# Mercury Oil Warning Module Failure and Design Analysis



## pescadero (Mar 31, 2006)

Earlier this summer I had the joy of a failing oil warning module. I was getting a warbling sick cat sounding continuous alarm - but the motor was not over heating. Disconnecting the overheat sensor didn't fix it. Disconnecting the oil warning sensor didn't fix it...

Therefore - we've got a failing module.

This is the 5 wire module, part number 14857a8.

After a little searching I found that there have currently been _*17*_ revisions of this part -
14857 - > 14857a16

I pulled the module and started inspecting it - which lead me to find that the potting compound was separating from the plastic case... a likely source of moisture intrusion.

After pulling the potted block out of the plastic case one can see just how poor the potting job was - nice dime size hole on the bottom of the PCB completely uncovered. Poor flow control on the potting line.











I chipped away the potting compound to inspect the board, and see exactly how they were implementing the circuit.


















Wow... this is an amazing combination of unbelievably cheap build quality, and a circuit straight out of the late 1970's.

These things sell for $80-$100 new. I could produce them in quantities of 10 (not high volume) - with an actual high quality PCB... for probably a little under $10/unit.

This is a single sided PCB, unbelievably thin fiberglass, ultra thin (probably 1/2 oz Cu), with no solder resist... this thing is significantly lower quality than the $19 straight out of China Walmart DVD player PCBs.


...and the circuit is super old school - a comparator based delay for the startup beep (U2) OR'ed with a comparator based oscillator (U4/U5) driving an old school Darlington transistor array (TIP120) to provide a path to ground for the piezo buzzer.




















If anyone has a junk 14857 sitting around - I'd love to pull a few more apart and see if there is any failure commonality, and if they ever made any updates to the circuit.


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## Scout 2 (Dec 31, 2004)

I would check with some marina's and see if they have any laying around


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## ESOX (Nov 20, 2000)

I hate to say it, but nothing Mercury does comes as a surprise anymore. Is that a Tohatsu/Mercury?


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## Richw46 (Oct 20, 2019)

I'm amazed to find a recent posting about this issue. My 1995 Mercury 115 has a failed module and I cannot believe how much they are asking for this part. Initially I thought it was the switch in the bottom of the oil reservoir so I bought that for $20. When I turned the key I was still getting the beep-beep-beep alarm. I did a continuity test on the new one (in the reservoir) and it reads no resistance. I did a reading on the old one (dipped in 2 stroke oil) and it reads open, yet both produce the same signal.

I've lived without the warning buzzer (disconnected one of the wires from the oil tank probe) for 2 years. I don't like to do that but $140 is what I've found online for a new one; outrageous.


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## mso795 (Feb 24, 2006)

If you unplug the low oil level probe from the bottom of the oil tank and the beeping goes away. There is a magnet inside the tank that is glued to the float that came loose. You can glue it back in place. Google it. Mine did this last Spring.


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## Richw46 (Oct 20, 2019)

Everything I researched told me about the magnet problem. My tank is different. Mine has a short float to show you when the tank is full, a little metal pop-up in the filler area that has a cork float on it. It's travel distance is maybe 3". At the bottom is a place for the probe/switch to plug into an area built into the tank. There is no float with a magnet on it, like I expected to find. Mine is an early '95, I purchased the boat and motor in late 1994, September I think. There must have been a couple of different tanks they used.


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## mso795 (Feb 24, 2006)

If you unplug one of the wires to the low oil sensor and the beeping stops. It is that float. If you look inside of a empty oil tank you will see a molded plastic nipple that the oil sensor plugs into. On top is a metal round clip, the float is next. Last is the magnet. It does not show up on any of the diagrams. I have a 1998 125 and it is hard to see.


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## Richw46 (Oct 20, 2019)

I'm just noticing that you're close to Gun Lake. I used to fish there from the mid-50s to 2000. A good fishing lake but too crowded now.

Well you have given me hope mso795. Thanks for your help and quick responses.

I put a magnet close to the old switch/probe and a magnet near it and the circuit closed. So you're right and I seem to be wrong. Now I need to know a few things. When I took the tank off I didn't hear anything rattle around inside, so the magnet isn't loose. Where is it? Secondly, when I looked into the tank, the float shows when the tank is full. I can look down and see the float, but it's just on the other end of the pop up thing, which looks to be only about 3" long. How does the magnet get down to the bottom (normal operation) to close that nipple switch?

Most important question: how do you get the magnet and the cork float out of there? I'm attaching a picture of my tank. Is it the same as yours? Looks like it will be difficult to get any of the items out of there, repaired and back in.

I've uploaded a photo of the float at the top of the tank and whatever the yellow thing is at the bottom. There is a hole in the center but there is no rod in the tank. The circuit is obviously closed but I don't see anything to take out. The cork float has nothing underneath and is only 2-3" long. The cork doesn't extend downward beyond that.


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## FREEPOP (Apr 11, 2002)

Mercury's quality was doomed with the purchase of Force.
Mark up on parts is +400%


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## mso795 (Feb 24, 2006)

The magnet is glued to the bottom side of the float. When it becomes separated from the float it sits as low to the bottom of the tank as possible. The magnet position then tells the probe you are always low on oil. It is difficult to get to. I pried the ring off with a long screw driver. Putting it back on I found a pipe that would go over the nipple and taped the ring to it and pressed it back on. Yes I fish Gun lake a lot. Search on Google.


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## mso795 (Feb 24, 2006)

On Bing I found a video where a guy cut a oil tank in half to show how the float works.


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## Richw46 (Oct 20, 2019)

I fashioned a tool from some 12 AWG copper wire that has a hook on the end. I see it now. The star looking thing on the top is a metal retainer that goes over the center post. I'll have to find something that will let me pry up some of the tabs on that retainer so I can get those other things off and glue the magnet back to the cork. I have some mechanical fingers (flexible claw) that I should be able to use to put it back together.

I found the video, which helps understand how it work:





Thanks again for your help and persistence in putting me back on track before I wasted the price of the module.


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## Richw46 (Oct 20, 2019)

I got the magnet out, the top clip is somewhere down in the bottom. I'll find that but I'm having trouble getting the float out of the opening in the oil fill box. It seems to be too big no matter which way I turn it. Did you have to take the oil fill box off? It looks like it's a separate piece but it doesn't move when I try to turn it, like it's glued on the oil tank.


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## mso795 (Feb 24, 2006)

My filler opening was big enough to get it thru.


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## Richw46 (Oct 20, 2019)

I think they did away with the box on top of the tank and just went with a large opening. Looks like my choices are to either buy a new tank, go without the alarm or cut a hole in this one and then seal it up again after I've made the repair. I always add oil when I put gas in the tank so I think I'm just going to leave the alarm as is.


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## mso795 (Feb 24, 2006)

Mine does not have the extra tank on top. It is just one tall tank with a large fill opening.


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## wmohat (Dec 22, 2020)

pescadero: I also had one of these modules fail last year. I dissolved the potting compound in Methyl Ethyl Keytone, and got almost all of the potting off the PCB. (It dissolved the packaging on the three small diodes.....and took the label off the power darlington, but other than that, all parts are intact, and I can read all the values.) It's in pretty clean shape.

I *was* going to do a schematic, but then I ran into your posting, where you had 
entered the design into LT Spice. I probably would have had to do that also, to figure out
what the silly thing was doing. So, you saved me some time.

Anyhow, you had asked for other PCB assemblies to see if there was a common failure mode on these parts. I'm not sure what failed on my board, even now, as I didn't have the time to replace the diodes, and re-test while connected to my motor (which is under it's winter cover.) 

Given the number of revisions on this board (and the rather crude design), it's obviously been a headache that Mercury still hasn't solved. The power darlington is NOT what failed....I believe the LM2901 is what lets go, but (again) I haven't verified this. There is absolutely ZERO overvoltage and transient protection in this design, so it wouldn't take much of a transient to blow out that LM2901.....that's my guess anyhow.

**** If you would like to have me send you my PCB for you to analyze, just let me know.

--wmohat


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## wmohat (Dec 22, 2020)

pescadero.......one last detail. The unit I have is a 14857A8 version.

** did you ever get your LTSpice model to run, and oscillate?

--wmohat


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## kbird (Feb 26, 2021)

Richw46 said:


> View attachment 445717
> I think they did away with the box on top of the tank and just went with a large opening. Looks like my choices are to either buy a new tank, go without the alarm or cut a hole in this one and then seal it up again after I've made the repair. I always add oil when I put gas in the tank so I think I'm just going to leave the alarm as is.


did u ever figure this out im having same issue


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## ETW (May 18, 2020)

For what its worth... My "new" 92' 135hp didn't even have a problem with the alarm module or any part of the system. I just decided to delete the oil "injection" system completely when I realized that the pump is driven by a plastic gear buried in the crankcase. Upside: Never worry about hearing (or not hearing) that alarm again or starving the engine of oil. Downside: Pre-mixing. As far as decisions go it was the easiest one I made all day.


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