Fuel sender voltages?

[kevw]

I've started wiring in my Race Technology Dash2 (http://www.race-technology.com/dash2_2_31.html) and have a few questions (I'm gonna fiddle a bit more but maybe some of you will be able to answer some thoughts I've had while in work this morning).

The Dash2 has 4 analogue inputs that can be used for any sensors you want by configuring them via the PC software. I will be using these for fuel level, water temp, battery voltage and oil pressure.

I am fine with everything except the fuel level, I can gather stats about all the other sensors but it's not possible to vary the amount of fuel in the tank to gather stats about the fuel sender, especially with the cost of fuel at the moment lol. At this point I'm not willing to remove the fuel sender either. Does anyone have a fuel sender lying about that they could get me some info on???!

How does the fuel sender work? Does it vary voltage (potentiometer?) based on fuel in tank or does it vary resistance (rheostat?) based on fuel in tank?
If it varies voltage then all I need is some stats of what voltages equal to what fuel and I can program these in to the Dash2. If it varies resistance then I'm going to have to introduce some resistors and stuff to get a voltage reading as that's what the Dash2 is expecting.

Thanks in advance for any help :)

[Riggy]

i have a spare gsi fuel sender kev , ill bring it with me on sunday if you like :thumb:

[kevw]

OK, if I don't get it sorted by then I'll let you know, I'll bring a multimeter so we can get some measurements lol

Cheers dude

[Clean 2.0 nova]

As there is only 2 wires going to the sender it works as a variable resistor (or a potentiometer with the wiper tied to one leg).

I don't know what input is acceptable to your instruments but I assume its a high impedance input that 'reads' the varying analogue voltage input. I will also assume you can tell the instrument what voltage is a full tank (eg 8V) and what is an empty tank (eg 1V) as you tell me it is programmable from a pc. now a bit of ohms law coupled with the resistance range of the sender and your'e about there.

Right I've just measured a spare sender I have and when the tank is full the resistance is 40ohms, the tank empty = 280ohms. So if you used an in-line resistance of 80ohms (as that is the resistance of a standard instrument dial) you would get a voltage of 4.8V for a full tank and 11.2V for an empty tank (across the sender terminals) based on a 14.4V supply voltage.

As for the linearity of this then I measured very approximately 100 ohms for half a tank, giving 8V out - bang on half way between min+max voltage out. Using any other inline resistor than 80 ohms will lose your linearity.

I am an instrument engineer for one of my talents so I've probably gone completely overboard with the reply, but assuming you have a good feel for electronics I hope my input is of use.

Iain

[Riggy]

^^^ lol well thats sorted lol

[kevw]

I've probably gone completely overboard with the reply, but assuming you have a good feel for electronics I hope my input is of use.

Cheers Iain!

I think I understand what you mean, I'm OK with simple electronics, with your info I should be able to figure it out :)

For reference, the Dash2 can take upto 15V on any of the inputs, if you connect an input direct to a battery it will display the voltage and adjust to any change in voltage. From there you can "scale" it to display what you need based on the changing voltage. This requires really crazy equations, luckily for me you can enter values into a table and it creates the equation for you!

Thanks again

Kev.

[Clean 2.0 nova]

Thanks again

Kev.

Don't mention it.
It is fairly straight forward to make a circuit that will convert the 11.2V-4.8V signal to a, say 0-12V signal by using a 741 op amp or two. If you run into difficulties making it run from the 11.2-4.8V signal then give me a shout.
If you looked at voltage across the 80 ohm resistor a full tank would be 9.6V and an empty tank 3.2V (put the 80 ohm resistor at the negative side of the sender if you want this voltage with reference the chassis).

Iain

[mowgli]

I think you should measure the resistance of the fuel gauge as well, because I always thought the circuit was a potential divider (if 1986 o levels are still valid)

[Clean 2.0 nova]

I think you should measure the resistance of the fuel gauge as well, because I always thought the circuit was a potential divider (if 1986 o levels are still valid)

I did - thats where I got the 80 ohms from!

If you don't use 80 ohms then although empty and full maybe displayed correctly, half a tank will not. I'm sure there are many out there that would argue that even on a unmolestered nova the half way reading is far out - but that is probably to do with tank shape and lots of other factors combined.

http://i184.photobucket.com/albums/x198/iaingra/fuelsender.jpg
In reality the 80 ohm resistor is fitted directly across the gauge. Also the polarity maybe the other way on a standard nova (one wire of sender connected to chassis) but can't remember.