit.Or a ground somewhere.
Could be the plug(harness)on the motor.You said the other side was rotting.

 

[TwoFish]

Re: Overheat Alarm

The latest on the alarm.

It only seems to go off in rough weather. I was out a couple of weeks ago in good conditions and the alarm didn’t go off. I was out last weekend in 1.5 to 2 meter seas and the alarm sounder again. The longer the motor ran the louder the alarm became but not to full volume. When the motor sat for about 20 minutes the alarm would be quieter and then build up as the motor ran. Lots of spray about and a rooster tale close behind the motor as we could only travel at 9 knots.

Here is the bit that is confusing me. I managed to get the engine cover off and disconnect the tan wire from the overheat sensor. The alarm still made the same noise. I then disconnected the main wiring loom plug at the back of the motor and the alarm stopped as you would expect as this cuts the power to the motor.

The weather calmed off by the time we returned and the alarm was silent again. All I can think of is water is getting sucked into the back of the motor at slow speeds in rough weather and causing a short somewhere. The controls at the front of the boat are kept dry by the cabin so I’m thinking it has to be down the back somewhere.

As the alarm is silent again all I can think of doing is removing the cowling and spraying a mist of water over the engine to see if this makes the alarm go off. Is it OK do this? I don't want to add to the problem.

Any suggestions would be appreciated.

Jerry the impeller was changed about nine months ago. At this stage I’m not thinking is an overheating problem. The other half of the plug could well be the problem as you say.

Thanks

Dean

 

[pnwboat]

Re: Overheat Alarm

If I were a betting man, I would suspect a wiring problem. Most likely between the wiring loom plug at the back of the motor and the ignition switch at the helm. I would suspect that it's not water that is causing the problem, rather the wiring loom being jostled around during rough weather. The Tan sensor wire that goes to the helm is probably pinched against the engine block, or against a ground wire in the loom somewhere between the engine and the helm. If all else fails, disconnect the wire at the buzzer that goes to the motor and disconnect the same wire at the motor end at the terminal block. Run a separate wire from the buzzer to the terminal block screw that the Tan wire from the sensor is connected to. This will eliminate the wiring harness.

 

[jerryjerry05]

Re: Overheat Alarm

If you disconnected the sender and it still buzzes.Then there is a short somewhere.
When they install accessories such as biminis or canvas tops or rod holders.
They could have possibly run a drill through the wiring harness.
One of the new boat riggers did that and the motor started turning over with out the key.
Or the harness needs to be replaced.