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Lack of Power to Fuel Pump


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So I need some help.   I'm helping my Grandson build his first MJ.  We found a almost perfect 2WD minor rust ran great some standard issues like Steering column falling apart. We found a MJ 4x4 donor and did a 2wd to 4x4 swap and fixed the steering column. Now we have no power to the Fuel Pump.  I know the Pump is good cause I can hot wire it.  Ran a continuity check all the way back to the resistor block and the wiring is fine. also checked the relay and it seems fine. When we turn the key everything works but the fuel pump, If I hot wire the pump the truck runs. 

I can't figure out what we would have done that is not telling the relay to send power to the pump.  I know and checked the connector and switch block on top of the steering column I haven't removed it yet cause everything works but the pump when we turn on the key.  any ideas?    

 

Thanks in advance. 

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Swap fuel pump relay with known good relay. Use the relays right next to it. If there is no change, take two (2) voltage measurements: Remove fuel pump relay and measure at relay block.

  1. Pin 30: Hot at all times (battery voltage).
  2. Pin 86: Hot with Key On (battery voltage).
     
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Its a 88 MJ with tilt column, 4.0 L with Peugot trans and transfer case.     

We took apart the column down to the three bolts for the tilt that were extremely loose, put Loctite  on them and tightened them up reassembled Steering Column.   We have done a ton of work since then and we can't honestly remember if the fuel pump worked after we did the Column or not we believe it did but not sure. 

 

I'll do the relay swap if that doesn't fix it I'kll go to the Switch aliment.  

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These might help you out.

There are two separate circuits for the fuel pump, the primary one that goes through the ballast resistor while the engine's running; and one that bypasses the resistor to deliver full power during cranking. The primary one takes over after a couple seconds running. I'm not sure offhand what mechanism swaps out the circuits though.

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39 minutes ago, rhollen said:

Pin 86 is 14 v at switch on

Pin 86 show normal at this time.

40 minutes ago, rhollen said:

pin 30 is 0v switch off goes to 12v at switch on then drops to 4v and stays thier. ?  

Pin 30 Hot at all times.

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Pin 30 receives Battery voltage from a Fusible Link. Examine all fusible links for bubbles, frayed insulation, located off the starter relay. If good, disconnect C103 (flat 6 pin connector) located on the right side of engine, near rear of engine compartment. Measure for B+ on Pin B (Red) using the side that goes back towards the battery.

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6 hours ago, HOrnbrod said:

This thread deals with the same Renix system fuel pump problem you are having with the fuel pump. Ignition switch....

Fuel pump is tied to the run circuit. 

 

I read through that thread.......don't know what to say. 

 

The guy was rather lost on the subject.

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3 hours ago, rhollen said:

Got it!  Too many weird readings went back to basics and ran a second ground to fuel tank. Darn thing started right up.  

Thanks everyone for your help

I have the neg cable from the battery running to a ground post, other circuits are tied to the ground post. I have a ground post at the rear of the truck that I added when I wired for trailer brakes. I will tie the fuel pump, tail lights to that same ground post in the near future (bed is coming off and new lights). 

 

If you want to end your chassis ground nightmare.......run directly to ground. 

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3 hours ago, rhollen said:

Got it!  Too many weird readings went back to basics and ran a second ground to fuel tank. Darn thing started right up.  

Thanks everyone for your help

 

Good thinking and a good example of KISS.  :beerchug:  Everything back aft including the fuel pump in an MJ is grounded to a single sheet metal screw behind the left tail lamp. A good improvement is to that ground point to a larger 10AWG-6AWG wire behind the lamp and run it down to a solid ground point on the frame.  I used a trailer hitch bolt for mine.

FuelPumpGndSmall.jpg.ea3c8e5b92e6798f0ee98b186c588c84.jpg

 

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