Temp sensor alarm on 1995 Mercury 125hp

[acesario]

I have a new to me 1995 Mercury 125 outboard. The motor ran fine for the first half dozen or so outings, but over the last few trips has started emitting a temperature alarm (long constant alarm). At first this was intermittent... coming on for a minute or a few minutes, then going off. Some times it would come on after starting the boat, letting it warm up at the dock and pulling away or just randomly while out fishing. Last night it started right away and continued for a long time until I headed to the dock.

I took it to a boat shop in the area, but the alarm did not sound.

I have read as much as I could find online, and determined just a few things...
-The temp sender unit appears to be in the block, accessed from the back by the top spark plugs (starboard side)
-This can be tested by either grounding it (somehow to the block), or by heating it up directly.
-Tan wires are for temperature (sensor, alarm etc)

Ideas:
Maybe I can just replace the temp sender unit and fix the problem?
Maybe I can replace it with a a temp gauge since this boat does not have one?

Any advice or better ideas?

[wiredog]

Did you ever think that you might need a new water pump impeller?? Wiredog

[acesario]

Nope, never thought of that. Why would the impeller cause the alarm to go off? The motor is peeing a steady stream.

[Burr]

I'd replace the water pump impellor and re-evaluate. Water Pump impellors are normal maintenance, and when they quit pumping water with the pressure they are supposed to generate, the warning signs are exactly what you've described.

There will still be water coming out the pee hole, but it's not generating enough pressure to be effective.

Since it's new to you, it's is suspect that it's a normal maintenance item that is simply due. Normal intervals would be once every 100 hours, or 3 years, whichever comes first - but that's really just a guideline. If the motor has been run in shallow water, pumped sand through the pump, which is common, it will need to be replaced much more frequently.

It's not real expensive. With some research it's something that can be done at home if you have 3 hands.

[blackxpress]

I would do a complete water pump kit (not just the impeller) and go from there. Even if the impeller is doing it's job you could have leaky gaskets which would cause low pressure. It doesn't cost that much and since you have to drop the lower unit anyway might as well do the whole job while you've got it apart.

[acesario]

Thanks for the advice guys... Impeller kit waiting for me at west marine, will replace this weekend with the help of a local boat mechanic. I'll help so that I can learn how to do it myself!

[acesario]

I replaced the impeller this weekend. The old impeller was in pretty good shape, although two of the rubber splines were reversed (curled to the left instead of to the right). Otherwise, the project went as expected, I got a bit hung up on how to install the face seal, because there was not a seal installed previously. I found a few posts on this, and installed it 'lips down' as pictured. Everything else went ok with the installation.

After installing, I took the boat to the water and the motor started pushing a nice stream of water, and what looks to me like a much more steady stream than it did before. So I sat and let it idle for half an hour and putted around the harbor, filled up with gas and waited around to go fishing... no alarm yet...
Just as I was rigged up and ready to go out salmon fishing the alarm started to sound. ( I have a video recording on my phone, will post it later if I can). This alarm is a constant buzzing beep... almost a static sound.
Shut everything down, removed the hood to check and see if I could feel any heat... but none. When I started up the boat again... the alarm started right up with it, and then went off again within a few minutes.

See pictures and video here: https://picasaweb.google.com/1093441...069/MercRepair

Narrowing down the problem
I decided to eliminate the temp sensor from the picture, so I cut the wire just above the sensor on the back of the motor (color=tan with blue line) removed the sensor and put it in the glove box. The sensor I removed is a spring with a small, flat battery sized piece of metal behind the a rubber seal with the wire hole in it. I assume this is the correct sensor, as it was pointed out to me by a boat mechanic. After this.... turned on the boat, no alarm.
Figured I would continue testing, so we went out just in front of the break wall close to the docks and started trolling... no alarm for about an hour then the alarm started up again. In hind sight, it was shortly after turning on the radio... but not right away The alarm went off and on intermittently, 5 minutes on then off , 10 minutes on then off (but maybe quieter than it had been before)

Maybe a Short?
I shut down the radio here and there and it seemed like when I had the radio off, the alarm would never sound. But it would continue after I had shut off the radio for a while.... so not a direct connection but I never heard the alarm start when the radio was off. I installed that VHF radio and a new antennae myself a few weeks ago and it was a difficult installation because of how tight the area under the steering console is. I also had to pull the screws from the bank of switches, attach from the front side. I wonder if I didnt create a short between the radio and the alarm. I also notice that my ACC switch turns on the nav lights. I don't think it did that before, but the boat is new to me so I could be wrong.

Next Steps
I think my next step will be to solder the wire back together and reinstall the sensor.
Then check the electric control panel and back out whatever I did with the radio and and remove it, then see if the alarm still sounds.

I would also like to replace my temp alarm with a temp guage. Does anyone happen to know if this is possible? If so, what kind to get?

thanks
-ac

[ChuckD]

I wouldn't have cut the temp sensor out completely. Reinstall it.

It would be shocking to see any correlation of this motor alarm and the boat's radio, or any radio for that fact..

The engine is getting hot. If the water pump was installed correctly I would then replace the thermostat(s). I had to replace both thermostats on my '94 V6 Merc. Very easy and cheap.

Good luck

[acesario]

Originally Posted by ChuckD

I wouldn't have cut the temp sensor out completely. Reinstall it.

It would be shocking to see any correlation of this motor alarm and the boat's radio, or any radio for that fact..

The engine is getting hot. If the water pump was installed correctly I would then replace the thermostat(s). I had to replace both thermostats on my '94 V6 Merc. Very easy and cheap.

Good luck

The only correlation between the alarm, and the other electronics on the boat would be a short in the elec system.

How could the engine getting hot be the cause if the temp sensor is uninstalled, yet the alarm continues to sound ???
My thought is that by removing the temp sensor from the picture, it eliminates overheating as a cause of the alarm going off. It also removes a fault temp sensor as a possibility.


See the below update....
===

Had to work late last night but I did get a chance afterwards to test the temp sensor wire and repair it. I simply grounded it by touching it to the motor and when turning on the ignition switch, it beeped continuously. Once I had the wire repaired and sensor replaced I got the test beep only as expected.

Tonight I will test the oil sensor in a similar fashion... it is a tan wire running under the motor to the oil tank. Then I'll look around the throttle mechanism and horn wires as suggested, and remove the throttle cover for inspection.
When I hooked up the radio and my sonar/gps unit I didnt touch anything tan or tan/blue stripe nor the purple wire... I wasnt even down there by the throttle except to pass the sonar cable past it. I also tied a few things together with zip ties here and there. And come to think of it, this problem DID occur the first time before I had my radio (but I after I had the sonar/gps) installed.

I will also look into the thermostat. Thanks for the tip.

I had a somewhat similiar issue recently with my 1995 115 Merc. Overheat alarm was chirping intermittently and at a very low volume. I got a service manual and traced all the wires and it turned out my problem was a loose connection where the temp sensor wire connects to the switchbox.

Pulled my controller apart and the alarm has two wires going to it. Purple for power and tan/blue for ground. The only two grounds in the system are the temp sensor or the alarm module (which is triggered by the low oil sensor) If temp sensor isn't the problem then I would look for a loose connection or frayed wire. Barring that the alarm module might be going bad.