Fuel pump voltage only on START

[89redpioneer]

Trying to get my latest 88' MJ back in service. Sold my old '89 MJ a few years back and picked up the current 88' MJ around same time. Had set with fuel system taken apart for a few years before I got it.

 

Was not running when I picked it up, PO had said it had been running but needed new tank and pump as they were full of rust. Tank was pretty shot and old pump was caked with rust.

 

So new tank and pump with old sending unit. Sending unit is working fine.

 

Everything back together but still no start.

 

I checked voltage at pump harness. If remember correctly the pump should turn on for a couple seconds when turned from OFF to RUN. I am getting nothing. At RUN getting -0.0011 V. At START I get around +11...  V.

 

I assume since I get voltage on START that my ground is ok. Of course I may be off here, you know what happens when you assume.

 

Current state:

Ballast resistor bypassed

Fuel pump relay replaced(I believe it is the front of the 4 in the block)

 

Just thought I would check it anyone knew the solutions off the top of their head on this one. If not, I probably spend some quality time with the wiring diagrams to see if I can figure the issue out. There has been some not so kosher wiring work done by some of the PO's so might be a little more involved.

 

Thanks,

Chris

 

 

[Eagle]

Getting voltage on the hot side tells you nothing about the ground.

[omega_rugal]

Check the ballast resistor and see if there`s 12v when you put the in ON, remeber it will last only a few secs if you don`t engage the starter, listen for the fuel pump relay to kick in when you put the key in on, check the relay

 

basicly you must trace back the fuel pump wire all the way back to the relay

 

 

I assume since I get voltage on START that my ground is ok.

 

perhaps but for now find out why there`s no power with the key on

 

worst case scenario: a fried ECU... did you check for spark yet?

[HOrnbrod]

Getting voltage on the hot side tells you nothing about the ground.

 

.....unless you use the meter ground lead on the connector ground wire, not a close-by piece of metal.

[89redpioneer]

I checked the voltage across the + and ground on the fuel pump harness connector with pump disconnected, not grounded piece of metal on frame or anything.

 

Kind of what I figured, I will trace check the voltage at ballast resistor and trace back to relay.

 

Hadn't checked for spark yet as I wasn't getting fuel so figured I would work that one through first then move to spark.

 

Starting to think there is some sketchy wiring. There are about 4 or 5 red wires wrapped together and connected to 1 wire which is then connected to starter relay. From the diagram that doesn't look right as looks like there should be fusible links for most of those individually connected, if I reading it correctly.

 

Thanks for the pointers I will check those out. At least verifies what I was thinking.

 

Will post what I find out so maybe will help others.

 

Chris

[89redpioneer]

Ok looks like the issue is related to the ghetto wiring from a PO.

 

Found wire for fuel pump relay orphaned and taped off.

 

After some checking with wiring diagram, the twisted bundle of wires includes fuse links X, Q, H and I am sure at least another all with just a wire(not fusible link) to starter relay.

 

Will separate those with new fusible links and new fuse link G and see how things look then.

 

Thanks,

Chris

[Jeep Driver]

http://comancheclub.com/topic/50485-no-power-to-fuel-pump/page-5

 

 

Go to page 6, post 90

 

This is what happens when people get stupid with wiring............referring to POs, not you two guys. 

 

 

Sort that out before you lose your entire harness. 

 

Best to you.

[89redpioneer]

Ouch! I guess I will have to be extra diligent correcting before powering things up again then.

 

Will be on hold for a few days as I couldn't find the correct size fusible links at any auto part stores around.  

 

Smallest I found was 16 gauge and few circuits seem to call for 18 or 20 gauge fusible links. I had already ordered them, but based on seeing what happened to the other poster I better wait to get the correct size.

 

Thanks for the info.