94 Z-71 5.7 Need help troubleshooting the fuel system.

[crazy_dave]

I went out to start my truck after about a week of sitting and she just wouldn’t go. It turns over great with plenty of spark and if you put a little go go juice in the TB it starts right up.
I can’t hear the fuel pump priming with the key on engine off. But there is a (click) sound at what should be about the end of the prime cycle, 4 or 5 seconds.
I had a hole in my gas tank so I thought this would be a great time to change the tank and pump. So Off with the bed!
Put in a new tank and fuel pump and (click) nothing. Same **** different day. Both the new and the old fuel pumps will run with 12v applied directly to the motor. On the 94 the fuel fuse and relay are located on the passenger side firewall under a black cover. According to the Haynes manual I have the fuse should be a pink wire. I have a gray and gy/wh strip. I’m getting about 11.50V on the gy/wh strip. I’m also getting about 9.somthing volts on the pinkish/ orangeish wire of the relay.
Im thinking there is an electrical problem somewhere inline. So if this makes sence to anyone please lend a hand in the proper way to test the system from one end to the other.
I keep running across posts referring to testing the ALDL somehow. Any thoughts on how to do this?
Any and all help is greatly appreciated.

[crazy_dave]

Ok I’m narrowing it down. The fuel pump isn’t getting any power. I retraced the line all the way back to the relay and fuse and still don’t have any juice. I can (hotwire) the fuse next to the relay and get the fuel pump to work and the truck starts fine.
The question I have is where does the fuse normally get power from? It looks to me like it goes to the computer. Could my computer be shot or does it get a signal from somewhere telling it when to power?

[ChevyTech]

The fuel pump fuse under the hood on the passenger side of the firewall is in the wire between the fuel pump and the fuel pump relay. It will only have power during the prime cycle, or if the engine is running. If you are turning the key on with the engine off, then walking around the vehicle testing for voltage, there will be no voltage.


It sounds like your fuel pump relay is not working.

If the fuel pump relay is bad the computer should have stored a trouble code 54.

A code 54 happens when the computer does not see power on the circuit from the fuel relay to the fuel pump, when the computer is sending power to the relay to apply the relay.

Have you checked the ECM for codes?

If you have unplugged the fuel relay and turned the key on there could be a false code set by doing this.


The ECM controls the fuel pump relay.

The truck has a fuel pump relay that supplies power to the fuel pump when:
- Anytime the ECM receives a distributor reference pulse. (Engine running / engine cranking)
- The key is turned on engine not cranking – relay applied for 2 to 20 seconds (most trucks 2 seconds), depending on emissions equipment. Trucks over 8500 GVW and some other vehicles have and electronic Fuel timer module that allows the pump to run up to 20 seconds.
The key must be turned off for a certain amount of time before it will prime the system again when turned back on.


When the engine has oil pressure a redundant circuit supplies power to the fuel pump. This circuit goes through an oil pressure sensor/sender. This circuit is independent of the fuel pump relay.

[crazy_dave]

Put a new relay in and still no power. Bench tested both new and old relay and they seen to work fine.

[ChevyTech]

On the 94 the fuel fuse and relay are located on the passenger side firewall under a black cover. According to the Haynes manual I have the fuse should be a pink wire. I have a gray and gy/wh strip. I’m getting about 11.50V on the gy/wh strip.

The fuel pump isn’t getting any power. I retraced the line all the way back to the relay and fuse and still don’t have any juice.

So did the fuel pump have power when you wrote the first post, and now it doesn’t?

The gray wire with a white tracer is the wire to the fuel pump from the relay.

Don’t overlook the ground wire to the fuel pump.

Sometimes the connector to the fuel pump sending unit gets poor connections.

[crazy_dave]

The only time I have power to the pump is when I apply 12v directly to it. Be it at the pump its self or all the way back to the fuse/ relay. Then it starts and runs fine. I just cant seen to get any power to it the way its suppose to.

[speedygonzales]

So is the relay on and does the contacts (common and normally open) have power when it is on?

If the relay is off, track down the coil supply lines. If it is on and nothing is coming out the contact side, track down the contact side of the relay.

[crazy_dave]

Originally Posted by speedygonzales

So is the relay on and does the contacts (common and normally open) have power when it is on?

If the relay is off, track down the coil supply lines. If it is on and nothing is coming out the contact side, track down the contact side of the relay.

Ill give it a shot.




Found this online.

the computer will supply voltage for the first 2 seconds to the fuel pump then shut off intill the engine is turned over. the sensor that tells the computer the engine is turning over is in the distributor.so if you have voltage for 2 sec it could be the pickup in the distributor. other than that it could be the computer.?


How would I test the pickup? also I dont have any service/ check engine light on at any time.

[ChevyTech]

also I dont have any service/ check engine light on at any time.

Are you saying that the light does not work so you can’t check the trouble codes?

Are you saying the light will not come on when you turn the key on without cranking the engine?

If the light works.

Check the computer for trouble codes

If you need instruction how to check codes, ask us, or do a search. It can be done without a scan tool on your 1994.

How would I test the pickup?

If it has spark then the pickup coil is working.

There is a systematic way to find the problem if you are willing to take the steps to troubleshoot and test.

[crazy_dave]

I’m not sure if the light works or not. I was just saying that there is nothing indicating that there is a code.

I’ve found posts saying that you can (pin out) certain terminals on the test port and read the flashes of the test light to find out the codes. But I’m not sure how to do this please show me the steps.

[ChevyTech]

The only way to know if there is a code, or not, is to check for codes.

Here are some web sites with the basics on retrieving codes, on OBD1 systems, without a scan tool or code reader.

Poor drawing of ALDL but ok instruction + codes
http://www.cfm-tech.com/gm_tbi_trouble_codes.htm

OBDI terminal identification
http://robertpowersmotorsports.frees.../ALDL_plug.jpg

OBDI illustration and instructions
http://www.troublecodes.net/GM/

Good explanation
http://www.syty.org/old/d&e.html

[crazy_dave]

First two links worked, Thank you.All the trouble codes from the first link are coming up as invalid as well
The 3rd
OBDI illustration and instructions

NOTICE: This domain name expired on 04/20/2011 and is pending renewal or deletion.

[ChevyTech]

Sorry about the bad link – I am surprised, as this was a heavily used site for codes early on.

Let me know any codes there were, and I will let you know what they mean. Many sites only have the short list of codes and don’t include transmission codes…

Did it blink out code 12 three times and then blink out other codes?

Give me some more detail on what you saw.

[crazy_dave]

O.K I put a jumper wire from the A to B ports on the ALDL plug. According to the service engine flashes I have a code 12 and a code 54. It blinked 12 three times then 54 three times. I have no idea what this means but hopefully someone can help me out.
Also I ran 12v into the G port (fuel pump) on the ALDL and the pump did turn on.

[ChevyTech]

Originally Posted by ChevyTech

A code 54 happens when the computer does not see power on the circuit from the fuel relay to the fuel pump, when the computer is sending power to the relay to apply the relay.

If you have unplugged the fuel relay and turned the key on there could be a false code set by doing this.

Originally Posted by Crazy Dave

Also I ran 12v into the G port (fuel pump) on the ALDL and the pump did turn on.

This proves that the wire from the fuel pump relay to the fuel pump and the inline fuse are able to carry power.

Originally Posted by Crazy Dave

I’m also getting about 9.somthing volts on the pinkish/ orangeish wire of the relay.

The power wire to the relay is Orange and comes from a fusible link that gets power from the junction block on the firewall by the relay. It should have battery voltage on it. If you only have 9 volts there is a problem with a connection. You should retest this.
If it is low, look at the nuts on the firewall junction block for being loose or resistance/corrosion.

The relay ground wire is black with a white stripe. It should be connected to ground.

The control wire from the computer is Dark Green with a white stripe. It should have power for at least two seconds when you first turn the key to the on position.

These wire colors are from a 1993 manual but I think they will match.

[crazy_dave]

[QUOTE=ChevyTech;4651165]If you have unplugged the fuel relay and turned the key on there could be a false code set by doing this.

QUOTE]

Thats how I checked the relay so it maybe a false code. is there a way to reset the codes to see if comes back?
I will retest the wires tonight with a helper and see what happens. The relay mounts with metal tabs are they grounded as well, could it have a bad connection there?

[ChevyTech]

is there a way to reset the codes to see if comes back?

Clearing codes on GM OBD1 systems:
Make sure the key is OFF, and preferable in your pocket when disconnecting power from the system.
Remove the ECM fuses for at least tens seconds, then reinstall the fuses.
Or disconnecting the battery for tens seconds will also clear the codes.
Some GM information states 30 seconds.

The relay mounts with metal tabs are they grounded as well, could it have a bad connection there?

No;
The mounting tab just holds the relay.

All the ground electricity goes through the ground wire.

[crazy_dave]

I had to do a double at work yesterday, so no update. I’ll work on it over the weekend and report back asap. Thank you for all the help!

[crazy_dave]

Originally Posted by ChevyTech

The power wire to the relay is Orange and comes from a fusible link that gets power from the junction block on the firewall by the relay. It should have battery voltage on it. If you only have 9 volts there is a problem with a connection. You should retest this.


The relay ground wire is black with a white stripe. It should be connected to ground.

The control wire from the computer is Dark Green with a white stripe. It should have power for at least two seconds when you first turn the key to the on position.

These wire colors are from a 1993 manual but I think they will match.

1)I grounded out my test light and according to that I have NO power coming to wires in that part of the harness with or without the key on engine off.

2) Test light with 12V shows the ground is good.

3)Dark Green with a white stripe again no power to any of the five wires.

[ChevyTech]

It sounds like the truck has a least one burnt fusible link.

Look at the fusible links on the passenger side of the fire wall at the junction block.

One of these fusible links feeds the fuel pump relay and also feeds part of the computer.

On a 1993 it would be a gray fusible link, connecting to an orange wire, connected to the center stud of the junction block.

[crazy_dave]

On appearance the fusible link looks fine. No burn marks or surface indication that they are bad. Is there a way to test them?

[ChevyTech]

Double check all the tests you did about getting power to the fuel pump relay to make sure you get the same result.

Test for power at the eyelet end of the fusible link and this will also make sure the test light is working.

I have found several trucks with connection problems at the junction block studs when the nuts were not all that loose, but I have also found many with loose nuts.

Fusible links are pieces of wire that burn to protect the circuit. Feel all the wires at the junction block to see if you find a bad one. They could be melted off inside and hard and stiff near the ends.

If you have power on the eyelet end that connected to the stud and no power is making it to the Fuel relay you should probe the wire just after the fusible link and see if there is power there. This way if the link if bad you will be probing the wire were you are going to be connecting the new fusible link.

I have looked at a 1993 schematic to help you.
I know in 1994 the set timing connector was moved inside the cab so there are some differences in the harness.
I don’t have a 1994 schematic to look at.
If you don’t find the problem with what I have given you, then you will need to look at a schematic for a 1994 truck.

[crazy_dave]

I found it!!!!!!!!!!!!!!! The orange wire going into the fuel pump relay had a tiny pinhole about ¼ before the plastic connector. The wire was corroded to the point where it was just green dust a good 3in into the harness and where it connected to the metal terminal inside the connector. I jerry rigged a new wire in place and everything run as it should.
Now for the final (hopefully) question. Where can I get a new metal connector tab that snaps into the plastic housing of the fuel pump relay connector part of the wiring harness?

[speedygonzales]

That hole in the wire is more likely the result of someone probing the wire to look for voltage. Let this be a lesson to those who do this that it will bite you eventually.

However, the ones that do it are usually working on someone else's vehicle and have the adage of "AMC".

[crazy_dave]

Originally Posted by speedygonzales

That hole in the wire is more likely the result of someone probing the wire to look for voltage. Let this be a lesson to those who do this that it will bite you eventually.

However, the ones that do it are usually working on someone else's vehicle and have the adage of "AMC".

That’s exactly what it looks like!