1994 Mercury 40HP issue

My alarm is going off constantly in the "run" mode. At first I thought it was the low oil sensor but its still going off with it unplugged. I took the oil tank off as well and have tried various ways to get it to turn off (like hold tank upside down so sensor is at its high position, cross the wires). I read that all you have to do is unplug the sensor to make it go off but that's not happening in my case. I am kind of lost at what to do now. Could the overheat sensor be faulting instead?

SN: OG137497

Like you have done already....unplug it and see!

The low oil and overheat run off the same box so it might be the overheat going off. My 60/45 jet had the overheat go bad last year and I don't run the oil injection anymore so I had to disconnect both sets of wires. You could have a bad overheat sensor or the box going bad. You should be able to test the sensor before you spend money on either one. I bought a new sensor only to have the alarm keep going off so I just disconnected it altogether. I keep an eye on the water telltale to make sure it's pumping ok, but I rarely run more than 15 minutes at a time so I'm not worried about the overheat.

FYI - this is what my Merc service manual for a '97 50hp says about the Warning Alarm circuit. The text in blue color tells you how to troubleshoot it ... and I would imagine/guess (but can't confirm) that the alarm logic is similar throughout all Merc OBs - models and years.

Warning Module
When ignition key is initially turned on, warning module will briefly provide a self test of its engine overheat warning system – a brief steady BEEP tone, followed by a self test of its low oil warning system – a brief BEEP-BEEP tone.

- If the oil level in the oil reservoir drops to less than 7.5 fl. oz. (225ml), the low oil (float) sensor in the oil reservoir will signal the warning module to activate the warning horn.

- If the powerhead temperature should exceed 300-deg F (65-deg C), the overheat temperature sensor in the water jacket cover will signal the warning module to activate the warning horn.

If warning horn sounds a continuous “beep”, the engine overheat sensor may be faulty. Disconnect tan/blue overheat sensor lead at at bullet connection. Turn ignition key to “ON” position.

- If horn still sounds a continuous “beep”, the warning module is faulty. Replace module and retest.

- If horn stops sounding, the engine overheat sensor is faulty. Replace and retest.

DaleH said:

...

- If the powerhead temperature should exceed 300-deg F (65-deg C), the overheat temperature sensor in the water jacket cover will signal the warning module to activate the warning horn.....

Click to expand...

Good info Dale, the following question is not a jab at you...

Does anyone else see an issue with the F and C conversion?

212F ~ 100C
32F ~ 0C
300F ~ 149C
65C ~ 149F

At least it's not Chinglish.

:twisted: :twisted: :twisted:

SumDumGuy said:

Good info Dale, the following question is not a jab at you...

Does anyone else see an issue with the F and C conversion?

212F ~ 100C
32F ~ 0C
300F ~ 149C
65C = 149F

Click to expand...

WOWZA ... yowza ... that's not 300-deg :shock: F! I surely did NOT pick up on that, just copied it out of the PDF. Here's the actual page below. And then look at the temps for the OMC overheat switch ... either OMC value listed is neither 149 nor 300 degrees F.

So ... will the REAL Mercury overheat temp alarm values please stand up ... :?: ?

The manual for my 2000 40hp Merc 4s says overheat alarm will sound at:

192.2*F [89*C]

With both heat sensor and tank sensor unplugged the alarm is still going off in one ear piercing constant tone. Great...

So I will have to buy a module or just remove it and hope for the best.

From the Mercury Parts Catalog:
32 14857A10 1 OIL WARNING MODULE ELECTRIC

Xaero said:

With both heat sensor and tank sensor unplugged the alarm is still going off in one ear piercing constant tone. Great...

So I will have to buy a module or just remove it and hope for the best.

From the Mercury Parts Catalog:
32 14857A10 1 OIL WARNING MODULE ELECTRIC

Click to expand...

Mine went bad as well so I just unplugged it for now. I keep an eye on the motor to make sure water is pumping through and I don't run all that long so I should be ok. I'll replace the module at some point in time.