I messed around with that ground strap ontop of the spare carrier crossmember and now the fuelpump is dead, I have 5v on (a) wire and i believe that was for the level sender which is "kinda" working, its an ultrapower pump with 35 miles on it so while i know it will fail probably soon it was fine before i messed with its wiring and i could not find 12v back there and im not quite sure what 'apply battery voltage' to the prime terminal means, just jump the positive lead with an alligator clip?

Anyways, Need a wiring diagram for the fuel pump, searched around and could not find one though im sure its out there im not very smart as you can tell.

 

There are 4 wires to the pump assembly:

Blk - the ground for the fuel pump, goes through a splice pack to the frame rail or crossmember
Blk - the signal ground for the tank level sender which does not go to the frame rail
Ppl - the 5V reference for the fuel level sender
Gry - 12V to run the fuel pump

When the truck is not running but only key on, the fuel pump only energizes for a few seconds so 12V is not on Gry for very long. Sounds like you messed up the pump ground.

George

 

sounds about right, i have an airtex pump with a broken return tap that i plugged in and it was also dead so it has to be wiring, does the now missing serrated washer underneath the ground eyelets have anything to do with it?

 

A good ground contact depends on two clean surfaces having solid and substantial contact. The more current that flows through the connection, the more important that contact is.

There are a couple of ways to check the ground. One is a resistance measurement between the ground socket in the connector at the pump to any clean solid contact on the frame. This however would only alert you to a big issue. A marginal connection may pass under this circumstance of no load.

The best way to check is a voltage measurements under the full load. First you check the voltage from the positive connector at the pump to any solid frame contact point and then if that is good, between the positive wire and the ground wire right at the pump. This is the actual (net) voltage that the pump sees. If that reading is significantly lower then you have a ground lift problem under a load which means that the ground wiring or connection is hosed. Any weak or corroded connection has an increase in resistance. When the current of the full load passes through that increased resistance there is a voltage drop across that resistance. Ohms law > V=I*R. That raises the ground and the pump sees a lower net voltage:

Battery (B) > wire/connection/corrosion resistance 1 (WCCR1) > Fuel pump (FP) > wire/connection/corrosion resistance 2 (WCCR2) > frame ground (FG)

Lets look at a defective ground connection. Assume the pump consumes 2 amps (I dont know what the spec is offhand):

B (12.6V) > Good WCCR1 (0.1 ohms) > FP > Corroded WCCR2 (5 ohms) > FG (0V)

2 amps through the small WCCR1 resistance of 0.1 ohms creates a 0.2V drop in the positive battery line but the 5 ohm resistance of the corroded connection looked OK with no load but at a 2 amp load causes a 10V drop (2A * 5R = 10V) so the pump sees 2.4V (12.6 - 0.2 - 10 = 2.4). The ground wire at the pump is 10V not 0V.

This is why higher power motors should be wired as 220V and why power transmission lines have very high voltages. The current is lower for a given wattage so voltages losses through wires and connectors is less. This also why wiring charts call for heavier gauge wire for motors as the current or wire length goes up so that the voltage losses through the wire is lower and keeps the net voltage to the motor in the safe range. If you try to run a motor long enough at a very low voltage you can burn it up or overheat it and cause early failure.

George

 

Definitely a bad ground then, because there are many things that this truck is and one of them is NOT clean, the entire undercarriage is all sorts of packed with mud so im sure that when i distrubed the ground a bunch of junk got between in, while i have tried cleaning it (several times) with a wire brush its not down to completely clean metal, it has that sortof dull gunmetal look that you see when you first start clearing surface rust off something, going to get some sandpaper and a pack of the serrated washers that were originally under there and probably rerun the groundstrap with a new eyelet.

 

Turns out - It was a broken wire, from the splicepack up to the groundpoint its a black 16 gauge wire it seems like
anyways, only had 18 gauge on hand and it ran just fine with a new eyelet, going to see about messing with it later but its no longer halfway in the road blocking my mailbox

 

Glad you got it running.

George

 

set everything into a 3/8th length of splitloom after the pic was taken but now i have 2 eyelets, one for the license plate lights and one for the pump, and i may relocate it elsewhere in the future or run a dedicated wire from the battery cable ground strap.


For anyone that runs into this issue in the future, check the eyelet and make sure both lines have continuity to the eyelet before perusing other issues. would've saved me 2 days of stress.

 

Be careful with adding ground wires. The entire frame is one big high quality ground as long as the battery, alternator, engine and load connections are solid at or near their respective locations. The only dedicated grounds that travel back to a different location rather that the closest practical frame connection are signal grounds for sensors which need separate very clean signals not effected by the noise of high current or inductive loads.

George

 

Never thought about that, since the OE ground point is working fine (just a broken wire, should've started this diagnostic clusterF with a sanity check) i'll just leave it as is, i do need to fix the fuel filler ground strap still(and probably just replace the lines all together because they're nearly rusted through) as i found part of the eyelet under the bolthead and the rest turned to dust.