75 3.0 V6 fuel sender output voltage

[Colin Edwards]

Hi All,

Slowly sorting a number of electrical gremlins.  The fuel gauge, regardless of how much juice I put in the tank, never goes much past 3/4 FSD.  I haven't run out of fuel (yet!) however after around 350k of driving the gauge seems to sit at 1/4 FSD.  I'd rather not pull the dash to bits yet and check the mechanical movement of the gauge.  So the question remains - what are the maximum and minimum output voltages expected from the fuel sender / transducer.  The 75 in question features the fuel tank directly behind the rear seats - within the boot!. 
The sender will probably need to come out given I believe the seal is leaking a tad - occasional sniff of juice within the boot.  At the moment I'd like to diagnose the fault without unnecessarily pulling things to bits.  Is a complete sender available as a replacement part?

[bonno]

Hi Colin
I have located a video on Youtube that enable the testing of both the gauge and sending unit. With respect to the type of ohm readings you should have full and empty,  if unable to obtain OEM spec,  then I would go from having a tank that is ½ full then record ohm reading and work from there he covers this aspect in the video.
Hope this helps
bonno
Gauge and sending unit video
  https://youtu.be/z4c8_NnBW4s .

[Colin Edwards]

Hi Bonno,

Thanks for the Youtube link.
 
Had a closer look at the electrical schematic for the 75 3.0.  The gauge is the three terminal dual coil / balanced coil type and the potentiometer in the sender is connected in parallel with one of the gauge coils.  Managed to source a NOS sender and measure the resistance.  330 ohm MT and 10 ohm full.  This makes sense.  With the tank empty, both coils are similarly driven causing the needle to hang around "E".  With the tank full, one coil is significantly weakened causing the needle to hang around "F".  A cunningly clever arrangement for the day.  With a full tank of fuel and the potentiometer running at possibly 14.4w the juice is used to cool the resistor!!!
I'll sit the NOS sender in a suitable container of juice and connect it to the car and see what happens to the gauge at various levels of fuel. 
Presently a pain in the bum to have to fill the tank at around every 200klm.

Colin

[GTVeloce]

Do you intend to use the NOS sender? Or was it just on loan? If not, when you pull the original out, check the obvious things like that it slides freely but also clean the contact points to ensure correct readings.

Until you get it fixed: If you know the tank is full when you are at the bowser (should be able to tell by tapping it) then you could just use the trip meter to keep an eye on your distance regardless of the guage i.e. if you expect 600km from a tank then don't go much past say 500km before filling up again.

[Colin Edwards]

I'll probably purchase the NOS sender given their scarcity.  Also chasing the occasional faint whiff of BP's 98 in the boot so a new seal will be handy.
I've cleaned and re-crimped the external electrical connections including the ground with no apparent improvement in gauge performance. 
If I install the new sender I'll have a go at pulling the original one apart more for curiosity than anything else.  Looking at the NOS sender though, the way its assembled at the base might make non-destructive disassembly a challenge!

[GTVeloce]

Is it similar to a TS sender? I know the tank is located in a different location but they may use similar or the same sender. If so, I have lots of photos of a dissembled one and therefore how they work.

[Colin Edwards]

Is it similar to a TS sender? I know the tank is located in a different location but they may use similar or the same sender. If so, I have lots of photos of a dissembled one and therefore how they work.

I haven't seen a TS sender.  See below images of the NOS sender I purchased.

[Colin Edwards]

Replaced the suspect fuel level sender.  Given the close proximity of the top of the fuel tank to the underneath of the parcel shelf the fuel tank needed to be unstrapped and laid back to access the sender.  Took the opportunity to replace the fuel hoses leading to and from tank.  This meant draining the tank!

Prior to doing this I ran the car for about 75klm after the gauge needle stopped at 1/4 full.  It would NEVER drop below 1/4.  Ended up draining around 25 L of fuel from tank - so the sender was way out given the tank was more than 40% full!

Made up a precision engineered tool to remove sender unit - see attached image.  Sender came out very easily and new one installed in minutes. 

Put everything back together including new hoses and copper gaskets on return line banjo.  With ignition ON and no fuel in tank the low level warning light came on - never seen that before!!  With about 5 L of fuel in tank still no needle movement.  Sender resistance around 340 ohms.  With about 7 < 8 L of fuel in tank about 0.5mm of needle movement.  315 ohms sender resistance.  This confirms the 7 L "fuel reserve" as described in Owners and Workshop manuals.  With around 23 liters of fuel in tank needle displays about one graduation below 1/4 full.  Adding a further 27 L of fuel brings the needle to a graduation above 3/4 full.  Problem now pretty well sorted.  I have confidence in the fuel gauge!

Close inspection of the original sender unit revealed the 3mm diameter hole on the base of the unit blocked with fibrous material.  Maybe this material has come of the float or guide or resistor inside the sender and settled at the base preventing a fuel level reading below 1/4 full.  When I have a bit of time to kill I'll sort out a way to non-destructively take the sender apart.

[bteoh]

Hi Colin,
Good write up on fixing this problem. Mine has a problem reading full. I really have to fill it right up to the brim in order to get the gauge to read full. Perhaps a faulty sender also?
Where did you get the NOS sender unit?
Cheers

[Colin Edwards]

Hello bteoh,

With my sender inverted and the internal float(?) sitting at its "highest" level, the minimum resistance of the sender I measured was around 10 < 15 ohms.  With your tank "full", see what resistance reading you get?

I'm not sure if the internal resistor is linear or logarithmic, however the resistance in inversely proportional to the amount of fuel in the tank.  I'm also not certain if the tank can be filled completely or the capacity is as published in the owners and workshop doco.   I put a measured in 23 litres in a dry tank then another 27 for a total of 50 litres.  I would have been at the pump for another week trying to get in any more!  The tank is not physically large and although it occupies around 60 liters of space in the boot, given the amount of baffling, void chambers and internal ribs, I'd be surprised if the tank could hold more than 55 litres.

Given the dual coil design of the fuel gauge, low voltage across coils can result in a low reading.  With 50 litres in my tank, the gauge only reaches around 7/8 full however the sender is working perfectly.  One of my many "next projects" is to re-engineer the +12volts feeds and grounds to the dashboard and related widgets.  If the gauge does not receive a supply of at least 12 volts it will read low, regardless of sender performance. 

Purchased the new sender from "The Italian Job" in Oakleigh, Melbourne. 

Colin.