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Topic: Fuel Pump Relay (Read 591 times) previous topic - next topic

Fuel Pump Relay

Does anyone know where the fuel pump relay is on my 1986 36' Grand Villa on an Oshkosh chassis with a Ford 460 carbureted engine?  I have found 3 relays behind the dashboard.  One is for the horn (I can feel and hear it click when sounding the horn). Another clicks whenever the ignition is switched to 'run'.  I thought this might be for the fuel pump but have my doubts and think it might be the master magnetic relay.  I have no idea what the third relay is for.

Also does anyone know if the oil pressure switch (which is part of the fuel pump relay control circuit) makes contact when there is no oil pressure or when there IS pressure? I have read that both types of switch exist.

Thank you in anticipation
Marty Day
1986 GV 36'

Re: Fuel Pump Relay

Reply #1 –
It has been a lot of years since I worked on those coaches but it is possible that the fuel pump relay is mounted on the fire wall accessed by raising the bed and I think that the oil pressure switch closes when the engine is running.
previous 1984 35 ft ORED 250 HP 3208 Cat       
previous 1998 40 ft U295 CAI 325 hp Cummins
previous 2003 40 Ft u320 build #6140 450 Cummins M11.                                                         
1999 Mazda Miata
Ron, Nancy, Tipper the cat, Max The dog
1997 U 270 36 ft build number 5174 8.3 Cummins

Re: Fuel Pump Relay

Reply #2 –
There may be a crash switch also .. Black box , 2 wires, with a pin that sticks out when crashed. No fuel pump  power when pin is out.  By pass with a fuse plugged into the wires.  That switch may run the actual relay. 

Re: Fuel Pump Relay

Reply #3 –
Thank you, Mike and Ron for your replies.

My problem is no fuel supply.

There IS a relay in the engine bay and, of course, it is nigh on impossible to follow the wires and I have been unable to disconnect the wiring harness as yet.  It is too hot to spend much time outside.  However I am wondering if that relay is actually the master magnetic relay.  It is rated at 30 amps and the wires are heavier duty than most wires.  I have no idea how many amps the fuel pump draws.

I have an Oshkosh manual with a wiring diagram which shows the fuel pump circuit as follows:
A 12 volt supply source with 2 wires on the same terminal (I shall call them L for Load and C for Control).
Wire L goes to the inertia switch, then to the load side of the relay and then to the pump.
Wire C goes to the oil pressure switch and then to the control side of the relay.

The 12 volt supply and inertia switch are behind the dash, on the driver's side. When I turn the ignition switch to run, I have 12 volts approx at the supply and the same voltage going into and coming out of the inertia switch. From the inertia switch, wire L disappears out of sight towards the passenger side i.e. to the right. (the 3 relays behind the dash are on the extreme left). Wire C disappears straight down a hole.

As the only break in the circuit is at the oil pressure switch, I am theorising that cranking the engine produces sufficient oil pressure to operate the switch, complete the circuit, energize the relay and run the pump. I am therefore assuming that the relay that clicks when the ignition is turned on cannot be the pump relay.  I have voltage on one side of the pressure switch connector.  I jumped the connector but the pump did not run.  I have dropped the fuel tank enough to disconnect the 2 pin connector from the pump.  I ran a wire from a battery to the pump and the pump hummed.

So the only missing piece of the puzzle is the darned relay!!  Also the hot wire to the pump does appear to come from the front of the coach.
Marty Day
1986 GV 36'

Re: Fuel Pump Relay

Reply #4 –
Jump the oil pressure switch and see what happens. It might be faulty.
1999 40 ft. U-320 wtfe build 5563 Chuck & Lynda's "Rollin' Inn"  2030 watts solar
prev. mh's 71 GMC 5 yrs. 73 Pace Setter 1 yr. 78 Vogue 5 yrs 81 FTX 40ft all electric 18 yrs. 1996 Monaco Signature 3 yrs.
2014 Grand Cherokee Overland
Dream as if you will live forever. Live as if you will die today.  James Dean

Re: Fuel Pump Relay

Reply #5 –
Run another ground to the tank connection.
 Maybe check under the step for a relay.  Get a 30 ft test lead and start at the tank connection or the Inertia switch and work back and forth until you find the relay.  OR  run a new wire  from the inertia switch. Making sure that t he tank circuit has a good ground.
 These have grounds all over ,that are simply sheet metal screws run into the frame .

Re: Fuel Pump Relay

Reply #6 –
Thanks again to all for offering suggestions.

The pump is grounded otherwise it would not have hummed (not sure it was pumping - I shall check that later) when I ran a wire from the battery positive to the pump positive.

I am now wondering if I have been blinded by all the stuff I have seen on the internet.  I am wondering if the relay that clicks when I switch the ignition to RUN is the pump relay because the diagram shows the relay as "Normally Closed" i.e. the contacts are touching.  So when I switch the ignition to RUN, the control side of the relay is energised, pulling the contacts apart and preventing the pump from running.  This would mean that the oil pressure switch was closed when there was no pressure.  When oil pressure builds during cranking, the switch opens, cuts off power to the control side of the relay and the load-side contacts reconnect thus powering the pump.  Or have I gone completely loopy???

I shall see what I can check out tomorrow.  If my theory is correct, if I disconnect the pressure switch wires and switch the ignition to RUN then the pump should work, indicating a bad pressure switch and hopefully not zero oil pressure.
Marty Day
1986 GV 36'

Re: Fuel Pump Relay

Reply #7 –
Quote from: Marty Day –
Thanks again to all for offering suggestions.

The pump is grounded otherwise it would not have hummed (not sure it was pumping - I shall check that later) when I ran a wire from the battery positive to the pump positive.

I am now wondering if I have been blinded by all the stuff I have seen on the internet.  I am wondering if the relay that clicks when I switch the ignition to RUN is the pump relay because the diagram shows the relay as "Normally Closed" i.e. the contacts are touching.  So when I switch the ignition to RUN, the control side of the relay is energised, pulling the contacts apart and preventing the pump from running.  This would mean that the oil pressure switch was closed when there was no pressure.  When oil pressure builds during cranking, the switch opens, cuts off power to the control side of the relay and the load-side contacts reconnect thus powering the pump.  Or have I gone completely loopy???

I shall see what I can check out tomorrow.  If my theory is correct, if I disconnect the pressure switch wires and switch the ignition to RUN then the pump should work, indicating a bad pressure switch and hopefully not zero oil pressure.
Oil pressure switches close with oil pressure.
1999 40 ft. U-320 wtfe build 5563 Chuck & Lynda's "Rollin' Inn"  2030 watts solar
prev. mh's 71 GMC 5 yrs. 73 Pace Setter 1 yr. 78 Vogue 5 yrs 81 FTX 40ft all electric 18 yrs. 1996 Monaco Signature 3 yrs.
2014 Grand Cherokee Overland
Dream as if you will live forever. Live as if you will die today.  James Dean

Re: Fuel Pump Relay

Reply #8 –
I am pretty sure that the pump relay is in the engine compartment might also check on the frame rails.
previous 1984 35 ft ORED 250 HP 3208 Cat       
previous 1998 40 ft U295 CAI 325 hp Cummins
previous 2003 40 Ft u320 build #6140 450 Cummins M11.                                                         
1999 Mazda Miata
Ron, Nancy, Tipper the cat, Max The dog
1997 U 270 36 ft build number 5174 8.3 Cummins

Re: Fuel Pump Relay

Reply #9 –
Thanks, Ron.  I found the relay a couple of hours ago.  It is behind the dash about 6 inches from the inertia switch in full view!  It was so difficult tracing the wire to it and I didn't know what it was as I am ignorant when it comes to relays.  It is rectangular with 3 screw connectors and fixed to a block of wood.  I need to disconnect all batteries and remove a tray of fuses that is covering the retaining screw(s)/bolt(s).  There might be a ground wire on the retaining screw/bolt but I am not sure.  If it is actually a 3 pin relay then the wire from the oil pressure switch must be a ground but I don't think it is.  I shall investigate tomorrow.
Marty Day
1986 GV 36'