Fuel Pump Not Powering On - 89 Comanche 4.0L

[AvianFlu]

Would appreciate any help that can be given, I've been pulling my hair out on this.

 

I have never been good with electrical stuff, and I recently purchased a '89 Comanche with the fuel tank out of it. I hooked up the fuel tank and tried to start the truck, but the fuel pump will not prime or pump when cranking. I've cleaned all grounds, swapped my relays around, bypassed the ballast resistor by setting a paperclip between the connectors, and still have nothing. The pump works when setting the battery up directly to the pump. I set a paperclip between the 30 & 87 on the fuel pump relay, and still nothing.

 

I checked the ground behind the tail lamp, and even cleaned it up, but still have nothing. I tested the rear lights and they work so I don't think the harness back there is part of the problem. I attempted to test voltage at the connector right before it plugs into the pump, and got 12v during the prime. I'm not sure I tested it right, so any guidance is helpful. I am getting 12v at the starter relay fuel pump ballast terminal connection.

 

I looked at the fusible link on the starter relay fuel pump ballast terminal & it appeared to be fine, I was also getting 12.4v at the link.

 

I'm sure I might be leaving some things out, so please ask any questions and I'm happy to answer.

I'm really at a loss of what to do at this point. Any advice or guidance?
 

[Ωhm]

Using a voltmeter or 12vdc testlight (preferred) check for B+ on the following pins (use battery negative terminal for ground):
D1_5:  B+ (Hot at all times)
D2_4:  B+ (Hot during KEY ON/CRANKING).
D1_6:  KEY ON (B+ (Hot for 2-3 seconds)) or CRANKING (B+ (Hot during CRANK)).
 

 

[AvianFlu]

16 minutes ago, Ωhm said:

Using a voltmeter or 12vdc testlight (preferred) check for B+ on the following pins (use battery negative terminal for ground):
D1_5:  B+ (Hot at all times)
D2_4:  B+ (Hot during KEY ON/CRANKING).
D1_6:  KEY ON (B+ (Hot for 2-3 seconds)) or CRANKING (B+ (Hot during CRANK)).
 

 

Hey Ohm, thanks for the response.

 

Here are the results from the tests:

D1_5: 12.4v

D2_4: 11.98v

D1_6: 12.1v

 

These results are all from key ON, I didn't test with crank. D1_6 Was hot for only the few seconds.

Please let me know where we can continue from here. Thank you!

[Ωhm]

With Fused jumper wire jump Fuel Pump Relay Pins_30(1)&87(4) again. Listen for Fuel Pump.

 

 

If no Fuel Pump action, disconnect C157.

 

 

With jumper still in place measure for B+ on C157_F(ORN), engine or IP side of harness.

[AvianFlu]

15 hours ago, Ωhm said:

With Fused jumper wire jump Fuel Pump Relay Pins_30(1)&87(4) again. Listen for Fuel Pump.

 

 

If no Fuel Pump action, disconnect C157.

 

 

With jumper still in place measure for B+ on C157_F(ORN), engine or IP side of harness.

It looks like I am not getting B+ at C157_F. I was only able to test a few times because the battery died (I've been cranking on it for a while now trying to figure this out.)

 

I tested engine & IP, not seeing any voltage. To clarify, as I'm terrible with electrical testing, I'm probing C157_F with the positive probe, and grounding the ground probe on a clean spot of the door. Will this give me accurate enough results? Sorry for needing to be spoonfed but this is all new to me.

[Ωhm]

Doors aren't the best place for grounding. Use under the dash and on the medal. Should see some grounding lugs there. Make sure Fuel Pump Ballast Resistor is good and correctly connected or properly bypassed. Measurement takes into account the Fuel Pump Relay is still jumpered.

[AvianFlu]

On 8/22/2021 at 1:49 PM, Ωhm said:

Doors aren't the best place for grounding. Use under the dash and on the medal. Should see some grounding lugs there. Make sure Fuel Pump Ballast Resistor is good and correctly connected or properly bypassed. Measurement takes into account the Fuel Pump Relay is still jumpered.

Sorry for the late reply, I had to let the battery charge all of yesterday.

 

Upon testing today, I was getting 10-11v on C157_F (Engine) when cranking & 12v when the key was turned to ON. This is with the ballast resistor bypassed with a paperclip in between the two connectors. I did my testing with the fuel pump relay plugged in, I can jumper the relay and test again if needed.

 

I did not get anything from the IP side of the harness, only from where the harness comes in from the engine. Should I have any voltage on IP side when it isn't plugged into the engine side?

 

 

 

 

 

[Ωhm]

With B+ on C157_F connect connector back up. Disconnect C139 at the Fuel Pump and measure for B+ at C139_C. If circuit test good measure for continuity between C139_A and a good ground. Looking for ≈0_Ω.

 

 

[Ωhm]

2 hours ago, AvianFlu said:

This is with the ballast resistor bypassed with a paperclip in between the two connectors. I did my testing with the fuel pump relay plugged in, I can jumper the relay and test again if needed.

 

No need if properly bypassed.

 

2 hours ago, AvianFlu said:

I did not get anything from the IP side of the harness, only from where the harness comes in from the engine. Should I have any voltage on IP side when it isn't plugged into the engine side

 

When disconnected should see NO voltage.

[AvianFlu]

On 8/23/2021 at 8:12 PM, Ωhm said:

With B+ on C157_F connect connector back up. Disconnect C139 at the Fuel Pump and measure for B+ at C139_C. If circuit test good measure for continuity between C139_A and a good ground. Looking for ≈0_Ω.

 

 

After testing, C139_C is getting 12v during prime, 10v during crank.

 

On testing continuity, I had good continuity on C139_A on engine side, but could not get any beep when testing C139_A on the fuel pump side. Is there where my problem is?

[Ωhm]

1 hour ago, AvianFlu said:

After testing, C139_C is getting 12v during prime, 10v during crank.

 

Were these voltage readings taken with C139 disconnected? Reason for asking is next step will be to connect C139, backprobe C139_C and take both measurements again. Looking for Voltage Drop on Fuel Pump Motor circuit.  Leave Fuel Pump Ballast Resistor bypassed.

 

 

 

1 hour ago, AvianFlu said:

but could not get any beep when testing C139_A on the fuel pump side. Is there where my problem is?

No. Possible to get a beep if you went from C139_A (fuel pump side) and case ground on the fuel pump itself.

[AvianFlu]

30 minutes ago, Ωhm said:

 

Were these voltage readings taken with C139 disconnected? Reason for asking is next step will be to connect C139, backprobe C139_C and take both measurements again. Looking for Voltage Drop on Fuel Pump Motor circuit.  Leave Fuel Pump Ballast Resistor bypassed.

 

 

 

No. Possible to get a beep if you went from C139_A (fuel pump side) and case ground on the fuel pump itself.

Backprobed on C139_C, 12v during prime, 10v during crank with and without being connected to fuel pump.

It did seem like the voltage went up for a little bit less time when the pump was connected, but not much, around .5 seconds.

Fuel Pump Ballast Resistor is still bypassed.

[Ωhm]

On 8/21/2021 at 7:06 PM, AvianFlu said:

The pump works when setting the battery up directly to the pump.

If the pump works using the battery and the vehicle wiring harnesses check out OK, then I would think the Fuel Pump should run just fine. Seems like I'm missing something.

 

Need more eyes on this.

[AvianFlu]

6 minutes ago, Ωhm said:

If the pump works using the battery and the vehicle wiring harnesses check out OK, then I would think the Fuel Pump should run just fine. Seems like I'm missing something.

 

Need more eyes on this.

I think I've found the issue, but I'm not sure the best way to remedy it. From pictures I've seen, it looks like C139_A(BLK) should ground out on the pump itself. However, mine is just hanging loose. I tried to ground it by just holding it on the pump test, but still didn't get any power. I tested continuity on the ground cable (which is just a loose copper wire at this point) on the tank and did not get any beep. I tested with the ground I used in the cab and did have continuity. I had continuity on one of the fuel rails, so I held the wire onto it to try and test but still did not get any power to the pump.

 

My question at this point would be: do I need a new fuel tank for that ground, or is there any place under the bed that I could ground on, or add a ground to?

[AvianFlu]

23 hours ago, Ωhm said:

If the pump works using the battery and the vehicle wiring harnesses check out OK, then I would think the Fuel Pump should run just fine. Seems like I'm missing something.

 

Need more eyes on this.

I’ve done some further testing tonight and here’s what I’ve done: created a ground for C139_A, then tested and got around 0.8 ohm. Tried to start by holding the cable on the new ground and got nothing.

 

I re-tested my pump with the battery and it still works, but I’m not sure it’s hooked up correctly. To start it, I had to hook B+ to C139_C, then B- to C139_B. Shouldn’t I have to hook it up as B+ to C139_C & B- to C139_A? 

[Ωhm]

1 hour ago, AvianFlu said:

To start it, I had to hook B+ to C139_C, then B- to C139_B. Shouldn’t I have to hook it up as B+ to C139_C & B- to C139_A?

 

C139_B on the fuel pump side is for the fuel level gauge in the dash. Using this terminal somehow your finding fuel pump motor ground. Maybe next step should be remove fuel pump/sender unit from the tank. Lets see which wire goes where.

[AvianFlu]

10 minutes ago, Ωhm said:

 

C139_B on the fuel pump side is for the fuel level gauge in the dash. Using this terminal somehow your finding fuel pump motor ground. Maybe next step should be remove fuel pump/sender unit from the tank. Lets see which wire goes where.

I see. Another note would be that I do not have a fuel level gauge at the moment, it's missing from my gauge cluster. Is it possible that this is causing a problem?

[AvianFlu]

On 8/26/2021 at 7:58 PM, Ωhm said:

 

C139_B on the fuel pump side is for the fuel level gauge in the dash. Using this terminal somehow your finding fuel pump motor ground. Maybe next step should be remove fuel pump/sender unit from the tank. Lets see which wire goes where.

I'm waiting to test since it's raining right now, but I think(?) I may have found the problem, would like your thoughts. I took the fuel pump out, and all of the wires seemed to be hooked up correctly. I cleaned up all of the connections with a steel brush, there was a lot of white powder on any metal parts of the pump.

 

What I think was the problem, is that the connector on the level sender was very close to the assembly. I believe that this may have been grounding on there, causing my issue. Part of what makes me think this is that there was some purple coloration in that area. I did run the sender dry on accident for a few seconds once when I didn't have enough fuel in the tank, might have happened then. I've moved the connector about 90 degrees further away.

[Ωhm]

On 8/25/2021 at 8:00 PM, AvianFlu said:

From pictures I've seen, it looks like C139_A(BLK) should ground out on the pump itself. However, mine is just hanging loose

 

When connected, both C139_C(Fuel Pump Motor) & C139_B(Fuel Gauge Sender) use C139_A to find vehicle ground. This must be repaired. I know you tried grounding it, but something ain't right. Question: When you power up the Fuel Pump have you tried to START the vehicle?

[AvianFlu]

On 8/28/2021 at 5:14 PM, Ωhm said:

 

When connected, both C139_C(Fuel Pump Motor) & C139_B(Fuel Gauge Sender) use C139_A to find vehicle ground. This must be repaired. I know you tried grounding it, but something ain't right. Question: When you power up the Fuel Pump have you tried to START the vehicle?

I only have one battery so I've only tested the fuel pump & engine separately.  If you mean that I've cranked the vehicle in the start position, yes.

 

[AvianFlu]

On 8/28/2021 at 5:14 PM, Ωhm said:

 

When connected, both C139_C(Fuel Pump Motor) & C139_B(Fuel Gauge Sender) use C139_A to find vehicle ground. This must be repaired. I know you tried grounding it, but something ain't right. Question: When you power up the Fuel Pump have you tried to START the vehicle?

I repaired C139_A, I created a ground and used an eyelet and a bolt to secure it. I also cleaned up the ground behind the tail light a bit more. I'm still getting nothing from the fuel pump. I backprobed C139_A on both ends and got very close to 0 ohms on both sides. (around 0.03 usually). I did my testing solo so I didn't crank while backprobing, but I don't believe it makes a difference I'm assuming.

 

Really at a loss of what to do at this point. I can attach photos I took of the fuel pump, but everything looked normal. Should I get another gauge cluster at this point? I mentioned before that the fuel gauge is missing, could this have anything to do with the problem?

 

Should I just stop messing with it and buy another pump?

[Ωhm]

The Fuel Pump/Sending Unit is mounted to the tank on a rubber gasket, so the FP/S unit itself must find vehicle ground. If your good here, then I'm not sure where to go since Fuel Pump runs when connected to a battery.

 

16 hours ago, AvianFlu said:

Should I get another gauge cluster at this point?

If disconnected (fuel level gauge), should have no affect.

 

16 hours ago, AvianFlu said:

Should I just stop messing with it and buy another pump?

Tough call. I won't spend your money, but seems like vehicle wiring checks OK.

[Andy in Pa]

I just went through something similar with my MJ, and it turned out to be the actual contact internal to the ECU was bad.  In my case, the pump would run, but at a very low speed.  What the ECU does is cycle the ground reference for 2 seconds when the key is first turned on to activate the relay and prime the pump, then shut off... THEN when you turn it to START position this latches to ground and the relay activates and the pump runs.  I tested absolutely EVERYTHING, and it all tested out perfectly.  I traced the wiring everywhere, including right to the ECU connector. 

 

My solution was to provide a ground reference to the wire that was supposed to ground through the ECU.  Works perfectly now, but the fuel pump runs constantly when the key is in the "Run" position.  Its not a problem on our Jeeps though because there is a return line back to the tank.

 

Also FYI, The ONLY place the fuel pump is grounded is behind the drivers side rear tail light.  You should see 0 ohms between that place and the ground reference on the pump assembly.  If you turn the key on and jump the relay and the ballast resistor and there is no 12v being fed to the relay, it could be the ignition switch mounted to the steering column, as that is where this 12V originates.

 

Search for my post, maybe something in there would be helpful to you...

 

A.

[AvianFlu]

8 hours ago, Ωhm said:

The Fuel Pump/Sending Unit is mounted to the tank on a rubber gasket, so the FP/S unit itself must find vehicle ground. If your good here, then I'm not sure where to go since Fuel Pump runs when connected to a battery.

 

Tough call. I won't spend your money, but seems like vehicle wiring checks OK.

I tried testing the pump again after I took it out tonight, directly with the battery, and got nothing at all. I'll order a replacement part today. I'm hopeful it was just the pump dying out, as when I took the pump out this last time, fuel came out from the hoses, but I never heard the pump once, and I hadn't run it with the battery at all at that point. I'm finding it likely it was always just too weak to really push the fuel unless you gave it the juice straight from the battery.

[Andy in Pa]

12 hours ago, Ωhm said:

The Fuel Pump/Sending Unit is mounted to the tank on a rubber gasket, so the FP/S unit itself must find vehicle ground. If your good here, then I'm not sure where to go since Fuel Pump runs when connected to a battery.

 

If disconnected (fuel level gauge), should have no affect.

 

Tough call. I won't spend your money, but seems like vehicle wiring checks OK.

Fuel pump is grounded through the connector for the fuel pump assembly outside the fuel tank.  That ground runs back to the ground lug behind the taillight.  The integrity of this grounding circuit MUST be 100% or the fuel pump won't run.  The rubber gasket only seals the assembly to the tank, there is plenty of metal contact (for good grounding) in place once the lock ring is locked.