strange fuel pump ground circuit issue
05-16-2012, 08:54 AM
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strange fuel pump ground circuit issue
hi everyone i'm new... i have a question about a cherokee a bought recently. heres the backstory
i bought a 97 4dr XJ 4.0/auto/4x4 from my GF's brother that wouldn't crank or start. The crank issue was a corroded neg cable and a crappy connection at the battery, so now it cranks, but still didn't start. I put in a CKS because it had no spark and no fuel pressure. at that point it would start and run on carb cleaner sprayed through the throttle body. My GF's dad swears it needed a fuel pump. But upon checking for power at the pump i come to find out that there is no power to the pump (actually no ground to the pump). If you grounded the green/white wire to an outside ground it had 12v. so i go up to the relay box and try jumping the relay, that does nothing. I started testing circuits in the relay box and find that i have 12v everywhere i need it.. EXCEPT if i use the relay ground for the fuel pump relay. then i only register 6.1v! so i isolated the ground wire at both ends. made jumpers to wire the relay to the relay box but skip the ground and ran that jumper wire to the body ground connection, and then back by the pump snipped he ground wire outta the harness to a clean body ground and got power to the fuel pump and found out is does indeed need a fuel pump. So i replaced the fuel pump and now have 48PSI fuel pressure in the rail but have NO SPARK! Also the fuel pump doesn't do the 2-3sec purge, it'll run as long as the key is on. I'm sure it has to do with isolating that ground wire but the ground circuit in the relay box doesn't have enough voltage to click the relay to complete the higher amperage circuit to the fuel pump. Has anyone ever seen an issue like that? I've checked, double checked and triple checked everything related.. few things I've ruled out
-ASD is fine. I've tested the connections and swapped relays it's all good
-fuel pump is obviously good as it has awesome fuel pressure
-main body grounds in the engine bay are fine as well.. I've poked around with the meter and get 12v using both of them in the engine bay.
i bought a 97 4dr XJ 4.0/auto/4x4 from my GF's brother that wouldn't crank or start. The crank issue was a corroded neg cable and a crappy connection at the battery, so now it cranks, but still didn't start. I put in a CKS because it had no spark and no fuel pressure. at that point it would start and run on carb cleaner sprayed through the throttle body. My GF's dad swears it needed a fuel pump. But upon checking for power at the pump i come to find out that there is no power to the pump (actually no ground to the pump). If you grounded the green/white wire to an outside ground it had 12v. so i go up to the relay box and try jumping the relay, that does nothing. I started testing circuits in the relay box and find that i have 12v everywhere i need it.. EXCEPT if i use the relay ground for the fuel pump relay. then i only register 6.1v! so i isolated the ground wire at both ends. made jumpers to wire the relay to the relay box but skip the ground and ran that jumper wire to the body ground connection, and then back by the pump snipped he ground wire outta the harness to a clean body ground and got power to the fuel pump and found out is does indeed need a fuel pump. So i replaced the fuel pump and now have 48PSI fuel pressure in the rail but have NO SPARK! Also the fuel pump doesn't do the 2-3sec purge, it'll run as long as the key is on. I'm sure it has to do with isolating that ground wire but the ground circuit in the relay box doesn't have enough voltage to click the relay to complete the higher amperage circuit to the fuel pump. Has anyone ever seen an issue like that? I've checked, double checked and triple checked everything related.. few things I've ruled out
-ASD is fine. I've tested the connections and swapped relays it's all good
-fuel pump is obviously good as it has awesome fuel pressure
-main body grounds in the engine bay are fine as well.. I've poked around with the meter and get 12v using both of them in the engine bay.
Last edited by 370; 05-16-2012 at 06:23 PM.
05-16-2012, 10:32 AM
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The chassis ground point for your fuel pump is ground G302. It is located behind your spare tire mount/trim panel. See pic below. The pic is a bit fuzzy but you can see the G302 I marked on the frame (this is a '99 XJ).
The ground is crap. Make sure the screw is tight, but don't over tighten it because it will strip out.
The ground is crap. Make sure the screw is tight, but don't over tighten it because it will strip out.
05-16-2012, 11:21 AM
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thank you very much CCken!
**ok checked that ground and it was clean but i cleaned it up a bit more and tightened it up** no change
**ok checked that ground and it was clean but i cleaned it up a bit more and tightened it up** no change
Last edited by 370; 05-16-2012 at 02:08 PM.
05-16-2012, 02:57 PM
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i found this but it's really not much help
and this diagram for the ASD relay which also shows the fuel pump relay and it's wiring again.. no ground circuits for the relays pictured.. also has an explanation of the ASD circuit
http://www.justanswer.com/jeep/1k7e2-going-crazy-recently-bought-97-cherokee.html
and this diagram for the ASD relay which also shows the fuel pump relay and it's wiring again.. no ground circuits for the relays pictured.. also has an explanation of the ASD circuit
http://www.justanswer.com/jeep/1k7e2-going-crazy-recently-bought-97-cherokee.html
Last edited by 370; 05-16-2012 at 04:59 PM.
05-16-2012, 03:06 PM
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well i traced the ground wire for the fuel pump relay.. it goes back to the PCM rear most plug 3rd pin from the rear on the middle row of pins.. tested the wire there and got 6.1v.. so do i have a bad PCM? i also checked the brand new crank sensor.. on the center pin and the left pin i get a reistance of 170K and as you hold the leads on it the resistance falls off.. the longer you hold them there the farther the resistance falls off.. is my brand new CPS bad? i was under the impression that the CPS didn't control the fuel pump but only the injector pulse.. i'm completely lost and i've just about had it with this thing
Last edited by 370; 05-16-2012 at 05:07 PM.
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05-18-2012, 08:14 PM
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05-18-2012, 10:44 PM
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Quick rule of thumb.
If you take a meter reading from the ground side of a device (like a fuel pump) to a good chassis ground, what you're reading is the voltage not converted into mechanical energy by the device, and the resistance of the chassis ground to the battery.
If you take a meter reading from the ground side of a device (like a fuel pump) to a good chassis ground, what you're reading is the voltage not converted into mechanical energy by the device, and the resistance of the chassis ground to the battery.
05-19-2012, 06:11 AM
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