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Help! Issues with fuel gauge on 1990

qblue90

Active member
Joined
Jun 2, 2008
Messages
43
Location
S.E. Michigan
Corvette
1990 Quasar Blue 6 spd,Z51,FX3,J55,GT7
Hi Guys!

My 1990 had a communications issue between the ECM and the CCM. After going through all the steps in the FSM ie. verifying wiring and grounds, we decided to replace the ECM. The replacement ECM fixed the communications issue, but immediately caused another to pop up with the fuel gauge. This was not a problem with the original ECM and started as soon as the new ECM was installed. By new ECM I mean new to me.. remanufactured.

The problem is this: once the fuel gets just below a half tank. Say 5 bars left showing and after I have driven about 5 miles, the bars will disappear one by one within a few minutes until the fuel gauge reads empty. Even though there is still 5 bars worth of fuel in the tank. If I shut the vehicle off and allow it to sit for an undetermined amount of time, the 5 bars will show back up on the gauge at start up. But they will disappear down to empty again when I drive for any short length. This happens with any amount of fuel below 1/2 tank and seems to be getting worse. It has even gone in the opposite direction - indicating a full tank- a couple times when it only had a 1/2 tank.

Has anyone else experienced this?

Does the ECM control the fuel gauge?

My thoughts are that the remanufactured ECM had this issue and it was not discovered and I need another ECM.

I would like to avoid having to check the sending unit if I could. Especially since my logic tells me that this was not happening with the original ECM and started immediately with the remanufactured ECM.

Any advise would be greatly appreciated.
 

ecss

Well-known member
Joined
Nov 27, 2004
Messages
649
Location
Windsor Mill, MD
Corvette
89 Brilliant Red Metallic
The Purple wire at the sender goes to the CCM.

The CCM sends the fuel level data which is displayed on the dash cluster LCD.
0 ohms = empty
around 50 ohms 1/2 tank
around 90 ohms Full

If you connect pin A to Pin G on the diagnostic connector the LCD will display will any error codes
for the CCM and ECM.

After that happens the display will show a 1.0
Press the Trip ODO button until you see 1.2
Then press the Eng/Met button and you will see 01 and fuel level (gallons . tenths)
Example 01 would be you are looking at parameter 01 and the data 5.7 would be 5.7 gallons

There is no mention the ECM can modify the fuel level displayed.

Verify what the CCM fuel level reading is.
Drive the car and when the problem happens.
Shut the engine off.
Short Pin A to Pin G
Turn the ignition On
Verify what the CCM fuel level reading is.
If it corresponds to the original reading something is telling the CCM to display the wrong fuel level even though the reading on the Purple wire is correct. If the dash display level has changed such as 1 gallon and the CCM reads 1 gallon I'd say the sender is bad or something causing the float to hang up on something such as the filter sock. Could also be the float has a pin hole leak causing weird readings.
 
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