Superfast1

Hi there, I've recently acquired another Lexus LS400. It's a 2000 but in pristine & like new condition w/ only 46K miles from one of our forum's members. It had a starter problem & I have replaced it w/ a brand new starter. While at it, I went ahead & did the timing belt, water pump, hydraulic tensioner, thermostat & serpentine belt as well. The old serpentine belt did show sign of age (lots of small cracks), however the old timing belt looks brand new even though it's 17 years old. Anyway, I've just noticed my fuel gauge isn't working properly as I've just filled up the tank & within 10 miles, it's already down a quarter on the display. Since everything on the instrumentation panel works including the fuel gauge, I think the issue is either w/ the sending unit or the fuel gauge needs a re-calibration. Has anyone ran into this issue on your fuel gauge before? Please advise. Thanks in advance.

Superfast1

If it's the sending unit then I may have to put a 10K Ohm potentiometer in parallel w/ a built-in resistor at the sending unit to ground then re-calibrate the gauge.

Yamae

Below might help you to troubleshoot. Since you have a good skill, I don't think I need to explain the details.
Hope your another LS400 would be fixed soon.

Superfast1

Hi Yamae,
Like yourself, I'm also a EE. I used to work for Hughes Space & Communication Group & now w/ Boeing, design & build satellites so troubleshooting is one of my favorite things to do. I should be able to figure out the problem & re-calibrate the fuel gauge if need be. I assume the connector you're showing is a sending unit's connector. If so, may I ask where's this connector located? Is it close to the fuel tank or up front? I chose the 10K potentiometer for hi-impedance so it won't load down the ckt & when it's in parallel w/ the built-in resistor, the sending unit will still see the resistance of the built-in resistor. Please advise. Thank you.

Banshee365

The connector is accessed from the trunk area after removing the linings.

Superfast1

Hi Banshee, I've surfed the net after & yes, the sending unit's connector is in the trunk behind the linings. Thanks.

Banshee365

Prepare to be wowed by the quality of the fuel tank and filler tube. They don't build them like this anymore.

Superfast1

Yamae, do you have the schematic of the sending unit & fuel gauge? Based on the diagram you showed, pin 2 is the output of the sending unit & it's connected to the fuel gauge, correct? If so & in order for the gauge to accurately display the amount of fuel in the tank, I'll have to vary this voltage between 4.38V & .48V to indicate full & empty, respectively & by default, the half full would be 2.43V ((4.38 - .48) / 2 + .48). Would like to get a schematic of the sending unit for a sanity check before I modify & insert the 10K pot into the ckt. Thanks

Superfast1

Btw, I've read somewhere Toyota uses a logic ckt board for the fuel gauge. I'm not sure if it's true. Do you know?

Yamae

What you understood was all correct.
I have a JDM schematic of the combination meter below. Here I send you an enlarged one picking up the fuel related part. I remember a USDM is mostly the same.
I added English names using blue letters. Ask me if you have any more question but I'm sure you are skilled enough to read the contents except Japanese expressions.

Yamae

One thing I'd better to inform you. The sensing input line Pin 12 (B) which is connected to the Pin 2 of the gauge is sensitive against the static discharge. You have to be careful not to give it any stress electrically. I suggest you to insert 100 Ohms or so resistor in series to that line to prevent any damage to the AD converter in the CPU while you are experimenting.

Superfast1

Hi Yamae, thanks for the schematic. As expected, pin 2 is the center tap of the fuel sender gauge's built-in potentiometer. Once the connector is disconnected, I can measure the resistance of it via pins 3 & 1 but I think it's in the neighborhood of a few hundred Ohms. It would be nice if I can get access to this built-in pot so I can make the adjustment with a screwdriver without using an external pot in parallel. Do you know if this built-in pot is accessible? Is it part of the fuel gauge behind the instrumentation panel or fuel sending unit which is inside the fuel tank? If' it's not easily accessible then I'll hookup an external 10K pot in parallel & make the adjustment from there. I have read some of your posts related to the ECU & am very impressed with your vast knowledge in electronics. You have been a big help and truly an asset to this great community. It's a pleasure knowing you.

Superfast1

Btw, in regards to the input/output being sensitive to ESD (Electronics Static Discharge), most of the A/D converters do have internal built-in resistors so the input/output is pretty well protected but it wouldn't hurt to add another resistor in series as an added protection. I normally use a 1K Ohms so there will be very little drop across the resistor but a 100 Ohm would do as well. Thanks.

Yamae

The sending unit is in the fuel tank and it is not easy to access. Your idea to use a potentiometer is easier and quicker to check the meter's function. When you apply 2.43V to the Pin 2, the fuel gauge should indicate 1/2. This should be the first step to distinguish the cause of the problem.

I know some about the car and behind scenes. I make it a rule to share those to keep our precious LS400s/Celsiors to keep on the road as long as possible.

Superfast1

Yamae, I had replace the fuel pump (just the fuel pump) on my 97 Lexus LS400 couple years ago & don't recall of seeing a ckt board and/or a potentiometer on the sending unit. Perhaps I didn't pay attention to it back then. Anyway, if I want to get access to the sending unit on my 2000, I'll have to remove the backseat, the round cover & a bunch of 8mm bolts so I think I'll go w/ the external 10K pot modification via the connector in the trunk & should be able to re-calibrate the fuel gauge that way. The fuel gauge is off but it's working & I have some hot assignments going on at the moment so probably won't be able to get to it for at least a few weeks. You have given me all the info I need to modify & re-calibrate the fuel gauge. I'll keep you posted once I can find the time to work on it. Thanks & have a great day.
John