merc 150 overheat buzzer but normal temp

[doc14520]

I'm a first timer so bear with me. I have a 1989 150 merc V6. Recently I've been experiencing a continuous warning tone from under the console. This has happened several times. Sat. we were running on plane for 10 - 15 min, slowed to idle, and the warning buzzer went off, continuous tone, and wouldn't stop, even after engine was shut down overnight. Temp guage never exceeded 140 F. Cylinder heads are warm but not overly hot. The first time this happened 3 -4 weeks ago, I took it in. They ran it for several hours and found nothing !<br />So today I got a new overheat sensor but before replacing it I disconnected the old one. Still tones (even with engine not running). Disconnected the oil fill sensor and oil flow sensors. Still tones.<br /><br />Disconnected the wire from the warning module to the buzzer, tone stops. touch that wire to the post several times, beeps. Turn off key, reattach the wire to the post and voila, perfect.<br /><br />Now I'm really confused. Warning module failure? Poor contact at the little post ? Anybody got any great ideas?

 

[garyj]

Re: merc 150 overheat buzzer but normal temp

doc14520,<br /> My guess is the sensor wire is chaffed somewhere and is being grounded. If the wire is grounded, the alarm will sound. Look closely for damaged wiring. If there is no damaged wiring, then I would suspect the warning module.<br /><br />Southern Catfisher

 

[doc14520]

Re: merc 150 overheat buzzer but normal temp

Thanks, Catfisher,<br />I'll go back and check that more carefully but my initial inspection didn't reveal any wire problems. Also, do you know which side the overheat sensor is on ? There are two sensors and it looks to me like the port one is the sender to the temop guage and the starboard one is actually the overheat sensor.

 

[garyj]

Re: merc 150 overheat buzzer but normal temp

doc14520,<br /> You are right: <br />Overheat sensor - starboard<br />Temp gauge sensor - port<br /><br />Good luck......<br /><br />Southern Catfisher

 

[KCLOST]

Re: merc 150 overheat buzzer but normal temp

I would remove the starboard side cylinder head "cover". It's a water jacket. About 20 bolts hold it on to the head. This will allow inspection of the recessed area of the head where the sensor is located. It could be full of water, which it shouldn't be. It should be dry and clean. As well as the sensor.

 

[doc14520]

Re: merc 150 overheat buzzer but normal temp

Thanks Kclost,<br />I'll try that if I can't identify the short and if changing the overheat sensor doesn't do it (turns out it was the temp guage sender I disconnected before. Obviously that wasn't the problem). I will have time to play with it again Tomorrow or Fri. and I'll post thye result. Thanks for the suggestion.

 

[KCLOST]

Re: merc 150 overheat buzzer but normal temp

First, try this.<br /><br />If it tones when the engine is not running, follow the wire from the overheat sensor. It goes to a junction (as I call it), held in place with a screw. It's just above the rectifier, on the starboard side of the motor. <br /><br />Disconnect only the overheat sensor wire, and see if the alarm goes away after truning on the ignition... <br />Let us know...

 

[doc14520]

Re: merc 150 overheat buzzer but normal temp

So here's the update. I disconnected the tan/blue oil alarm module wire at the post and the beep stoppped and stayed stopped when I reconnected it. The problem recurred a few days later. I then disconnected the overheat sensor (Black) wire from the same post and again the alarm stopped but restarted a few minutes later with the sensor still disconnected. One day and a new (and expensive) oil warning module later the problem seems to be solved. Murphy's law works! It's always the most expensive part!<br /><br />Here's the interesting part. My temp guage now reads differently (somewhat lower) although I didn't do anything to the sender, AND my tilt guage which hasn't worked since I bought the boat three years ago now functions perfectly ! Can anyone explain that?

 

[KCLOST]

Re: merc 150 overheat buzzer but normal temp

Look at the temp guage wire.... Is it one continuous wire from the sender to the guage? (it should be).

 

[doc14520]

Re: merc 150 overheat buzzer but normal temp

So the brown wire from the temp sender travels over the heads along with a bunch of other wires to the staboard side of the motor and inserts via wire connector to a similar wire which disappears into the harness (presumably to the console). I believe it's wired correctly.<br /><br />Engine's running great now. Does one evar calibrate the temp guage?