Code 54/ LOW VOLTAGE TO FUEL PUMP RELAY

[Pos94obs]

Truck is throwing code 54 which is low voltage to fuel pump relay. I think it causes my truck to not start sometimes. Anyways what could cause this. I got a new relay (the piece that mounts on the top of the firewall) and it fixed it for a bit but I guess it came back. In need of some ideas as what's causing it and some diagrams for this thing!

 

[Road Trip]

Truck is throwing code 54 which is low voltage to fuel pump relay. I think it causes my truck to not start sometimes. Anyways what could cause this. I got a new relay (the piece that mounts on the top of the firewall) and it fixed it for a bit but I guess it came back. In need of some ideas as what's causing it and some diagrams for this thing!

Hello Pos94obs,

I'm inferring from your username that you are troubleshooting a '94, please confirm.

Especially when working through an electrical system malfunction, it can really make a
difference if I'm working through an early bare bones W/T truck vs a later fully-loaded
Suburban. '88-'94 wiring, vs '95+ redesign with Underhood Fuse Block.

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Anyway, let's assume the implied '94 and see what the FSM has to say about your Code 54:

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In order to make sense of the wiring diagram so that we can troubleshoot with it, we first need to know the
goals were of the design engineers and how they solved the problems that they faced:

* The fuel delivery guys didn't want a failure in their engine supporting system to be the primary cause of
having customers stuck on the side of the road. So instead of wiring in the fuel pump relay so as to
be a single point of failure, they instead wired an engine oil pressure switch in parallel, so that even
if the fuel pump relay were to fail, eventually with enough cranking, once the oil pressure > 4 psi, power
would be supplied to the fuel pump through this backup switch and the truck will start.

* Well, the computer guys said that we should monitor this circuit, (via pin B2) and if it takes more than 1.5 seconds
worth of cranking to start we will flag this issue to the driver as a Code 54. If you study the circuit above
you should be able to see everything I just described, from the parallel power feeds to the splice that
feeds the fuel pump, as well as the monitoring path from that same splice over to B2 of the ECM.

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So, as a troubleshooter we need to identify the possible players, and then further organize the list in
terms of probabilities. Here's one possible list:

* Failed fuel pump relay (relay as perpetrator of failure)
* Good relay, loose/intermittent female connections in relay socket. (relay as victim)
* Good relay + good relay socket connections, but loose ground on engine interfering with proper relay control winding operation. (See diagram)
* Fuel pump relay good, intermittent connection between splice and ECM monitoring pin. (Falsely detected error, and hard starting root cause is ignition/elsewhere.)
* Fuel pump relay good, solid connection to pin E2, but ECM is flaky. (low probability if ECM otherwise symptom free, but is still a remote possibility.)
* Flaky power/ground at fuel pump. {But this does not explain Code 54}

That's my list, each one a possibility, but in decreasing order of probability.

In English, you replacing the fuel pump relay was correct. And the fact that it worked
for awhile before the failure returned tells me that the physical disturbance during the
relay replacement may have temporarily overcome one of more loose connections between
the relay & associated relay socket connections.

If you think about how many million potholes that have been hit over the years vs how that relay is
situated, it will give you an appreciation of how an intermittent problem can develop in this area.

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This issue is absolutely fixable, we just have to go through the process of elimination in order to tighten
up the problem description/focus and get a good fix in place.

Hopefully this gives you a big picture understanding and where to start. Understand all this is predicated
on the Code 54 being thrown, and that we are just going from the fuel pump relay onward. If we find nothing
in this area to be bad then we'll need to go upstream towards the power source of this circuit.

Food for troubleshooting thought.

Best of luck with your hunt --