MEP 002A Fuel Level Float Switch

[AndyJ]

I have been working on a low hour MEP 002A trying to get the various small problems resolved. The latest thing to pop up is a seeming problem with the fuel level float switch.

Whenever the master switch is placed in the RUN/AUX position, the breaker on the panel pops. I replaced the float switch and it ran about 2 hrs and is now doing the same thing.

All the fuel pumps seem to run as normal.

Disconnecting the AUX fuel pump with the float switch connected gives the same results- the breaker pops.

I can disconnect the fuel level float switch and the unit runs like a champ with no other problems.

Any insight into what is going on would be appreciated.

 

[PeterD]

Q: why/how did you determine the float switch was bad? Did you apply the same test to the new switch?

Here's my take on it (worth roughly $0.02!): when you replaced the switch you 'shifted' or moved a wire that is shorting out somewhere (perhaps in a harness/bundle) and the switch itself may not have been the problem. Then after two hours of running, things shifted back to their original positions and the short reappeared. I'd carefully inspect both the harnesses and the various connectors to see if there is any damage.

 

[Keith_J]

Either a wire short to ground (circuit codes P62B to P62C, then P63A) or the aux solenoid valve which is attached to the aux pump is shorted.

Using a volt-ohm meter, probe all three pins on the tank switch with the meter in the lowest resistance scale or 200 ohm scale. With J4 (the tank level switch Cannon plug) disconnected, all three pins should be over 1000 ohms to ground.

When the master switch is in the aux position, the aux fuel pump is on. The only time fuel can flow from the aux pump to the tank is when the float switch drops about 2-1/2". This closes the contacts in the float switch, energizing the solenoid valve which then allows the flow.

These pumps only draw current for a short pulse which moves the internal piston against a spring. As fuel is delivered by the spring force, the piston moves to close the internal switch, repeating the motion. This is why they "tick", every "tick" is the piston moving rapidly.

 

[AndyJ]

Either a wire short to ground (circuit codes P62B to P62C, then P63A) or the aux solenoid valve which is attached to the aux pump is shorted.

Using a volt-ohm meter, probe all three pins on the tank switch with the meter in the lowest resistance scale or 200 ohm scale. With J4 (the tank level switch Cannon plug) disconnected, all three pins should be over 1000 ohms to ground.

2 of the 3 pins have readings in the 12-1300 range. The third pin is nada

I put the original float switch back in and none of the pins have a reading

 

[AndyJ]

FWIW, I just tested both switches for continuity per TM 5-6115-584-12 and they both test good

 

[Keith_J]

1200 ohms pin to ground shouldn't be a problem. At 24 volts, a 1200 ohm resistance has current of 24/1200 amperes or 0.020 amperes. But if it is 12 ohms, it would be 24/12 ohms.

Yes, the third pin is open. This switch is a magnetic reed switch, encased in glass. If you have checked the wiring from Cannon plug to Cannon plug (switch to solenoid valve) and found no insulation break, look under the plugs. The out rings unscrew and there should be a polyethylene collar and a gray silicone rubber packing/strain relief. Since Cannon plug shells are metal, any break in insulation here could cause a short to ground.

 

[AndyJ]

If you have checked the wiring from Cannon plug to Cannon plug (switch to solenoid valve) and found no insulation break, look under the plugs. The out rings unscrew and there should be a polyethylene collar and a gray silicone rubber packing/strain relief. Since Cannon plug shells are metal, any break in insulation here could cause a short to ground.

The AUX fuel pump looks to have been replaced at some point with a Facet brand pump. The two wires going into the AUX solenoid were twisted together and wrapped with electrical tape instead of being properly spliced. Whoever did this replacement also reversed the two wires. I now have the correct wires properly connected. Bad news is it still pops the breaker.

 

[AndyJ]

Anyone know how I can check the AUX fuel solenoid to see if it is bad? That seems to be the next logical step.

 

[AndyJ]

Any additional ideas on tracing down this electrical gremlin? Sure would appreciate it.

 

[Keith_J]

The solenoid valve should have resistance between 35 and 70 ohms. Specification current draw is 1/2 ampere. Usually, these fail open but rust could have shorted out the coil.

 

[jbk]

have you tried disconecting the aux/pump, and solenoid wires?

 

[AndyJ]

have you tried disconecting the aux/pump, and solenoid wires?

Yes,

When both are connected the breaker trips.

If you disconnect the AUX pump power lead with the other two connected, the breaker trips. The pump does run.

If you disconnect ONLY the float switch connector, the breaker does NOT trip.

If you disconnect ONLY the AUX fuel solenoid, the breaker does NOT trip.

The fuel level float switch itself checked out OK according to the TM and to the advice Keith gave.

All wires have been traced and connections checked with no problems --other than the 2 wires being swapped which was noted and corrected.

 

[jbk]

theirs a new solenoid on ebay under military generators for $60.00 or delks surplus. sounds like thats the trouble. you will still be able to use it until parts arrive.

 

[Keith_J]

Like I stated, resistance should be in the 35 to 50 ohm range, allowing only a half ampere of current at 24 volts DC. Use the 10 amp setting on your volt meter.

Ohm's Law applies here, voltage is equal to current multiplied by resistance. Or V = I x R.

 

[AndyJ]

Like I stated, resistance should be in the 35 to 50 ohm range, allowing only a half ampere of current at 24 volts DC. Use the 10 amp setting on your volt meter.

Ohm's Law applies here, voltage is equal to current multiplied by resistance. Or V = I x R.

I get no reading (infinity) on any of the 3 pins on the solenoid.