Mechanics

NO POWER TO FUEL PUMP

1995 Jeep YJ • 150,000 miles

4.0 L motor
Manual transmission
20 gallon tank

Would just crank (it started and stalled after a second a few times making me think a fuel problem was present)
Fuel relay clicked, fuel pump fuse is good, battery was good
Swapped relay with starter relay and no change, swapped with the relay that's in the fuel circuit (automatic shut off? No change either)
Checked fuel pressure at rail, there was none
Fuel filter was not very old, and there were no obvious leaks
Fuel pump was not priming
I dropped the tank and took apart the fuel sending unit
Put 12 VDC on the pump and it functioned

When the key is on (during the first 2 seconds, no voltage at the fuel sending unit harness, however there are 7 volts at the fuel gauge sensor pin

I jumped the relay (the high current one.30 I believe) to 87
There is 12 VDC at the fuel harness when the pins are jumped
The fuel relay still clicks when the key is just turned on

Ideas would be appreciated
Avatar
Aaronmcd
August 14, 2012.



I am completely out of ideas on a solution for this fix. I have tried everything I know to test and diagnose my problem, and the Jeep still isn't running. Everything up to this point has led me to believe that the PCM is bad. I previously had a '94 wrangler that had this same problem. Running down the road one day and instantly lost power to fuel pump. The PCM was bad. I'm guessing that's what is wrong with this one. Any ideas would greatly be appreciated.

What I know: 1. Fuel pump is good (12V) connected to it and it runs.
2. Fuel gauge wire (tan/blk) has 7 volts on it and gauge itself is pegged full.
3. Ground is good.
4. When key is initially turned on, both the fuel pump and the ASD relay "click" and then there is an additional "click" immediately following, which I'm assuming is the ASD relay killing the power.
5. The jeep will run with the fuel pump wired directly to the battery, so I'm assuming the crankshaft sensor is good as well as any other sensor that could normally prevent the Jeep from starting or cause the ASD relay to kill power.
6. There is no check engine light on whatsoever and when I flash codes (ignition on-off, on-off, on-off), the only codes showing are 12 and 55, so not displaying any trouble codes.

I have literally tried EVERYTHING to fix this problem.
Anybody have any ideas?

Tiny
Yjfanatic
Oct 29, 2012.
A fuel pump will not set a code unless you are running lean. You need to see how much if any voltage is going to the pump. KH low sent you amessage on how to check if power is going to it. All you really need is a test light to see if you aregetting voltage or not. You need to see if the org/blk wire is gettingpwoer not the tan one. The tanoneis for fuel gauge. If you are getting power with the relay jumped you have a short in that wire someplace. If not then the pcm isn't turning the relay on or yu have a bad connection

Hmac300
Oct 29, 2012.
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