Guys,
I am in the proccess of hooking up my Z's fuel level sensor to the C5's cluster. I need to know the impedance/resistance of the stock C5 fuel level sensors. The C5 has 2 sensors 1 on the left tank and 1 on the right tank. I spent 3 hours lastnight to play with it with a bunch of resistors but I couldn't get it to work consistently. Any help would be greatly appreciate. I am very close to starting this thing up.

 

this is what the sensor sweep is, the same for both sides.....but don't forget, this info is fed directly to the ECM. The ECM controls what goes to the cluster.

5.1. Empty stop = 38.5-41.5 ohms.

5.2. Full stop = 247-253 ohms.

 

This is from the Corvette Service Information regarding fuel gauge diagnostics




Did you perform the Diagnostic System Check - Vehicle?
--
Go to Step 2
Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

2
Turn ON the ignition, with the engine OFF.
Install a scan tool.
With the scan tool, perform the Displays Test in the instrument panel cluster (IPC) Special Functions Output Controls list.
Does the gage move from 0 to its maximum value and back to 0 when you perform the Displays Test?
--
Go to Step 3
Go to Step 16

3
Disconnect C420, the fuel level sensor.
Connect the J 33431-C Signal Generator and Instrument Panel Tester between the signal circuit of the Fuel Level Sensor Signal Primary (left) and the low reference circuit of the fuel level sensors on the male terminal side.
Turn ON the ignition, with the engine OFF.
Important: Verify the J 33431-C resistance settings with a DMM.


Vary the resistance on the J 33431-C from 40-250 ohms.
With the scan tool, observe the Fuel Level Sensor Right Voltage parameter in engine control module (ECM) evaporative emission (EVAP) Data list.
Refer to Fuel Level Specifications in order to convert from resistance to Fuel Level Sensor Right voltage.
Does the Fuel Level Sensor Right voltage parameter change smoothly across the specified range?
0.7-2.5 V
Go to Step 4
Go to Step 10

4
Connect the J 33431-C between the signal circuit of the fuel level sensor signal secondary (right) and the low reference circuit of the fuel level sensors on the male terminal side.
Turn ON the ignition, with the engine OFF.
Important: Verify the J 33431-C resistance settings with a DMM.


Vary the resistance on the J 33431-C from 40-250 ohms.
With the scan tool, observe the Fuel Level Sensor Left Voltage parameter in ECM EVAP Data list.
Refer to Fuel Level Specifications in order to convert from resistance to Fuel Level Sensor Left voltage.
Does the Fuel Level Sensor Left voltage parameter change smoothly across the specified range?

 

Hi Oscar,
You are awsome. Thanks for the info. I understand the fuel sensors are sending signal to the PCM and the PCM sends it to the cluster. At this point my fuel sensor wires are not connect to anything. I will have to find ways to convert my stock sending unit to send out the correct signal 40-250 ohm. Thanks again.
Vinh

Edit: Oscar, There are 2 sensors but 1 gauge. How does the car knows which sensor to use to diplay the info? I plan to use one sensor only. What do I know with the sensor wire from the other tank?

 

The ECM uses info from both sensors to arrrive at a correct output signal. It crunches the input numbers with it's programmed calibration. That's why a "reflash" was used on some early C5's to correct a gas gauge issue, just to change the ECM parameters. I'm not sure how you would utilize one sender unless you figure out what signal would be usable at the IPC end of things and just bypass the ECM. Possibly you could "fool" the ECM by giving it the same signal for both inputs.

 

Upon further review......it looks like the IPC uses info from the ECM transmitted over the class 2 serial data, the IPC then pulls it's required info from that. Back to the drawing board!

 

Oscar,
Thanks for the drawing but unfortunately I couldn't read it. Time for me to bug my coworker again for the C5 manuals. I have a bigger problem with this fuel sensor and that is my Z's sending unit is opposite with the C5. The C5, according to you, 40 Ohm = Empty and 250 Ohm= Full. On the Z 90 = Empty and 8 = Full. I am screwed. Please let me know if you have any thought on how to inverted the signal. Thanks again for the help.
Vinh

 

Seems like you are going to need a special circuit to invert and scale the signal. One of these links may help:

http://data-acquisition.globalspec.c...nal_Converters
http://data-acquisition.globalspec.c...nal_converters

 

Frizzlefly,
Thanks for the helpful links. Yes all I need is a DC to DC converter.

On another note about how to nullify the 2nd tank's signal. I just thought about it lastnight and I think all I have to do is hook up a 40 Ohms resistor to the right tank's input and that should nullified it and I should be able to use just the left tank's signal.

 

You have to use both sensors. The way the fuel system works with the two tanks is like this:

When the car is off both tanks sit at the same level. When the car is started the fuel is sucked from the left tank and fuel is pumped from the right to the left tank. The PCM will see a full left tank and a lowering right tank signal. Then as you cross half full the right tank signal will be low and the left will lower.

So for example if you had a half full tank then the left tank would be full and the right tank would be empty. When the PCM sees the right signal lower than a specified level and the left tank higher than a specified level, then it sends the half full signal to the IPC.

I don't see any way to duplicate this with one sensor unless you built some kind of converter box probably with a PIC in it to replicate the dual signal.


Here is some more info I pulled from the service manual.

Fuel Gage
The IPC displays the fuel level as determined by the PCM. The IPC receives a class 2 message from the PCM indicating the fuel level percent. The fuel gage defaults to empty if:

The PCM detects a malfunction in the fuel level sensor circuit.
The IPC displays a low fuel message on the DIC.
The check gages indicator turns on.


System Overview




(1) Fuel Feed Pipe
(2) Auxiliary Fuel Feed Rear Pipe (left tank to jet pump)
(3) Auxiliary Fuel Return Rear Pipe (jet pump to left tank)
(4) Fuel Feed Pipe Check Valve
(5) Fuel Feed Pipe Tee with Orifice
(6) Fuel Feed Rear Pipe (to fuel filter/fuel pressure regulator and siphon jet pump)
(7) Fuel Return Rear Pipe
(8) Auxiliary Fuel Return Rear Pipe (jet pump to left tank)
(9) Fuel Filter/Fuel Pressure Regulator

2 fuel tanks store the fuel supply. An electric fuel pump attaches to the fuel sender assembly inside the left fuel tank. The fuel pump pumps fuel through the fuel feed pipe (6) and an in-line fuel filter (9) to the fuel rail. The rear fuel feed pipe (6) has an integral check valve (4) in order to maintain the fuel system pressure in the feed pipe. The pump provides the fuel at a pressure greater than what is needed by the fuel injectors. The fuel pressure regulator, part of the fuel filter (9), keeps the fuel available to the injectors at a regulated pressure. A fuel return pipe (7) returns the unused fuel to the left fuel tank. The fuel pump also feeds the fuel through a tee (5) with an orifice in the fuel feed rear pipe (6) and through the auxiliary fuel feed rear pipe (2) in order to supply the siphon jet pump inside the right fuel tank. The siphon jet pump transfers the fuel from the right fuel tank to the left fuel tank through the auxiliary fuel return rear pipe (3, 8).

 

70 LS1,
wow..that is some great info. Thank you very much. I need your help one more time. What type of signal does the PCM sends to the IPC for the fuel gauge? I am thinking of bypassing the PCM for this if I have no other ways to get around it. THanks again.
Vinh

I played with it some more this morning and no luck again. I thought if I put a 40Ohms resistor to the right tank then it will read the left tank only. But it still show empty or half tank that's it

 

I'm thinking your best bet is to bypass the PCM, and convert the signal. You could provide power to the gauge and vary the impedance to ground to simulate the level sensor. You should be able to figure out what range it's looking for. You do need to know the pin-out though.

 

Yup.. Monday I will get the C5 manuals from my coworker again to check out the wiring diaghram to the cluster and then I will attempt to bypass the PCM's function for this. Thanks for the replies and advices.
Vinh

 

You can't bypass the PCM unless you can get your hands on the proprietary GM code. And even if you could you wouldn't want to. The fuel level signal is a digital serial data signal and is passed to the IPC as a packet of information including tach signal, oil pressure, coolant temp, etc. Also all of the electronic modules in the car are connected by this serial data line so its not just engine info on the line. Information is passed between all of them from the door lock mechanisms to the ABS module to the BCM to the... You get the idea. This is the same line that runs to the Data Link Connector that the Tech II or other scan tools connect to.

You have to replicate the sending unit signals for the fuel meter to work.

Hope this helps.

 

Where did you get the IPC and how much was it?

 

Ahhh... Didn't know it was serial to the IPC. I wonder what actually drives the individual gauge though? Must be analog DC. Probably a D-to-A converter in the IPC sending this to the gauge. If you could figure that out, what drives the gauge, you should be able bypass the PCM and wire straight to it. The other gauges can still be driven by the serial data.

 

From reading the manuals I know for a fact that the VATs system uses the serial line to communicate with the PCM for the fuel. Other than that I didn't know the fuel sending level sender is using the serial data line too. That's sucks. I know the fuel level thing will not affect how the car running so if I don't get it working oh well. But I have a mentallity nothing is impossible so ...I will give it a try.

 

I'm using a vette IPC in my swap... I'll post some info in a day or so on my findings. Here is some other info too... IPC Stuff

 

Get rid of VATS on the PCM and you don't have to worry about the fuel cutoff. Only thing I'm still trying to determine is if we need the BCM for the serial data line and the "Wake up" signal it provides to the IPC. I'm looking into it now

 

It looks like each tank uses the 40-250 ohm sensor. You could probably build a box that senses converts your single in tank sensor to provide the two outputs to the PCM. You just need it to be 250 on each line when full, 250 on the left and 40 on the right when half and 40 on each when empty.

You should also know you are going to have multiple error messages on the text display as well as some of the idiot lights on all the time. You might be able to remove the bulbs for those.

This topic is rekindling my interest in using the cluster.