[Mk2] [Turbo] ohm/voltage reading of fuel sending unit full and empty.

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Joined: Fri Apr 29, 2016 10:33 pm

[Mk2] [Turbo] ohm/voltage reading of fuel sending unit full and empty.

Post by [email protected] »

Can someone please tell me what the full and empty readings for for the fuel sending unit.

Ohm reading and voltage if possible please. Fitting a dash 2 and want to compare to the standard reading
Pauln
Posts: 850
Joined: Mon May 08, 2006 10:15 pm
Location: Cardiff, South Wales

Re: [Mk2] [Turbo] ohm/voltage reading of fuel sending unit full and empty.

Post by Pauln »

The fuel level sender should give you the following readings measured at the connector after disconnection:
Raised (full tank) around 3 ohms
Lowered (empty tank) around 110 Ohms

When I last changed mine I also connected the sender up outside of the fuel tank with the float arm in both positions to check the meter was reading correctly. But if you do that, don't forget the needle on the fuel gauge takes some time to travel to the correct position.

Don't forget to keep the car well ventilated when changing the sensor.

Paul
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Re: [Mk2] [Turbo] ohm/voltage reading of fuel sending unit full and empty.

Post by [email protected] »

Pauln wrote:The fuel level sender should give you the following readings measured at the connector after disconnection:
Raised (full tank) around 3 ohms
Lowered (empty tank) around 110 Ohms

When I last changed mine I also connected the sender up outside of the fuel tank with the float arm in both positions to check the meter was reading correctly. But if you do that, don't forget the needle on the fuel gauge takes some time to travel to the correct position.

Don't forget to keep the car well ventilated when changing the sensor.

Paul


Thanks.

I'm struggling with converting it to a volt reading from ohm. The dash2 analogue wire for the sender is 5v.
Would I be right in assuming it's
0.63v full tank 55litres
4.3v empty
?

I'm going insane
Pauln
Posts: 850
Joined: Mon May 08, 2006 10:15 pm
Location: Cardiff, South Wales

Re: [Mk2] [Turbo] ohm/voltage reading of fuel sending unit full and empty.

Post by Pauln »

I'm afraid I've no experience of connecting this up to something like the dash 2, but you do need to remember there are two parts to the MR2 fuel sender.

You have the float arm that operates a wire wound variable resistor arm to provide the correct resistance to be interpreted by the std fuel gauge, and you also have the low level fuel sensor that operates the warning light on the std dash.

So you will need to identify the correct wires for the fuel level arm.

Hopefully someone with experience of the Dash2 can help, but looking at the instructions available on line the setup procedure does look a bit more involved than a conversion from resistance to voltage. V= IR is all well and good but it depends on how the dash operates:

Link

Good luck.

Paul
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Re: [Mk2] [Turbo] ohm/voltage reading of fuel sending unit full and empty.

Post by [email protected] »

I believe the low level light is separate. I need to figure out the resistance ohm. Convert it to a volt reading so the float up top and bottom we know the ohm reading. I just need to know what the 110ohm in the volt reading will be
Pauln
Posts: 850
Joined: Mon May 08, 2006 10:15 pm
Location: Cardiff, South Wales

Re: [Mk2] [Turbo] ohm/voltage reading of fuel sending unit full and empty.

Post by Pauln »

Well assuming you're running new cables in to run this on the 5v supply from your dash2, and assuming this is +5v, I guess if you could just connect the +5v to the sender, and you should in theory then be able to use a meter to measure the voltage returned between the other side of the sender and ground, with the float is in the up and down positions. If you don't connect the return feed from the sender to the dash that should be isolated, so the only thing to be careful of is that you don't exceed the current limit of the 5v supply.

But as I said, I really don't know anything about Dash2 so I could be wrong. The link I looked at seemed to suggest you might need an additional load resistor in the circuit to make it work correctly when you finally hook it up. Which is why I was hoping someone who knew more about setting this up could advise you, as there's always a risk of causing damage to the electronics if it's connected incorrectly.

Paul
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Re: [Mk2] [Turbo] ohm/voltage reading of fuel sending unit full and empty.

Post by [email protected] »

I do appreciate your help.
I am getting readings up on the display. I set to display from 55 to zero.
I'm just not getting accurate readings.
tiff_lee
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Joined: Thu Jun 04, 2015 2:18 pm

Re: [Mk2] [Turbo] ohm/voltage reading of fuel sending unit full and empty.

Post by tiff_lee »

The reason your readings are not accurate is because as it stands the voltage drop across your fuel tank sender (variable resistor) will be 5 volts (if you're using the Dash 2 reference voltage) regardless, it is the only resistor in the circuit so all that changes with fuel level is the current flowing through it.

On that note if you have simply wired it up like so:
Dash 2 5V reference > fuel tank sender > Dash 2 analogue input
it might be advisable to disconnect it, according to their website the maximum current flow on the reference output is 50mA, @3 Ohms (full tank) you will be pulling 1.67A!

https://www.race-technology.com/wiki/in ... FuelSender

You need to add an additional resistor as Pauln mentioned (2 wire sender)

https://www.race-technology.com/wiki/in ... ensorToDL1

this way as the resistance varies on the fuel sender so does the total circuit resistance, in turn the current flowing through the circuit will change and the volt drop across the added resistor will scale accordingly.

Not wanting to teach you to suck eggs but here is the math based on the above link recommendation of adding a 1k ohm and using Paul's values of 3 Ohm (full) and 110 ohm (empty)

V=IR
5=I*1003 (total circuit resistance for a full tank)
5/1003=0.0045A
0.0045*1000= 4.5V this will be the analogue input voltage on a full tank

5=I*1110 (empty tank)
5/1110=0.004985
0.004985*1000= 4.985V voltage for an empty tank

Edit: Just corrected the maths as calculated the volt drop across the sender itself and not the 1K load resistor!
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