Low voltage

[Diamondback]

I'm working on a friend's '90 2wd with the 2.9L. The truck's been sitting for a year or so and wouldn't start. We noticed the fuel pump wasn't priming and the fuel gauge didn't work, so we swapped in a new pump/sensor assembly into the gas tank. We turned the key and the truck fired right up. We drove it a few miles, it ran like a champ. So, we came home and shut it off. A week later we tried to start it. Again, the fuel pump wouldn't prime (however, the gas gauge works now). Every now and then we heard a slight groan from the pump, but it didn't run. So, we broke out the volt meter. There's only 6V getting to the pump. There's also 6V at the inertia switch. We swapped in a new battery and still only got 6V. You can hear the relays click when the key is turned, just something seems to be pulling down the voltage. Do you all have any idea what would be cutting the fuel pump's voltage down?

 

[almostclueless]

Find a wiring diagram, trace the voltage back to the relay connections, find out if the relay contacts are buggered or maybe somewhere upline.

 

[Hair]

seen it more than once.
fuel pump relay is shot!

 

[Diamondback]

So I swapped in a new fuel pump relay. There's still 6-7V at the fuel pump. I'm starting to suspect the harness deserves a closer look.

 

[Diamondback]

Ok, so we checked continuity from the fuel pump to the fuse box. Everything is fine that way. We pulled the engine codes and the only code was a failed MAP sensor. So, we replaced the sensor, now there's no codes and the fuel pump still doesn't prime.

Here's the interesting thing. When the code reader is plugged in, when you turn the key to "On", the fuel pump relay flutters for a second, as does the signal on the code reader. With the code reader unplugged, we checked the voltage at the relay. You turn the key to "On" and you hear the fuel pump relay kick, then it makes a quieter click half a second later. There's 12V at the relay until the quieter second click. It's like something is telling the fuel pump relay to let go a half second after it initially pulls in. Both the original and a new relay do this. Anyone have a clue what would cause that?

 

[MAKG]

Something has low (not zero) resistance. Grab the ohmmeter and check all the actuators. It's not uncommon for a solenoid coil to short internally, leaving low resistance. At least a few Ohms (generally considered 1-3) is necessary.

Another possibility is an issue in the alternator.

If you aren't testing voltage at the battery, FIRST try that. If it's still 12V, find the voltage drop and you'll find the culprit.

 

[almostclueless]

Check the ground for the fuel pump relay coil.

 

[Bob Ayers]

Check the ground for the fuel pump relay coil.

A open collector driver in the PCM provides the ground to pick the fuel pump relay.

 

[MAKG]

A open collector driver in the PCM provides the ground to pick the fuel pump relay.

Said another way, the fuel pump relay ground is the PCM ground.

 

[Bob Ayers]

Said another way, the fuel pump relay ground is the PCM ground.

How about: "the fuel pump relay ground is the PCM ground with a switch"

 

[chilort]

I've been helping Diamondback with this truck.

There is a tan with green wire that runs from the fuel pump relay so the eec. The eec obviously controls the ground side of the relay control as has already been stated.

The eec appears to open the ground side of the fuel pump relay control shortly after or exactly when it triggers a solenoid in the transmission. I know next to nothing about how Ford eec's work. So, why would the eec provide a ground for a second, and then disarm the fuel pump relay? And why would it do this and not provide some kind of error code? If it had some kind of over current protection like newer eec's that some manufacturers use, I would expect a code.

Since I know next to nothing about Ford products, we also had a very difficult time even trying to locate the eec. It was late in the day, so we gave up.

 

[Psychopete]

Only later 2.9Ls have a fuel pump monitor, yours may or may not have it, I am not sure. And it might not even be checking for low voltage, but if it just any voltage at all. Maybe someone can go into this with more detal.

The computer controls the fuel pump relay, and that's just how it does it.

The computer is located under the passenger side kick panel.

Pete

Edit:
It only turns on for a second to prime the system to 40PSI. It makes sense to me that the fuel pump shouldn't run if the trucks not. IMO.

 

[MAKG]

It runs for TWO seconds past the last detected ignition event.

And even ancient EEC-IV PCMs detect loss of the ignition monitor, so it's something else.

 

[chilort]

Thanks for the great info. I'm used to cars that turn on the fuel pump and leave it on, so this is very helpful.

I am guessing the 6v we see at the fuel pump is parasitic.

Sounds to me like the new fuel pump is bad since it never does anything within the two seconds it gets voltage. I guess we could put 12v on it to make sure though.

 

[MAKG]

DISCONNECT the fuel pump and see if it's 12V across the connector that way. If it is, it's a bad fuel pump. Otherwise, it's something else.