This will be long but im going present what ive found and hope someone can fill in the pieces im missing.
I have a 97 cheyanne. Originally a 4.3, previous owner swapped in a vortec 5.7. It ran fine, everything worked for couple years as it should,. I recently put 5.3 in and combined harnesses. Everything works but the fuel gauge but hang in there, this led to my finding things that don't make sense.
The wiring all looks original, so whomever did the swap, to 5.7, seem do it right. All wiring looks OE. I read 97 used a buffer module for the fuel gauge. So that has me thinking the obs harness and pcm shoukd be 96-97.
But in trying to figure out the fuel gauge issue, i began finding things that dont mesh, with the information i used to do the LS swap.
I used lt1swap for the obs pinouts to determine what I keep and remove.
Strangely enough, I ended up with three green wires, all marked unused on the pinout list, that turn out to go to the ac monetary switches. That doesn't bother me, as I've bypassed them to keep wiring simple.
The confusion arose, when I wanted figure out the fuel gauge, it's pegged past full nearly to 3 o'clock position. Which shoukd indicate ground issue.
So Following wire hatness from fuel pump/frame rail to a square 4 pin connector under the brake booster. It has 4 wires, Black/white goes to ground. Gray has power, green wire, goes nowhere, it went to obs pcm but under the pinout list. It shows that pin as unused. Then there's a purple/white wire. This wire goes to the buffer module behind glove box. At the buffer module, there's 3 purple/white wires. One, is the wire from the square firewall connector, going into the buffer module. Second wire went to obs pcm but based on pinout list, is unused. The third is the output, and this wire goes to the abs module, that seems odd.
Now bare in mind, all wiring looks untouched, so this is how it apparently was OE. But I can't find any diagrams explaining why this is so. It conflicts with what I'm finding. I would have thought, this purple/white wire would go to the c100 connector but no, abs module.
This is where things get stranger. I figured a fuel gauge wire would go to the c100 connector, id find it then back track. . So I looked it up, sure enough, fuel gauge gas C-2, I went to C-2, theres supposed be a purple wire for the fuel gauge. There's nothing there, just a factory rubber plug.
No other pinouts on the c100 indicate anything to do with the fuel gauge.
So that's a puzzler, even more confusing, as before I pulled the 5.7 and harness out, the gauge worked. Everything looks OEM, so I've no idea how the gauge was wired to work or how it went through the firewall or why all of a sudden it doesn't work. Of all things, whether or not fuel gauge worked, wasn't on my radar, doing the swap and its turned to be the only hangup.
I'd be fine bypassing the buffer. I did disconnect all wires at the buffer module and join the in/out purple/white wires but it didn't get it working. So I've since put it back to stock.
There's no identifying or apparent markings on the obs pcm. It's black. So my assumption was it was 96-97 but it may explain more if it were 98, although I looked at pinouts and c100 for both. I presume, and both show a purple gauge wire on the c100. Yet it's not there.
I guess the green wire at the 4 pin connector that goes nowhere, may need ground but from what I've read. That wire was for a tank pressure sensor.
For the record, when I wrote the list shown the pinout unused. It wasn't in terms of. A list to build a stand alone harness. It was a list of factory pinouts and wire color or whether that slot was empty.
I'm at a loss, any input appreviated.
I have a 97 cheyanne. Originally a 4.3, previous owner swapped in a vortec 5.7. It ran fine, everything worked for couple years as it should,. I recently put 5.3 in and combined harnesses. Everything works but the fuel gauge but hang in there, this led to my finding things that don't make sense.
The wiring all looks original, so whomever did the swap, to 5.7, seem do it right. All wiring looks OE. I read 97 used a buffer module for the fuel gauge. So that has me thinking the obs harness and pcm shoukd be 96-97.
But in trying to figure out the fuel gauge issue, i began finding things that dont mesh, with the information i used to do the LS swap.
I used lt1swap for the obs pinouts to determine what I keep and remove.
Strangely enough, I ended up with three green wires, all marked unused on the pinout list, that turn out to go to the ac monetary switches. That doesn't bother me, as I've bypassed them to keep wiring simple.
The confusion arose, when I wanted figure out the fuel gauge, it's pegged past full nearly to 3 o'clock position. Which shoukd indicate ground issue.
So Following wire hatness from fuel pump/frame rail to a square 4 pin connector under the brake booster. It has 4 wires, Black/white goes to ground. Gray has power, green wire, goes nowhere, it went to obs pcm but under the pinout list. It shows that pin as unused. Then there's a purple/white wire. This wire goes to the buffer module behind glove box. At the buffer module, there's 3 purple/white wires. One, is the wire from the square firewall connector, going into the buffer module. Second wire went to obs pcm but based on pinout list, is unused. The third is the output, and this wire goes to the abs module, that seems odd.
Now bare in mind, all wiring looks untouched, so this is how it apparently was OE. But I can't find any diagrams explaining why this is so. It conflicts with what I'm finding. I would have thought, this purple/white wire would go to the c100 connector but no, abs module.
This is where things get stranger. I figured a fuel gauge wire would go to the c100 connector, id find it then back track. . So I looked it up, sure enough, fuel gauge gas C-2, I went to C-2, theres supposed be a purple wire for the fuel gauge. There's nothing there, just a factory rubber plug.
No other pinouts on the c100 indicate anything to do with the fuel gauge.
So that's a puzzler, even more confusing, as before I pulled the 5.7 and harness out, the gauge worked. Everything looks OEM, so I've no idea how the gauge was wired to work or how it went through the firewall or why all of a sudden it doesn't work. Of all things, whether or not fuel gauge worked, wasn't on my radar, doing the swap and its turned to be the only hangup.
I'd be fine bypassing the buffer. I did disconnect all wires at the buffer module and join the in/out purple/white wires but it didn't get it working. So I've since put it back to stock.
There's no identifying or apparent markings on the obs pcm. It's black. So my assumption was it was 96-97 but it may explain more if it were 98, although I looked at pinouts and c100 for both. I presume, and both show a purple gauge wire on the c100. Yet it's not there.
I guess the green wire at the 4 pin connector that goes nowhere, may need ground but from what I've read. That wire was for a tank pressure sensor.
For the record, when I wrote the list shown the pinout unused. It wasn't in terms of. A list to build a stand alone harness. It was a list of factory pinouts and wire color or whether that slot was empty.
I'm at a loss, any input appreviated.
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