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I have a 1989 comanche eliminator. Inline 6, 4x4, AX15, 3 inch lift. Been driving fine. Just bought it a couple weeks ago. Has a bit of high idle. Leaks here and there. Smallest amount of coolant in oil but it does have 268k miles. Gonna do the head gasket and such soon. I drove it and parked it, no problem. Blinkers stopped working but was going to look in to that. Been following Cruiser54's tips for sprucing up grounds, adding some, etc. Will keep doing them as i go. The next morning after i parked it in my swamp of a driveway, it wouldnt start. Normally i turn the key and the pump primes, its loud. I can hear it while driving but it seemed fine. Definitely noticeable. This time, no prime. So being before work i didnt do much besides bang on the tank and switch the relays around, nothing. After work, i checked ballist resistor, 1.1 ohms, bypassed anyways to no avail. The relay was swapped around with all the rest so that rules out the relay. I checked for power at pump, i get 6 volts off of the plug for pump with key in on position. I'm thinking that's kinda low. I also get 6 volts if i hook up the wire (bypassed ballist resistor) to my meter and ground it on body. So I'm sure its getting to the pump but maybe not enough? I also did Cruiser54's tip of adding a independent ground on the pumps harness just to be sure as well as clean ground behind taillight housing. I have power on pin 30 of relay and switched ignition power on pin i believe 85 (trying to remember what pins are what). I tried jumping power to the pump and that didn't work. I also removed the pump (before i added the extra ground point to it) and hooked it to the battery for a split second, and it did run. So I'm positive the pump itself is okay. Right now I'm working on tracing wires and connections and sprucing up plugs, etc. I tried looking around other forums, etc for ideas but I'm not sure at this point. Any ideas or thoughts would be greatly appreciated.

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Do you have a wiring diagram for it?  I'm on my phone, and don't have my FSM.  However I'd go back to the relay and ballast resistor and see what you're getting for voltage there.  I think there's a bad connection somewhere and you're only seeing phantom power.

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the ballast resistor is bypassed by the computer at startup so that the pump will see full voltage for cranking.  after a couple seconds of running it'll switch to resistor-only and carries on like that 'til shutoff.  

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27 minutes ago, Pete M said:

the ballast resistor is bypassed by the computer at startup so that the pump will see full voltage for cranking.  after a couple seconds of running it'll switch to resistor-only and carries on like that 'til shutoff.  

 

Yeah.  The other thing is that when you key it on it only runs the pump for a couple seconds to prime the system as long as you don't start the engine.  A helper or long test leads may be needed to check the voltage because of this.

 

I noticed he said he tried jumping power to the pump, so it may well be that it is just the pump is dead and the voltage reading was irrelevant.

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So i tired jumping the pump with a hot battery wire and still nothing. How can that be? Both pumps run out of the tank hooked to the battery. But not in the tank? Should i get a whole new sending unit and pump? I'm at a loss. And does anyone know where fusible links are? I think they are below the starter relay, but bot entirely sure.

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So i figured out that when i pull the whole assembly out of the truck is works jumped like that. But as soon as i put it in the tank. It acts like a dead short to ground and the wire tries to burn up. I think i have a short in the fuel pump assembly wires that when it touches the metal tank it makes it inoperable. I inspected the harness and it looks great. But i think where the harness metal touches the tank with the lock ring and such there is a short. I'm at work right now. When i get home I'm going to take it out of tank, put my meter to ground and the metal of the assembly and have a buddy touch the wire to battery for a second and see if i get voltage. If so i think the assembly needs replacing. Thoughts?

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Ill post a few pics of the ground set up i did for it and the jumper after work to show you guys. Ill also snap a photo of what i believe are fusible links and see if anyone knows if I'm right. Thanks for the help though. Really appreciated.

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1 hour ago, AzraeL said:

So i figured out that when i pull the whole assembly out of the truck is works jumped like that. But as soon as i put it in the tank. It acts like a dead short to ground and the wire tries to burn up. I think i have a short in the fuel pump assembly wires that when it touches the metal tank it makes it inoperable. I inspected the harness and it looks great. But i think where the harness metal touches the tank with the lock ring and such there is a short. I'm at work right now. When i get home I'm going to take it out of tank, put my meter to ground and the metal of the assembly and have a buddy touch the wire to battery for a second and see if i get voltage. If so i think the assembly needs replacing. Thoughts?

That's what it sounds like. 

 

As for the links, once they burn, they're gone, if you have 12vs anywhere else, the links are good. 

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With fuel pump sender assembly out of the tank and fuel pump motor disconnected, ohm out all three (ground, power, and fuel level) wires to the sender frame. The ground wire only should read continuity. Also ohm out the two legs of the sender potentiometer while moving the arm slowly up and down. It should read the full value of the pot (~100 ohms or so) and decrease slowly w/o skipping to continuity. I think the pot sender wire (or the pot winding itself) is shorted to the pump frame in one of it's swing positions and killing your 12V supply.

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So the ground and positive 12v wire have continuity to the sender frame where it meets the tank. And the potentiometer (float) reads normal, no jumps to continuity. It also reads from the harness to the sender frame as well as the two leads on the arm itself. So new sender unit/assembly? 

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So new pump assembly i just bought reads the same as old. Except the float arm has a larger ohm scale. .001 to .089 on old, .001 to .117 or so on new. And i have continuity on power wire pin to pump assembly frame where it meets tank. And continuity on ground. Testing in the tank when i get home. At least i know the assembly and pump are all new.

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