Fuel Quantity Used

[jharden1]

Anyone know how the DIC measured fuel used? Is it calculated, mass flowed, volumed, throttle position?
Thanks in advance.

[h45wt]

Not sure about the fuel used but MPG is computed based upon throttle position and other data. When you use a power adder the throttle position is lessened so the DIC reports you get better MPG than you actually do.

[Kennedy]

Whithout getting into great detail, it's the time/pressure/orifice deal. The ECM knows that at "x" psi a pulse of say "y" microseconds delivers a fuel rate of "z". Then RPM, injection events per rev etc are used to calculate use versus miles driven.

[rob from bc canada]

It must be a pretty intricate algorithm (formula?) that they use.

The available range appears to respond rapidly when you go uphill, or coast, and also responds when you fill up the tank.

So the fuel remaining must be a calculation based on rate of consumption (for instant response), cross checked periodically with the actual amount remaining in the tank - to avoid showing an inaccurate range when nearing bottom of tank.

The average consumption responds quickly when first reset, and much more slowly when it has been running for a while.

Again, however, it does not appear to be a simple division of fuel consumed by distance travelled since last reset. The mpg reading seems to go up (or in my case litres per 100km goes down) too quickly on a downhill stretch, even after running for quite a while.

It appears to weight more heavily towards recent average as compared to overall average since reset.

[jharden1]

Whithout getting into great detail, it's the time/pressure/orifice deal. The ECM knows that at "x" psi a pulse of say "y" microseconds delivers a fuel rate of "z". Then RPM, injection events per rev etc are used to calculate use versus miles driven.

Thanks, I thought that might be the case. :-)

PS. I hope the family is well!

[FJ40INTOW]

I do know that BEFORE I had my injectors replaced my DIC would say that I used 18 gallons of fuel when the pump said I put in 22 gallons. After replacement it is usually maybe 1/2 gallon difference.

[Duramaster]

Here JHarden1:


This is pasted from the GM SI web site for 2003 Description and operation.

:) :)


DIC Fuel Information / Reset Capabilities
The available DIC Fuel Information and reset capabilities are as follows:

• Range - Can not be reset

• Avg. Fuel Economy - Can be reset

• Inst. Fuel Economy - Can not be reset

• Engine Oil Life - Can be reset


Range
The range display is the estimated distance that the vehicle can travel under current fuel economy and fuel level conditions.

This range is calculated from odometer information and class 2 messages.

Fuel Range is capable of displaying values from 0-999 mi or km.

Display appears as: RANGE: XXX MILES (English), RANGE: XXX km (Metric).

If the range is less than 40 miles, RANGE: LOW will be displayed.


Average Fuel Economy
The Average Fuel Economy (AFE) value is calculated by the cluster based on Trip Distance and Trip Fuel Used.

Pressing the Select button for longer than 1 second while the Average Fuel Economy is displayed in the DIC will reset to 0.0.

Average Fuel Economy is capable of displaying values from 0.0-99.9.

Display appears as: AVG ECON XX.X (English) MPG, AVG ECON XX.X L/100km (Metric).

Trip Distance and Trip Fuel Used accumulated values are stored in memory when the ignition is turned OFF.


Instantaneous Fuel Economy
Instantaneous Fuel Economy values are calculated by the cluster from odometer information and class 2 messages.

The cluster calculates instantaneous fuel economy based on distance and fuel.

Instantaneous fuel economy is capable of displaying values from 0.0 - 99.9.

Display appears as INST ECON XX.X (English), INST ECON XX.X L/100km (Metric).

Values for the Instantaneous fuel economy are not stored when the ignition is turned OFF.

[jharden1]

Thank you!

[rob from bc canada]

OK, now that brings up the real question :D

What are class 2 messages???

I keep wondering how the computer cross checks the calculated fuel consumption against actual volume remaining in the tank.

It obviously takes into account, because it knows when it gets filled up, and even though my actual consuption is off by 5% compared to what the DIC thinks, yet the range appears to remain accurate as the tank nears empty.

I've also wondered how the average consumtion displayed relates current mileage with average. Even after not resetting it for a long time, it seems to weight the current consumption higher thatn the average.

[Duramaster]

Does this help Rob?

:::::::::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::::::


Data Link Communications Description and Operation
Circuit Description
The data link connector (DLC) allows a scan tool to communicate with the class 2 serial data line. The serial data line is the means by which the microprocessor-controlled modules in the vehicle communicate with each other. Once the scan tool is connected to the class 2 serial data line through the DLC, the scan tool can be used to monitor each module for diagnostic purposes and to check for diagnostic trouble codes (DTCs). Class 2 serial data is transmitted on a single wire at an average of 10.4 kbps. This value is an average; class 2 uses a variable pulse width modulation to carry data and depending on the message it may operate faster or slower. The bus will float at a nominal 7 volts during normal operation. Each module can pull this lower during the transmission. The bus is not at battery positive voltage or ground potential during normal operation. When the ignition switch is in RUN, each module communicating on the class 2 serial data line sends a state of health (SOH) message every 2 seconds to ensure that the module is operating properly. When a module stops communicating on the class 2 serial data line, for example, if the module loses power or ground, the SOH message it normally sends on the data line every 2 seconds disappears. Other modules on the class 2 serial data line, which expect to receive that SOH message, detect its absence; those modules in turn set an internal DTC associated with the loss of SOH of the non-communicating module. The DTC is unique to the module which is not communicating; for example, when the inflatable restraint sensing and diagnostic module (SDM) SOH message disappears, several modules set DTC U1088. Note that a loss of serial data DTC does not normally represent a failure of the module that set it.

On some vehicles, if the powertrain control module (PCM) is unable to communicate with the vehicle theft deterrent (VTD) system after the vehicle has started, the PCM will consider the VTD system to be malfunctioning. The PCM will enter a fail enable state and will command the security indicator to illuminate. When the PCM is in a fail enable state, the vehicle will NOT stall or stop running. If the PCM is in a fail enable state when the ignition is switched OFF, the PCM will remain fail enable until communications with the VTD system has been restored. When the PCM is in a fail enable state, the VTD system is NOT active and the vehicle will start. This feature is NOT available on all GM vehicle lines.

Data Link Connector (DLC)
The data link connector (DLC) is a standardized 16 cavity connector. Connector design and location is dictated by an industry wide standard, and is required to provide the following:

Scan tool power battery positive voltage at terminal 16.
Scan tool power ground at terminal 4.
Class 2 serial data at terminal 2.
Common signal ground at terminal 5.
Class 2 Serial Data Line
The class 2 serial data line on this vehicle is a star configuration. The following modules communicate on the class 2 serial data line:

The body control module (BCM)
The driver information center (DIC)
The electronic brake control module (EBCM)
The HVAC control module
The inflatable restraint sensing and diagnostic module (SDM)
The instrument panel cluster (IPC)
The memory seat module (MSM) w/A45
The powertrain control module (PCM)
The rear wheel steering control module w/NYS
The transfer case shift control module w/4WD
The transmission control module (TCM) w/allison transmission
The vehicle interface unit (VIU) w/UX2
Entertainment and Comfort Serial Data Line
The entertainment and comfort serial data line does not communicate with the scan tool via the DLC. entertainment and comfort serial data line is used by the following modules to exchange information between the system components.

Radio
Remote Playback Device - CD Changer
Vehicle Interface Unit (VIU)
In order to diagnose the entertainment and comfort serial data line, refer to Diagnostic Starting Point - Entertainment in Entertainment.

[rob from bc canada]

Thanks for that - does it help - well, sort of. That tells me how the devices communicate.

I suppose some genius must have spent quite bit of time writing the various "algorithms" as they seem to be known in computer language, which actaully interpret the signals and derive the output messages to display.

In particular, I'm amazed how accurate the range calculation is, given the inherent inaccuracy and non-linear display of the fuel guage itself.

[jharden1]

I like to find out where all this happens on the gauge. I was finally able to manage them so I could observe the gauge and message. The message does come on at 40 miles range remaining, the fuel gauge light came on at 50 miles remaining. The range displayed moved up and down 2 or 3 miles according to driving condition, stopped in traffic vs driving.