New to the forum, this is my first technical post. I have a 460 in a 1996 Jamboree Sport RV. It cranks and will run on ether, fuel pump has voltage, but is not running. So I dropped the tank and replaced the pump assembly, the new pump runs when I put 12volts to it with the power probe, the old pump does not run with 12volts. The voltage to the connector for the pump is 6.8 volts and will not run the pump. Batteries are new and showing 12+ volts. The volt meter on the dash shows about 10 volts. I replaced the EEC two years ago. Am I missing something?


Thanks.

 

Get a schematic or EVTM for your year and trace the electrical circuit that feeds the fuel pump. You'll find there is a relay that might be the issue of insufficient current flowing to the pump---voltage present shows continuity but not the amperage capacity that can start and run the pump itself.

Also check all wiring and connectors leading to the pump---they too can become corroded further impeding current.

 

Thanks for the response JWA.


I have picked up a Haynes manual for the schematic. The only relay in the fuel pump electrical line is in the power distribution box. I borrowed a relay test jumper and have low voltage coming off the relay. I have tested the relay and swapped it out as well. I put a power probe (12volts) from the fuel pump connector and can see 12 volts all the way back to the relay. The power probe will run the pump from the power distribution box.


I just took the ignition starter switch off and will replace this evening. I have 12 volts into the ignition starter switch, but not coming from the relay. So I am thinking the power drop is between ignition and power distribution box. If this doesn't do it, the only place left is the power connection into the power distribution box. I took all grounds off and wire brushed them and cleaned connectors in the line.


Anything else to consider? Thanks again.

 

Ignition starter switch didn't do it. Where is the main ground to the engine block? Can the Power Distribution Box be removed and/or can the connections underneath be checked?

 

The underhood power distribution box theoretically can be removed although I have no idea how tough or complicated that would be---never had to address this.

Sounds to me like you're on the right path but we still have a lack of current to the pump connector. Frankly I'm a bit stumped why this is but think a wire in that circuit is frayed, a terminal corroded or something amiss somewhere---duh right?

Perhaps inspecting the underside of the power distribution box is the only thing remaining. Because you have operation before and after that box strongly suggests it figures greatly into this.

Sorry I can't be more specific or helpful.

 

Thanks again for your thoughts JWA. It is reassuring that I am on the correct path, however weird that path may be.


I got the Power Distribution Box loose, but can't get it out without breaking the A/C line and disconnecting the brake lines. SO, I am planning on running a line from the yellow side of the ignition switch to the fuel shutoff switch on the passenger side. This tested as the connecting line (that is ignition activated) that runs to the fuel pump fuse and relay in the PDB. I will put a fuse and relay in the line on the passenger side wall next to the fuel cutoff switch. I will take the fuse out of the PDB to avoid sending current from both lines.


What do you think?


Jim


Just for back story, the RV is backed into my driveway with the 24 foot race car trailer hooked up. I can't start prepping the race car until I can get the RV out of the way, hook up the F250 and move the trailer. Now it is raining cats and dogs.

 

I'm thinking to agree with your intended work around Jim---at this point not much else to do without major surgery.

While its little consolation the factory electrical relay/fuse boxes tend to be robust and I've not seen a lot of threads or comments similar to yours. All I'm saying is despite this relatively minor glitch the rest of your system should be long lived.

(I didn't want to post this yesterday---it being Friday The 13th and all. )

 

If I may add some fuel to the fire here........
I'm more familiar with the 5.8 in this era.

Your working on a '96, 7.5, which is still OBD1.

First, the ECM controls the fuel pump relay.
Key on, the fuel pump pressurizes the fuel rail, and shuts off the pump relay.
Run, the fuel pump should be running. If the ECM senses a problem, it will shut the pump relay off. This is through the MAP sensor.

I don't understand the reason to run a jumper to the Inertia Switch, that can be by-passed with just connecting the wires together. It acts as a spst switch. With key on, you should have power to ground on one leg.

As you found out, the Power Distribution Box is almost impossible to get out, as it is buried under many other items.

Check out this link, it covers the fiction of the Fuel Pump Relay.
Yes, it's for up to a 5.8, yes it's for a independent relay, where our's in integrated into the PDB. But, it covers the basic of the relays function.

Also, look on that site for the test of the MAP sensor. (All Tutorials: 4.9L, 5.0L, 5.8L)

As JWA wrote, you could have a problem with the wire run from the PDB to the fuel pump, either in the wire itself, or at a connector, that's causing the low voltage at the pump. It might come down to running a new wire from the harness of the PDB to the fuel pump harness itself.

 

What!!!

Little superstitious here or what??

You didn't get near any mirrors yesterday............did you???

 

This is the Mac tester that I used. Just attached leads while it is plugged into PDB.



Thanks for the responses, JWA and Wildman. Just an recommendation for the relay tester that I borrowed. The voltage from the fuel pump activation (30) is low at the relay. Wildman, that link is great, this tool makes is easy to perform the test in place.

 

Yea, that looks like a nice test kit for the relays

It's great that you can "borrow" testers like that, I too have a ASE mechanic that I can "borrow" the special tools from
Of course, when his overhead door springs break, It's payback time to him.

Glad to help out, I just thought your getting into the wrong direction on this problem, and tossed you a couple of other options to check first before tearing everything apart.

 

You said you can not get power through the starter relay. That to me sounds like the starter relay is not wired right. The power for the fuel pump does NOT go through the starter relay. It is hooked on the battery side and is hot at all times. The only wire on the other side of the starter relay is the wire going down to the starter solenoid mounted on top of the starter.

Maybe this will help:

/

 

Subford, the only relay that I have been working with is the fuel pump relay. If it was confusing, I apologize. I wanted to go through the ignition switch so the power would turn on and off with the key. Since I had the test kit I did check every relay in the PDB, as well as the two spares that I keep in the vehicle.


I got everything wired in yesterday and included a lighted on/off switch, an inline fuse, and the relay pigtail. I now have 12.7 Volts to the pump and can run the pump. I put a gallon of gas in the tank, and the engine starts and runs. I will re-install the tank and dash today, I hope.


Does anyone want to see a diagram and/or photos of this ham fisted mechanic work? It is not pretty.


Thanks to all for their help and interest.


Jim

 

The Ignition switch is all ready overloaded and you can not load it any more or the the plug will melt or the switch will go bad.
The PCM Power relay is a relay that is operated by the Ignition switch to the carry the load so the Ignition switch does not have to. When the Ignition switch is on the PCM Power relay is on and when the Ignition switch is off the power relay is off.
The fuel pump relay carries the load of the fuel pump motors so no other switch or relay has to.

Yes I would like to see how you did it and if you are powering the Fuel Pump with the ignition switch as you seem to be doing I think you are going to have problems down the road.

 

Tap at the ignition switch




Line (blue) to fuse, on/off switch, relay




On/off switch




Relay


Thanks again for the information. This is what I have done. I took the lead from the second yellow which I think is the dedicated line to the PDB for the fuel pump relay. Then fed that to a fuse, on/off switch, relay, then to the fuel shutoff.