3/4 = 1/2 & 1/2 = Empty - Fuel Sending Unit 98 Impreza

 

I have not pulled the unit yet, but it's kind of strange that it's reading so high.  The fuel low level light does come on but it's still showing half.

 

I'll pull the unit when I get time and clean the contacts and make sure the float is free.

 

Larry

Pull the sending unit and test it, there should be a spec sheet on the sending unit on it giving an ohm spec and where the float is. If that checks out to be all okay and within specs check the resistance of the sending unit wiring harness, there should be little to no resistance through the wires. Another spot to check would be the connector for corrosion 

I think there are two sending units used and it sounds like one of them is showing near empty. I think the sensors are in series with each other if I remember correctly.

Pulled both sending units tonight and cleaned the slip ring with a little WD-40 and finger pressure.  

 

Both read 500 Ohms Empty and O Ohms at Full.

 

Time will tell if that takes care of it.

 

Smooth resistance changes as the float moved full scale.  No Opens.

 

Larry

Sounds like you got it taken of.

I think there are two sending units used and it sounds like one of them is showing near empty. I think the sensors are in series with each other if I remember correctly.

That is right, on the 4WD models at least, 2WDs have one sending unit, although if my memory serves me correctly they are wired in parallel not in series. The low fuel light is independent of the level sensor and is simply a temperature sensor that pics up the change in temp when the fuel level drops below a certain amount. (If you remove the sensor that has the fuel pump assembly attached to it, you will see the low fuel sensor, it's the small metal canister about 1/2" long that's attached to the side of the level sensor.) If one of them goes bad, it throws the gauge off, either will be full = full and 1/2 = empty, or 1/2 = full and empty = empty, or something screwy in between depending on how they fail.

Cleaning the sensors helped but I don't think it fixed the fault.  The Low level sensor light came on today, with 3/8 of a tank.  It went back off in a bit.  I have not filled the tank yet to see how much gas it will take.  I'm going to run it until the low level light is on steady or I'm at E.

 

I think they are wired in series as when I disconnected either one of them the tank went to E.

 

Larry

Did you try to clean and put some dielectric grease on the connectors?   I now remember my 01' lego pulled the same thing once before and that fixed to problem with mine.

Too much resistance in the system causing a high reading on the gauge 

Contacts looked good.  I did not clean them but will.

Thanks, Larry

 

  I'm going to run it until the low level light is on steady or I'm at E.

 

you might want to carry a an extra gallon of gas with you.

my son's 95 lego had gauge issues.

reading E at about 10 - 12 gallons gone, not unusual for these cars.

but occasionally, the light would come on and then go off, not to come on again.

he ran out of gas more than once.

and then learned to fill up at 250 miles regardless of the light or gauge.

 

FYI

maybe a clogged jet pump?

Bad ground

 as nipper (i think it was him) described it,

the light is independent of the gauge.

and is triggered by a ''thermistor''? or something like that.

when it is exposed to the air in the tank, instead of the fuel, the temp changes, and it triggers the light.

 

or so i have read.

Edited September 10, 201312 yr by johnceggleston

Fuel level senders are wired in series. Generally the lower the resistance the higher the gauge is. You can pull the signal wire off of one of the senders and ground it to see what the gauge does. If the gauge spikes to the top (full)high reading = low resistance. If it spikes to the bottom (empty) low reading = low resistance.

 

Figure out how the gauge works and then you know if you need to look for a short to ground between the senders and guage or a bad sender ground.

At just over a 1/4 tank now.  I'm going to see how much gas it takes in the AM.  That will give me a good idea if the fuel level is now correct after cleaning the resistors of the sending units.

 

Thanks Larry

Anyone know where the ground point is for the fuel sending units?

 

98 Impreza L Wagon.

 

I don't have the wiring manuals with me.

 

Thanks Larry

I would have to think the metal flange that seals to the tank is grounded.

IIRC it's on the floor pan under the rear seat, but if not, follow the wire harness and you should find it.

Thanks, that's what I was thinking.  It took 13.6 gallons yesterday, the gauge was just over a 1/4 and the low fuel light was on.  It only went up to 3/4 so there is something going on.  The fuel pump side may not be the original unit as it did not want to set flat in the tank when I pulled it to clean the sensor.  the front was up a 1/4" or so, it felt like the filter was up against the bottom of the tank.  

 

I'm thinking the OP may have changed it out trying to fix the problem.  

 

I'll have to check the PN when I get time to ensure it's the correct unit.

Check the resistance values of each of the sensors while the tank is full and see if each one is indicating the correct resistance value for a full tank. One of the sensors may not be correct. Of course if a incorrect part was used that would change things, like you say.

Not that my car is the best example of a properly functioning gauge, but when it hits E I can drive another 25 miles before the light comes on, then I fill up wih about 13 - 13.5 gallons. When its full the needle pegs on the high side of full for between 40 and 60 miles before it starts to come down. (Depending on how hard I've driven the car).

 

Did you pay any attention to the floats on the senders? Are they the old hollow brass/plastic barrel type or are they foam? I know the hollow type used to crack and the float would sink.