Okay so long story short, I was driving one day with a friend down a paved road and my Cherokee just flat out shut off on me at 55MPH and left me stranded. This was almost 3 months ago. The Jeep was just filled with fuel not even 20 min prior to our drive, so yes it had fuel.

I attempted to start her again and again, with no luck. She cranks over strong but will not start. We notice that the fuel pump is not priming/turning on upon turning the key so we swap the fuel pump fuse and another matching one (cant remember exactly) that's in the engine bay. Still nothing. We tow it back home and start again the next day.
I picked up another fuel pump fuse from Autozone, and replaced the old fuse in case both were bad, but had no luck. Next week i buy a brand new fuel pump and gas tank, install them and still nothing. Crank but no start or fuel pump prime. At this point we brought out the Multimeter and got to work. We checked the fuel pump relay for power, and it had the correct 12V initial for the first 3 seconds, then it would step down to like 6v like it should. We tested both the old and new fuel pumps with an external power supply and both work (lol). I cleaned all of the grounds in the engine bay. I used a paperclip to jump in-between pins (If i remember correctly) 37 and 80 on the fuel pump fuse relay inside the engine bay, and the pump ran continuously.... I also want to add that when trying to turn the key to get an engine diagnostic code (PRE OBD1) I get no blinking engine light, and that i had just taken the whole top end apart and replaced the head gasket (and cleaned the top end too!) and it ran just fine. I am totally lost here guys. I have been throwing parts at it since then with no luck. I appreciate any help and will respond to any questions you may have when i can. I will be deploying to Afghanistan soon and would like her fixed obviously before that happens. THANKS!

All in All, these are the parts i have replaced with BRAND NEW PARTS:
MAP Sensor (known good)
Fuel Pump (known good)
Crank Shaft Position Sensor
Gas tank
Fuel Pump Fuse Block
Fuel Filter
Throttle Position Sensor

1992 Jeep Cherokee Limited 4 door,
AMC 4.0L HO, 155k miles.

 

Have you tested to see if you have spark? Have you rented/borrowed a fuel pressure gauge from Autozone and verified fuel pressure? Have you tested the Cam Position Sensor (it's in the distributor)?

 

Check wiring too.

Did you check in your tank for water? I filled up once and in about 10 minutes it would barely run. I had got a crapload of water in the tank.

 

By jumping the 87 & 30 terminals of the fuel pump relay with a jumper wire or clip.. I believe this would indicate to you this relay you were jumping is the reason for the problem of the fuel pump not priming or working. You said you put a paper clip jumping terminal 37 with 87. It is supposed to be terminal 30 with 87. It would be good to be sure. It would seem logical that if jumping the fuel pump relay makes it run like it did, that power is not being switched on by the relay to be sent to the fuel pump.

The pump tested fine when you ran it providing power to it, so if you continue to do that backwards away from the pump like that, applying battery voltage to the green and black wire (It should be a green and black colored wire for most all years?). You can use the black wire leading back from the fuel pump as a ground and connect it to the negative battery terminal with a wire. You will eventually run into where the problem is that won't allow your fuel pump to prime or operate.

The ASD relay would be able to produce the same symptoms. You could test it like you can the fuel pump relay. Or, you can take the relay out and apply battery voltage to it once you have it pulled out of the PDC. If it energizes you will know by hearing it click when you apply power to it. You can test the relays for continuity too. It they do not click or continuity is not present, it indicates faulty operation of the part. The ASD can shut both spark and fueling down. If you test and find you also have no spark, it would indicate potentially the ASD to be the reason. Both of the relays - the fuel pump and ASD relays have fuses to check.

Once either relay is energized it sends power to the pump. It could be in the fuel pump assembly connector at the tank. Although, the testing should lead you to it. If jumping the relay does not work when you are providing power through it via the jumper wire, then maybe it is because the relay is not recieving the power? That could be from the ignition switch? If not, you would work your way to the relays from there..

The ground for the fuel pump should be hidden behind the drivers side door paneling. It should be ground #G302 (or G304). You have to expose it by removing the drivers door paneling in order to see it, or to clean it. You can load test the ground.. By applying battery power to the ground wire and measure the voltage after the electrical load. I believe it is less than 100 milli-volts is needed to qualify as a good ground.

From what I know, a properly operating circuit should show 12 volts up to the load and less than 100 mill-volts after the load. A short should result in a blown fuse though. You can test for a short throughout.. Continue applying power and ground on positive and negative sides of the circuit independently until the affected area of the circuit is located. 12Volts and less than 100 milli-volts.

 

And recheck the crankshaft position sensor, especially if it's a WalMart sensor.

 

Well, the parts cannon got some use.....

 

Could be a cracked solder joint in the infamous auto shutdown relay (ASD) located in the relay box behind the battery. That's what my issue was. You can remove and test the relay with a couple of jumper wires to the battery to see if it clicks. $28 new from the dealer.

 

Thank you for your time, and your detailed reply. I just picked up a nice Fluke multi meter and will go attack it tomorrow. The army sent me to basic electronics theory school 3 years ago, and if were being honest i dont remember much, those were long... monster pounding, eyes rolling back, standing up so i didnt fall asleep and get yelled at days. A good free education Squandered away.. oh well. lol

 

Thank you for your service.

 

You could try swapping around the different relays..

 

Definitely double check that crank sensor. My Sport dropped dead on the road and I coasted into a parking lot. I bought the lesser-costing sensor and replaced it. It got me three miles closer to home and also failed. I spent the big bucks ($75.00) and replaced it again. That was a couple years ago now. So far, the sensor is working fine. When a truck ignition just dies like that suspect the crank sensor 1st and don't go cheap.

 

Yes, even with an SUV, like a Cherokee.


NGK/NTK sensors only in these beasts. Even a quality brand that might work fine in other vehicles is often no good in a Cherokee (which is an SUV, not a truck).

 

'Crank but no start or fuel pump prime.'


It is easy to see and verify wether your crankshaft position sensor is okay or not, to help eliminate it as the reason.. But, a faulty CPS sensor is not going to shut off the fuel pump.

I would verify and make sure the fuel pump truly is not priming. You can take off the gas cap and stand with your ear to it listening while a helper turns the key to the starting position for you. Or, you can use a DVOM to check for and see if you are getting 2 seconds of power at the fuel pump during the priming cycle as your helper turns the key forward into the starting position for you? You can check the pressure at the fuel rail for getting a proper prime too.

 

That was the first thing we tried on the side of the road, and i bought a new high quality fuel pump relay fuse the next day and installed it just to make sure both fuses were good.

 

Cruiser54, "Thank you for your service."

It is my pleasure and an honor Sir.

Thank YOU for paying your taxes so i can blow it on my jeep