Fuel Gauge Issue

Hello,
I've owned my 2005 V6 DC SR5 for about three weeks now. In that time I've noticed a few oddities with the fuel gauge on this truck. The first time I put gas in the truck was on my 2.5 hour long ride home from buying it. I looked down and the gauge indicated a little more than 1/8th of a tank (no low fuel level light) I decided it was time to fill up. I knew from my research that the truck has a 21 gallon tank so I was slightly surprised when I filled the truck up with 20.8 gallons of fuel, upon fill up the gauge went straight to full.

Fast forward one week and I had depleted about 3/4 of the fuel in the tank driving around. I happened to have my lovely wife beside me and our Christmas tree in the bed when the truck shuts off at 70mph on the highway. I coast to the shoulder and proceed to check the fuel gauge. It shows I have exactly 1/8th of a tank and about 365 miles on this tank (no low fuel light). Long story short I add a couple gallons of fuel from a kind motorist with a fuel jug and the truck starts right up. I drive to the gas station and add 19 gallons to fill the truck. It was fuel starvation that caused the truck to stall.

Being a mechanic in a past career I concluded that the fuel level sending unit must be getting tired and since the truck has over 200,000 miles with an unknown age fuel pump assembly. I order a new pump from the local store. After dropping the tank things got strange.

The pump I bought is a Delphi FG0919, complete fuel pump assembly. Having learned the hard way in the past I decided to test the old fuel level sending unit when I removed it to see if the arm was binding or maybe the variable resistor contacts had visible damage from years of service. I found some minor discoloration on the contacts but all else looked great otherwise. When I plugged the old sending unit (Original Denso Unit with Toyota branding) back into the chassis harness and swept the arm/float assembly I found that the gauge was accurate above 1/4 of a tank. If the arm rested on the very bottom stop of the pump assembly (the out of fuel position) the gauge would only indicate 1/8th of a tank with no low fuel level light. I felt good about this finding but went a step further to test the new Delphi unit, I found that the Delphi unit produced the exact same results as the factory Denso pump, arm sitting on the bottom of the tank indicates 1/8th of a tank and no low fuel level light. So maybe its not the sending unit?

With the new pump (and my own gas can in the back) I deliberately run the truck out of fuel, don't worry the pump has a lifetime warranty, this time with a brand new fuel level sending unit the truck runs out of fuel just above a 1/4 of a tank at 375 miles and with no low fuel level light.

I know there has been mention of this exact issue in the forums before but I've seen nothing about a resolution or remedy other than use a scan gauge, track fuel level by mileage or see a psychic advisor.... etc.-etc.

I'm the kind of person who wants the truck to work as designed, all those options will work but I'm not looking for a work around. I want to fix my gauge, it obviously worked when new... what changed?

As far as electrical circuits go this one is pretty simple, all based on resistance, when I get to a working ohm meter (mine took a union break) I intended to measure the resistance of my old sending unit to see what readings it produces.

That leads me to my question, has anyone fixed this issue on their truck? I'll admit that my Delphi unit could be bad even though its out of the box new. It may also have the wrong resistance scale, but the fact that it duplicates the readings of my OEM pump makes me suspicious of the chassis wiring, the gauge cluster and all the grounds in between.

Questions,
-Is the low fuel level light supposed to come on with the other lights when you turn the ignition on? I've never seen this light on my truck.
-Is there an instrument panel self test on these trucks to see if all the instrument panel lights are working?
-Is there a common ground issue on these trucks that has been identified which could alter the resistance between the gauge and the sending unit?
-What activates the low fuel level light? This is a two wire sending unit so I'm guessing when the needle reaches a certain value or resistance the light will be activated.

I intend to diagnose this issue until I figure it out and will report back my findings so others can fix their gauges too!

I know the gauge won't drop linear to the gallons used but I want my gauge to indicate empty when the tank has a few gallons left like most folks on TW indicate theirs do, I also want my low fuel level light to function.

Sorry for the essay! Thanks in advanced for your input

 

Does it even have the low fuel light it should have 3 wires on the plug of the sending unit .

Not all trucks had the low fuel light

Don`t know about how the gauge reads I fill up around every 100 miles put my 10 or 12 gallons in

 

If it has a low fuel level light I’ve never seen it. I know for a fact the new and the old sending unit connectors only have 2 wires.

In my experience the low fuel level light sometimes has its own sensor with one wire that will trigger the light when the sensor is uncovered.

The Toyota owners manual shows an indicator for a low fuel level sensor but I have yet to see it illuminate.

Anyone else out there with a base V6 SR5 package that has a functioning low fuel level light?

 

Is there a way to "mechanically" adjust the rheostat on the new sending unit. Typical calibration procedure on some devices I've worked on in the past. To calibrate ZERO - remove the rheostat from the mount - turn rheostat until ZERO is represented - reinstall rheostat with mechanics in the ZERO position.

I've never seen a sending unit. So just a thought.

 

The resistance scale on these sending units are pretty much fixed. It’s literally a resistor strip with a hinged wiper arm that moves across the scale depending on the fuel level in the tank. The float provides the buoyant force that moves the arm. I’ll take some pictures of it when I get home and post for everyone. I appreciate the replies gentlemen.

 

Pictures would be nice. I know a little bit about this type of control system. Just don't know how it applies to automotive.

I doubt that the voltage input is a full 12V since this is inside the gas tank. Should be something supplying power to the slide-wire input. This is the "reference" signal. If this signal is compromised, the output will be too. Depending on which direction the slide works, this could be something that need to be looked at.

 

I agree, I do not know what the reference voltage is supposed to be but I’ll do some digging on that, most ECU related sensors use 5v but I’m not sure if this input is processed by the ECU or not. Another variable is the anti-slosh module, it dampens the oscillations from sloshing fuel by averaging the input readings to make a smooth input. I’ve driven quite a few 90’s model Chevrolet’s where the gas gauge looks like a windshield wiper arm going crazy!

 

I'm thinking that, being that it's submerged in gasoline, the signal will probably be in the millivolt scale. In my world, I've seen anything from millivolt to full line voltage.

Interesting - slosh dampening?

I started driving in the late 70's. I remember those gauges.

 

Here’s the resistor strip, it’s a two wire arrangement. Slight discoloring from age is suppose. I hear sulphur and other chemicals in fuel can bridge the gap on the contacts over time. No adjustment that I see. Float appears to be solid not hollow like some so I doubt it’s a factor. I’ll post more pictures later this evening

 

Only possibility I see would be bending the float arm. Not sure if repositioning the arm "mount" is possible? Or even necessary? You mentioned that installed, the new float would only read 1/4? so bending the float arm my make a difference. But - if the best you could get was 1/8, I would suspect the reference signal?

Would be nice to know what direction is FULL / Empty. IE: Reference HIGH / LOW. -- If ref. HIGH = EMPTY, might need to look at the signal.

 

Bending the arm may improve accuracy some but it won’t change the resolution. The scale will still be limited to F through 1/8th of a tank.
All I can do by bending the arm is change the indication on the gauge for the fuel level.

I took this screenshot off another forum, it lists the ohm reading for each position on the sending unit. Unless I can come up with a factory service manual before that time I’ll compare my resistance readings to these indications and go from there.

 

This is the first spring loaded fuel pump assembly that I’ve seen. It apparently can be used on different shaped tanks and still be at the “bottom” of the tank. The sealing flange (white plastic disc at the top of the assembly) sits about 1/2” proud of the top of the tank. You have to push the flange down against the spring pressure to get the threaded lock ring started.

 

Two pairs of wires here, one pair for the pump and one pair for the fuel level sending unit. I unplugged the pump for key on testing purposes when the unit was removed from the tank, that kept the pump from running while dry. Pulling the fuel pump fuse or relay would accomplish the same thing.

 

The molex connector on this pump is configured for a maximum of 5 wires, mine of course only uses 4.

If the trucks with low fuel level lights have a 5th wire for the light I’d be surprised as there is no option to select a pump assembly with or without the provision for a light. I bet the anti-slosh module commands the light to activate at a certain resistance value.

But, then why do some Taco owners report the light coming on at 1/4 of a tank or higher?!?

I think the bold red V on the flange may stand for victim... ie. me!

 

Here’s the assembly installed in the tank. The lock ring on top is threaded. Apparently Toyota makes a special wrench to loosen it, I used a wide blade pry bar and a hammer. I recommend marking the lock ring position on the old fuel pump flange so you can set the same torque on the lock ring.

Also, to keep the arc of the fuel level float arm clear the pump has an indexing tab. That assures the pump is oriented in the right direction upon installation. Those are the highlights of the fuel pump assembly and the fuel level sending unit.

 

For clarification, with each pump assembly removed from the fuel tank; I plugged the sending unit into the chassis harness and I swung the float arm from its lowest position to its highest position. I did this on both pumps multiple times and they both indicated full when the arm was all the way up, and both indicated 1/8th of a tank with the arm all the way down. Not once did a low fuel level light illuminate.

That should isolate the arm binding, float not floating, or any other mechanical issue. The problem is electrical.

 

Lastly the needle on the fuel gauge falls to below empty on the scale when the ignition is turned off so I don’t think there is a dead spot in the gauge motor or a binding needle!

 

I only suggested bending the float arm because you stated that you were getting 1/8 out of the tank. And only 1/4 in the tank.

I don't like making assumptions when electrical troubleshooting. Your resistance values may be correct. But without verifying with known good documentation, we can only assume.

Never seen a vehicle show LOW FUEL @ 1/4 tank. Most show "LF" @ ~50 miles remaining. Which would be closer to the 1/8 mark. For this reason, I would say that it's possible that the "LF" indicator may still work.

Since you have comparable readings between old and new units, agree that the issue isn't with the unit itself. Not sure where the reference signal is provided. But that would be the next focal point. Also, take a close look at the connector body(s). Make sure that the connecting pins are tight and no corrosion. week connections can change resistance. Although, assuming that the scale is going in the direction you indicated, a bad connection would migrate towards EMPTY.

 

I totally agree about investigating further up the circuit. I’ll need to burn some fuel out before I drop the tank again, don’t want to wrestle with 15 gallons of fuel in the tank! I greatly appreciate the feedback.

I should’ve clarified but in researching my problem I’ve ran across several posts about wayward low fuel level lights coming on when they shouldn’t be. Ideally the engineer who designed the system would’ve set it up for the low fuel light to come on 50-75 miles before actual fuel exhaustion!