So as many GRM swaps go, I'm into seemingly uncharted territory... I'm putting an older gas tank into a newer vehicle.
tank is from like an '87-'89 TBI Suburban. It has a simple 3 wire fuel sending unit with in tank pump.
purple = level sender, black = chassis ground, grey = 12V power.
The truck however is newer, being a 2003 Silverado 3500, it uses a 4 wire harness on its factory tank.
now in terms of making it run, it seems pretty straightforward, black = chassis ground for pump, grey = 12V pump power, and purple = level sensor... but the black with white stripe is a "low reference" which I understand is a ground, but is sourced back from the pcm.
I'm still a few days off just hooking up to see what happens, but why would they isolate the level signal ground to be sourced from PCM vs. the simpler way it used to be on older vehicles? If I just put that to chassis ground will the PCM get mad? As an aside, I also know the level signal resistance on purple is different from those model years, but I see they make some adapter boxes to compensate for that so gauges work correctly.
The reason for the dedicated ground wire back to the PCM is that it eliminates the possibility of differing voltage drops to ground that the old simpler way was susceptible to:
https://www.haltech.com/news-events/ecu-grounding-the-dos-and-donts/
You might get away with just putting it to chassis ground, or it might throw the fuel level reading off...
If the two tank senders both operate the same way it might work. Go back a few more years, and the signal is different, although I could be wrong. Older stuff, you are grounding the bimetal strip in the fuel gauge. Newer stuff, there is a 5volt reference from the ecu. One is not compatible with the other.
Does the wiring diagram show the tank sender signal wire going to the ecu, or the dash cluster?
Streetwiseguy said:
If the two tank senders both operate the same way it might work. Go back a few more years, and the signal is different, although I could be wrong. Older stuff, you are grounding the bimetal strip in the fuel gauge. Newer stuff, there is a 5volt reference from the ecu. One is not compatible with the other.
Does the wiring diagram show the tank sender signal wire going to the ecu, or the dash cluster?
Ah, good thought. I guess once I have a battery back in it I can check for 5V on it. I don't actually have a wiring diagram for it, so am flying a little bit blind, just finding what I can on the internet.
The fuel level sender is much more precise with its grounding because it doesn't go to a gauge, it goes to the PCM. It goes to the PCM because the PCM needs to know the fuel level in the tank so that it can accurately perform the evap leak tests. If the fuel level is higher or lower than the reading the PCM gets, that can look like a leak, or a purge flow issue, depending on which way it goes. So, it needs to be a lot more precise than the shoulder-shrug that a fuel gauge can be.
Pete. (l33t FS) said:
The fuel level sender is much more precise with its grounding because it doesn't go to a gauge, it goes to the PCM. It goes to the PCM because the PCM needs to know the fuel level in the tank so that it can accurately perform the evap leak tests. If the fuel level is higher or lower than the reading the PCM gets, that can look like a leak, or a purge flow issue, depending on which way it goes. So, it needs to be a lot more precise than the shoulder-shrug that a fuel gauge can be.
ah, that's good input. Thank you.
So I guess that begs the question... what's the workaround?
This sender level sensor reads 0(empty)-90(full)ohm... as did the rest of GM vehicles from ~'65-'97. The newer GM vehicles ~'98+ (like this truck is) use a 240(empty)-33(full)ohm range. If this sender only has chassis ground available, I wonder if I send that level sender signal to a conversion box like this one from Speedway Motors: Fuel Gauge Interface Module which can then send output of the correct range (in theory)... and feed that back to the PCM.
It'd be nice to actually have the EVAP system work, and a functional stock fuel gauge on the dash. Otherwise I imagine I'd have to resort to hacking up the sending unit in some fashion to adapt to the newer style signal level setup.
