so i have a 1990 f350, crewcab dual fuel. it runs on propane or gas. propane runs fine so its not a spark issue. did a fuel pressure test and got 0 at the shreader valve. so here is what ive done so far:

-checked inertia switch
-tested fuel pump relay. relay tested ok.
-verified fuel tank selector switch was working, tested ok.
-jumpered eec test plug to ground to put fuel pump in constant on. relay clicks but no FP sound and no pressure.
-jumpered pins 3 and 4 on relay plug to provide direct power to fuel pump. tried both tanks, no sound and no pressure.
-verified voltage at tank selector switch, gets 10.5v and directs in appropriately.
-found harness wires to each pump on each tank and verified voltage at each(near the tanks). both get about 8.5v when selected.

now that i am at this stage is it safe to assume the pumps are bad? is 8.5v enough to hear something happen at the pump? i find it unlikely that both pumps have gone bad.

a little more info: truck has a 5.8l 351 engine with a 5spd transmission. the truck has been sitting for a while, about 2-3 years. we start it up every now and then to move it somewhere and always fires up easily on propane. the no-run on gas started right before it was parked. it had no fuel in the rear tank so at the time we didnt bother trying that tank. there was(and still is) about 1/4 of a tank in the front tank. can non-use lead to pump failure?

also, do i need to drop the tank anyways to get rid of the old gas?

thanks for any help or insight.

 

definitely sounds like you have dead pumps you say the rear pump stopped about 2 years ago before you parked the truck now I would be guessing front pump is also dead now or stuck from sitting around but if possible can you unplug one of the pumps and plug in a tested working one im sure they need to be grounded also to work so you may even just have an earth problem I had the read pump go in my truck a few months ago it would start and run fine but as soon as you touched the throttle it would die as it would not deliver any flow but it sounds like your problem is worse then mine and yes the tank has to be completely removed to get at the pump well on the rear tank anyway as that's the only one I have done be careful when removing fuel line clips you also have to remove a hose from the inside of the tank to the pump and also be careful with the filler hose it is a real pain to get that back in without kinking it and causing a filler splash back problem if you have a standard exhaust running past the rear tank and coming out behind the rear wheel it also is a pain to get tank in out it a fun job to do by yourself on the ground good luck with it I would definitely try to connect 12 volts to pumps before removing tanks if you can maybe try earthing out the tank with a wire from the earth on your battery first

 

Sounds like an open fuse link to me.
When checking voltages in the fuel system do not use a non loading meter as they will tell you you have some voltage when you do not.
Always use a test-light in the fuel pump circuit with a bulb in it.
The voltages you are reading are coming from the computer and not the fuse link.
You should have near battery voltage all the way to the fuel pump.

 

thanks, i was thinking the voltage reading was funny. but when i test the fat yellow wire at the FP relay i get a full 12v, is that ghost voltage from the computer too? is the fuseable link connected to the starter solenoid? thanks.

i will see if i can dig up a test light.

 

NO. As far as the ghost voltage on a meter I have not seen it above 10 volts with a fully charged battery.

Yes.

Sounds like the voltage (current) is not making it passed the fuel pump relay. Check the wiring under the relay socket.

 

ok. so the yellow wire going to the relay has full voltage and the fuse link is good. i used a test light to check the voltage/current at the tank. i have tried this with the relay bypassed(yellow jumped to brown) and with the relay itself.

i get no voltage reading at the tank with the relay open and the test light does not light. with the relay closed or the harness jumpered i still get a reading of 8.5-9v at the tank and the test light does light. this tells me im getting some kind of useable voltage at the pump. i get the same when i do this on either front or rear tanks.

still seems like low voltage at the pump. i think im gonna try feeding it full 12v at the tank and see what happens.

 

i would say bench test the pumps out of the tanks and if you still get nothing with direct 12v then they are dead

 

yeah thats the plan. i tapped the wires and fed 12v directly to each pump, i heard a click/small thud but heard no whirring ad got no pressure. my guess is they are stuck somehow so the next step is to pull the tanks.

thanks for all the help.

 

During 1990 ford was trying to get rid of the problematic fuel reservoir/tank selection valve and just put valves in the fuel pumps. my wife's truck had a dead front pump and a rear pump on its last leg, some how the rear pump was feeding the front tank and I had nothing but problems till I changed the front pump and have not had any problems.