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Fuel pump problems

2K views 15 replies 0 participants last post by  bradtx 
G
#1 ·
I have a 91 5.0 with a t-5 in a 66 coupe. I am not having this problem
all the time but it is more frequent now. It seems to be effected by
the cold some how because it did this last winter, summer was fine, and
now its happening again.

Usually when I turn the key to on the pump will run for about 2
seconds then stop. Lately when I go to start the car the fuel pump will
not run. I know the pump is good because I had to temp my pump up with
a wire from my battery (trunk mounted) so I could get home friday
night. Saturday morning everything was fine, started like it was
suppose to. I checked for a relay for the fuel pump and the guy who did
the swap cut it out. There is a wire straight from the computer to the
pump.

Any one have any ideas?

Evan
 
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#2 ·
Sounds like someone created a zoo for you. The PCM is going to have a hard
time with the current draw of the pump.... this is why a relay was in the
circuit to start with. The control circuit of the relay draws a fraction of
the current of the pump.

HTH.


"Evan" <Blindthieves@hotmail.com> wrote in message
news:1126837330.858021.269050@g47g2000cwa.googlegroups.com...
> I have a 91 5.0 with a t-5 in a 66 coupe. I am not having this problem
> all the time but it is more frequent now. It seems to be effected by
> the cold some how because it did this last winter, summer was fine, and
> now its happening again.
>
> Usually when I turn the key to on the pump will run for about 2
> seconds then stop. Lately when I go to start the car the fuel pump will
> not run. I know the pump is good because I had to temp my pump up with
> a wire from my battery (trunk mounted) so I could get home friday
> night. Saturday morning everything was fine, started like it was
> suppose to. I checked for a relay for the fuel pump and the guy who did
> the swap cut it out. There is a wire straight from the computer to the
> pump.
>
> Any one have any ideas?
>
> Evan
>
 
G
#3 ·
So you are saying I should install a relay so the pump does not suck
all the power from the pcm?
Last friday I drove the car all day without a problem. Then I go see a
movie, and when I come back, the pump wont run. The car runs just fine
once its started and the pump will literaly work one minute and not the
next.

-Evan
 
G
#4 ·
"Evan" <Blindthieves@hotmail.com> wrote in message
news:1126848387.952540.96680@f14g2000cwb.googlegroups.com...
> So you are saying I should install a relay so the pump does not suck
> all the power from the pcm?
> Last friday I drove the car all day without a problem. Then I go see a
> movie, and when I come back, the pump wont run. The car runs just fine
> once its started and the pump will literaly work one minute and not the
> next.
>
> -Evan
>


Next time it won't run, hit the bottom of the tank with your shoe. If it
runs, change the pump.

Al
 
G
#5 ·
As has been mentioned, try the old whack test on the tank to be sure that
there is not a fuel pump concern.... however, it is not a matter of "sucking
all the power".. Electrical circuits are designed to carry a finite amount
of power.... especially when it comes to computer related circuits. If you
consistantly draw 15 amps across a circuit designed to carry 5 amps,
something is gonna give. Your PCM was designed to control a low current
device (the relay) which, in turn, controlled a high current device (the
pump). When we power up an electrial device, it is so much more than
stringing a wire from here to there... many things have to be taken into
consideration if we want to prevent little things like expensive failures or
fire.

I'm not saying you should or shouldn't install a relay to control the
pump.... I am saying that, if the circuit is connected as you describe, you
are lucky that the PCM still works....

"Evan" <Blindthieves@hotmail.com> wrote in message
news:1126848387.952540.96680@f14g2000cwb.googlegroups.com...
> So you are saying I should install a relay so the pump does not suck
> all the power from the pcm?
> Last friday I drove the car all day without a problem. Then I go see a
> movie, and when I come back, the pump wont run. The car runs just fine
> once its started and the pump will literaly work one minute and not the
> next.
>
> -Evan
>
 
G
#6 ·
I will try the wack a tank test and see if that works or not. The fuel
pump was replaced about 2 years ago. Its a 88 Ranger high flow pump.
Thats what the previous owner installed and told me in would work fine.
Is this sufficent to run the 5.0?
I will look into installing a fuel pump relay too. What are the
chances that the pcm relay is starting to fail?
 
G
#7 ·
"Evan" <Blindthieves@hotmail.com> wrote in news:1126887323.003647.86790
@g14g2000cwa.googlegroups.com:

> I will try the wack a tank test and see if that works or not. The fuel
> pump was replaced about 2 years ago. Its a 88 Ranger high flow pump.
> Thats what the previous owner installed and told me in would work fine.
> Is this sufficent to run the 5.0?
> I will look into installing a fuel pump relay too. What are the
> chances that the pcm relay is starting to fail?
>
>

What kind of idiot would bypass the relay and run it from the PCM, And why
do I doubt this , anyway.

What do i recall from my distant memory of the wiring that makes me wonder?

Hmmm?

anyway:
your check engine light wont work (you SHOULD have that wired in... even if
it's underdash), and the pump wont run on "key-on" but you've already
covered that.
 
G
#8 ·
"Jim Warman" <mechanic@tenalpsulet.net> wrote in message
news:7WtWe.282190$on1.182628@clgrps13...


Jim, I always read your posts; I usually learn somethin'.

I mean that in a non-**** way. : )

GMC in NC
 
G
#9 ·
It's not just power draw that will cause a problem but thefeedback from the
electric motor form the fuel pump could cause other problems as well as blow
up the PCM.


"Evan" <Blindthieves@hotmail.com> wrote in message
news:1126837330.858021.269050@g47g2000cwa.googlegroups.com...
>I have a 91 5.0 with a t-5 in a 66 coupe. I am not having this problem
> all the time but it is more frequent now. It seems to be effected by
> the cold some how because it did this last winter, summer was fine, and
> now its happening again.
>
> Usually when I turn the key to on the pump will run for about 2
> seconds then stop. Lately when I go to start the car the fuel pump will
> not run. I know the pump is good because I had to temp my pump up with
> a wire from my battery (trunk mounted) so I could get home friday
> night. Saturday morning everything was fine, started like it was
> suppose to. I checked for a relay for the fuel pump and the guy who did
> the swap cut it out. There is a wire straight from the computer to the
> pump.
>
> Any one have any ideas?
>
> Evan
>
 
G
#10 ·
Evan, If the Ranger pump worked fine in the past, then don't worry. A
relay designed to start an electric fan will do the job. The biggest
danger is the high voltage spike that's caused to initially start the
pump's motor. This should be isolated from the PCM.

bradtx
 
G
#11 ·
On Fri, 16 Sep 2005 06:49:39 GMT, "Jim Warman"
<mechanic@tenalpsulet.net> wrote:

>As has been mentioned, try the old whack test on the tank to be sure that
>there is not a fuel pump concern.... however, it is not a matter of "sucking
>all the power".. Electrical circuits are designed to carry a finite amount
>of power.... especially when it comes to computer related circuits. If you
>consistantly draw 15 amps across a circuit designed to carry 5 amps,
>something is gonna give. Your PCM was designed to control a low current
>device (the relay) which, in turn, controlled a high current device (the
>pump). When we power up an electrial device, it is so much more than
>stringing a wire from here to there... many things have to be taken into
>consideration if we want to prevent little things like expensive failures or
>fire.


I don't think the pcm can power the pump for even one second. There
must be something else going on.


>
>I'm not saying you should or shouldn't install a relay to control the
>pump.... I am saying that, if the circuit is connected as you describe, you
>are lucky that the PCM still works....
>
>"Evan" <Blindthieves@hotmail.com> wrote in message
>news:1126848387.952540.96680@f14g2000cwb.googlegroups.com...
>> So you are saying I should install a relay so the pump does not suck
>> all the power from the pcm?
>> Last friday I drove the car all day without a problem. Then I go see a
>> movie, and when I come back, the pump wont run. The car runs just fine
>> once its started and the pump will literaly work one minute and not the
>> next.
>>
>> -Evan
>>

>
 
G
#12 ·
"bradtx" <b2723m@aol.com> wrote in news:1126965499.274700.297790
@g49g2000cwa.googlegroups.com:

> Evan, If the Ranger pump worked fine in the past, then don't worry. A
> relay designed to start an electric fan will do the job. The biggest
> danger is the high voltage spike that's caused to initially start the
> pump's motor. This should be isolated from the PCM.
>
> bradtx
>
>


Uhh... that would be a high CURRENT spike to start the pump, the high
VOLTAGE spike comes when it's turned off... results from the pump winding's
collapsing field.

not to be a nanny or anything but it's important.

And this is why I dont beleive now that the thing DOESNT have a relay...
never mind I cant figure how it would be wired.
 
G
#13 ·
RT wrote:
> On Fri, 16 Sep 2005 06:49:39 GMT, "Jim Warman"
> <mechanic@tenalpsulet.net> wrote:
>
>
>>As has been mentioned, try the old whack test on the tank to be sure that
>>there is not a fuel pump concern.... however, it is not a matter of "sucking
>>all the power".. Electrical circuits are designed to carry a finite amount
>>of power.... especially when it comes to computer related circuits. If you
>>consistantly draw 15 amps across a circuit designed to carry 5 amps,
>>something is gonna give. Your PCM was designed to control a low current
>>device (the relay) which, in turn, controlled a high current device (the
>>pump). When we power up an electrial device, it is so much more than
>>stringing a wire from here to there... many things have to be taken into
>>consideration if we want to prevent little things like expensive failures or
>>fire.

>
>
> I don't think the pcm can power the pump for even one second. There
> must be something else going on.


I don't either. And if it did, it wouldn't last long. The PCM isn't
designed to output that much current. I think someone is pulling our
leg or is just very confused/misinformed about what is going on with
their car.

Cheers,
 
G
#14 ·
And we're getting into another, quite popular misconception. Other than a
regulated reference voltage that is supplied to some sensors, it is
extremely rare for a PCM to supply power to any device. For most actuators,
the PCM simply supplies a ground... be it a relay, solenoid, ignition coil,
fuel injector... anything... all the PCM requires is a good ground.

Add that modern automotive computer systems utilize voltage divider circuits
which offer a certain amount of over-current protection. Voltage divider
technology uses a small resistor within the PCM. This resistor is used to
aid in systems monitoring and self-testing at the same time as they reduce
current flow in that particular circuit.

Semiconductor technology is still far from perfect... a device rated at 2
amps may be able to carry far more before failure simply because of clean
room conditions at the time the semiconductor was "grown". Many
semiconductors, especially computer CPUs, are tested after menufacture to
see what it is they wound up with. Diodes and switches have several
characteristics... Peak inverse voltage - forward current... many more that
I don't remember or possibly understand.

There is much more to these "simple circuits" than first meets the eye.
 
G
#15 ·
Add that relays used in PCM controlled circuits should include a clamping
diode or resistor to eliminate the high inverse voltage induced when the
relay coil shuts down.

A message for all.... electricity is probably the most simple thing to
understand on the modern vehicle if we only open our minds to it's very
basic and unalterable truths. Properly performed voltage drops are the best
diagnostic aid when faced with electrical problems.... Kirschoffs laws are
very simple to understand as is Ohms law...

While the engines an transmissions in our cars change at an alarming pace,
electricity will always work the same....


"Backyard Mechanic" <pettyfog@yaywho.com> wrote in message
news:Xns96D48A9FA5522pettyfogery@207.115.63.158...
> "bradtx" <b2723m@aol.com> wrote in news:1126965499.274700.297790
> @g49g2000cwa.googlegroups.com:
>
> > Evan, If the Ranger pump worked fine in the past, then don't worry. A
> > relay designed to start an electric fan will do the job. The biggest
> > danger is the high voltage spike that's caused to initially start the
> > pump's motor. This should be isolated from the PCM.
> >
> > bradtx
> >
> >

>
> Uhh... that would be a high CURRENT spike to start the pump, the high
> VOLTAGE spike comes when it's turned off... results from the pump

winding's
> collapsing field.
>
> not to be a nanny or anything but it's important.
>
> And this is why I dont beleive now that the thing DOESNT have a relay...
> never mind I cant figure how it would be wired.
>
 
G
#16 ·
"Uhh... that would be a high CURRENT spike to start the pump, the high
VOLTAGE spike comes when it's turned off... results from the pump
winding's
collapsing field.

not to be a nanny or anything but it's important." - Backyard Mechanic

It's GOOD to re-read one's post, I should do it once in a while!
Thanks.

bradtx
 
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