mjtjnj Posted September 7, 2008 Share Posted September 7, 2008 I've been having intermittent stalling. When I started the new engine for the first time, it ran great for 20 mins then stalled. We noticed the fuel pump wasn't humming, so I unplugged the harness at the gas tank, cleaned the contacts, and we were back in business. Since then, I can drive the truck for a bit, and then it dies, and the pump doesn't hum. Let her sit for 20-30 mins (about the time it took for the AAA guy to come! :eek:) and the pump hums and it starts again. Is this typical of a dying pump, or should I consider the relay? If I do replace the pump, I guess I should replace the strainer while I'm in there, and definitely the lock ring, right? Thanks! Tom Link to comment Share on other sites More sharing options...
JeepcoMJ Posted September 7, 2008 Share Posted September 7, 2008 first, what year is your truck? have you replaced the ballast resistor? it's located on the left hand side of the engine bay, made of ceramic with two wires connecting to it. by the sounds of it, it's heating up the resistor then the resistor expands the spring inside and therefor doesn't make contact. it's around $2 at the parts store. if you buy one, buy two. never hurts to have a spare Link to comment Share on other sites More sharing options...
Eagle Posted September 7, 2008 Share Posted September 7, 2008 If you replace the fuel pump, yes you should replace the "sock" and the lock ring gasket (O-ring). And please let us know if that solves the problem, because my '88 is doing exactly the same thing and I haven't solved it yet. I tried bypassing the ballast resistor and that didn't help, so I'm trying to think of ways to trouble shoot on the side of the road before I spring for a new fuel pump that may or may not be the answer. Link to comment Share on other sites More sharing options...
mjtjnj Posted September 8, 2008 Author Share Posted September 8, 2008 If you replace the fuel pump, yes you should replace the "sock" and the lock ring gasket (O-ring). And please let us know if that solves the problem, because my '88 is doing exactly the same thing and I haven't solved it yet. I tried bypassing the ballast resistor and that didn't help, so I'm trying to think of ways to trouble shoot on the side of the road before I spring for a new fuel pump that may or may not be the answer. Sorry it's an 87 2.5, so I don't have to worry about the fuel rail. Would a bad ballast resistor cause the fuel pump to stop running? Link to comment Share on other sites More sharing options...
mjtjnj Posted September 8, 2008 Author Share Posted September 8, 2008 If you replace the fuel pump, yes you should replace the "sock" and the lock ring gasket (O-ring). And please let us know if that solves the problem, because my '88 is doing exactly the same thing and I haven't solved it yet. I tried bypassing the ballast resistor and that didn't help, so I'm trying to think of ways to trouble shoot on the side of the road before I spring for a new fuel pump that may or may not be the answer. Sorry it's an 87 2.5, so I don't have to worry about the fuel rail. Would a bad ballast resistor cause the fuel pump to stop running? OK, did a little reading up on the ballast resistor. Where is it on a 2.5L? The 4.0's were on the drivers' side. Which of the 3 relays is the fuel pump relay on a 2.5? :popcorn: Link to comment Share on other sites More sharing options...
Pete M Posted September 8, 2008 Share Posted September 8, 2008 If I remember right, 87s didn't have the ballast resistor. That was introduced in 88 after complaints about the loud fuel pump or something like that. Link to comment Share on other sites More sharing options...
mjtjnj Posted September 8, 2008 Author Share Posted September 8, 2008 If I remember right, 87s didn't have the ballast resistor. That was introduced in 88 after complaints about the loud fuel pump or something like that. Ah, that makes sense. Mine is loud, which I don't mind, because I know when it's working. I'm just stumped if it's a dying pump which fails after a while or a funky relay? Love these trucks! LOL. Link to comment Share on other sites More sharing options...
Pete M Posted September 8, 2008 Share Posted September 8, 2008 Swap the relay with the one next to it and see if anything changes. Also, the pump gets its ground at the back of the truck. There could be corrosion in the connector located right behind the drivers side rear tire, or at the grounding location itself behind the drivers side tail light housing. Link to comment Share on other sites More sharing options...
mjtjnj Posted September 8, 2008 Author Share Posted September 8, 2008 Swap the relay with the one next to it and see if anything changes. Also, the pump gets its ground at the back of the truck. There could be corrosion in the connector located right behind the drivers side rear tire, or at the grounding location itself behind the drivers side tail light housing. Good thinking, Pete, all the relays are the same so I did just what you said. Interesting results. The fuel pump relay is the middle of the 3, so standing at the passenger fender, I swapped it with the one to it's right. Got in, turned the key, heard the hum and drove off. Before, I was getting 15-20 mins. before it would stop. Today? 1/4 mile! Boom. No fuel pump. I got under the car, unplugged the harness and put a test light on the orange (+) and black (ground) leads. I had power, but no pump! So I swapped the left relay into the center spot, and put the right one back. Nothing. Put the original fuel pump relay back where it belonged and HUMMMM - drove her the 1/4 mile home. Going to mow the lawn now, so I'll clean up the ground behind the taillight later, but I'm leaning more toward pump problems at this moment. Would you agree? What are the other 2 relays for??? A/C? ? Thanks a lot Tom Link to comment Share on other sites More sharing options...
Pete M Posted September 8, 2008 Share Posted September 8, 2008 Cleaning grounds and wire connectors is free. Definitely wise to try that first. :D Link to comment Share on other sites More sharing options...
mjtjnj Posted September 9, 2008 Author Share Posted September 9, 2008 this is why this site is the best, and Pete is a genius. Look what I found behind the driver's taillight (and I took the time to change out all the stripped screws, a bad bulb, and put in new screws with grease on them, so it was a project in a project). The funny thing about this is, I vaguely remember finding a long strand of wire hanging from the car, probably a year ago. I figured "MEH" just some old redneck rigged remains of a trailer harness, and cut it off, never thinking it was my FUEL PUMP GROUND. And the thing ran! When it died, I assumed the 2.5, which leaked/burned/blew by more oil than it used gas, had kicked the bucket, jumped a tooth on the gears, whatever. Well, on the post mortem, the timing marks lined up, and it turns out it was probably my fuel pump, which worked enough to drive it around a bit, and then sometimes not at all. How's that for a kick in the @$$? Oh well, got a fresh engine, bored over with roller rockers! :headpop: It's still slow! EAGLE! Check your grounds! Link to comment Share on other sites More sharing options...
Pete M Posted September 9, 2008 Share Posted September 9, 2008 While I may or may not be a genius, this particular item I've learned the hard way when I was splicing in new wiring at the back of the truck and suddenly my truck wouldn't start. :roll: Don't forget to check the condition of the connector in the wire harness behind the tire. Corrosion there caused me to lose my brake lights one day. Link to comment Share on other sites More sharing options...
fiatslug87 Posted September 9, 2008 Share Posted September 9, 2008 What are the other 2 relays for??? A/C? ? This is from the '87 MJ Electrical Manual: They are the Power Latch Relay, Fuel Pump Relay and the A/C Clutch Relay from front to back. Link to comment Share on other sites More sharing options...
mjtjnj Posted September 12, 2008 Author Share Posted September 12, 2008 Well, I thought I'd seen the last of the fuel pump gremlin, so as I was driving to the boneyard to get a turn signal socket today (had the morning off), the fuel pump conks out again. :headpop: I get her going enough to stall out in my driveway, and tear into the fuel tank harness. I'm getting voltage at the harness (my brother has my volt meter, so I'm using a test light). I give 12 volts to the connector to the pump with a jump starter and ....it works. I decide the harness pins aren't making good contact, so I cut the plugs off, strip the wires, and twist them together. NOTHING!! Really frustrating. I have a good ground now. The pump works. I figure if I'm getting a test light on, I'm getting 12 volts (relay is on or off, I figure). Pete what were you referring to BEHIND the tire? Aside from getting behind the taillight to access the ground, which has now been replaced, all my work is in FRONT of the tire. Then again, if I've re-routed the ground, that connector is moot now, I guess. Thanks Tom Link to comment Share on other sites More sharing options...
Pete M Posted September 12, 2008 Share Posted September 12, 2008 When are you testing for 12 volts? During cranking? With the key turned? The fuel pump should get power for only a brief moment after the key is turned to the run position (meaning not all the way to start). Which wire are you checking for voltage? If I remember right, I believe the one in the middle is for the fuel level sensor and puts out some voltage (4 or 5 maybe?), but not a full 12 volts. Link to comment Share on other sites More sharing options...
mjtjnj Posted September 12, 2008 Author Share Posted September 12, 2008 When are you testing for 12 volts? During cranking? With the key turned? The fuel pump should get power for only a brief moment after the key is turned to the run position (meaning not all the way to start). Which wire are you checking for voltage? If I remember right, I believe the one in the middle is for the fuel level sensor and puts out some voltage (4 or 5 maybe?), but not a full 12 volts. Hey Pete, Testing the orange (+) and black (-). I'm testing with the key in "on/run" position. When the pump works, I get that momentary "hum" when I turn the key on. When it's not, I get silence. The other (middle) wire of the harness, as you correctly stated, is for the fuel gauge sender. I have to get my volt meter back, but with the key in the on position, I'm getting voltage to the orange (fuel pump +). I'm guessing the pump gets 12V when you turn the key on, then what? Does gas continue in on a siphon effect, or does the pump get voltage continuously after startup, just less?? Link to comment Share on other sites More sharing options...
Pete M Posted September 12, 2008 Share Posted September 12, 2008 As I understand it, when you first turn the key to "run", the pump turns on for a short bit to make sure the fuel pressure is up to spec, then it shuts off. When you turn to "start", the fuel pump should get full voltage. After the engine has started, the fuel pump should get full voltage until you kill the ignition. If you're screwing around with checking voltage, you can disconnect the wire that engages the starter and then you're free to have someone hold the key to "start" without wasting your battery on cranking the truck indefinitely. Link to comment Share on other sites More sharing options...
mjtjnj Posted September 12, 2008 Author Share Posted September 12, 2008 Think it's worthwhile to run all new wires from the relay back, from the sound of it?? :mad: Link to comment Share on other sites More sharing options...
Pete M Posted September 12, 2008 Share Posted September 12, 2008 Until I figured out the real issue in my truck (totally unrelated to your problems unfortunately), I was prepared to wire the pump to a switch in the dash (through an aftermarket relay). Link to comment Share on other sites More sharing options...
mjtjnj Posted September 13, 2008 Author Share Posted September 13, 2008 I'm leaning that way, Mr. M! :cheers: Link to comment Share on other sites More sharing options...
fiatslug87 Posted September 13, 2008 Share Posted September 13, 2008 Hey mjtjnj, I can send you a PDF of the '87 MJ electrical manual if you want. Just pm an e-mail address. If anyone else wants one just send an address. Link to comment Share on other sites More sharing options...
mjtjnj Posted September 13, 2008 Author Share Posted September 13, 2008 FS- The electrical troubleshooting manual? I have that, just not too swift at deciphering it....but thank you! Tom Link to comment Share on other sites More sharing options...
mjtjnj Posted September 13, 2008 Author Share Posted September 13, 2008 Played with it some more this morning. It initially worked, and when I turned the key on, it pumped a few cc's into the cup I had, then stopped. When I cranked it, it filled the cup pretty briskly. Hooked the line back up, started her up, ran for a minute and stalled. No re-start. I have a relay/switch for foglights lying around -- I'm going to bypass the whole thing and see what happens. If it works, I'll make it permanent. If it continues to conk out, I'm replacing the pump. Tom Link to comment Share on other sites More sharing options...
mjtjnj Posted September 13, 2008 Author Share Posted September 13, 2008 Does anyone know what regulates the pump coming on momentarily in the "on" position, then stopping, and coming back on during cranking? Not sure that's my problem but I think this is a more complicated circuit than the diagrams are showing. I'm tempted to take it to a mechanic, which would be the first time since I've owned it. I hate doing it, and I am new to my town, so I don't trust anybody. There was a great electrical guy near where my parents live, but that's 40 miles away, and would involve a AAA tow, so half a day! :mad: Link to comment Share on other sites More sharing options...
Pete M Posted September 13, 2008 Share Posted September 13, 2008 I believe the computer turns on the relay which turns on the pump. Link to comment Share on other sites More sharing options...
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