Low Fuel Warning

[paulo57509]

Not looking for answers, this should have been in the "What are you working on today" thread.

LS400 - In the process of repairing small dash panel that is delaminating, I re-installed the remaining lower dash panels in order to be able to drive the vehicle.

Reconnected the battery and the low fuel light was on, even with the gauge showing ~1/4 tank. Drove to the fuel station and filled the tank. Took ~18 gallons which is close to 3/4 tank. The "Range Approx" display read substantially lower than normal; normal hovers around 390 miles.

Tonight, after driving 305.1 miles, this:

Three possibilities, all labor intensive:

1. Gauge is bad.
2. Sender is bad.
3. Gauge cluster is bad.

I can't imagine that anything I did caused any of the three possibilities. Before I tear into this, I'm going to go back and recheck and review my work; I hope it's something stupid. I just wish it wasn't +100F outside.

 

[BrandonV]

I'm going to go with 3. I've seen bad electrolytic capacitors on Lexus gauge clusters before cause weird issues.

 

[GMBoy]

I'm going with the cluster as well because the sender is working fine - the gauge is reading properly.

 

[BlueOvalFitter]

Not looking for answers, this should have been in the "What are you working on today" thread.

LS400 - In the process of repairing small dash panel that is delaminating, I re-installed the remaining lower dash panels in order to be able to drive the vehicle.

Reconnected the battery and the low fuel light was on, even with the gauge showing ~1/4 tank. Drove to the fuel station and filled the tank. Took ~18 gallons which is close to 3/4 tank. The "Range Approx" display read substantially lower than normal; normal hovers around 390 miles.

Tonight, after driving 305.1 miles, this:
View attachment 111053
Three possibilities, all labor intensive:

1. Gauge is bad.
2. Sender is bad.
3. Gauge cluster is bad.

I can't imagine that anything I did caused any of the three possibilities. Before I tear into this, I'm going to go back and recheck and review my work; I hope it's something stupid. I just wish it wasn't +100F outside.

Do like I do, when I tap on my dash the light for my odometer/mileage indicator light comes on like it's supposed to. 🤷‍♂️

 

[paulo57509]

Do like I do, when I tap on my dash the light for my odometer/mileage indicator light comes on like it's supposed to. 🤷‍♂️

I don't do it but it's the go-to fix for when the tach and/or speedo needles stick (chronic known issue with most every LS400). It has to be a lot more than a tap.

With age, instrument lube that Toyota uses tends to get tacky with elevated temperatures and causes the needles to stick.

 

[paulo57509]

I heard back from the resident (ClubLexus) electronics (EE) from Japan with direction; if not the sender, then instrument panel. He did mention to try a "reset" first - disconnect the battery for ~10 minutes and reconnect, the go-to for almost everything digital.

 

[mk378]

"Low fuel" lights are usually connected to a separate sensor inside the gas tank. It's mounted alongside but not the same as the sender that controls the dial gauge.

If you go under the back seat and unplug the wires to the gas tank, then turn the key on, the gauge should read empty and the low fuel light should be out.

 

[Gladtojoin88]

I don't do it but it's the go-to fix for when the tach and/or speedo needles stick (chronic known issue with most every LS400). It has to be a lot more than a tap.

With age, instrument lube that Toyota uses tends to get tacky with elevated temperatures and causes the needles to stick.

Our old ls @(90) and sc ( 94) got beat every morning lol

 

[paulo57509]

I got this from the forum moderator of a Lexus board I frequent. Same diagrams that appear in the NA FSM. This gentleman knows the Toyota Celsior electronics inside, upside down and inside out.

Translated, he just confirmed my SOP; by process of elimination the problem will become evident. This is a ways down on the to-do list for the short term, I'm going to have to live with it.

BTW, he did mention doing a "reset". Translated to English, "Disconnect the battery for 10 minutes and see if it goes away." It did not.

 

[joekingcorvette]

I know a lot of Corvette's have fuel gauge problems and one of the easy fixes is to put in a can of Chevron Techron and use it for a couple tanks of fuel. There is something in the gas tank that sticks and the Techron will free it up. Worth a try anyway.

 

[paulo57509]

Finally got cool enough outside to crawl around in the trunk to get to the fuel level sensor.

Testing the sender yielded the following results:

Conditions:

• Tank was physically about 3/8 full based on mileage driven.
• Gauge read just under 3/4 full.
• Distance to empty read 168 miles.

Test 1 - Unplug harness from sender; turn key ON.

• Expected Result - Gauge indicates empty.
• Test Result - Gauge didn't move.

Test 2 - Jumper terminals 2 and 3 on harness side, turn key ON.

• Expected Result - Gauge indicates full.
• Test Result - Gauge reads empty (low fuel light and text warning on instrument panel).

I didn't see the need to further diagnose the system. The combination meter will have to be removed and tested/repaired. I had a new, OEM sending unit so it was installed before re-assembling the trunk. There are test points for the gauge; I'll check these before sending the cluster out.

The low fuel warning is part of the gauge itself. There is an odd, spring loaded arm attached to the sensor assembly. It moves the entire resistor/float ~2-3 degrees. When the sender is installed, this arm is in contact with the tank wall. I can only assume it's function has something to do with calibration that works with the distance-to-empty or some other function.

 

[paulo57509]

I'm finally getting around (kind of) to the fuel gauge. The FSM describes tests for the fuel gauge to determine its health. So, I purchased an entire gauge cluster from an ebay seller 😬, figuring I'll just do a bench swap of the gauge. At least in the seller photos, the fuel gauge shows empty and a low fuel warning when the cluster is powered up.

A few days ago, I was browsing the dealership's parts ordering section. Of all the cluster parts that are NLA, the fuel gauge is still available 😧. And it's $100 less than what I paid for the used cluster.

I have a fall-back if the use gauge tests bad. And I have used, NLA cold cathode tubes I can put away for a rainy day.