Misfires, fuel/air issues. Where to start?

[Zimmerly]

If I need to, can I jump the orange wire from a key-on wire, using an in-lin fuse, if I need to get to work in a pinch and figure it out tomorrow? ...if NEED BE anyway?

 

[Road Trip]

If I need to, can I jump the orange wire from a key-on wire, using an in-lin fuse, if I need to get to work in a pinch and figure it out tomorrow? ...if NEED BE anyway?

Short Answer: Yes.

Emergency backup strategy: Remove fuel pump relay. Fused jumper wire to pin for gray wire. (To fuel pump.)
Fuel pump starts running. (Assuming jumped from Hot At All Times source.) Close hood, start truck normally,
drive to day job.

Turn key off, go under hood & physically Disconnect/remove jumper. Listen for the fuel pump to go quiet.

Not recommended if you are feeling forgetful. Best case scenario, if you leave the jumper in place, the pump
will run continuously & completely discharge your battery while you are working. Worst case could be...a lot worse. :0)

We'll get this figured out. More to follow.

Safe travels!

 

[Zimmerly]

Short Answer: Yes.

Emergency backup strategy: Remove fuel pump relay. Fused jumper wire to pin for gray wire. (To fuel pump.)
Fuel pump starts running. (Assuming jumped from Hot At All Times source.) Close hood, start truck normally,
drive to day job.

Turn key off, go under hood & physically Disconnect/remove jumper. Listen for the fuel pump to go quiet.

Not recommended if you are feeling forgetful. Best case scenario, if you leave the jumper in place, the pump
will run continuously & completely discharge your battery while you are working. Worst case could be...a lot worse. :0)

We'll get this figured out. More to follow.

Safe travels!

Back again. For sunset's sake, I jumped the constant hot wire going to my fuel pump relay, from the "heated seat" fuse spot in my box (unused), using a fuse in between (It looks a little sketchy but it was only for the sake of testing the o2's before it gets dark).
I then pulled both rear o2's, and got it running. The relay is obviously good since it's transferring power to the gray wire internally as long as the orange is given power. I was finally able to read the o2 data. B2S2 is still reading a constant .45 (volts?) When unplugged. I plugged B2S2 back in and the numbers skyrocketed for . Unplugged again and back to a steady .45... The other 2 seem to be dancing just fine (I think?). Funny thing is, B2S1 still isn't reading rich.
...I'm still not sure where the power went to my orange wire, or where the wire disappears to inside the loom, and I can't find any info besides the svc manual diagram stating it goes to "ecm-b"... I'll try to dig for more info, unless someone can point me in the right direction..?
Though I made sure the relay is cutting pump power on shutoff, I will be SURE to pull my jumper after I park. (I'll be wrapping exposed connector ends in tape. Gonna dive back into this wiring to get it fixed after work tomorrow.
Besides that, what are your thoughts on the o2 data?

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[Road Trip]

...I'm still not sure where the power went to my orange wire, or where the wire disappears to inside the loom, and I can't find any info besides the svc manual diagram stating it goes to "ecm-b"... I'll try to dig for more info, unless someone can point me in the right direction..?
Though I made sure the relay is cutting pump power on shutoff, I will be SURE to pull my jumper after I park. (I'll be wrapping exposed connector ends in tape. Gonna dive back into this wiring to get it fixed after work tomorrow.

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GOOD troubleshooting. Like the creative use of the hot side of the (unused) heated seat power source.
The only safety-related suggestion would be for you to change from a 30A jumper fuse to a 20A one.
(ie: The same amperage as the ECM B fuse what was originally protecting this circuit. Even so,
I genuinely appreciate that you incorporated a fuse into your ad-hoc troubleshooting jumper. (!)

Re: Tracing the break causing the unpowered Orange Wire to this relay socket? We had a flurry of
replies, so maybe you missed my reply #28? Those 3 fuses share a power bus, and if the Horn blows
and the A/C still comes on, then the missing power is between the input of the ECM B fuse and the
bus itself. On the other hand, if all 3 fuses are dead, then we need to look at the RED wire that's
supposed to be connected to the shared power bus. Etc.

It's always fun to work with someone who will not take Whoa for an answer. Obviously you are
comfortable around vehicle wiring harnesses, so I have a positive outlook on this troubleshooting
session ending up in the win column.

Anyway, I've got to run a couple of errands, but later this evening I'll post a couple of photos from
my fully operational 'known good' '99 C2500, and lay out some things to look for during tomorrow's
fuel pump relay troubleshooting session. (And bonus points when you discover where the power was
lost that you take a clear picture for others to see how these old girls can suffer power distribution
failures...and more importantly, how they get fixed. (!)

Safe travels. You do not want to snatch Defeat from the jaws of Victory, so minimize your exposure
to driving while jumpered. :0)

EDIT: I will also take a moment to discuss the latest O2 test results. Lot's of excitement in your
engine bay to be sure.

Cheers --

 

[Zimmerly]

GOOD troubleshooting. Like the creative use of the hot side of the (unused) heated seat power source.
The only safety-related suggestion would be for you to change from a 30A jumper fuse to a 20A one.
(ie: The same amperage as the ECM B fuse what was originally protecting this circuit. Even so,
I genuinely appreciate that you incorporated a fuse into your ad-hoc troubleshooting jumper. (!)

Re: Tracing the break causing the unpowered Orange Wire to this relay socket? We had a flurry of
replies, so maybe you missed my reply #28? Those 3 fuses share a power bus, and if the Horn blows
and the A/C still comes on, then the missing power is between the input of the ECM B fuse and the
bus itself. On the other hand, if all 3 fuses are dead, then we need to look at the RED wire that's
supposed to be connected to the shared power bus. Etc.

I'm not sure if you got my reply to this, but disregard those 3 fuses being dead. I erroneously thought they were hot at all times. Not so. They're on with key, and work just fine. It's only that orange constant hot going to the FP relay. I don't know how this plays into your strategy, but I was almost hoping for a loose connector to all 3 of those. I saw that they share a connection somewhere... but disregard.

I will switch to a 20a fuse for that jumper. Luckily I work less than 10min away, so I'm not sweating it too much.
I'm fairly comfortable with wiring- I rewired my entire cab when i stripped it down, including a replacement dash. I work on zero turn mowers at work too, and mess with relays when I think they can be of use in whatever application... Actually just configured a double-relay system to work with a reverse polarity rocker switch to power a windshield wiper motor in our new closeable "grass flap" attachment at work (switch takes too much power when sending it to the unit. And no, I didn't design or build the attachment, just the switching system) , so although I'm not a pro by any means, I have the general concept and a little more than beginner knowledge when it comes to wiring.
Thanks for sticking with me on this! I appreciate it very much. I look forward to getting this figured out.
Have a good night, Road Trip.
-Jacob

 

[Road Trip]

Anyway, I've got to run a couple of errands, but later this evening I'll post a couple of photos from
my fully operational 'known good' '99 C2500, and lay out some things to look for during tomorrow's
fuel pump relay troubleshooting session.

To recap, at this point you have got your Fuel Pump Relay operational by jumpering in good power
to load input pin #87. But in order to help future readers of this thread to make better use
of the wiring diagram in reply #26, here's a photo of the Underhood Fuse Block in my '99 C2500,
where I've removed the Fuel Pump Relay and labeled the socket pins:

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NOTES:

* I was always curious about the automotive relay pin naming convention. Turns out, just like the size of most vehicle radios,
this can also be traced to a German DIN standard. (Electrical DIN 72552, good informational article here: Relay Pin layout)
* And if you cut classes the day that Relays were being taught in automotive electrical, here's a great concise primer on relays: (12 volt planet)
* If you look closely at the underside of the relay, the pins are labeled by the manufacturer. This helps me tie physical reality to the wiring diagram.
* Since the screen snap in reply #26 doesn't blow up all that well, I've included a .pdf file where you can zoom in and really see the relay circuit details
in the FSM for yourself. (See attached.)

****

Later on I'll post a couple of underneath photos showing of the fuse block showing how the power/wiring is organized.
But right now I've gotta help a friend with some wiring, sparky-style. :0)

Cheers --

 

[Road Trip]

Later on I'll post a couple of underneath photos showing of the fuse block showing how the power/wiring is organized.

I've attached 5 more pics that will hopefully give GMT400 owners a little more familiarity with how the power
flows through the newer style Underhood Fuse Blocks. Certainly not enough detail to redesign this important Power
Distribution component, but instead enough info to be able to troubleshoot where/why there is missing input power
to a particular fuse or relay socket.

Attachments:

1) My previous photos of the in service 'known good/flown good' Underhood Fuse Block in my '99 C2500
suffered from bad lighting/poor depth of field. This macro photo should allow you to zoom in on the details.

2) The area between the fuse block and the firewall is usually out of focus. Here I wanted to show where the
main 12v connection to the battery is made. If you're helping a neighbor with an old GMT400 where the
entire vehicle power system is flaky (and you've already verified the connections at the battery as well as the
various infamous grounds...then I would focus on the quality of this 12v connection.

3) Here's a shot I took at a recent Treasure Yard visit where someone decided to cut the fuse block out...but
then changed their mind and left it? This gave me the opportunity to take a detailed pic of the wiring underneath
the fuses & relays.

4) Here's what the FSM has for documentation showing the physical location of each connection referenced in
the Power Distribution wiring diagrams. NOTE: In order to make out some of the smallest print you may have
to open up your local copy of the .pdf file to this page. (And it's still hard to read even when zoomed up.)
FWIW I've marked up the diagram in order to highlight the power bus that brings the power from the positive
battery cable to the rest of the fuse block.

5) Here's a shot of my fuse block split open so that you can get another view of the power bus implementation.
Note the location of the inboard blade on the large Maxi fuses compared to the power bus underneath.

****

If there are any questions about any of this please don't hesitate to ask. Once there's too much structural
rust we have to retire the old soldier from the public roads, but I am of the opinion that we shouldn't give
up on one of these GMT400s because of a stubborn electrical problem. This electrical stuff can be troubleshot
& fixed so that the truck is once again DD reliable. Or better. :0)

The Underhood Fuse Block is simpler than it looks. Lots of details, but no high tech hidden inside.

 

[Road Trip]

...I'm still not sure where the power went to my orange wire, or where the wire disappears to inside the loom, and I can't find any info besides the svc manual diagram stating it goes to "ecm-b"... I'll try to dig for more info, unless someone can point me in the right direction..?

Alright, now that we've laid a little troubleshooting foundation on the Underhood Fuse Block,
let's see if we can take your missing power on your orange wire to the Fuel Pump Relay and
see where the FSM would have us look.

Here's a close up on the wiring diagram between the ECM B fuse and the Fuel Pump Relay:

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Given that the other loads on the ECM B fuse are working, I'm pretty sure that there is power at connection 'G12'. (Output side of the fuse.)
But in order to be 100% sure it would only take a second or two to pull the fuse & verify the power with a voltmeter.

Assuming that power is good at G12, then the problem description tightens to an open in the orange wire connected to the output of the ECM B
fuse in the Underhood Fuse Block to Splice S101 out in the wiring harness, and from there back into the Underhood Fuse block where it's
connected to pin #87 of the Fuel Pump Relay.

Stating it as concisely as possible, we need to find & fix the open in the wiring between G12 and B1 in the fuse block -- and in order to do so,
we need to go out onto the wiring harness. Including the location of splice S101:

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Translating FSM metric-speak for us Yanks, S101 is ~5 inches away from the EBCM (Electronic Brake Control Module)
wiring harness breakout.

And here's the final piece of the puzzle -- the component location view for all involved:

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To summarize, at a casual glance it looks like the missing power problem is entirely within the fuse block itself?

But a closer look at the wiring diagram (that includes interpreting the significance of the dashed lines
denoting entering/leaving an electronic box or fuse block) ...your missing power on your orange wire
is between 2 pins inside the fuse block, but the drawing shows that the power wire goes outside of the fuse
block, fans out at splice S101, and then one of the orange wires from that fanout comes back into the
fuse block and connects to the Fuel Pump Relay socket.

Looking forward to you finding/fixing this missing power problem and
reporting back with what you discovered.

Happy Hunting --

 

[Road Trip]

Stating it as concisely as possible, we need to find & fix the open in the wiring between G12 and B1 in the fuse block -- and in order to do so,
we need to go out onto the wiring harness. Including the location of splice S101:

Rereading this, I feel good about the diagnosis, but at the same I felt bad that even the best FSM illustration I
could find for the pinouts in the Underhood Fuse Block can be extremely difficult to read. (For example, pin G12)

I've looked high and low across the interwebs for a better illustration, and in one of electronics parts distributors
web sites I found this Delphi layout for the Underhood Fuse block. I took the liberty of highlighting/completing the
alpha-numeric identification per what I had from the FSM:

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Now if you are looking for pins G12 or B1, between the above and the page from the FSM (see attached) you should
now be able to successfully locate the corrent physical position. (And if this seems a bit intimidating, just drag out
your old Battleship game and practice your 'letter-number coordinate' skills for a game or two. (Battleship Instructions)

By the time you complete a couple of games this will be a piece of cake. Who knew that playing Battleship in your youth
was building valuable GMT400 fuse block navigation skills? :0)

This is my last post on all the wiring stuff. My next reply will focus on the O2 sensor results you posted.

Hope this proves helpful.

 

[Zimmerly]

Wow, thank you for all your help, Road Trip. If only everyone was as fortunate to have as much help as you have given me. I truly appreciate the lengths you've gone though to troubleshoot and document all of this for myself and others to learn from.
To address your posts-
I'm right with ya every step of the way and understand fully. I've been referencing those same schematics that list the S101 junction, but was unable to pinpoint where exactly the connection is... until now. I kept looking back at this one, scratching my head and thinking "there HAS to be more detail on S101, and where exactly it is.":

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Everything in my mind came together when you found this:

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I'm anxious to hunt this connection down. Knowing the region of this wire location gives me a lot of relief.
Thing is, I came home from work sick today, for the first time in years. Some sort of stomach bug. I'm hoping I have enough energy and motivation later on to go out and do some wire hunting. Having the fuel pump/sender wiring put back together will be a big win for the week in my book. The o2 sensors should be en route to be delivered any day now. I'm crossing my fingers, in hopes that new sensors will either fix or help fix this lean/misfire issue. I'm not holding my breath though, because I wouldn't be surprised if I have to replace my intake gaskets after all when the rest is taken care of...
But progress is progess, and you have been such a big help in this. Thanks again!

-Jacob