Dash2 Fuel reading - PLEASE HELP

cdnav8r
Posts: 4
Joined: Sun Jun 08, 2008 12:50 pm

Dash2 Fuel reading - PLEASE HELP

Postby cdnav8r » Tue Sep 23, 2008 10:13 pm

Guys,
I have been fighting with the fuel stuff for months and do not seem to be able to get anywhere.

As I understand it the Dash 2 and DL1 can only accept incoming voltage which I can then create an equation for in order to end up with a desired
output. The problem I have is with the fuel level or more specifically the fuel sending unit. The fuel sending unit takes a resistance reading and sends that to the instrument cluster which displays the appropriate fuel level. I have established a table that matches the specific fuel level with 2-5 ohms being full, 25-40 ohms around half a tank, and around 100 ohms empty. All of this is with a constant .5mV voltage on the line.

I have been told that a "voltage divider" can correct this problem by allowing for the conversion of the resistance to a specific voltage allowing me to create a table that the Dash2 can read. I located a voltage divider that Race Technology Al thought would work but it turns out it can only output millivolts so we are back to square one.

Has anyone out there successfully dealt with this before or have any ideas on a solution? I just can't imagine with the number of vehicles that have resistance based fuel sending units that this has not been a problem before for someone with the Race Technology hardware.

Any help would be greatly appreciated!

pault
Posts: 64
Joined: Fri Nov 02, 2007 4:53 pm
Location: St Charles, IL

Solution

Postby pault » Wed Sep 24, 2008 7:39 pm

This should be straight forward. You need to connect the 5V DL1 supply through a resistor to the sender. The wire that goes to the sender also goes to the DL1. The resistance value will determine how much power it takes and the voltage range that the DL1 will see.

If you select a 100 ohm resistor, when full you will be taking 50mA out of the 5V supply and getting 0V at the DL1.

When half full you will be taking 33mA out of the supply and getting 1.66V at the DL1.

When empty you will be taking 25mA out of the supply and getting 2.5V at the DL1.

If you go with a 100 ohm resistor it should be at least a 1/2W device. Selecting a smaller series resistor will require more current from the supply and more power dissipation, but will give a a larger voltage swing.

The equation is V (@DL1) = 5 * R1 / (R1 + R2)

R1 is the current resistance of the sender, R2 is the series resistor value.

I don't know the power capability of the 5V supply on DL1, but it is in the specs.


Paul

faraday
Posts: 267
Joined: Sun Feb 25, 2007 1:18 am

Postby faraday » Thu Sep 25, 2008 1:28 am

:?: Perhaps our colleague hopes to have the original instrument cluster still hooked up to the sender.

That appears to "read" the current through the sender unit, by controlling the voltage.
In R-T land, we sense voltage; users need to understand that it is not the actual sensing element that is different, but the circuitry at the reading end.

I assume that it is not possible to build a circuit that allows both reading devices to be connected at once.
Therefore, the original fuel gauge can't be used as well as the R-T Dash (for reading fuel level), unless a dedicated sensor is fitted as well as the original. :(

Will an electronic expert please confirm this.

pault
Posts: 64
Joined: Fri Nov 02, 2007 4:53 pm
Location: St Charles, IL

Postby pault » Thu Sep 25, 2008 7:07 pm

I must have missed that he wanted to keep the instrument cluster still working. The important question for making both systems work is what type of gauge is in there or is it an integrated unit?

Standard needle gauges are current meters, more current, more needle deflection. They also tend to operate on the high side of the "sender". Which means that the battery is hooked up to one side of the meter and the other side of the meter is connected to the sender and the other end of the sender is grounded. In this case you have two options.

Option 1. You would need a small ohm (like 0.1) resistor in series between the gauge and the sender. The differential signal between both ends of this resistor must be amplified and level shifted before being connected to the DL1. So this is also possible, though not simple electronics.

Option 2. If you don't mind having to mentally recalibrate the reading you see on the fuel gauge (like half full now really means full), you can insert a 25 ohm resistor between the gauge and the sender and tap in between it and the sender. As this resistance value of this new resistance gets smaller the instrument cluster will be more accurate, but you may run into issues with power dissipation in the resistor. Also potentially having the DL1 not read accurately when the tank is empty as the voltage to the DL1 may exceed 12V (alternators run the system voltage above 12V). You also need to be sure that the DL1 input you are using is one of the 3 that support 12V signals (as opposed to the ones that only support 5V inputs).

If the instrument cluster is some digital arrangement, then the chances of it operating on current is just about zero. Analog-to-Digital voltage converters are very common, while I've never heard of an analog to digital current converter. So in this case you should be able to tap directly into the wire between the instrument cluster and the fuel sender.

If you measure the voltage on the wire going into the sender while filling the car up you can tell if the DL1 will work directly. If the voltage changes, then you should be alright. If the voltage doesn't vary then you need either option 1 or 2.

Paul

dstevens
Posts: 11
Joined: Wed Jul 18, 2007 9:03 pm

Postby dstevens » Sat Sep 27, 2008 3:55 am

If the original cluster is still connected then usually you can just read the voltage on the sensor output, assuming that the ground levels are the same.

A more detailed explaination is that the level sensor (like temperature sensors) has a variable resistance. To read this resistance, the dash/ecu pulls one side of the sensor high (usually to 5V) and grounds the other side of the sensor. The 'sense' line of the sensor then is between the pull up resistor and the sensor. This may be internal to the dash/ecu, but the 'sense' line will run to the sensor. The pull up resistor value is usually close to the mid level reading of the sensor in order to maximize resolution.

I have connected up fuel level sensors in this way to dataloggers without any problems. If the OP has tried this then the first step is to look at the factory wiring diagrams and try to see how the dash reads the sensor. I would expect one side of the sensor to be grounded and the other to run to the dash - which should produce a voltage we can directly read with the DL1 / dash2.


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