Andy in Pa Posted June 19, 2021 Share Posted June 19, 2021 Hey gents, this problem is driving me nuts, so I am hoping you can help me out. MJ is a 4.0L 5 spd short bed. Very rust-free rig, that has been running great for a while. Suddenly it started running poorly and died on my daughter. We got it home, and I started out by checking spark, confirmed all seems to be working well, then moved on to fuel. The fuel pump relay is buzzing, and the pump is running, it just doesn't sound healthy at all. It never stops as long as the key is in the "On" position. Just keeps running/buzzing. Please see below for the list of things I have done so far: 1. CHeck/cleaned spark plugs. Noticed a few looked a bit white, so possibly lean, others were gray/black so OK 2. Checked spark plug wires, all good and pretty new. 3. Pulled distributor cap/rotor. Also pretty new and they look fine. distributor is also less than a year old. 4. Pulled and changed the relay 5. Removed and checked the coil and mounting block. All looked good 6. Jumped the ballast resistor 7. Cleaned the relay block with brakeleen and let dry. 8. Checked and confirmed good the ground going to the pump (grounded behind the drivers side taillight) 9. Checked fuel pressure. Its at less than 20psi (expected this, but was hopeful) 10. Replaced fuel pump with a new Bosch unit 69302 11. Changed the fuel filter No change after ANY of this... I did some research and do have some schematics, so went back to the fuel pump relay. I tested ohms at the Orng w/Tr wire, expecting to see a ground reference (its supposed to see ground at the ECU) but could not get it to reference ground with the key on or off. Orng wire had .7 ohms, which I am guessing is fuel pump motor windings resistance. Yellow wire was 12vdc with key on, red wire was 12vdc all the time. It seems to not be picking up a ground reference at the ECU. Does the sync sensor or CPS have anything to do with that? Its not cranking, so I am not sure how the CPS would come into play yet unless it provides some sort of gatekeeper signal to the ECU. I am proficient with electrical tools, meters and such, is there some way to check and troubleshoot a problem with the sync sensor, CPS and/or the ECU? Does the computer need to see a signal from somewhere to provide the ground reference for the fuel pump relay? Don't know how the logic works with this, but am willing to learn and figure this out. Link to comment Share on other sites More sharing options...
Ωhm Posted June 19, 2021 Share Posted June 19, 2021 Seems like your ECU (pin A5) is faulty. Just can't hold the relay coil to ground with KEY ON. I'm not sure how to test. Check for continuity between FP relay (C216_2(85)) and ECU C202_A5. Link to comment Share on other sites More sharing options...
Andy in Pa Posted June 19, 2021 Author Share Posted June 19, 2021 i just jumped a ground reference right to the fuel pump relay on pin 2, turned the key to ON and no buzz, and the fuel pressure came right up to just under 40psi. Yep, there is something faulty in that circuit. I will see if I can confirm continuity to that pin on the ECU. I hope I don't have to swap the ECU out. Seems like it may be hard to find one for a 5 spd. Link to comment Share on other sites More sharing options...
Jeep Driver Posted June 19, 2021 Share Posted June 19, 2021 2 hours ago, Andy in Pa said: yet unless it provides some sort of gatekeeper signal to the ECU. This is precisely how the 2.5 operates. Have someone turn the key to crank and see if a ground is generated. I'm just curious. Link to comment Share on other sites More sharing options...
Jeep Driver Posted June 19, 2021 Share Posted June 19, 2021 BTW, The fuel pump is grounded at the driver's tail light. If the ground there has failed....they all do.......the ground circuit cannot be completed and thus why the relay is buzzing. Link to comment Share on other sites More sharing options...
cruiser54 Posted June 20, 2021 Share Posted June 20, 2021 Good point JeepDriver. Here's something else that might help, and needs doing anyway. CRUISER'S MOSTLY RENIX TIPS IMPROVING THE FUEL PUMP GROUND JANUARY 3, 2016 CRUISER54 54 COMMENTS The fuel pump and fuel tank sending unit ground at a sheet metal screw up behind the spare tire on an XJ, and behind the driver’s taillamp on an MJ. Not only is a sheet metal screw a lousy way to ground things, this ground path is long and travels through some connectors that are prone to corrosion and moisture. Locate the black wire on the HARNESS side of the fuel pump/sender 3 wire connector. Remove a 6″ length of the split loom covering. Strip back about 1/2″ of insulation from the BLACK wire. Take your new ground wire, preferably at least 14 gauge and 12 to 18 inches long as needed, strip it about 3/4″, and wrap it around the exposed part of the harness plug wire.Solder the connection. Tape it up and reinstall the split loom covering. At the other end of your new ground wire, add a crimp on eyelet. Attach the eyelet under a bolt that goes directly into the chassis. Be sure to clean the attaching point til shiny and apply OxGard to the contact surfaces. Link to comment Share on other sites More sharing options...
scaleless Posted June 20, 2021 Share Posted June 20, 2021 He says he already checked the fuel pump ground. Also, the fuel pump ground would not affect the fuel pump relay - the relay is grounded, as Ohm said, at ECU pin 5. Relays usually buzz when they receive unstable power, which causes the coil to vibrate or even engage/disengage rapidly. My first expectation with a buzzing relay that didn't shut off would be a weak short to ground somewhere on the fuel pump relay ground wire. That doesn't explain why it dies on the road, however. If you're ok with cutting and splicing back wires, I would cut the fuel pump ground close to the ECU. If it keeps running, it's a wiring issue. If it stops buzzing, it's the ECU or connector. If there isn't a break in the wire somewhere, it's very likely that there is a failing transistor in the ECU. I find it more likely than a corroded or broken ECU connector with how they're designed, but I may be wrong. Link to comment Share on other sites More sharing options...
cruiser54 Posted June 20, 2021 Share Posted June 20, 2021 Understood. I was thinking along the lines of leaving no stone unturned. As for the ECU, you can using any Renix 4.0 ECUs. there's even some advantages. Why not refresh the ECU connector? Link to comment Share on other sites More sharing options...
cruiser54 Posted June 20, 2021 Share Posted June 20, 2021 CRUISER'S MOSTLY RENIX TIPS RENIX GROUND REFRESHING OCTOBER 30, 2015 SALAD 62 COMMENTS The Renix era XJs and MJs were built with an under-engineered grounding system for the engine/transmission electronics. One problem in particular involves the multiple ground connection at the engine dipstick tube stud. A poor ground here can cause a multitude of driveabililty issues, wasted time, failed emission tests, and wasted money replacing components unnecessarily. All the components listed below ground at the dipstick tube stud: Distributor Sync Sensor, TCU main ground, TCU “Shift Point Logic”, Ignition Control Module, Fuel Injectors, ECU main ground (which other engine sensors ground through, including the Oxygen sensor, Knock Sensor, Cruise Control and Transmission Sync signal. All extremely important stuff. The factory was aware of the issues with this ground point and addressed it by suggesting the following: Remove the nut holding the wire terminals to the stud. Verify that the stud is indeed tightened securely into the block. If the whole stud turns, you can use a 7/32″ six point socket or wrench to hold it so the nut can be removed. Worst case, cut the wires and remove the stud and nut. Install new terminal eyelets on the wires when going back together. Scrape any and all paint from the stud’s mounting surface where the wires will attach. Surfaces must be clean, shiny and free of any oil, grease, or paint. Inspect the wire terminals. Check to see that none of the terminals are crimped over wire insulation instead of bare wire. Be sure the crimps are tight. It wouldn’t hurt to re-crimp them just as a matter of course. Sand and polish the wire terminals until clean and shiny on both sides. Apply a liberal coating of OxGard, which is available at Lowe’s and other stores. Reinstall all the wires to the stud and tighten the nut down securely. While you’re in that general area, locate the battery negative cable which is fastened to the engine block just forward of the dipstick stud. Remove the bolt, scrape the block to bare metal, clean and polish the cable terminal, apply OxGard, and reattach securely. Another area where the grounding system on Renix era Jeeps was lacking is the engine to chassis ground. There is a braided cable from the back of the cylinder head that also attaches to the driver’s side of the firewall. This cable is undersized for its intended use and subject to corrosion and poor connections at each end. Remove the cable end from the firewall using a 15mm wrench or socket. Scrape the paint off down to bare metal and clean the wire terminal. Apply OxGard. Reattach securely. Remove the other end of the cable from the rear of the head using a 3’4″ socket. Clean all the oil, paint and crud from the stud. Clean the wire terminal of the cable and reattach securely with a liberal coating of OxGard. 2 STRONG suggestions regarding the ground system: I prefer to add a #4 gauge cable from the firewall to a bolt on the rear of the intake manifold, either to a heat shield bolt or fuel rail bolt. A cable about 18″ long with a 3/8″ lug on each end works great and you can get one at any parts store already made up. NAPA has them as part number 781116. A further improvement to the grounding system can be made using a #4 cable, about 10″ long with 3/8″ terminals at each end. Attach one end of this cable to the negative battery bolt and the other end under the closest 10mm headed bolt on the radiator support just forward of the battery. NAPA part number 781115. For those of us with Comanches, it’s very important to remove the driver’s side tail lamp assembly to access the ground for the fuel pump. Remove the screw holding the black ground wire. Scrape the paint from the body and corrosion from the wire terminal. Add a 10 gauge wire, with an eyelet on each end, from that grounding point to a bolt on the frame. Better yet, on both Cherokees and Comanches, complete Tip 29 for the best fuel pump grounding. Be sure to scrape all mounting points to bare metal and apply OxGard also. If you want to UPGRADE YOUR GROUND AND BATTERY CABLES with custom made parts, contact Paul at www.jeepcables.com Link to comment Share on other sites More sharing options...
Andy in Pa Posted June 20, 2021 Author Share Posted June 20, 2021 Thanks guys... yes the ground behind the rail light is cleaned up, tested and good to the fuel pump. Didn't fix the problem. Cruiser, I have done 99% of the ground refreshes you recommend on your page. All are good. I am going to recheck the multi ground lug on the dipstick tube. Also good info on the ECU, thanks for that. More testing to do today, I'll update when I have more info. Like I said, when I jumped ground to the pin 2 on the relay, everything worked properly. I really need to check the integrity of that ground connection to confirm its the ECU. My concern is that there is no logic (either relay or software) needed from the sync sensor or cps that could be holding this ground reference from working at the ECU. Link to comment Share on other sites More sharing options...
Ωhm Posted June 20, 2021 Share Posted June 20, 2021 Yes loss of CPS signal would cause a NO START condition. Along with no FP relay action there would also be no spark from the ICM. Since jumping FP relay pin2 straight to ground vehicle START and RUNS just fine means CPS is good. SYNC SIG has no bearing with this problem. Even if you show good continuity between FP relay and ECU, disconnect ECU_C202 and with KEY ON ground pin_A5. Does Fuel Pump motor TURN ON? If yes, suspect ECU, otherwise check wiring. Link to comment Share on other sites More sharing options...
cruiser54 Posted June 20, 2021 Share Posted June 20, 2021 Make sure there isn't a wire NOT connected to the dipstick stud. Could be dangling in the general vicinity. Link to comment Share on other sites More sharing options...
Andy in Pa Posted June 21, 2021 Author Share Posted June 21, 2021 8 hours ago, cruiser54 said: Make sure there isn't a wire NOT connected to the dipstick stud. Could be dangling in the general vicinity. I checked and re-crimped two of the wires today on that stud. Actually when I replaced the battery I hooked it right up to that stud as well. Each ring terminal holds two wires, so a total of 4 grounds connect there. Its clean, tight and 100% now. Relay still buzzing. I did not have the time to pull the lower valance off the dash to get to the ECU. Should be able to confirm the A5 pin tomorrow. If it turns out the ECU is bad, I will probably just ground that wire at the relay and call it good. Seems like the much easier solution than scrambling all around to a dozen junkyards and pulling ECU's that I know nothing about. Link to comment Share on other sites More sharing options...
cruiser54 Posted June 21, 2021 Share Posted June 21, 2021 Don't have to pull anything to access the ECU. Remove it bracket and all using a 10mm ratcheting wrench. Link to comment Share on other sites More sharing options...
Andy in Pa Posted June 21, 2021 Author Share Posted June 21, 2021 14 hours ago, cruiser54 said: Don't have to pull anything to access the ECU. Remove it bracket and all using a 10mm ratcheting wrench. Thanks for the pic, I will have to go look again. Are those screws at the top, bottom or side (closest to the firewall?) Never had to pull an ECU. Link to comment Share on other sites More sharing options...
cruiser54 Posted June 21, 2021 Share Posted June 21, 2021 You'll see them up there. Link to comment Share on other sites More sharing options...
Andy in Pa Posted June 23, 2021 Author Share Posted June 23, 2021 On 6/21/2021 at 5:11 PM, cruiser54 said: You'll see them up there. Lol... so at the top huh? Can't even get my hand in there let alone see them. Gonna have to get someone with smaller arms/hands to check it out... Link to comment Share on other sites More sharing options...
cruiser54 Posted June 23, 2021 Share Posted June 23, 2021 I have big hands and have no trouble. Link to comment Share on other sites More sharing options...
Andy in Pa Posted June 23, 2021 Author Share Posted June 23, 2021 OK, so I pulled the lower valance and could get right to the bolts much easier. Tested the wire, and it's perfect from the relay all the way to the connector at the ECU. This means a bad ECU in this case. Who has a known good ECU that they want to part with? Any concerns about just grounding that wire at the relay? Link to comment Share on other sites More sharing options...
Ωhm Posted June 23, 2021 Share Posted June 23, 2021 48 minutes ago, Andy in Pa said: Any concerns about just grounding that wire at the relay? With KEY ON only, fuel pump motor will not cycle OFF when key is in the ON position and ECU sees no CRANK signal. Link to comment Share on other sites More sharing options...
scaleless Posted June 23, 2021 Share Posted June 23, 2021 34 minutes ago, Andy in Pa said: Who has a known good ECU that they want to part with? I've got a couple that I would part with for cheap if you sent me your broken one. Link to comment Share on other sites More sharing options...
Andy in Pa Posted June 23, 2021 Author Share Posted June 23, 2021 12 hours ago, Ωhm said: With KEY ON only, fuel pump motor will not cycle OFF when key is in the ON position and ECU sees no CRANK signal. I don't really think that is a big deal, do you? It has a return line, so its not like the pump will be pushing against a blocked line. Link to comment Share on other sites More sharing options...
Ωhm Posted June 23, 2021 Share Posted June 23, 2021 Yea, your good. Just running the pump with KEY ON. No more listening to the radio using just KEY ON. Link to comment Share on other sites More sharing options...
Andy in Pa Posted June 23, 2021 Author Share Posted June 23, 2021 30 minutes ago, Ωhm said: Yea, your good. Just running the pump with KEY ON. No more listening to the radio using just KEY ON. Hmmm... good point. I wonder if it comes on in accessory mode too. Might have to test that out. I can't even HEAR the new pump run, even with the ballast resistor jumped. Its gets pressure though!! Link to comment Share on other sites More sharing options...
Ωhm Posted June 23, 2021 Share Posted June 23, 2021 8 minutes ago, Andy in Pa said: I wonder if it comes on in accessory mode too. No. Its a IGN_SW_I1 feed. Hot (B+) only with KEY ON/CRANK. Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now