Runs Out of Fuel When Gauge Reads 1/4 Tank

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Document ID# 1645676
2004 Cadillac CTS
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Runs Out of Fuel When Gauge Reads 1/4 Tank keywords 3.6L 4.6L 5.7L connector DIC display
DTC electrical fluctuation inaccurate LH2 LS6
LY7 low MIL module P0445 P0453 P0461 P0463
P1172 P2066 P2068 #PIC3094A - (Apr 11, 2005)
Table 1:
Sender Tech 2
Readings
Table 2: Fuel gauge function
Runs Out of Fuel When Gauge Reads 1/4 Tank
2004-2005 Cadillac CTS
2004-2005 Cadillac SRX
2005 Cadillac STS
The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in the PI.
Condition/Concern:
Customers may experience an out-of-fuel condition when the gauge reads about 1/4 tank. If the vehicle
is not in a failed condition, it may be necessary to run the vehicle out of fuel to duplicate the concern.
Observe the Left Sender voltage reading on the Tech2 as the fuel level decreases - the sender should
decrease to 0.73 V as the LH (secondary) reservoir is depleted. If the sender does not go down to 0.73V,
the ECM does not recognize a change of state, and the gauge may not reflect the last quarter tank of fuel
as it is consumed. If the LH sender does not go down to 0.73V and the right sender decreases to 0.73 as
the vehicle runs out of fuel, there are two known conditions:
1. Poor fuel transfer from the LH secondary side to the RH primary side. This failure mode can be
identified by the presence of fuel in the left reservoir when the tank is removed. The condition is
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well-documented in bulletin 04-06-04-077 and is repaired by replacement of the primary fuel
module.
2. LH tank reservoir bottom bowed up, preventing the LH sender from registering below 0.74 Volts
as the fuel is consumed. This prevents the ECM from recognizing an empty left reservoir, and no
change of state occurs. This failure mode is identified by no fuel in the secondary (left) reservoir
and is repaired by replacement of the fuel tank.
Recommendation/Instructions:
The chart below identifies the Tech2 sender voltage readings at various positions. The data was taken
from a sample 2004 CTS. The second chart outlines the function of the change of states and gauge
function.
Fuel Gauge Reading LH Secondary Volts RH Primary Volts
Fuel Remaining gallons State
Full
2.53
2.53
16.9
1
3/4
1.75
2.53
12.5
1
1/2
0.73
2.53
7.7
2
1/4
0.73
2.20
5.1
4
1/8
0.73
1.78
3.5
4
Empty
0.73
0.94
1.9
4
Low Fuel DIC
0.73
0.73
1.7
4
Out of Fuel
0.73
0.73
1.0
4
Note: Directions from driver's seat
Primary side is the filler-neck side
Sender sweep is 40 to 250 ohms for both senders:
(40 ohms @ 0.73 V, 250 ohms @ 2.53 V)
Stage 1
LH Secondary Sender
RH Primary Sender
Gauge input
Full or readable range 2.53 - 0.74 V
Full 2.53 V
Reads Secondary 1st half
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Stage 2
Empty < 0.73 V
Full 2.53 V
Calculated gauge position
Stage 3
Readable Range
Readable Range
Spillover - takes last known
Stage 4
Empty < 0.73V
Readable Range
Reads Primary last quarter
Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is
resolved without completing every step, the remaining steps do not need to be performed.
GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They
are written to inform these technicians of conditions that may occur on some vehicles, or to
provide information that could assist in the proper service of a vehicle. Properly trained
technicians have the equipment, tools, safety instructions, and know-how to do a job properly
and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle,
or that your vehicle will have that condition. See your GM dealer for information on whether
your vehicle may benefit from the information.
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Document ID# 1645676
2004 Cadillac CTS
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