[umbertob]

I just bought a 2007 RR sport and my fuel light is on with 3/4 tank left.

So the needle on the fuel gage indicates you have 3/4 of a tank but the low fuel lamp is on? It's got to be some sort of malfunction in the instrument cluster pack software... I ran out of gas a few times before - as my dear, dear friend goosey points out - so I was interested in finding out how the cluster calculates and displays how much gas is left in the tank. This is the description of the fuel gage system operation, straight off the service manual:

Fuel Level Gage

The fuel level gage displays the fuel tank contents. When the ignition is off, the pointer returns to the empty position.

The instrument pack is connected to two tank level sensors. One sensor is located at the front of the fuel tank and the other is located at the rear. Each sensor uses a float operated MAgnetic Passive Position Sensor (MAPPS) for measuring the fuel tank contents. For additional information, refer to Fuel Tank and Lines (310-01A Fuel Tank and Lines - 4.2L)
For additional information, refer to Fuel Tank and Lines (310-01B Fuel Tank and Lines - 4.4L)

Each sensor is supplied with a reference current from the instrument cluster. The instrument cluster measures the returned output from each sensor which is proportional to the amount of fuel in the tank and the position of the float arm. The inclination of the vehicle, derived from a CAN message from the Electronic Park Brake (EPB) module, is also used to calculate the fuel tank contents. The instrument cluster uses the fuel level sensor signals and the EPB module inclination signals to accurately calculate the volume of fuel in the tank and display this on the fuel level gage. The instrument cluster monitors the signals and updates the fuel level gage pointer position at approximately 20 second intervals. This prevents the pointer moving continually due to fuel movement in the tank due to cornering or braking. When the ignition is off, the pointer needle returns to the empty position.

A warning lamp is incorporated into the fuel level gage and illuminates when the fuel tank volume is at or below 12.5 liters (2.75 Gallons) on petrol models and 14 liters (3.69 Gallons) on diesel models. This is accompanied by a chime from the instrument cluster and a message alert the driver to the low fuel condition.

The fuel tank content is converted into a CAN signal by the instrument cluster and transmitted on the CAN. This is received by Fuel Fired Booster Heater (FFBH) control module (if fitted) to suspend operation of the FFBH on diesel variants when the fuel tank content reaches 14 liters or less.

The following table shows the fuel tank fuel quantity and the respective pointer positions.

Fuel Tank Quantity (Liters) Needle Pointer Position
4.91 Empty
12.5 Low fuel (low fuel lamp illuminated - Petrol engines)
14 Low fuel (low fuel lamp illuminated - Diesel engine)
25.6 Quarter full
45.8 Half full
67 Three quarter full
81.05 Full

 

[umbertob]

Kinda like this?

That's the Tire Pressure Monitoring System warning light. Normally, when that amber icon illuminates there is an accompanying message appearing in the message center below (look for an "i" icon while you are driving) to identify the problem, like a TPMS fault, a tire low on air, etc. The low fuel LED is on the bottom of the fuel gage.

 

[umbertob]

You guys saw this thread on the General Chatter, I assume? viewtopic.php?f=9&t=34199 Gotta love that OGC logo... Maybe we should move the TPMS icon there, it does look like that from a distance.

 

[umbertob]

Rear tires should be 42, front ones 38 (cold PSI.) Also, the warning light may not extinguish right away, you may need to roll around a bit before the module reads the new pressure from the sensors in each wheel.