Hi all, new user with a question about a fuel pump problem I'm having... Occasionally the pump will not come on with the key as I prepare to start car. The motor will turn over but not start. This has happened many times and I have dropped the tank a couple of time to check the pump and by so doing it the pump will work fine for months when I put it all back together but I'm not sure I'm on the right track. As I sit here this morning drinking my first cup of coffee and waiting for the gas tank to drain (I curse the engineer who thought saving the 50 cents by NOT putting in a drain plug was a good idea) and having to put a small plastic tube down the vent tub to drain the tank. Anyway, I'm thinking there must be a pressure switch somewhere to tell the pump to turn on or off. So before I dump a couple hundred dollars into a new pump anyone know where the pressure switch is located, what kind of voltage should I see and or have any suggestions as to where to go next. thanks Walt

 

Problem is you don't know how the system works.
Here it is.
At key on, the PCM puts out a ground to the 'pump relay' through a crash sensor switch, for about two seconds to pre-pressureize the fuel rails, then stops.
At cranking, the crank shaft sensor detects the rotation and tells the PCM to turn on the pump full time.
In this line up you have a crash sensor, a relay, the harness and the connector on top of the tank.
An intemittant at any point will cause your issue.
The power for the pump is a seperate fused source to the pump relay and is passed through to the pump.
Should do some testing before taking things apart to try to prove the 'source' of the problem.
Use voltmeter, maybe?
Test for fuel pressure at the rail, as well to see what it is when ithe pump does run..
Good luck.

 

Hi Bluegrass, knew there was more to it but it appears now that it is even more than I thought. That was part of my request for more information thinking it was just a simple pressure sensing unit that either said the pressure was correct or that the pump needed to run to get it higher. I was hoping to get by with just asking others more knowledgeable than I am about it and get some direction just as you have done so I could check a few things and find what is wrong. I'm still putting my money on the pump being the problem. Using a voltmeter on the the connection above the tank that goes to the pump. Came up with 11 - 12 volts with key on and gas filter removed so line open and not pressurized but still no pump, cleaned the connector but still nothing so I dropped the tank removed the pump and just as before hit it with battery voltage and it works. Connect it back on the connector and with each turn of the key it runs. Don't want to do more than "bump" it since it is already not working the way it should and could cause further harm.

But, I'm interested in checking out the other components you mentioned especially the PCM. Without having to buy some Ford manual where can I get more detail on what and where the items are located. First, what is the PCM where located when you say fuel rails what exactly are you talking about the fuel lines?? or what??? I don't remember this level of detail in the owners manual.

thanks Walt

 

You won't see any of this in the owner's manuel.
Closest you can come without a shop manuel is a Haynes book on 97 to 03 Ford F150.
In it are wire drawings in the last chapter. Even then all is not there.
There may be minor changes over the years as the truck were changed.
The book is only meant for limitds use by an owner because it get to complex otherwise to educate at the same time help with a repair function.
.
Fuel rails are those that supply the fuel injectors on the top of the motor.
The fuel regulator is also there.
The pump is able to output a pressure as high as 100 psi up to the regulator that knocks it down to about 38 psi for the injectors.
I would agree that if the pump does not push gas out of the open line that it is not running or is broke.
Good luck.

 

thanks again Bluegrass, I will look around for the Haynes book, maybe on Ebay? The truck is getting old and starting to give me troubles. Transmission, ABS, odometer display light just to mention a few. So I need something to help me keep it running for a few more years.

Walt

 

my problem is i have 7 volts to the fuel pump with key on. is this normal. i just put a new fuel pump in.

i moved the relays around and it still won't start.

checking the fuse now but i don't think that's the problem.

help

joey

 

I think it should be 12 volts........

I think you just hijacked a thread - what year is YOUR truck?

 

started a new thread here. https://www.f150online.com/forums/v8...mp-issues.html

it's a 1998

 

Just for the record... Bluegrass, you were RIGHT ON! I bought the new pump $175 bucks, hooked it up to the connector at the pump and NOTHING!!! Tried it three times in a row ( key on, key off) with no action at the pump. Took the old pump and hooked it back up and it worked three out of three times. Put the new one back in and yes it worked just fine. Guess I should have paid more attention when you pointed me to the PCM. On another post you told that person the PCM is on the passenger side behind the kick panel. Wife handed me a disk today she got somewhere??? that is a Ford workshop information disk that also mentions the PCM as a possible problem area. It doesn't tell me where the PCM is located but thanks to you I know that already and I guess I have some more places to look to find the actual problem.

So, Bluegrass or others, the PCM is it just a relay (on or off) with the key turned to ignition should show voltage for that second or two? right! So that I should be able to check pretty easily, But here is the question, how come the problem only seems to show up on ignition start and not while driving?? If it is the relay shouldn't I also experience a stalled engine once in a while if it is a relay or connection problem??? Is there anything else in the loop that might be the gremlin under the hood. The Ford disk shows a EEG power relay and a fuel pump monitor also in the fuel pump circuit could they be part of the problem too. What are they, where are they and how do I check them.

thanks for all the help thus far and for any additional help

Walt

P.S. how come I can't post attachments?? Have the Ford schematic but not able to put in post.

 

The pump circuit operates from a ground application.
....In the PCM, there is a solid state switch that is triggered by inital application of power to the PCM.
....This switch will only apply ground to the PCM terminal for about 2 second due to the 'time constant' designed into the switch circuit trigger input.
....When the crank begins to turn, the crank sensor feeds rotation (rpm) signal to the PCM that pulses the trigger of the switch circuit at a rapid rate such that the pump remains on full time as long as the crank is turning.
.
The reasons this is done this way are several.
1. To not run the pump full time when not needed even with the key to run.
2. To stop the pump if an accident has happened and not feed the possability of a fire if the engine stops with the ignition on.
3. To provide a way to test fuel pressure without running the motor and to re-prime the fuel rails should the vehichle run dry of fuel, by cycling the ignition switch on and off a half dozen times.
.
Moving on; this output ground is supplied to the pump relay through a crash sensor switch, again for fire protection in case of hard front hit.
....The pump relay (slave relay) has battery setting on it's winding waiting for the ground to be applied. It's operated contacts pass 'seperate' fused battery to the pump, through the harness.
....The reasons as given above in addition, is because the pump draws at least 7 amps of curent. The PCM cannot handle nor was ever designed to handle that much current given all the other external hardware it has to control that would generate to much heat in the PCM.
....So bottom line is to ground switch every point possible for long term relibality.
....The coils and injectors are all ground switched out of the PCM. They all take a good amount of current in the total.
Look for an intermittant pump ground, relay or switch contact along the both parts of the circuit. Look at supply fuse for being intermittant in it's holder etc.
I gave you the whole general circuit discription, now go and try to sectionalize the fault to which part, then run to cause down.
Good luck.

 

Thanks again Bluegrass, Haven't had much luck. The truck IS starting every time I turn the key (can hear the pump pressurize the system). Spent couple hours yesterday trying to trace circuit and clean every connector or ground I run into along the way. Haven't really found a problem and the placement of the PCM terminal connector located on the back of the firewall hidden behind the battery box scares the heck outta me. All those wires I just know I will never get it back together if I try and take it off to clean terminals. The PCM is the aluminium box inside the cab against the firewall, right?? I'm guessing the crash box is working OK cause I disconnected it from the circuit and nothing worked as expected. It has three wires a green and black and a brown. the brown is really thin 22 gauge? so I'm thinking it is for the light to say it needs to be reset.

Bottom line, not sure what to do next. Maybe fixed but doubt it. Might have to bring to Ford dealer and let him try and figure it out.

Thanks Bluegrass and I hope doorman you have better luck than I did. Only good thing out of all this is I did find this site, good advice and a sounding board where I could get help. Figure Bluegrass saved me 175 bucks by not wasting the money on a fuel pump I didn't need. Yep Advance Auto (free plug to them) cause they took back the pump no questions asked. I didn't use it but auto parts stores used to be picky about electrical stuff being returned

thanks

Walt

 

An intermittant is tough to find.
Unless it becomes a hard fault to pin point, don't waist money on another shop because they are at the same point you are.
Just keep chaseing the circuit at different points until it no longer is a problem.
The PCM harness is a large connector that has a bolt to keep it in place on the PCM.
Should be no problem removing it for inspection and reseating.

 

Hi Bluegrass, yep that is the one the block connector with the small bolt head. I just see me putting it on wrong and bending a pin somewhere on the terminal block. But maybe I will get the courage to give it a shot if you think it isn't that bad.

I know about the intermittent short issue. I have that with the ABS system. I have the light on and the mechanic I took it to said that the code said it was one wheel sensor but when he checked wheel in question it showed it was good. He said he could trace the problem for 90 dollars an hour but couldn't promise he would find the problem.

thanks for all the help

Walt

 

bump - with a follow up question

Truck fuel pump has been working fine. Every turn of the key primes the pump circuts and you can hear the pump run for the second or two. So I still have no idea what is really wrong.

So here is the issue. When the truck wouldn't start cause the pump wouldn't prime with the first turn of the key I didn't know all this so I tried to use either by squirting the starting fluid into the air stream after separating the line after the air filter and spraying for a second or two of spray into the intake. Any other time on any other vehicle I have has had a positive reaction and most times I could keep a vehicle running on the either, Not so with the Ford. So is there some circuit as part of the starting, when I do the first turn of the ignition to prime the pump and the pump doesn't come on and I turn the key further to crank the engine wouldn't you think it would start with the either??

I was thinking of putting in a fail-safe switch that would hot wire the pump so if the PCM didn't turn on the pump with the key then I could, but if there is something else in the circuit that isn't allow the ignition to work either than I guess I'm hosed.

Thanks

Walt

 

Just to review, be sure it's the fuel pump not supplying fuel to the injectors and be sure you don't have an ignition problem.
If it is lack of fuel, the motor will respond to the Either or other starting fluid as long as there are no other issues.
Both are dependent on the crank position sensor for ignition and restarting the fuel pump.
Look at the possibility of a poor ground for the pump circuit.
Good luck.