Fuel pump...can I run a dedicated power line

dls322

Member
Apr 4, 2005
149
0
16
Mt. Penn, PA
Hi everyone,
I have been having issues with getting power to my fuel pump. There must be a short some where cause i'm not getting a full 12 volts to the pump. I've been over this system for a while now and a was thinking about running a dedicated power line. Can i just run a line from the battery, with an line-fuse, to a new relay, then tie in a manual switch, then run the line directly to the fuel pump? I know i will be by passing the inertia switch.....which i already have bypassed and that the pump will run continiously until i turn it off at the switch. As far as i know the computer just provides a ground to complete the circuit to turn the pump on, this dedicated line would bypass the computer as well.
Can i do this or will there be complications? Any ideas would greatly help, i'm getting tired of pushing her in and out of the garage each day.

Thanks
-Dave
 
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Do it right. Bypasing the computer control of the fuel pump is asking for trouble.

Fuel Pump Troubleshooting for 86-90 Mustangs

Clue – listen for the fuel pump to prime when you first turn the ignition switch on.
It should run for 5-20 seconds and shut off. To trick the fuel pump into running,
find the ECC test connector and jump the connector in the lower RH corner to
ground.
attachment.php

If the fuse links are OK, you will have power to the pump. Check fuel pressure –
remove the cap from the Schrader valve behind the alternator and depress the
core. Fuel should squirt out, catch it in a rag. A tire pressure gauge can also be
used if you have one - look for 37-40 PSI. Beware of fire hazard when you do this.

No fuel pressure, possible failed items in order of their probability:
A.) Tripped inertia switch – press reset button on the inertia switch. The hatch
cars hide it under the plastic trim covering the driver's side taillight. Use the
voltmeter or test light to make sure you have power to both sides of the switch

B.) Fuel pump power relay – located under the driver’s seat in most stangs built
before 92. On 92 and later model cars it is located below the Mass Air Flow meter.
C.) Clogged fuel filter
D.) Failed fuel pump
E.) Blown fuse link in wiring harness. All the applicable fuse links are in the wiring
harness up next to the starter solenoid.
F.) Fuel pressure regulator failed. Remove vacuum line from regulator and inspect
for fuel escaping while pump is running.

The electrical circuit for the fuel pump has two paths, a control path and a power
path.

The control path consists of the inertia switch, the computer, and the fuel pump
relay coil. It turns the fuel pump relay on or off under computer control. The
switched power (red wire) from the ECC relay goes to the inertia switch
(red/black wire) then from the inertia switch to the relay coil and then from the
relay coil to the computer (tan/ Lt green wire). The computer provides the ground
path to complete the circuit. This ground causes the relay coil to energize and
close the contacts for the power path. Keep in mind that you can have voltage
to all the right places, but the computer must provide a ground. If there is no
ground, the relay will not close the power contacts.

The power path picks up from a fuse link near the starter relay. Fuse links are like
fuses, except they are pieces of wire and are made right into the wiring harness.
The feed wire from the fuse link (orange/ light blue wire) goes to the fuel pump
relay contacts. When the contacts close because the relay energizes, the power
flows through the contacts to the fuel pump (light pink/black wire). The fuel pump
has a black wire that supplies the ground to complete the circuit.

Remember that the computer does not source any power to actuators, relays
or injectors, but provides the ground necessary to complete the circuit. That
means one side of the circuit will always be hot, and the other side will go to
ground or below 1 volt as the computer switches on that circuit.


Diagram courtesy of Tmoss & Stang&2birds
fuel-alt-links-ign-ac.gif

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host)
for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel pump, alternator, ignition & A/C wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Computer,. actuator & sensor wiring
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Now that you have the theory of how it works, it’s time to go digging.

Look for 12 volts at the Orange/Lt. Blue wire (power source for fuel pump relay).
No voltage or low voltage, bad fuse link, bad wiring, bad ignition switch or ignition
switch wiring or connections. There is a mystery connector somewhere under the
driver’s side kick panel, between the fuel pump relay and the fuse link.

Turn on the key and jumper the fuel pump test connector to ground as previously
described. Look for 12 volts at the Light Pink/Black wire (relay controlled power
for the fuel pump). No voltage there means that the relay has failed, or there is a
broken wire in the relay control circuit.

Check the Red/black wire, it should have 12 volts. No 12 volts there, either the
inertia switch is open or has no power to it. Check both sides of the inertia
switch: there should be power on the Red wire and Red/Black wire. Power on the
Red wire and not on the Red/Black wire means the inertia switch is open.

The Tan/Lt Green wire provides a ground path for the relay power. With the test
connector jumpered to ground, there should be less than .75 volts. Use a test
lamp with one side connected to battery power and the other side to the
Tan/Lt Green wire. The test light should glow brightly. No glow and you have a
broken wire or bad connection between the test connector and the relay. To test
the wiring from the computer, remove the passenger side kick panel and
disconnect the computer connector. It has a 10 MM bolt that holds it in place.
With the test lamp connected to power, jumper pin 22 to ground and the test
lamp should glow. No glow and the wiring between the computer and the fuel
pump relay is bad.

If all of the checks have worked OK to this point, then the computer is bad. The
computers are very reliable and not prone to failure unless there has been
significant electrical trauma to the car. Things like lightning strikes and putting
the battery in backwards or connecting jumper cables backwards are about the
only thing that kills the computer.
 
Thanks.
I was thinking that it would work. Do you think the computer will be effected since i'm bypassing it, will it get confused and think that the pump isn't running? Also any ideas on what size inline fuse to use?
 
Sure, but why add a switch, you already have an ignition switch, just use that :nice:

Not a good idea. The computer shuts the pump off after about 5 seconds when you turn the key to Run but don't start the engine.
This keeps the pump from running continously anytime the ignition switch is in the Run position. This does several things: it saves
the battery, and reduces wear on the fuel pump. More importantly, if you had a leaking injector and left the pump running continously,
you would flood the engine. The other thing that would happen is that it would wash all the oil from the cyinder walls of the cylinder
with the leaking injector.

As you can see there are good reasons why things work the way they do. Bypasing computer control of the fuel pump on an EFI car
is a bad idea.
 
Not a good idea. The computer shuts the pump off after about 5 seconds when you turn the key to Run but don't start the engine.
This keeps the pump from running continously anytime the ignition switch is in the Run position. This does several things: it saves
the battery, and reduces wear on the fuel pump. More importantly, if you had a leaking injector and left the pump running continously,
you would flood the engine. The other thing that would happen is that it would wash all the oil from the cyinder walls of the cylinder
with the leaking injector.

As you can see there are good reasons why things work the way they do. Bypasing computer control of the fuel pump on an EFI car
is a bad idea.

I completely agree, I would (and have spent many hours doing so once) restore the stock function myself rather than running a new circuit. My post was basically in reguard to 'will it work'. Good looking out the the words of wisdom :)
 
Jrichker.....I understang that the relay has 2 sources, the power and the control. I assume the power should read at 12volts, but what should the control side read on my meter?
I originally ran a fuel pump cutoff switch between the red/black wire. My switch is fine and I even reconnected the red/black to double check. Something must be loose because i know the relay is getting power when the switch light is on... originally i was getting nothing when i pushed her into the station, then i was packing it in for the night and i'm about to push her out of the garage (i work on her up at my fire station) I turn the ignition on and the replay light is on. I go over to the inertia switch and i'm getting 11.7volts. But the pump still is not priming.
The car was running fine when i drove her into the station on new years day. I had the interior gutted at the time, and that day i put my new carpet in the car....i get everythign back together and I have no light at my fuel pump cut switch......this is where i'm at now. Since something is screwy i was just thinking about running a new dedicated wire, but if you think thats a bad idea then i won't.
My stang was rear ended a month and a half ago. The interia scitch was shot so i by passed it, (i spliced the wire going into it into the one leaving the switch, i believe it was a red/black and i get 11.7 there now (when the relay is lit)...this was the same setup that was in her when she was running prior to the carpet isntall) the tank was ruptured so i fixed it, it took the pump out and replaced the filter and tested the pump....no problems, pump was fine. I just think it's odd that she drove fine till i put the carpet in.
Sorry about the long post but i wanted to make sure i fully explained everything.
Any suggestions?
Thanks!!
 
Please read the description again and look at the diagram.

The red/black wire on the relay should have 12 volts. The tan/green side will have 12 volts when the computer turns it off. When the computer turns the relay on, the tan/lt green wire will go to less than 1 volt.

Jumpering the test connector tan/lt green wire to ground will turn the fuel pump relay on when the ignition switch is in the Run position. Use this feature to aid you in troubleshooting the wiring. The computer grounds the tan/lt green wire to turn on the relay. Grounding the tan/lt green wire at the test connector for test purposes won't hurt anything and does not disturb the computer.
 
If you're getting 11.7 at the inertia switch your relay is working.

The only thing left downstream is the connector after the inertia switch, the pump itself, and ofcourse the ground comes last.

Check out those 3 things, it's one of those.
 
Success, I finally figured it out. After dropping the tank and checking every other connector we found it. I calledmy buddy over who is a mercedes mechanic to get his imput. The problem: The orange blue wire was corrroded and pulled out of the pin harness under the drivers side kick pannel. It was weird cause it was showing power at the relay and the relay was working. What a hassle. Anyway, i fixed it and she fired right up.
Thanks everyone for your input and for the diagrams, they made a big difference!!