After head gasket change, no start. C1500

[Skylar M]

at relay location, only getting 5v to the fuel pump relay. This is with key off though. Should the reading be 12v?

 

[Road Trip]

at relay location, only getting 5v to the fuel pump relay. This is with key off though. Should the reading be 12v?

After digging through several manuals, I am puzzled by the fact that I couldn't find
a go/no-go voltage listed for the control side of a Fuel Pump relay?

Furthermore, I know that the VCM (new name for gas-engine ECUs) is fed battery voltage,
and then turns that into a regulated +5v for use by all the internal logic. Therefore, it's
theoretically possible (but not probable) that +5v regulated could be sent out to control the
relay?

Guess what? All the FSM troubleshooting flows I opened up (2-3 different manuals) only state
that 'a trouble light will illuminate' when checking for power at the Fuel Pump relay socket.

Searching the forum for "fuel pump relay voltage" I found someone having a similar issue,
and here's what he discovered: (+12V measured at fuel pump relay after finding bad fusible link)

So, I'm inferring from the above that they were troubleshooting the voltage on the power side of
the relay. But for me I am still not sure about the control signal voltage from the ECM to the
control side of the relay?

****

Given all this, I am going to take a Fluke to the chore truck and report back with some definitive
readings. (I've got a VA appt for some routine stuff today, so I may not be able to gather/share my
findings until tomorrow.)

At any rate, if you read through the thread that I linked to above between now & then, possibly
you will be able to make some progress on this issue? (Keep in mind that they were troubleshooting
a '94, and that there might be will be implementation detail differences between this and your '98?)

So as of right now I feel like I've shared more questions than answers in this area. More later after
I see/document what's happening on a known-good truck.

 

[Road Trip]

For what it's worth, here's the Fuel Pump Relay circuit that we're currently troubleshooting as seen on p.1693 in Vol. 3 of the '98 FSM .pdf file:

You must be registered for see images attach

When I get back I'll add some real-world voltage measurements to this schematic.

Note: There is the matching "Fuel Pump Relay Diagnosis" for the 5.0/5.7 motors
starting on p. 1746 of the same manual. Quite possibly if you follow the factory
procedure you can get this sorted out faster than waiting for my voltage checks?

At any rate, fortunately the entire circuit in question fits onto a single page. No
doubt that we can figure this out. Speaking of which, which pin on the fuel pump
relay did you get that +5v reading?

 

[Skylar M]

Pin 87. If I use a jumper wire to pin 30 from 87, it does turn on and run. But now after that, I checked again and it is reading 12v at pin 87.
Now the code reader does connect. It pops 2 codes that I had before this as well.
P0453
P1351
Also the fuel pump does not turn on and run with the relay installed still.

 

[Road Trip]

Pin 87. If I use a jumper wire to pin 30 from 87, it does turn on and run. But now after that, I checked again and it is reading 12v at pin 87.
Now the code reader does connect. It pops 2 codes that I had before this as well.

OK, this is good progress! Now, before we press on, only to have this 12v > 5v
issue come back and haunt us down the road (in the rain, where there's no cell
phone coverage, etc) let's look around for something like this lurking on the underside
of your Underhood Relay/Fuse Box. (Intermittent fuel pump relay failures traced to this bad wire)

On the one hand, you physically disturbing the Starter Relay and going from 5v->12v
would lead me to believe that the problem is a associated loose/burnt/marginal connection on
the underside?

On the other hand, we also have to explain why you are now getting codes again?
(Which makes me think that the intermittent power connection was affecting the
VCM, and the nonfunctional Starter Relay was just a derivative symptom of that?)

For what it's worth, I've flipped my own Underhood Relay/Fuse box in order to help
someone else with a horn wiring question, and it's easy to do.

Don't hesitate to take a good gander at *all* of the wires/connections under that box, for
after 26 years of current flow x road vibrations stuff tends to loosen up/heat up...and then
give you flaky behavior like 5v readings where 12v is expected.

At any rate, good find, and thanks for the status update. And after you get the 12v
steady/robust in this area, then on to the next troubleshooting challenge. Good stuff!

 

[Road Trip]

Also the fuel pump does not turn on and run with the relay installed still.

You mentioned in the original posting that the relay has tested good.
Assuming a good relay, then we need to focus on the control side
between pin 1 of C4 on the VCM, through the relay's control coil, and
finally at the G103/104 grounds on the engine itself?

In English, with the pin 87 > pin 30 jumpering you performed and
the fuel pump responding, our troubleshooting is now an electrical/control
issue under the hood instead of in the vicinity of the gas tank...

 

[Skylar M]

I forgot to check earlier, but pin 85 (the 2 sec prime when turn key to on) has no power in doing so. Pin 86 (constant ground) is grounded and good with test light check.
Yes it’ll be under the hood because it was all working before the head gaskets.
i have checked g104 and g103 and is good. Besides it looks like 3 wires are grounded on g104? I have 2. And then 2 on g103? I have 1.

 

[Road Trip]

For what it's worth, here's the Fuel Pump Relay circuit that we're currently troubleshooting as seen on p.1693 in Vol. 3 of the '98 FSM .pdf file:

You must be registered for see images attach

When I get back I'll add some real-world voltage measurements to this schematic.

Sat, 13-Jan-2024 status update:

Currently in rain/sleet delay, awaiting drier weather. (working in driveway)

Haven't forgotten! :0)

 

[Skylar M]

So I actually had to change out the ground connector end because it broke off (due to checking it again) coming from the engine harness on g104. Now my truck does not crank and the engine just whines when I try to turn over. The code reader is now reading error - no connect again. Also made sure the engine still turns from the crank bolt and it’s good.

 

[Schurkey]

Now my truck does not crank and the engine just whines when I try to turn over.

Failed starter drive?