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Juan Crespo
Juan Crespo, ASE Certified Technician
Category: Jeep
Satisfied Customers: 1513
Experience:  A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
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I have a 1998 Grand Cherokee 4.0. I had the battery unhooked

Customer Question

I have a 1998 Grand Cherokee 4.0. I had the battery unhooked while doing minor bodywork on front. Hooked up the battery and will not start. I was told the crank sensor would go bad so replaced it with no help. I checked further and saw I have spark but no fuel pressure. I replaced fuel pump and still will not start. A shot of starting fluid it will crank for a second. Tjhis is where I'm at.
Submitted: 10 months ago.
Category: Jeep
Expert:  AZTechBrian replied 10 months ago.

Try swapping the fuel pump relay with another one in the power distribution center and see what happens.

https://www.dropbox.com/s/q8drb10c16lo2kz/fpr.jpg?dl=0&preview=FPR.jpg

Customer: replied 10 months ago.
Sorry I already tried that
Expert:  AZTechBrian replied 10 months ago.

does the relay click when you turn the key to the on position? Have you checked all fuses?

Customer: replied 10 months ago.
I check all fuses under the hood. The relay does click
Customer: replied 10 months ago.
I did try to scan it just now and will not link to pcm.
Expert:  AZTechBrian replied 10 months ago.

OK. For some reason my information on this jeep is lacking, but i believe there is an interior junction block, have you check that?

Expert:  AZTechBrian replied 10 months ago.

it's behind the passenger side kick panel I believe.

Customer: replied 10 months ago.
No fuses blown
Expert:  AZTechBrian replied 10 months ago.

fuse 11 in the junction block supplies the fuel pump as well as the pcm

Customer: replied 10 months ago.
It is good.
Expert:  AZTechBrian replied 10 months ago.

Did you remove the fuse block when you did your body work? Any possibility of a damaged wire?

Customer: replied 10 months ago.
No Header panel in front by headlight and grill area. Of course I checked everything in that area.
Expert:  AZTechBrian replied 10 months ago.

ok. if all fuses and relays are fine and there is no damaged wiring, I am going to open this up to the others. I am out of ideas.

Expert:  Juan Crespo replied 10 months ago.

Hi there. Perhaps I can help.

As you probably know, your Jeep's engine needs fuel, spark and compression to start and run - the fact that it will momentarily run on starting fluid tells us compression and spark are OK. That leaves lack of fuel as being the cause of the no start.

Now, fuel is delivered to the injector rail at about 50 psi by the fuel pump. The PCM then pulses the injectors open so the right amount of fuel can get into the combustion chambers. So we ask ourselves, why no fuel? Is it because of no fuel pressure or no injector pulse?

You state fuel pump relay clicks; does that mean we have good fuel pressure? You need a fuel pressure tester to check it out. If you don't have access to one, those are available as loaners from most major auto parts retailers. What about injector pulse, is it or not there? While you're at the auto parts store, you might also want to pick up a Noid light (usually less than $20) to check injector pulse.

Once I have that information, I'll be able t0 formulate an accurate diagnosis for you.

Best Regards

Customer: replied 10 months ago.
It doesn't have any pressure. I can not hear fuel pump kicking in. As I stated I tried to check PCM but scan tool could connect.
Will check on a noid light.
Customer: replied 10 months ago.
Also gas gauge does not show any fuel and I know it has at least 5-8 gallons in it. Not sure if that has anything to do with it.
Expert:  Juan Crespo replied 10 months ago.

It might all mean the PCM is faulty, I can't tell at this point. I'm trying to get you to follow a diagnostic path that is sure to pinpoint the failure. Otherwise, we'll be shotgunning the problem - a very risky and potentially costly proposition.

Have you checked whether the clicking fuel pump relay means it is actually sending current back to the fuel pump? If you have, and found good current at the pump, the pump or its connector is bad for sure.

Customer: replied 10 months ago.
Ill check that out this evening.
Expert:  Juan Crespo replied 10 months ago.

Good. I look forward to hearing from you.

Best Regards.

Customer: replied 10 months ago.
No current to fuel pump.
Expert:  Juan Crespo replied 10 months ago.

OK. That means one of two things: there is a bad connection between the relay and the pump, or the relay is not clicking as you thought it was.

You need to remove the relay from the PDC and check voltage and ground with a voltmeter at its socket cavities (see attached graphic). With the ignition switch turned to START, cavities 16, 17 and 18 should have battery voltage and cavity 20 should have ground.

Please let me know the results as soon as you finish those tests so we can continue.

Customer: replied 10 months ago.
17 does not have any power to it at any time. 16 and 18 do. I used the ground on the battery I had 12 volts. I notice if I used 20 as ground only around 6 volts.
Expert:  Juan Crespo replied 10 months ago.

OK. Can you jumper 16 and 17 and turn the ignition on to see if the pump runs? If it does, can you try to start the engine and tell me if it does start?

Customer: replied 10 months ago.
It will not start, no fuel pressure.
Expert:  Juan Crespo replied 10 months ago.

If there is current at the pump with relay cavities 16 and 17 jumpered, but still no fuel pressure, the pump is bad. If there is no current at the pump with relay cavities 16 and 17 jumpered, then there is an open circuit on the Dark Green/White wire somewhere between the relay and the pump.

Please let me know what you find.

Best Regards.

Customer: replied 10 months ago.
With them jumpered green with black stripe has 5 volts. Green with yellow stripe had 5 volts. Green with white stripe no voltage at all. I checked it with relay and have same results. I powered pump and it works fine.
Expert:  Juan Crespo replied 10 months ago.

OK. I think we have a wiring or diagram problem. Check for battery voltage at fuse 16. Disconnect the battery and check for continuity from fuse 16 to cavity 16. If OK, then check continuity from cavity 17 to fuel pump. If there is none, then check from cavity 19 to fuel pump. If there is continuity from cavity 19 to the fuel pump, then reconnect the battery and put a jumper between cavities 16 and 19 which should send battery voltage to the pump. Please let me know what you find.

Customer: replied 10 months ago.
Have voltage at 16 fuse.I have continuity from fuse 16 to relay 16. I do not have continuity to 18. relay 19 does not have a prong ( checked all other relays and they dont either).
Expert:  Juan Crespo replied 10 months ago.

Thanks for that information. Is there continuity from cavity 17 to the fuel pump Dark Green/White wire? If there isn't, can you check and see which cavity does have continuity to the fuel pump Dark Green/White wire?

Customer: replied 10 months ago.
It does not have any continuity to any of the relay cavity.
Expert:  Juan Crespo replied 10 months ago.

In that case the Dk Green/White wire has to be disconnected from the relay box (PDC) or broken somewhere between the PDC and the pump. Fuel pump relay cavities should be connected as shown below:

Cavity 16 - to fused power (B+) Dk Green/Black wire

Cavity 17 - output to fuel pump Dk Green/White wire

Cavity 18 - fused ignition (hot in START and RUN) Orange wire

Cavity 19 - empty

Cavity 20 - relay control from onboard computer (PCM) Dk Blue wire

Battery voltage is always present at 16. When ignition is turned to START or RUN, voltage goes to 18. PCM grounds 20 and relay is energized (clicks), sending voltage to 17 and on to the fuel pump.

Start your search by removing the PDC and inspecting the wiring connected to it from the bottom. Isolate the Dk Green/White wire and make sure it is in fact connected to the PDC. If it is, then you could either trace and find where it is broken, or cut it right there and run a new wire to the fuel pump.

Please let me know if you any other questions.

Best Regards.

Customer: replied 10 months ago.
Sorry have not had time to check anything the last few days. Ok I checked the wiring behind few box and didn't see anything loose. I then ran a jumper wire from the fuel pump with no luck. I jumped same jumper wire to hot and fuel pump worked. I did tell you the other day it ran just fine ( I only let it run a second). Today I cranked it up with the jumper wire and thought I will let it run a little. It dies after 2-3 seconds and I notice the tach and other gauges never move at all. So the other day i probably didn't let it run long enough to shut off. Also I might have made a mistake on 20 having a ground as I checked all cavities again and it appears it has 10 volts with key off but doesn't after key is on.
Customer: replied 10 months ago.
Sorry I checked fir ground again with a better tester and 20 has a ground for a couple of seconds after key is turned on.
Expert:  Juan Crespo replied 10 months ago.

This is the way it's supposed to work;

As soon as the ignition is turned on, a transistor inside the PCM switches the signal to cavity 20 in the relay socket from voltage to ground to energize the relay which then allows voltage to go to cavity 17 and turn the pump on, then it switches back to voltage to de-energize the relay and turn the pump off if it doesn't get a crank signal within 2 seconds. Can you focus on this one task for now and check to see if that's how it's working?

Customer: replied 10 months ago.
With tester in cavties in 16 and 20 it shows around 10 volts with key off. Switch on it holds voltage the same then about 2 second then drops.
Expert:  Juan Crespo replied 10 months ago.

Please do not use a transistorized circuit as a ground for a tester; a low-impedance tester will need much more amperage than the transistor/driver can handle and will burn it up. Once that happens, the PCM will be useless.

So here are the basic rules for working on computer circuits:

  • Always use high-impedance (10 KOhms or more) test equipment when working with transistorized/computer circuits
  • Always use good grounds like the battery negative post, engine block, body and chassis to check circuit voltages

Taking those rules into consideration, you need to find out whether the PCM is keeping the relay energized and the pump on not only for 2 seconds when the ignition is turned on, but also when the ignition switch is turned to START and the engine is cranking. Please tell me what you find.

Customer: replied 10 months ago.
It is not energized at any time.
Expert:  Juan Crespo replied 10 months ago.

In that case, if power to the relay is OK, then either the PCM is not getting voltage from the ignition switch (check engine light won't even turn on in the dash); or the circuit from the PCM (connector C3, terminal 19, Dark Blue wire) to cavity 20 is open (possible broken wire or bad connection); or it is getting voltage but the transistor that switches the signal from power to ground is burned.

With that, I've given you the only three logical causes for the fuel pump relay not to be energized. As you test the circuits, compare your results with what I've written and then you can take the necessary actions.

Please let me now if you have any other questions.

Best Regards.

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