what size resistor

gelmonkey · Post by **gelmonkey** » Mon Jun 29, 2015 2:02 pm

Hi All
After paying out for two Auto meter fuel sender units and both of them not working properly I have decided that it may be better to go back to the original Cavalier sender unit and then put a resistor inline to get the correct Ohms reading and of course correct fuel level reading (Ive run out twice much to my embaresment)
The Cavalier uses 0 to 90 ohms range for full and empty and the Auto Meter uses a 33 to 240 ohm range.
Is there a formula for working out the resistor size or do I need a Potentiometer to work this out?

Thanks in advance

cheers
P

ChrisJC · Post by **ChrisJC** » Mon Jun 29, 2015 4:13 pm

There's no answer that will give you both ends of the scale working properly.

If you got a 150ohm resistor in series, then the total resistance would be 240ohms at one end, but 150ohms at the other, so you'd get a reading of 1/2 for a full tank, but a reasonable reading for empty.

You could use a 33ohm resistor instead, so full would work properly, but when the tank is empty you would have 123ohms, which is again a reading of about 1/2 a tank, so you'd probably run out all of the time.

Chris.

Ian Anderson · Post by **Ian Anderson** » Mon Jun 29, 2015 6:05 pm

Hi Paul

Give ETB a ring, they should be able to get something with the right numbers
http://www.etbinstruments.com/Sensors_ancillaries.htm

Ian

gelmonkey · Post by **gelmonkey** » Mon Jun 29, 2015 6:30 pm

Thanks guys
Need to sort something out as getting narked off with the fuel reading being wrong all the time.
My mate Malc has got a potentiometer he is sending down so hopefully I can work from that as well as talk to ETB and consider what Chris has said also.
Bloody cars!!

cheers
P

r2d2hp · Post by **r2d2hp** » Mon Jun 29, 2015 8:25 pm

Hi Paul

Something loke this might work. http://www.ebay.co.uk/itm/Fuel-Gauge-Wi ... 3a7f14fa8b

kiwicar · Post by **kiwicar** » Mon Jun 29, 2015 8:54 pm

Hi Paul
go with

I was going to say "what you need is an op amp and a power transistor", honest, and look the wise Reg has found you one of each in a nice box.
How are you mate? on the mend I trust

Best regards
Mike

gelmonkey · Post by **gelmonkey** » Tue Jun 30, 2015 8:04 am

Hello guys
I am sure you lot have become my full time but distant care assistants

Reg that looks right up my street mate.
I was looking at changing the gauge and sender as Auto Meter do a 0 90 ohm range gauge which is now available .
It may well have been when I bought all of the gauges but was not aware of it at the time
I have found a brand new sender unit on E bay so I reckon that with the magic box and new sender I could finally have that little bug ironed out.
With regard to my mending I am on the right track now.
Understandably still sore but am off the crutches and can manage about 30 mins on my feet before it starts to hurt again so thats good.
Going back to work on Thursday come hell or high water but only doing light duties.

Thanks guys you are stars.

cheers
P

DaveEFI · Post by **DaveEFI** » Tue Jun 30, 2015 8:44 am

Fuel gauges are generally a form of volt meter. It looks like the sender produces a higher voltage than needed, so an attenuator that also matches impedance should work. Basically three resistors.
There are online calculators for the values.

Eliot · Post by **Eliot** » Tue Jun 30, 2015 8:49 am

I made a self calibrated LED fuel gauge because my petrol tank is triangle shaped.
Essentially there are 8 voltage comparators, each one compares the voltage from the sender with a reference voltage set using a multi-turn pot.
Once built you go to the petrol station with an empty tank and get an assistant to fill the tank up, but stopping at known increments - at which point you twiddle the pot to make the led come on, then carry on to the next one.

http://www.mez.co.uk/ms13-new.html

I'm happy to solder one up for you if you are uneasy with the electronics side.

DaveEFI · Post by **DaveEFI** » Tue Jun 30, 2015 10:23 am

Sounds an excellent idea, Elliot. The standard one on my SD1 is distinctly non linear - it gets through the bottom half much quicker than the top half.

stevieturbo · Post by **stevieturbo** » Tue Jun 30, 2015 12:39 pm

DaveEFI wrote:Sounds an excellent idea, Elliot. The standard one on my SD1 is distinctly non linear - it gets through the bottom half much quicker than the top half.

It's fairly normal for senders or tank design to be non linear, to give the gauge more resolution as fuel runs lower.

There are a couple of places that make custom senders in the dip tube style, not sure about lever arms though. The dip tubes arent expensive though.

I got mine from Fozmula.

But if you're matching an OEM sender to a different gauge, the fuel gauge wizard is probably the handiest way

Blown v8 · Post by **Blown v8** » Tue Jun 30, 2015 4:35 pm

I think this is what you need
http://www.carbuildersolutions.com/uk/e ... nFw.mailto

richardpope50 · Post by **richardpope50** » Tue Jun 30, 2015 6:25 pm

https://www.spiyda.com/magento/index.ph ... d-mk2.html

gelmonkey · Post by **gelmonkey** » Wed Jul 15, 2015 7:16 pm

Hi All
My apologies for not coming back to you sooner.
After looking at various options I have ended up with the fuel wizard and new sender.
The sender is an odd ohms reading unit but the guys at FW have calibrated the unit to the sender and the guage for me already.
Having lost 10 whole days because of what happened at the Pod work stacked up very quickly and I have been unable to either fit the unit or have much time to come on here.
Thank you to all who offered opinions help and taking the time to look at the options for me.
Thank you again guys.

cheers
P