Low Voltage to Fuel pump

[[email protected]]

Happy 4th ,I checked my Fuel pump yesterday after install a new pump. I am only getting 8 volts tested before the pump at harness. ???????
Any help would make a good Day Thanks in Advance

 

[GoToGuy]

Battery has full charge? Ensue wherever your testing has a good ground. Check and clean grounds.

 

[Schurkey]

I checked my Fuel pump yesterday after install a new pump. I am only getting 8 volts tested before the pump at harness.

Pump is running? How did you ground the voltmeter? Did you also check for voltage on the ground side, close to the pump?

First Guess: Corroded connections, corroded wires, multi-strand wires with some strands broken.

Check voltage progressively closer to the battery, when you get to "good" voltage, you'll know where the problem(s) are.

When I tested my Luminas, ('92 and '93) absolutely every connection added about .2 volts of voltage drop About 2 volts total of VD on the supply side, about 1 volt on the ground side. 3 volts of VD altogether. Pump running on 11 volts when alternator is charging at 14 volts. There was no "one place" where a problem was found...it was EVERYWHERE. GM used undersized wire for the fuel pump harness; the metric equivalent to 16-gauge wire when the load and length would have demanded 12 gauge wire.

 

[[email protected]]

Thanks Schurkey I ordered a Racetronic upgraded pump harness should do the trick. I used it on my 96 Impala ss and worked out

 

[Schurkey]

Did you ever check the voltages including the ground side, like I suggested?

 

[rdldrdld]

I have a similar problem on 93 c1500. The fuel pumps are not lasting very long before totally going to crap. We have 12v at the relay, which appears to go to the ECM, then to the pump. We have 8.5v in the wire going to the pump. It drops to 5.6v when the pump is running. I am curious if the low voltage is is ruining our fuel pumps. We have tried 2 ECMs, new relay, new fuel tank with fresh fuel, and this is the 3rd or 4th fuel pump in a short time. I know it is a good ground because we ran a ground wire directly off the battery. We checked continuity on the pump power wire to the relay and the relay appears to be acting correctly.
Some info suggests this 8.5v is alright, while others say it is a problem.

 

[Schurkey]

We have 12v at the relay, which appears to go to the ECM, then to the pump.

No. The ECM supplies power through the relay, which then supplies power to the pump from another source.

We have 8.5v in the wire going to the pump. It drops to 5.6v when the pump is running.

The GM wire harness uses undersized wire; some amount of voltage drop (VD) is to be expected, on both the supply side and the ground side.

OTOH, 5.6 volts with the pump running is crazy low, especially if the engine is running and the alternator is charging at 14.x volts as normal.

I am curious if the low voltage is is ruining our fuel pumps.

Seems VERY likely.

We have tried 2 ECMs, new relay, new fuel tank

You need fresh wire with connections that aren't corroded.

I know it is a good ground because we ran a ground wire directly off the battery.

What is the voltage on the ground wire with the pump and engine running, as close to the pump as you can access, when the ground is connected as usual?

Some info suggests this 8.5v is alright, while others say it is a problem.

8.5 volts with the pump running would be bad enough. 8.5 without the pump running is horrible.

Figure that system voltage is 14+ with the engine running. Allow 2 volts of VD on the supply side, and one volt of VD on the ground side. The pump should be operating on 14+ - 3 = 11+ volts; and ideally it'd be higher due to less VD.

 

[rdldrdld]

It is 8.5v in the engine compartment and right at the pump with either the normal ground or battery ground. The ground is good..

If we were to run new wires, where would the wires be replaced? There is a connection from the relay to the purple fuel pump power wire that we are not getting traced out. It is a different color at the relay. I don't remember if the relay was orange or what is was.
BTW. Appreciate the info.

 

[Schurkey]

Download the service manual set for your vehicle, the wiring diagrams are in there.

Trace the wiring back to the battery. Somewhere in there, there's faulty connections/poor conductors. THAT'S what needs to be repaired.

 

[Road Trip]

We have 12v at the relay, which appears to go to the ECM, then to the pump.

Greetings rdldrdld,

Schurkey is correct. The ECM is wired to the 'control side' of the relay. When the ECM
enables current passing through the coil, the switch contacts close, and battery power (at full
battery voltage) is sent first to an inline fuse, and then down to the fuel pump.

Here's the circuit taken from the Factory Service Manual:

You must be registered for see images attach

ECM Pin A1 controls the relay. ECM pin B2 monitors the voltage sent to the fuel pump in order to set
the DTC 54 code.


OK, instead of having to troubleshoot intermittent loss of power or blowing of the fuse, the problem description
is that the power is steady, BUT there is a measurable voltage drop between the Battery positive terminal and
the input pin to the fuel pump?

Good news, we can make a series of voltage checks, at each connection down the circuit, and at some point
we will observe the difference between the 12.6v (battery at full charge) vs. the 8.5v measured at the pump?

What are the voltage measurements at each of the following locations? Suggestion: write them down as you go:

* Battery Positive terminal voltage to ground: ______volts

* Voltage at Battery Junction Block Terminal: __________
(ie: Voltage drop between Battery and Terminal Block on firewall.)

* Voltage at Pin A at input to fuel pump relay: _________
(ie: What is voltage drop across Fusible Link?)

* Voltage at input to Inline Fuse: ___________
(ie: What is the voltage drop across the relay/relay socket?)

* Voltage at input to Fuel Pump: ___________
(ie: What is the voltage drop from inline fuse output to fuel pump positive pin?)

NOTE: In order to make these measurements, here's an illustration of physically
where the relay and inline fuse are found:

You must be registered for see images attach

****

Your truck is a '93, so we're working with 31 year old connections now. There have
been documented cases of large voltage losses across wounded (but still working)
fusible links. There have been documented cases where the new relay is good internally,
but the associated connections in the relay socket are loose/corroded/marginal electrical path.
The inline fuse socket could be corroded/marginal electrical path. And the wire down to the
fuel pump could have had the insulation pierced/damaged, and all the wires inside are
corroded?

The silver lining in this cloud? It would be one thing if you were trying to track down a
missing 1/2 volt. But since you have measured a 4 volt drop (no load) this has to show
up if you take the series of measurements listed above.

And 5.6 volts running is not good. And if every single connection somehow manages to
be good, then if was me I would upgrade the wire diameter, especially between the output
pin of the inline fuse and the input to the fuel pump.

And it's simple electrical math: P = I x E. In English, the higher the voltage supplied in
order to perform a given amount of work (watts) the lower the amperage needed to do so.
Amperage = heat. I'm oversimplifying a bit, but nobody lowers the voltage to an electric
motor in order to make it last longer. (!)

We have 8.5v in the wire going to the pump. It drops to 5.6v when the pump is running. I am curious if the low voltage is is ruining our fuel pumps. We have tried 2 ECMs, new relay, new fuel tank with fresh fuel, and this is the 3rd or 4th fuel pump in a short time. I know it is a good ground because we ran a ground wire directly off the battery. We checked continuity on the pump power wire to the relay and the relay appears to be acting correctly.
Some info suggests this 8.5v is alright, while others say it is a problem.

If you were to make those careful voltage measurements, starting at the battery terminal
and ending up at the fuel pump power in pin, I think that you should be able to find where
the unwanted voltage drop is occurring. And if you come back here and post your results
(along with a photo of the corroded wire or damaged connection) this would be a big help
to many members of the GMT400 community.

Let us know what you discover.

Happy Hunting!

Cheers --