Mike574

Long story short. Jeep dies. No fuel from pump not turning on to prime. I check ground and power at harness leading to the pump. Ground wire to chassis good. 2 second 12 volt to power wire when key turned on. 12 volt constant with relay jumped. So I buy a precision brand pump before dropping tank I plugged it up...won't kick on. Precision I understand is a airtecs reboxed so I think it's dead out of the box. I return it and buy a Delphi from Napa. Same test plug it in turn key it kicks on. I drop the tank put in the Delphi turn the key primes , starts runs like ****. Key off on nothing. back to checking ground ,power jumper relay wont kick on. Next day go out cycle key. It starts working but as I cycle the key I noticed different tones coming from pump. It would sound weak but then after cycling key at one point it sounded to really take off. I start it fires up nice idles a few seconds and dies. As it sits I got power to the green wire 2 seconds with key constant with relay jumped. Ground continuity test from pin to chassis= good. Did I get two different bad pumps??? All power and ground test I have unplugged the pump and used the pins on the harness. I'm at a dead end here.

jordan96xj

Given what you have observed, I would next want to inspect the pump harness connector. Sounds like something is not making consistent contact there. Also double check that the connector is fully seated and locked. I recently did mine, and the connector appeared to be fully seated, but after experiencing similar troubles to your post, I realized it was not fully connected/locked.

Mike574

I have not locked it each time I have pulled it apart and reconnect it except for initially, thinking I wouldn't need to crawl back under it. I'll double check of course thanks for your time.

Mike574

. I made sure it was pushed together and locked. No go.

Mike574

So I hot wired the pump. Straight 12v. Pump ran good Jeep ran good. Jumping pins 87/30 won't run pump. I get about 5v to the green/wht wire with the relay jumped. Pin 30 gets 12v. FP ground is good. Not priming either. What next?

gat

I would guess that your voltages at the terminals of the relay are lower than battery or the relay is sucking voltage before delivering it to the pump. It could be wiring between battery and power distribution center or wiring between power distribution center and pump.

You could try to use a schematic to find the coloring of the wires and check them by eye and hand.

Mike574

Worked on more today. I also don't see power to terminal 86. From what I have read there should be 12v switched with ignition in run position. But that still wouldn't explain why the pump won't run with 30 and 87 jumped. I even tried batt+ to 87 even though I have 12 at 30 I thought I'd try. If 87 is a straight shot to the pump I can run a new wire out of the pcm straight to pump. Other than not having power to 86 everything points to a broke wire between pcm and pump.

gat

I believe that 86 would be a connection to 85 and 85 is a ground to the negative terminal of the battery.
I don't know whether your 1998 is like my older ones or not. Is there a fuse by the relay for that circuit? If so, then pull it to visually check and use a meter to check for continuity. I don't know whether the fuse is after or before that Relays connection. If it is after, then that could explain why jumping 30 to 87 doesn't activates the pump. Also, if it isn't running, then the pump only primes in the on position for the ignition terminal. If you aren't testing during that time, then there shouldn't be a connection to ground to make the relay jump to 87 from 87a.

gat

I don't think that the power control module has anything to do with the pump, but I may be wrong. I believe that the pump just does that, pumps. There is a valve that controls the pressure. I don't believe that there is any need for a computer to control the pump. All of that is handled with the injectors.
If full battery voltage at the pumps directly works, then the pump works. The full battery voltage is present at 30. Somehow going from 30 to the connector at the pump there is a loss.

gat

I searched for a pinout for a 1998 and stumbled across this, https://www.cherokeeforum.com/f2/what-color-wire-powers-fuel-pump-114943/index2/. When you connected battery positive directly did you also connect battery negative directly? Maybe you have a short to ground on the 12v line.

Mike574

Yeah positive and negative both from an external battery. Checking for a short could be a good idea. I'll do that but I would think if it we're that severe I'd be blowing fuses. I'm just confused because of the lack of power to terminal 86 from ignition and the fact that jumping 30 ( definitely got 12v) to 87 dosent turn the pump on. I could see my ignition switch being bad. The kid that owned it before me use to toggle the heck out of it because the check valve in pump was bad. So that might solve power to 86. But then there's the fact I can't force power back to the pump by jumping relay.

gat

Check continuity from 87 to each of the power ends of the connector to the pump. If it doesn't sound or it doesn't sound and reads ohms, then you have corrosion or an open in the opposite order.

Mike574

Yep I'm going to do that this evening. 60 mph winds today but hey I need my Jeep.

gat

I learned something that I wanted to know for years on today.
https://m.youtube.com/watch?v=0M9piFUSDwo
That little thing by my air box is a ballast resister for the lower voltage line to the pump. It is a different circuit than the starting pump power. If your later model is the same way, then there should be a similar resistor somewhere that controls that voltage. I'm guessing that that circuit is a run only circuit. The 12v with the relay is a start only circuit and is activated to prime in the on position.
How does the contact look on the pin 87 with a light?

Mike574

That's what I'm try to figure out. If my 98 does run on a lower voltage it is controlled by the computer. No ballast on a 98. That being said I need to figure out how that works because as far as I know there is only one power wire to the pump. The wiring diagram shows a straight shot to the pump from pin 87 on relay. So how does it get power after the prime cycle when I don't see the power wire tied into anything from the CPU? Pin 86 is hot from ignition and 85 is ground supplied by the CPU which activates the relay. ( The ground from CPU).