Fuel pump problems

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.

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

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....

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?

"Evan" wrote in news:1126887323.003647.86790 @g14g2000cwa.googlegroups.com:

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.

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

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

GMC in NC

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, 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

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

"bradtx" wrote in news:1126965499.274700.297790 @g49g2000cwa.googlegroups.com:

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.

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,

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.

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....

"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