2003 Johnson 150 (J150PXSTM) Overheat/Temperture Switch Question

[Oreo]

This weekend we got a extremely low tide down at our camp in Dulac, La due to a high (25-30 mph) Northeast wind driving the water out of the marsh. When we returned from a morning of fishing we had 14-16" of water in our canal that run in back of the camp. Well once I got part way in I was committed and continued to the camp kicking up mud the whole way. 2/3rds of the way in I got a engine temp warning horn and water temp light. I shut down the engine, tilted it to check the lower unit, recranked it and continued the last 100 feet without any further warning horn however the water temp light stayed on.

The water at our docking area was about 3' in depth and I pulled the pee frog nut from the back of the engine and recranked the motor allowing the water pump to flush the indicator area of any mud or other debris. Water pump is pumping strong, however light still on. Thinking maybe there maybe some debris still in engine I attached the hose via flush port and flushed engine for about 5 minutes. Turned on key, all indicators did self check and Water temp indicator on, crank motor, instant horn for 10 seconds then light only. Shut down engine and let sit for several hours. Time for evening fishing motor has been sitting for 3 hours, turn on key, engine self check cycles water temp indicator on, start motor instant warning horn for 10 second and then water temp indicator only.

There is no way the engine is overheating that quickly and I now suspect that I have another issue. Take boat out to main canal and work throttle up slowly. Engine is in self preservation mode limiting RPM's to around 1500 or so. Head to landing and trailer boat to camp. We pulled the thermostats they look good, flushed housing again this time with thermostats, springs and caps off. Everything looks good. Reinstalled and looked for temp sensors/switches in head area. Found and unplugged starboard side (single wire) connector still get water temp indicator. Unplug port (dual wire) no water temp indicator. Plugged back in water temp indicator. So I pulled the switch from head and with it in my hand and plugged in I do not get a water temp indicator but if I ground it to powerhead water temp indicator comes on.

It appears to me that the port switch (dual wire) is shorted however I am not completely familiar as to how this system works. Does this switch short or pop with a single overheat much like a thermostat on a copier fuser/heater unit? Just wanting a couple opinions about what I have found and if I am on the right track. If so I can replace this myself in 3 minutes and can be fishing again next weekend.

Thanks in advance.

 

[jhebert]

Re: 2003 Johnson 150 (J150PXSTM) Overheat/Temperture Switch Question

A temperature operated switch is designed to operate ON and OFF in response to temperature. It is not a one-time-only device. The sensor on the port cylinder head probably has two circuits. One likely actuates at a lower temperature to signal that the engine has reached operating temperature so the engine timing for warm up can be restored to normal; the other actuates at a higher temperature to signal an over heat condition.

It sounds like it has malfunctioned.

 

[Texasmark]

Re: 2003 Johnson 150 (J150PXSTM) Overheat/Temperture Switch Question

We had a similar problem on a 150 Merc. Our failure was not a dead short, rather about 180 ohms which produced an weak visual indicator and horn.

I was going to dissect the sensor in an attempt to understand how a SPST switch can fail at 180 ohms, but for some reason didn't.

Replacing the sensor solved our problem.

These things do happen.

Mark

 

[Oreo]

Re: 2003 Johnson 150 (J150PXSTM) Overheat/Temperture Switch Question

Thanks guys for the response. Gonna order the port side switch and replace. I do have another question. The port switch has an aluminum case and easily slides into the head where the starboard switch appears to be encased in rubber and is solid in the head. By solid I mean it won't easily slide out for inspection, seems to be stuck. Is there a tool for removal or just use the old poke and pry method? Was thinking about just replacing both since it would be around $100 to do so.

Over the years I have become a bit more conservative in my yank and pry methods. I'm told it is a sign of old age setting in...

 

[Texasmark]

Re: 2003 Johnson 150 (J150PXSTM) Overheat/Temperture Switch Question

Per what I have heard about the two sensors, the one that you think you have to pry out may be for a temp indication meter....just a guess. You'd think that a V engine would have a sensor in each head if it has a stat in head as Merc does.

However, you never know what's in the mind of an OEM designer, nor his/her experience with reality.....we used to hire them right out of college too and it took 2 years of internship to be able to trust them.....no brag, just fact. During that internship, we put them on the assembly line so that they could see what bad engineering/design decisions did for the product and those trying to produce it.

Mark

 

[angus63]

Re: 2003 Johnson 150 (J150PXSTM) Overheat/Temperture Switch Question

Stbd side has overheat sensor
Port side has dual sensor for operating and overheat
Different part numbers
Port side has tan lead and striped lead
Stbd side just tan lead
As a marine mechanical design engineer working hard for a recent corporate takeover , unfortunately the rush to make monthly and quarterly numbers by public corporations is the biggest culprit for unproven,unqualified designs from reaching the assembly process. Showing good numbers to the stockholders has become JOB 1, with quality and refinement relegated to a distant second.