Early 03 fuel Sending unit wiring/issues
Thread Starter
Le Mans Master
Joined: Jun 2016
Posts: 5,426
Likes: 3,971
From: Cincinnati, OH
Early 03 fuel Sending unit wiring/issues
I just got done doing a Racetronix fuel pump and wiring harness. I converted to E85.
0411 P59 PCM (I was using a P01 PCM as stock early 2003 C5)
"truck" OS loaded for E85 capability
Racetronix 340 pump kit
After doing the swap, my fuel gauge only reads 1/2 full. I went to the gas station and the car won't take any gas. So I suspect it is full but the gauge is only showing 1/2 since it instantly clicks the pump off of 3 different gas stations. It will puke some fuel filler as if it's full. I've NEVER had filling issues due to filler neck/pressure issues in the past. I have no codes in the DIC and shows 73mi to empty.
Does the sending unit need to be wired a particular way? Will wiring it "backwards" cause it not to work? I have found equal sources saying it doesn't matter and that it does matter.
If the sending unit is dirty or bad, I suppose I need to get into the DS tank regardless...
I remember draining 8 gal of gas out before pump swap, putting 8 gal back in. Driving maybe 20mi total going home then back to the shop, added 1 gal E85 to verify sensor worked. We did 5-6 dyno pulls, added 4 more gal E85 and then added another 8 gal E85. So I estimate it's very close to being full on a "19 gal tank". Which makes me believe it's not filler neck issues, it's sensor issues.
Tomorrow some Techron goes in fingers crossed.
0411 P59 PCM (I was using a P01 PCM as stock early 2003 C5)
"truck" OS loaded for E85 capability
Racetronix 340 pump kit
After doing the swap, my fuel gauge only reads 1/2 full. I went to the gas station and the car won't take any gas. So I suspect it is full but the gauge is only showing 1/2 since it instantly clicks the pump off of 3 different gas stations. It will puke some fuel filler as if it's full. I've NEVER had filling issues due to filler neck/pressure issues in the past. I have no codes in the DIC and shows 73mi to empty.
Does the sending unit need to be wired a particular way? Will wiring it "backwards" cause it not to work? I have found equal sources saying it doesn't matter and that it does matter.
If the sending unit is dirty or bad, I suppose I need to get into the DS tank regardless...
I remember draining 8 gal of gas out before pump swap, putting 8 gal back in. Driving maybe 20mi total going home then back to the shop, added 1 gal E85 to verify sensor worked. We did 5-6 dyno pulls, added 4 more gal E85 and then added another 8 gal E85. So I estimate it's very close to being full on a "19 gal tank". Which makes me believe it's not filler neck issues, it's sensor issues.
Tomorrow some Techron goes in fingers crossed.
Last edited by smitty2919; Jul 5, 2023 at 10:45 PM.
Thread Starter
Le Mans Master
Joined: Jun 2016
Posts: 5,426
Likes: 3,971
From: Cincinnati, OH
I came across this on the forums:
The C5 Corvette has two fuel tanks, left and right hand. The left hand tank contains the electric fuel pump that supplies fuel to the engine. The right hand tank has a pump, which transfers fuel from the right tank to the left tank. The pump in the right hand tank has no moving parts and is not really a pump. It is a siphon device that uses fuel pressure from the left hand pump to start a siphon that transfers fuel from the right hand tank to the left hand tank. In order to create this siphon, the fuel line that goes to the engine splits and a small amount of pressure is routed to the right hand tank to start the siphoning. The fuel that is being siphoned is routed through another hose to the left hand tank. If the left hand tank is full, the fuel transfers back to the right hand tank through the large filler tube located at the top of the tanks.
The idea is to always keep the left hand tank full whenever there is fuel in the right hand tank. When the fuel gauge reaches a half tank, the right hand tank is empty and the left hand tank is full. Both tanks have a float and sender that measures the fuel level in the tank. These signals go to the Powertrain Control Module (PCM) which interprets the information and sends a signal to the Body Control Module (BCM) which then sends a signal to the fuel gauge in the Instrument Cluster.
Because of the variation in tanks and sending units, the PCM doesn't always read the actual fuel level in the tank. Above or below preset voltage limits, the fuel level is estimated. The voltage limits for each sending unit, right and left, are set to represent "full" and "empty." When one or both tanks have exceeded the preset limits, either "full" or "empty," the system is estimating the actual fuel level. This happens when both tanks are full, gauge is "full;" when the right tank is empty and the left tank is full, "half" on the gauge, or when both tanks are empty, gauge is at "empty."
This estimating strategy and the physical layout of the system results in an unusual fuel gauge characteristic that may be noticeable to some people. When the fuel level is at or near half tank, the right hand tank is empty and the left hand tank is full, so the system is estimating the actual fuel level. Because of the siphoning system, when the car sets without running, the fuel level equalizes in the two tanks. When the car is first started, the system is actually reading the fuel level in both the right and left hand tanks. After running for several minutes, the fuel in the right hand tank has been transferred to the left hand tank, leaving the right hand tank empty and the left hand tank full. The change in the fuel levels results in the system changing from reading the actual fuel level to estimating the fuel level. This change results in the fuel gauge reading actually indicating more fuel than when the car was first started.
One of the common concerns with the Corvette is the fuel gauge goes to "empty" intermittently. First, let's discuss why this happens. As previously mentioned, the system estimates the fuel level at certain times. When the signal from the left hand sender is above the preset voltage limit, the system estimates the left hand tank fuel level to be full. When the signal from the right hand sender is below the preset voltage limit, the computer estimates the right hand tank fuel level to be empty.
In this situation, left tank full and right tank empty, the computer estimates the fuel level and sets that gauge at half tank. Once the fuel level in the left hand tank goes below the preset voltage limit, less than full, the computer begins reading the actual fuel level and setting the gauge accordingly. At this time the computer expects to see the voltage signal from the right hand tank stay below the preset limit that indicates empty. The problem comes from the fact that the signal from the right hand tank does not stay below the preset limit. When the computer sees the right hand signal voltage exceed the preset empty limit, it assumes that there is a problem in the fuel transfer system, which is a possibility. When this condition exists, the computer software turns on the Check Gauges light and commands the fuel gauge to the empty position. The logic for this is, if there were a problem with the fuel transfer system and fuel was not being transferred from the right hand tank into the left hand tank, you could have the gauge indicate half tank, when then the left hand tank would be empty and the right hand tank would be full. In this situation you would run out of gas and be walking.
On some Corvettes the situation that is occurring now is the result of the right hand sending unit being attacked by compounds within reformulated gasoline. This condition manifests itself as an erratic voltage signal from the right hand sending unit. In the above scenario, left tank full and right tank empty, gauge at half, the computer looks for the voltage signal from the right hand tank to stay below a preset level. The erratic voltage signal caused by the reformulated gasoline causes the computer to think there is fuel in the right hand tank when there is not. When this happens the computer software turns on the Check Gauges light and commands the fuel gauge to empty. GM has issued a revised computer software, for 1999 through early 2002 models, that raises the right hand preset voltage empty limit to prevent the erratic voltage signal from causing this situation. While it has proven to be an effective solution, it has not corrected the concern in all instances.
Another possible resolution is to use Techron fuel system cleaner. This has also worked in many, but not all cases.
So it seems reading 1/2 full on the gauge with no CEL would suggest the left DS tank sending unit is working but the right PS tank is not reading correctly (showing empty or float stuck down) when in fact both tanks are full.
The C5 Corvette has two fuel tanks, left and right hand. The left hand tank contains the electric fuel pump that supplies fuel to the engine. The right hand tank has a pump, which transfers fuel from the right tank to the left tank. The pump in the right hand tank has no moving parts and is not really a pump. It is a siphon device that uses fuel pressure from the left hand pump to start a siphon that transfers fuel from the right hand tank to the left hand tank. In order to create this siphon, the fuel line that goes to the engine splits and a small amount of pressure is routed to the right hand tank to start the siphoning. The fuel that is being siphoned is routed through another hose to the left hand tank. If the left hand tank is full, the fuel transfers back to the right hand tank through the large filler tube located at the top of the tanks.
The idea is to always keep the left hand tank full whenever there is fuel in the right hand tank. When the fuel gauge reaches a half tank, the right hand tank is empty and the left hand tank is full. Both tanks have a float and sender that measures the fuel level in the tank. These signals go to the Powertrain Control Module (PCM) which interprets the information and sends a signal to the Body Control Module (BCM) which then sends a signal to the fuel gauge in the Instrument Cluster.
Because of the variation in tanks and sending units, the PCM doesn't always read the actual fuel level in the tank. Above or below preset voltage limits, the fuel level is estimated. The voltage limits for each sending unit, right and left, are set to represent "full" and "empty." When one or both tanks have exceeded the preset limits, either "full" or "empty," the system is estimating the actual fuel level. This happens when both tanks are full, gauge is "full;" when the right tank is empty and the left tank is full, "half" on the gauge, or when both tanks are empty, gauge is at "empty."
This estimating strategy and the physical layout of the system results in an unusual fuel gauge characteristic that may be noticeable to some people. When the fuel level is at or near half tank, the right hand tank is empty and the left hand tank is full, so the system is estimating the actual fuel level. Because of the siphoning system, when the car sets without running, the fuel level equalizes in the two tanks. When the car is first started, the system is actually reading the fuel level in both the right and left hand tanks. After running for several minutes, the fuel in the right hand tank has been transferred to the left hand tank, leaving the right hand tank empty and the left hand tank full. The change in the fuel levels results in the system changing from reading the actual fuel level to estimating the fuel level. This change results in the fuel gauge reading actually indicating more fuel than when the car was first started.
One of the common concerns with the Corvette is the fuel gauge goes to "empty" intermittently. First, let's discuss why this happens. As previously mentioned, the system estimates the fuel level at certain times. When the signal from the left hand sender is above the preset voltage limit, the system estimates the left hand tank fuel level to be full. When the signal from the right hand sender is below the preset voltage limit, the computer estimates the right hand tank fuel level to be empty.
In this situation, left tank full and right tank empty, the computer estimates the fuel level and sets that gauge at half tank. Once the fuel level in the left hand tank goes below the preset voltage limit, less than full, the computer begins reading the actual fuel level and setting the gauge accordingly. At this time the computer expects to see the voltage signal from the right hand tank stay below the preset limit that indicates empty. The problem comes from the fact that the signal from the right hand tank does not stay below the preset limit. When the computer sees the right hand signal voltage exceed the preset empty limit, it assumes that there is a problem in the fuel transfer system, which is a possibility. When this condition exists, the computer software turns on the Check Gauges light and commands the fuel gauge to the empty position. The logic for this is, if there were a problem with the fuel transfer system and fuel was not being transferred from the right hand tank into the left hand tank, you could have the gauge indicate half tank, when then the left hand tank would be empty and the right hand tank would be full. In this situation you would run out of gas and be walking.
On some Corvettes the situation that is occurring now is the result of the right hand sending unit being attacked by compounds within reformulated gasoline. This condition manifests itself as an erratic voltage signal from the right hand sending unit. In the above scenario, left tank full and right tank empty, gauge at half, the computer looks for the voltage signal from the right hand tank to stay below a preset level. The erratic voltage signal caused by the reformulated gasoline causes the computer to think there is fuel in the right hand tank when there is not. When this happens the computer software turns on the Check Gauges light and commands the fuel gauge to empty. GM has issued a revised computer software, for 1999 through early 2002 models, that raises the right hand preset voltage empty limit to prevent the erratic voltage signal from causing this situation. While it has proven to be an effective solution, it has not corrected the concern in all instances.
Another possible resolution is to use Techron fuel system cleaner. This has also worked in many, but not all cases.
So it seems reading 1/2 full on the gauge with no CEL would suggest the left DS tank sending unit is working but the right PS tank is not reading correctly (showing empty or float stuck down) when in fact both tanks are full.
Thread Starter
Le Mans Master
Joined: Jun 2016
Posts: 5,426
Likes: 3,971
From: Cincinnati, OH
Both sensor resistance seems to check out
DS is 247 ohms
PS is 253
I'm pretty confident the tanks have a good mount of fuel in them so the resistance values check out. Both sensors are getting power and the ground circuits check out OK.
If I unplug the PS tank only, dash gauge still reads 1/2 tank, no fuel warning present.
If I unplug DS tank only, dash gauge reads Empty and "low fuel" message appears.
Both unplugged, gauge goes to Empty and "low fuel" message appears
Almost as if the PS tank is not being recognized by the PCM.
DS is 247 ohms
PS is 253
I'm pretty confident the tanks have a good mount of fuel in them so the resistance values check out. Both sensors are getting power and the ground circuits check out OK.
If I unplug the PS tank only, dash gauge still reads 1/2 tank, no fuel warning present.
If I unplug DS tank only, dash gauge reads Empty and "low fuel" message appears.
Both unplugged, gauge goes to Empty and "low fuel" message appears
Almost as if the PS tank is not being recognized by the PCM.
Race Director
Joined: Oct 2004
Posts: 12,864
Likes: 4,667
Thread Starter
Le Mans Master
Joined: Jun 2016
Posts: 5,426
Likes: 3,971
From: Cincinnati, OH
Yes. More info here:
https://www.corvetteforum.com/forums...uel-on-c5.html
Specifically two OS have been confirmed to work 12587603 and 12592425. I can't 100% say that if you find a 04Z VIN, it will have one of those two OS's
https://www.corvetteforum.com/forums...uel-on-c5.html
Specifically two OS have been confirmed to work 12587603 and 12592425. I can't 100% say that if you find a 04Z VIN, it will have one of those two OS's
