'94 4.0L Fuel Pump Problem

[OldBlue61]

Sorry to post a new thread on this because I'm sure the answer lies in here someplace, but I'm kinda stumped and I can't figure this out. So, a year and a half ago I replaced my fuel pump on my '94 4.0L manual trans SuperCab. I don't remember where I got the pump, but I remember at the time I was crunched for cash, so I bought a cheap one. Well, a couple of days ago the truck wouldn't start, acting like it was starved for fuel. Let it sit a bit (it wasn't warm) and it finally started, so I pulled it in the barn.

The next day I figured I'd start with a fuel pressure check, and with my gauge on it I got 30psi and slowly dropping, which from what I understand is low. So this time I went to NAPA and dropped some coin on what I hope is a good pump. So, prior to pulling the bed and the pump from the tank, I turned the ignition on and heard the fuel pump come on for a second. Ok, time to pull it.

Got the new pump installed today, and low and behold - nothing. Pump doesn't. I'm thinking a bad pump (usually pretty rare for NAPA stuff), so I reinstall the old one, and nothing from it either. So, I guess it's possible to have something happen electrically at the same time, so I'm going to start with testing the fuel pump relay, looking for a fuse for the pump (I'm sure there is one somewhere, right?), and the inertia switch. I'll start there w/ my multimeter and see what I get. I'll look in my service manuals for the fuse location too to get started. Any help would be appreciated.

 

[OldBlue61]

Well, here's the inertia switch in all it's glory, just as I found it under that felt-like cover under the dash. I've owned this truck for 7-ish years, so it's been hanging there like that as long as I've owned it at least. I've got .57v on the pink wire and zero on the green.

 

[OldBlue61]

And then here's the fuse box under the hood. I know the middle of the 3 relays is for the fuel pump, but is one of these fuses for the pump? Or is it under the dash? Let me go look in the manual...

Yup, the 20amp mini fuse on the left in that panel is good. Any ideas on why I have no power to the inertia switch, assuming I should?

 

[OldBlue61]

Should I be checking for voltage at the pump? Is there a wiring diagram or guide for what the voltages should be at what wire?

 

[OldBlue61]

Well, I guess I’ll try replacing the relay. It’s only $20, so not like that’s a deal breaker. I’ll pick up one tomorrow and see if that works.

 

[SenorNoob]

How did you measure power at the inertia switch? Both of those wires are "hot". I've found it difficult to get a good ground for my meter. The grounding screw for the PCM is usually ok. Have to scratch some paint off first?

The connector to fuel pump is usually touchy as well. Good luck!

 

[sgtsandman]

And then here's the fuse box under the hood. I know the middle of the 3 relays is for the fuel pump, but is one of these fuses for the pump? Or is it under the dash? Let me go look in the manual...

Yup, the 20amp mini fuse on the left in that panel is good. Any ideas on why I have no power to the inertia switch, assuming I should?

View attachment 125359

I had a perfectly good looking fuse that killed power to my fuel pump on the 2011. After spending all day checking wire continuity and power all over the place in the truck, I replaced the fuse because I remembered seeing in a vehicle repair video about there being some brown stuff on the pins being the problem. I replaced the fuse and the truck fired right up. It was a very frustrating day that cost me a day of vacation and a lot of labor, all for a fuse that otherwise looked just fine.

 

[SenorNoob]

We had a similar experience a work a couple weeks ago. 24v to the whole machine was down because of a fuse that looked good.

 

[OldBlue61]

Thanks for the input everyone. When I swap out the relay I’ll change that 20amp fuse as well, just in case.

When I tested the inertia switch, for the ground I ran a jumper to the seat bolt and then verified it was a good connection by checking for continuity.

 

[rumblecloud]

If you were getting some power to the pump prior to its removal and re-install of the new, I would prolly start there with a good cleaning of the connector and contacts (if you haven't done that already). There's also a grond back there somewhere, maybe on the fuel filler neck.

I know I'm stating the obvious here but...

Good luck with it. I have to pull the bed and drop my tank this summer for some cleaning and strap replacement. Not looking forward to that.

 

[OldBlue61]

Is there a diagram anywhere that shows what each of the four wires are for and what they should be? I gotta assume one is a ground, one for the fuel level, one for power, and the last? It would help in the trouble shooting of it.

 

[Terrys87]

Even cheap fuel pumps will last a while and are not a real issue. Just dont run your tank on empty as that will ruin a pump faster then anything else. The fuel actually help keep the pump cool. On any truck that has an intank pump, i do not go below 1/8th tank.

I have had the Inertia Switch to go bad. Even when I could shake it , reset it, if I remember correctly, the contacts in mine were corroded.

 

[sgtsandman]

Thanks for the input everyone. When I swap out the relay I’ll change that 20amp fuse as well, just in case.

When I tested the inertia switch, for the ground I ran a jumper to the seat bolt and then verified it was a good connection by checking for continuity.

That's fine if you do that, just do one at a time and try to start the truck. That way, you know what fixed it and it keeps you from spending anymore money than you have to.

Too many people get impatient and unload the parts cannon. Spending more money than they had to and never found what what the true problem was. We all are guilty of it but if we can save someone else some money and help fix the problem, all the better for everyone.

 

[sgtsandman]

Is there a diagram anywhere that shows what each of the four wires are for and what they should be? I gotta assume one is a ground, one for the fuel level, one for power, and the last? It would help in the trouble shooting of it.

I don't know if @85has the same wiring that you do, but he might be able to help. My trucks are too new be be a reliable source.

 

[SenorNoob]

A diagram I have here shows 2 black wires as ground. 1 black/pink from inertia switch. And 1 yellow/white from gauge.