Coolant temp sender bad?

[mlumetta]

I have a merc 4.3 carb with alpha 1, built in 2008 with 140 hours. The temperature gauge shows 160F warmed up at idle and rises to 170F at WOT.. This is how I always remember the boat performing except now I get an over temp alarm intermittently at WOT. The temp sender on my engine is Mercury part # 805218T.

1) I am reading 400 ohms at 170F, does that sound correct?

2) Can someone supply me with a chart showing a few resistance versus temperature points?

3) What temperature should the coolant over temp alarm sound (its a fresh water cooling system with a 160F thermostat)?

Thank you,

Mike

 

[thumpar]

Are you sure the alarm is going off for temp? It can go off for other things too. The alarm for temp goes off around 200+ degrees. Check your lube monitor and oil pressure.

 

[alldodge]

I have a merc 4.3 carb with alpha 1, built in 2008 with 140 hours. The temperature gauge shows 160F warmed up at idle and rises to 170F at WOT.. This is how I always remember the boat performing except now I get an over temp alarm intermittently at WOT. The temp sender on my engine is Mercury part # 805218T.

1) I am reading 400 ohms at 170F, does that sound correct?

2) Can someone supply me with a chart showing a few resistance versus temperature points?

3) What temperature should the coolant over temp alarm sound (its a fresh water cooling system with a 160F thermostat)?

Thank you,

Mike

:welcome: to iboats Mike

Are you sure it's not overheating?
When is the last time the impeller was changed?

 

[Fun Times]

Hi there, welcome to iboats!

Water Temperature - Ohms Reading approximate temp to resistance chart from a Mercruiser service manual
210°F (100°C): 185
160°F (70°C) : 450
100°F (38°C) : 1,800
70°F (20°C) : 3,400
40°F (4°C) : 75
20°F (-7°C) : 13,500
0°F (-18°C): 25,000
-40°F (-40°C): 100,700


Typically you should hear the warning sound starting around 196°F.

Are you sure/what makes you believe it's temperature related?

First thing to suspect for temperature related issues is the sea water impeller or any blockage in the cooling system due to debris. Also check the thermostat itself.

Since it only happens at WOT, Keep in mind that your system should also sound a warning horn if you hit the rev limiter that should sound at about 150 RPM higher then your max WOT RPM range allowed. If memory serves me correctly, on your engine model that should be either at 4850 or 4950 RPM.

Let us know what you find, Good luck.

Wow^^^ Formatting really needs some work. Check your PM box.

 

[mlumetta]

Thank you all for your help.
That resistance chart is what I was looking for and confirms that sensor is good.
I know the engine is not over heating because I have double checked actual temperature with a non-contact thermometer over the thermostat housing. I was blaming it as a temperature sender because I eliminated the low lube switch from the circuit and oil pressure is good. The engine will not run if the oil pressure sender is disconnected, so I can't fully rule out a bad oil sender.
I have on occasion hit the rev limiter (cuts spark) but no alarm sounded.. Of course I backed off the throttle immediately. Are you positive the alarm sounds when the rev limiter is hit? Maybe the alarm only sounds when the rev limiter is hit for a few seconds straight? Keep in mind the engine is carb, so the alarm is handled by the ignition module, not an ECM.
Thanks again,
Mike

 

[thumpar]

Just remember that the senders for the gauges and switches for the alarm are separate.

 

[mlumetta]

Yes, I'm aware that the alarm has it's own temp sender and oil pressure switch unrelated to the senders for the gauges.

My understanding (from Mercury literature) is the alarm will only sound from over temp, low oil pressure, and low outdrive lube.

To find which fault sounded the alarm, I temporarily disconnected the low lube switch (reservoir is full). So that means I'm down to temp or oil pressure.

I then determined the my temperature gauge was correct by double checking with a non-contact thermometer. The engine is only at 170F during WOT.

I would be suspicious of the low oil pressure switch but I believe that is required to keep the fuel pump on (I unplugged the oil pressure switch while running and it stalls out as soon as the carb starves of fuel.) The engine is still getting plenty of fuel when the alarm sounds, so I have a hard time believing it's the oil pressure switch.

Any suggestions? Does anybody know exactly what temp sets of the alarm? Could it be as low as 170F? The Ignition module that sounds the alarm is Mercury part # 861251T 1 .


Thanks,

Mike

 

[thumpar]

When my impeller went the alarm sounded at around 200 degrees F. Mine is different than yours so I am not sure if it trips at the same point but should be close. My temp will normally hit just a tick under 175 degrees F with no alarm.

 

[Fun Times]

Are you positive the alarm sounds when the rev limiter is hit?

Rethinking it through, I'm having second thoughts about if the rev limiter sounds the warning horn on the later model carbureted engines. I was going off of a fading memory believing I recalled hearing the horn while at the rev limiter on a carbureted engine while prop testing, but it may have been an EFI engine. I can't find anything in writing to back what I thought I recalled in the past. I was thinking since Merc recently has put the yellow wire from the coolant sensor into the ICM, that the gray wire going into the ICM would sound the warning horn as well for the limiter. Sorry for the wild goose chase I may have put you through. Again, the temp horn should sound right around 196+/- 2 degrees.

~~Mean-Best-Timing (MBT) Spark Advance During light load cruising, the ignition module maintains optimal ignition timing by
making small spark advance adjustments. At a given RPM, the module will add a small amount of advance and wait to see if there
is an RPM change. If RPM increases, it will increase timing more. The module will continue to advance timing until it no longer
gets an increase in RPM. Conversely, if it senses an RPM drop, it will start to retard some of the spark timing. Between 1200‑4000
RPM the ignition module can add approximately 10‑15 degrees of spark advance to the base spark timing curve.
NOTE: The Audio Warning System is also connected into the ignition module circuit. If the audio warning system becomes
activated by the closing of one of the audio warning system switches, the MBT feature is deactivated.



http://forums.iboats.com/forum/engi...ard-engines-outdrives/595348-4-3l-temp-sensor

 

[mlumetta]

I took the boat out and ran it hard.. No alarm, so it's "fixed". The only thing I did recently was unplug the oil pressure switch connector and then re-plug. I'm curious if connector corrosion was the problem.

Please answer this:

With the engine running, I removed the oil pressure switch connector. The engine ran for an additional 15 seconds before it stalled. My assumption is that the fuel pump shut off when I disconnected the oil pressure switch. However. during that 15 seconds without the oil pressure switch in the circuit, the alarm did NOT sound. When the engine sputtered from lack of fuel and finally stalled, the alarm sounded until I tuned the key off.

If the fuel pump shut off because the oil pressure circuit went open, why didn't the alarm sound?

Answer: Because the oil warning horn has a separate switch for the low oil psi...Under 4 psi. See item number 34, http://www.mercruiserparts.com/Show_...cal+Components

 

[mlumetta]

Years later but I wanted to post the solution in case anyone else had that intermittent temperature alarm issue I did.

A while before the problem occurred I had replaced a stuck open thermostat with a non-mercury replacement (same temperature, just a different manufacturer) . Curious if the new thermostat was acting up, I decided to purchase a mercury thermostat. I placed the new and old thermostat in the a pot of water and started heating them up simultaniously. Both thermostats started opening at the same time but the mercury thermostat opened MUCH larger.

Never had another alarm since installing the Mercury thermostat.