Fuel Pump Issues

[SoCalManche]

Hey there folks,

I'm going to make this as short and sweet as possible. Ironically, I was driving to the machine shop today to drop off the block from my TJ. I hit a red light, and as it turned green, I started to move, the engine cut, and since then has not turned over. Cranks, but no turn-over because the fuel pump is shorting somewhere.

 

1986 Jeep Comanche 2.5L TBI

- I have spark and air; fuel not pushing.

- Turns over when adding a small amount of fuel in the throttle body, but dies after about 1-3 seconds.

- Fuel pump has been directly checked via battery to see if it isn't burnt; starts right up.

- Fuel pump does not sound off / prime when ignition is turned on.

- Get 12 volts in two out of the three relays that are on the passenger side fender near the battery; can someone please tell me which relays are which? The most forward relay isn't getting voltage.

- Get 12 volts on starter relay also.

- When we checked the pump with the ignition on and pulling each of those relays out, the voltage wasn't changing, and staying at 8 volts.

- I do not have a fuel ballast, or at least I have searched high and low and one does not seem to exist.

 

That's all I got. And no, I have not refreshed the ground behind the taillight housing. I will be doing that in the morning once I get some daylight. Any and all questions or comments are appreciated and welcomed. 

 

 

[SoCalManche]

Oh yes, and an important note I forgot; I'm almost certain it died because I ran out of fuel. The needle wasn't on E, but close. I understand that that could cause some issues.

 

[gogmorgo]

Splash some gas in her and try again. I don't have any experience with the 2.5 TBI but the later systems will only prime so many times in a set interval if you keep turning the key on and off.

 

Some terminology for you: "Cranking" and "turning over"

mean exactly the same thing. It's when the starter engages the flywheel and rotates the engine.

"Firing" is when spark ignites the air/fuel mixture. Sometimes  "coughing" is used to describe firing without starting, but it's hardly a technical term.

One more, "shorting" means current is flowing through a curcuit jin a way not intended, either to another circuit altogether or straight to ground. Symptoms include activating the wrong electrical parts, magic smoke escaping, and blown fuses. How did you establish the pump is shorting somewhere?

I'm not trying to be an @$$ here, just trying to help clarify your statements. We're not standing next to the truck, so all we know is what you tell us, and if you say something other than what you intend to convey, it can make that challenging. 

 

Sounds to me like your engine is turning over just fine, and will fire with gas sprinkled in the intake. That's a red flag for fuel delivery problems, yes. Your fuel gauge isn't a precision instrument, and now it's 33 years old, and may not function as intended. It may no longer go all the way to empty. In my experience driving around with the gauge showing bear empty is a great way to run out of gas. 

Where were you measuring to get 8 volts? Were you grounding to the harness or back to the chassis? If measuring to the chassis increases voltage, or there's high resistance between the wiring ground and chassis, you've got a ground fault.

In addition to the ground behind the taillight there's also a connector behind the rear driver's wheel which is known to have issues, and so can added-on trailer wiring. 

[SoCalManche]

Thanks for the info. Apologize on the mis-terming. So I have an update:

 

Got back to the truck and sprinkled some fuel in the injector. Go to try to start and now it's not even attempting to start as if it has no power. Battery has 12 volts, starter relay has 12 volts, the key buzzes when in the ignition and door is open, but as soon as I try to fire it up, it goes dead. Thoughts?

[Ωhm]

Bad battery. 12vdc at battery doesn't mean it can work a starter. With DVOM connected across the battery, what does the voltage drop too, when IGN SW is in the CRANK position?

[Minuit]

If you have 12.0 volts at the battery, the battery is pretty much dead and will not start the truck. You want to see 12.4 to 12.8 volts on a charged battery.

[SoCalManche]

I wouldn't say the battery is bad considering it's new and let me rephrase -- when ignition is ON, it reads 12.4 volts.

[Jeep Driver]

14 hours ago, SoCalManche said:

- When we checked the pump with the ignition on and pulling each of those relays out, the voltage wasn't changing, and staying at 8 volts.

You've got a wiring/cable/ground problem somewhere. 

[Ωhm]

3 hours ago, Ωhm said:

With DVOM connected across the battery, what does the voltage drop too, when IGN SW is in the CRANK position?

 

[SoCalManche]

Good question. I will have to get back to you on that when I get to the truck next, which will most likely be tomorrow.

 

On the same note, the truck has no issue starting up when I throw some fuel in the throttle body. It cranks live and well, and then shuts down once I burn whatever gas was thrown in from up top.

 

I did a back-probing on the three-pin connector (in the B-slot with the red lead) that connects to the fuel pump three-pin connector. Regardless of Ohms setting (I was doing 200 but switched through all of them as I'm a novice with a DVOM), I had high resistance when grounding to chassis. I did this with the ignition switched off.

 

So my question is, is that more of a relay issue? Or specific wires?

[gogmorgo]

You should have basically zero resistance between ground and a clean part of the chassis. If you have any resistance there, then that's your problem. I bet you've got near battery voltage between the fuel pump's hot wire and a clean ground. 

Corroded wires can play havoc like that. Your voltmeter puts out so little voyage and current that you could have one tiny strand of wire or grain of green death making the connection with low resistance, but it's not enough connection to take the current necessary to power the load. Clean up the taillight ground and then the connector behind the wheel. 

 

Also worth mentioning, those three wires (and I can't tell you which is which) should be your 12v hot, the fuel pump ground that should have near zero resistance, and the feed to the fuel gauge, which I imagine will have at least some resistance.

[SoCalManche]

On 4/20/2019 at 2:33 PM, gogmorgo said:

You should have basically zero resistance between ground and a clean part of the chassis. If you have any resistance there, then that's your problem. I bet you've got near battery voltage between the fuel pump's hot wire and a clean ground. 

Corroded wires can play havoc like that. Your voltmeter puts out so little voyage and current that you could have one tiny strand of wire or grain of green death making the connection with low resistance, but it's not enough connection to take the current necessary to power the load. Clean up the taillight ground and then the connector behind the wheel. 

 

Also worth mentioning, those three wires (and I can't tell you which is which) should be your 12v hot, the fuel pump ground that should have near zero resistance, and the feed to the fuel gauge, which I imagine will have at least some resistance.

When you say connector behind the wheel, what are you referring to exactly?

[gogmorgo]

This one. The Renix connectors are different from the one on my '91 but they're in the same place. 

Its it's the connector for the taillight harness, right before it goes through the rear splash panel. 

[SoCalManche]

 

 

 

1 hour ago, gogmorgo said:

This one. The Renix connectors are different from the one on my '91 but they're in the same place. 

Its it's the connector for the taillight harness, right before it goes through the rear splash panel. 

I do not have that connector it seems. I felt up between the frame and bed where the wires go, but nothing. The other picture shows the only connectors I have. 

 

I have traced the wires into the cab and up to the connector point under the dash, next to the hood latch release. I have had a continuous resistance of 39-40 ohms. 

[SoCalManche]

[SoCalManche]

Okay, so I have a fun update that may be the problem: 

 

I tested out the relay for the fuel pump and it's acting strange. The relay that was there when I got the truck constantly clicks when turning key into ON position. If I plug in a new relay, nothing happens. And I tried it with three different new relays. So maybe the socket is corroded? I'm not to keen on the electrical side of things, but this experience is definitely making me learn.

 

[Jeep Driver]

'86 harness is different from the ' 91 harness. 

 

Pull the driver's tail light out, there should be a wire that is screwed to the sheet metal, clean that ground. 

[SoCalManche]

19 minutes ago, Jeep Driver said:

'86 harness is different from the ' 91 harness. 

 

Pull the driver's tail light out, there should be a wire that is screwed to the sheet metal, clean that ground. 

Already have. That ground is a shiny as metal can get, with some connector grease.

[Jeep Driver]

2 minutes ago, SoCalManche said:

Already have. That ground is a shiny as metal can get, with some connector grease.

Alright, well, don't use grease between the terminal and the sheet metal. 

 

 

Can you see fuel emitted from the ejector while cranking the engine?

[SoCalManche]

1 minute ago, Jeep Driver said:

Alright, well, don't use grease between the terminal and the sheet metal. 

 

 

Can you see fuel emitted from the ejector while cranking the engine?

No, but I've usually been working on it alone so I've had to crank. But the fuel pump does not prime when ON and like I just mentioned, the original fuel relay constantly cycles via clicking and never stops until I turn the ignition off. It goes all the way to the fuel pump with the clicking. Engine will turn over easy if fuel is poured into TBI.

 

Also, doesn't Cruiser say to use Oxgard?

[gogmorgo]

Screw the ground fastener down, then put your anticorrosive agent on top if you care to use one. 

"Turn over" is what happens when the starter is engaged. It's irrelevant to fuel being present in the throttle body. Sounds like the word you're looking for is "fire", which is what happens when the air/fuel mixture is ignited inside the cylinder. 

 

40 ohms is WAY more than you should be getting to a chassis ground. That's well over ten times the resistance you'd expect through the fuel pump itself... Although it is the resistance you'd see through the fuel gauge sending unit at around 1/2 tank. Are you measuring through the chassis side of the sending unit connector, of through the sending unit itself?

[SoCalManche]

3 minutes ago, gogmorgo said:

Screw the ground fastener down, then put your anticorrosive agent on top if you care to use one. 

"Turn over" is what happens when the starter is engaged. It's irrelevant to fuel being present in the throttle body. Sounds like the word you're looking for is "fire", which is what happens when the air/fuel mixture is ignited inside the cylinder. 

 

40 ohms is WAY more than you should be getting to a chassis ground. That's well over ten times the resistance you'd expect through the fuel pump itself. 

On my multimeter, its showing 40 ohms when it's set on "200." That's the lowest setting I have.

 

As for "turn over" vs "fire," wouldn't the starter need to engage in order for the cylinders to fire? Regardless, my issue is the fuel pump relay socket I'm assuming. No?

[SoCalManche]

8 minutes ago, gogmorgo said:

40 ohms is WAY more than you should be getting to a chassis ground. That's well over ten times the resistance you'd expect through the fuel pump itself... Although it is the resistance you'd see through the fuel gauge sending unit at around 1/2 tank. Are you measuring through the chassis side of the sending unit connector, of through the sending unit itself?

Sorry, I didn't see the rest of your question. 

 

As far as how I've measured it, the ignition is OFF, I disconnect the two three-pin connectors, one coming from the truck cab, and the other being the fuel pump itself. Once the fuel pump is disconnected, I take the other connector and back-probe the middle pin, and ground out to the chassis. That's how I get 40 ohms.

[gogmorgo]

The point is "turning over" has nothing to do with what's happening in the engine, just the fact that it's turning, driven by the starter. It's already turning over as soon as you hit the key. 

I got distracted and hit send before finishing that post. Went back and edited it. It sounds like something funky is happening with the relay, sure, but if you've got 40 ohms resistance through the fuel pump ground, that's a problem, and it needs addressed. 

[Jeep Driver]

1 minute ago, gogmorgo said:

The point is "turning over" has nothing to do with what's happening in the engine, just the fact that it's turning, driven by the starter. It's already turning over as soon as you hit the key. 

I got distracted and hit send before finishing that post. Went back and edited it. It sounds like something funky is happening with the relay, sure, but if you've got 40 ohms resistance through the fuel pump ground, that's a problem, and it needs addressed. 

I suspect he's got a problem with all his grounds.