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:
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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:
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****
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 --