MEP-803A just smoked the "Fuel Float Module" electrical box to right of fuel tank.

[ian99rt]

Well my awesome MEP-803A's fuel tank gaskets were in need of replacing and I just replaced the smaller no name batteries with $440 worth of Optima 34/78 batteries (currently sitting at 25.3VDC .

I went to test out the Aux fuel pump and put the system to "PRIME & RUN AUX FUEL" without starting the engine. I was holding my finger over the Aux fuel fitting (expecting to feel a vacuum, but did not hear/feal the aux fuel pump running, only the main fuel lift pump I believe was running) when I heard a lovely electronics POP and saw the blue smoke come out of the small electronics box to the right side of the fuel tank that the Low fuel float system connects to.

I removed the box and investigated and found a MOSFET that popped. I had just verified the 2 float switch circuits were working correctly.

From what I can tell this simple circuit board simply converts the 2 float reed switches to a likely higher current signal for the Aux fuel pump relay and low fuel warning system. I'm trying to find a good wiring schematic for the whole genset but can't seem to find it in the TM.

Before I spend $90 on a used one from fleabay is there any other things i should investigate to ensure I don't let all the smoke out of the next one?

 

[Scoobyshep]

That should be an easy component to replace.

 

[ian99rt]

That should be an easy component to replace.

Are you referring to replacing the MOSFET or the module? I could replace the MOSFET if I could find a part # on it. Question is what killed it.

 

[Scoobyshep]

Are you referring to replacing the MOSFET or the module? I could replace the MOSFET if I could find a part # on it. Question is what killed it.

Yes the MOSFET. They arent too many variations out there. There has to be a diagram in one of the TMs. As for what smoked it, its usually an overload or short

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[ian99rt]

Scoobyshep:

I did find the schematic in the TM albeit its super tiny in the .pdf from GreenMountain.

So it looks like the module's MOSFET directly controls +24VDC power to Aux Fuel pump. Maybe the pump is bad and smoked the module.... I'll investigate the pump tomorrow (found a guy on ebay selling 2 new Facet 40194 for $60 and got them coming already). I'll see if i can identify the popped MOSFET and find a replacement before buying a whole new module.

 

[Guyfang]

Yes the MOSFET. They arent too many variations out there. There has to be a diagram in one of the TMs. As for what smoked it, its usually an overload or short

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No diagram. Its a cheap, toss away part. Non repairable at any level.

 

[Guyfang]

Scoobyshep:

I did find the schematic in the TM albeit its super tiny in the .pdf from GreenMountain.

So it looks like the module's MOSFET directly controls +24VDC power to Aux Fuel pump. Maybe the pump is bad and smoked the module.... I'll investigate the pump tomorrow (found a guy on ebay selling 2 new Facet 40194 for $60 and got them coming already). I'll see if i can identify the popped MOSFET and find a replacement before buying a whole new module.

View attachment 803170

The module rarely goes bad. If you can repair it, its cheaper then a new module. Did you do anything else to the set other then what you wrote above? The pumps should have no influnce on the module. Rather the FL-1 and FL-2. Like I wrote above, there was never a need to break down the schematics that far, its a toss away part. There are better schematics in the forum here.

 

[ian99rt]

The module rarely goes bad. If you can repair it, its cheaper then a new module. Did you do anything else to the set other then what you wrote above? The pumps should have no influnce on the module. Rather the FL-1 and FL-2. Like I wrote above, there was never a need to break down the schematics that far, its a toss away part. There are better schematics in the forum here.

Can you link to the better schematic?

 

[Guyfang]

Do you have the TM's?

 

[ian99rt]

Do you have the TM's?

Thank you so much!

 

[Guyfang]

Dont thank me. Thank the Forum. Lots of people here worked hard to improve and check them. A member, Dieseladdict did that fine art work.

 

[ian99rt]

So for everyone's reference if a similar issue occurs here is the replacment Darlington Transistor that controls the Aux Fuel Pump:

Part #: MJE800 (obsolete), replaced with MJE800G

https://www.mouser.com/ProductDetail/ON-Semiconductor/MJE800G?qs=HVbQlW5zcXXyyLGkvlgkjA%3D%3D

I have 2 spliced spade connections for the aux fuel pump to the normal all white wiring. I marked one set of connectors with sharpie to make sure i put them back the same. After stripping some heat shrink back I can see this connector pair is the pumps black (negative) wire yet the opposite connection ( assumed positive on the genset side) has 0.4 ohms to chassis ground and the negative leg of the pump was connected to a circuit that is currently open.

Head scratcher is how did I somehow accidentally swap the polarity on the pump side and cause the transistor to smoke when I purposefully marked the connectors before disconnecting?

 

[Guyfang]

After almost 30 years of fixing gen sets, I will say, "SHI* happens".

 

[ian99rt]

I can confirm that after replacing the blown transistor with the MJE800G unit in the link above the module is fully functional again. I had to buy a used unit to get the part # off of the transistor so now i have an extra (will get it conformal coated at work).

 

[fa35jsf]

Nice. I just did the same thing, however it was because I wired in a second fuel pump off the same module.

Thought I may need an external pump to pull fuel from a drum 15’ away and push it through a fuel/water separator and filter before going into the unit. Not only was that unnecessary, but it put too much draw on that little transistor that is only rated at 4A.

Live and learn.

I also learned that if you jumper the #1&3 pins on the wire harness that should plug into the module, you can get the aux pump working again. Of course it won’t automatically shutoff, and low fuel protection doesn’t exist with the module removed, but it is handy if you still have to transfer fuel from a 55gal drum into the unit.


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