Fuel level/fuel gauge question

[coyotes]

Ok so, when the low fuel light comes on, is it getting a reading from where the position of the needle is on the cluster, or the actual level in the tank? When I fill my tank all the way, the gauge only goes to 3/4th tank on the cluster. Therefore when the low fuel light is on due to the needle reading at 'E', it is reading from the position of the needle and not the actual fuel in the tank?

Sorry for the winded, dumb question, but I would rather be safe than sorry and not run out of gas while driving. I am guessing when my car is on E, I still have somewhere around 1/4 tank left.

 

[marvinmadman]

Actual level in the tank. Its a separate sensor on the FP assembly.

 

[coyotes]

ok, so then it is just my gauge that is incorrect… at least when the tank is full. The fuel light seems to come on around the time the needle is close to the 'E'. But when the tank is full it only goes to 3/4 tank on the gauge.

 

[marvinmadman]

The arm could be bent. Search. There was a thread recently that shows good pics of how it should look.

 

[turbohf]

you have a fuel light? damnit. mine has never worked if its there.


btw. my pump (denso/supra) sucks air at 15.1gallons.

 

[toybreaker]

Quoting coyotes:

Sorry for the winded, dumb question, but I would rather be safe than sorry and not run out of gas while driving. I am guessing when my car is on E, I still have somewhere around 1/4 tank left.



Not a dumb question at all!

Many vr4's suffer from this malady.

I think many floats / float arms are damaged / bent / tweeked during pump swaps, but there are also other causes.

fwiw, cleaning the ground point on the pump hanger on the tank has "cured" (or at least improved) a few I saw with this problem.

Next time you fill it up, unplug the harness at the tank and check the resistance on the sender wire, and post up the result.

That will help us narrow down the cause. /ubbthreads/images/graemlins/smile.gif

 

[r4pt0x]

The gauge uses the delta R from the sensor to indicate the fuel level change. There is no reference like "X Ohms equals fuel level Y".

I also forgot about that when installing the evo fuel pump to my FWD galant: I dropped the fuel tank, then unplugged the sensor but reconnected it AFTER lifting up the fuel tank - so it also showed a lower level than i actually had left.

Only accurate solution:
fill up (or empty) the tank, unplug the sensor, open up the dashboard, set the needle to full (or empty) and reconnect the sensor.

 

[iceman69510]

Hmmm, never thought of that.

 

[toybreaker]

Quoting r4pt0x:
The gauge uses the delta R from the sensor to indicate the fuel level change.



... I'm a dumbass, so I'm not exactly sure what this means. /ubbthreads/images/graemlins/blush.gif

Unless I am mistaken (which does happen, a lot /ubbthreads/images/graemlins/blush.gif ) I *believe* the actual construction of the gauge as used in a Galant VR4 is a "wheatstone bridge"

The variable leg in the bridge is the sender in the fuel tank.


Quoting r4pt0x:
I also forgot about that when installing the evo fuel pump to my FWD galant: I dropped the fuel tank, then unplugged the sensor but reconnected it AFTER lifting up the fuel tank - so it also showed a lower level than i actually had left.





I have swapped quite a few instrument clusters due to vss failures, and the fuel gauge always "seems" to register just fine (as in the same level as the previous gauge) after a moment or two.

It must be noted that the gauge is heavily dampened (reaction time slowed down) so as not too fluctuate wildy as the fuel shift in reaction to the vehicles motion.

It will take several minutes for it to equalize.

Quoting r4pt0x:
There is no reference like "X Ohms equals fuel level Y".




The factory service manual lists resistance for the float / sender at certain positions.

I have used this set of specs to dial in many a sender.

In fact, so far, all the problems I have found in the fuel gauge system have been in the float arm assembly (bent /damaged), the float wiper (broken wire strands in the winding), or high resistance on the ground side of the circuit.

Quoting r4pt0x:
Only accurate solution:
fill up (or empty) the tank, unplug the sensor, open up the dashboard, set the needle to full (or empty) and reconnect the sensor.



While I like your posts, (you sound like a saavy cat /ubbthreads/images/graemlins/smile.gif ), I would still suggest troubleshooting things the old fashioned way first.

It's worked for me, every time. /ubbthreads/images/graemlins/smile.gif

 

[r4pt0x]

The gauge reacts to the change of the sensor resistance.

It's correct the manuals give exact values for the resistance in upper and lower position so you can test if the potentiometer within the sensor is broken, but it doesn't say the gauge directly translates these values /ubbthreads/images/graemlins/wink.gif

The gauges are dampened, yes - but the gauge at my car never went higher than a bit over the 3/4 mark even when the car was standing for several days - the sensor wasn't touched at all because it's a seperate unit to the fuel pump on 2WD cars.


I once found the explanation i posted in one of the workshop manuals for a 4WD car with 2 sensors - must have been a colt/mirage 4wd or evo ~4-5 but i wasn't able to find that exact page now. I remember searching half a day back then for this when an VDO sensor with similar resistance range as the stock one did'nt work properly and always sent the indicator up to max. (We needed a dip-tube sensor because the tank was filled up with foam for motorsport use)
The instruction given there, was to disconnect the gauge before moving the tank or replacing one/both sensors, so the fuel level will be indicated correctly afterwards. (But the problem back then was a faulty VDO Sensor...)

There's also a difference between the gauges that stay in position with ignition off, and those dropping to E when switching ign off.