NEED HELP - 1973 FJ40 gas gauge & Chevy S10 Gas tank PROBLEMS!!!

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We have a 1973 fj40 that we are restoring. We are using a 1994 Chevy S10 gas tank with the Chevy S10 sending unit. When it is hooked up the OEM gas gauge he's not registering.

Are there any modifications that we need to do to the gas gauge to read the Chevy S10 gas tank and sending unit?
 
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I went through this trying to get the stock sender to work with an aftermarket gas gauge. The short story is Toyota used a ohm setting that isn’t compatible with most aftermarket or US OEM equipment. You have to match up the high/low ohm readings. Maybe someone has a work around.
 

Downey

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8/72 and older Landcruisers work on 110 ohms empty, 3 ohms full, but with a 7 volt drop
9/72 and newer Landcruisers work on 120 ohms full, 17 ohms empty, at a full 12 volts.
A Landcruiser registered in the USA as a 1973 was no doubt built in Japan in 1972, so check your production date.
 
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Also, this is weird, the FJ40 fuel gauge shows empty when the S10 Tank is full and full when it is empty. Why would it be showing opposite?
 

Downey

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Possibly for the same reason some sending unit manufacturers make a 0-90 and a 90-0 ????. Any stock Landcruiser sender you used in a GM tank will need to be modified for depth, plus make sure the mounting does not interfere with the travel of the float arm (i.e. float hitting some wall of the tank).
 
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I messed with this a lot, I don't know what year it all ultimately changes. I assume the latest models of Toyotas are different, but clear up to about 2000 at least the sender and gauges work on the principles that Downey noted on all Toyotas. High ohms when empty and low ohms when full. The sender creates the resistance via the rheostat windings that the float arm sweeps over. The arm in the gauge basically heats and deflects depending on the power coming through it. Chevy stuff is 90 ohms full and 0 ohms empty. That is why the gauge reads backwards.

I tried a few things to make this work in a different situation. I was trying to use the factory sender in an FJ80 Auxiliary tank to light three LEDS to denote empty or full. You could buy off the shelf stuff that would work with the 0-90 sweep of most aftermarket senders and gauges, but nothing for Toyota. There is no combination of resistors that can make the ohms right at the gauge. There is however, at least, two products I found that are electronic wizardry that will make any sender work with any gauge. Google fuel sender convertors, see if anything pops up, they were kind of pricey for my project, so I didn't get any. That was the only way I found to do it properly.

Guess you could change the E and F on the face of the gauge, it will be kind of right most the time.
 
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I messed with this a lot, I don't know what year it all ultimately changes. I assume the latest models of Toyotas are different, but clear up to about 2000 at least the sender and gauges work on the principles that Downey noted on all Toyotas. High ohms when empty and low ohms when full. The sender creates the resistance via the rheostat windings that the float arm sweeps over. The arm in the gauge basically heats and deflects depending on the power coming through it. Chevy stuff is 90 ohms full and 0 ohms empty. That is why the gauge reads backwards.

I tried a few things to make this work in a different situation. I was trying to use the factory sender in an FJ80 Auxiliary tank to light three LEDS to denote empty or full. You could buy off the shelf stuff that would work with the 0-90 sweep of most aftermarket senders and gauges, but nothing for Toyota. There is no combination of resistors that can make the ohms right at the gauge. There is however, at least, two products I found that are electronic wizardry that will make any sender work with any gauge. Google fuel sender convertors, see if anything pops up, they were kind of pricey for my project, so I didn't get any. That was the only way I found to do it properly.

Guess you could change the E and F on the face of the gauge, it will be kind of right most the time.
 

Engineer8000

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According to the great @bj40green write up, 9/72 to 9/73 cluster still had a 7v regulator in the fuel gauge, and used the 9/72 to 9/79 sender, 83320-60050. This is what I have in my 10/72 truck. This sender reads 120 ohms empty and 17 ohms full. I tested a spare sender and these numbers are correct. I also recorded how the sender is wired. The level gauge is basically a wiper that moves up and down a coil of resistance wire as level changes. When full, the wiper is at the top of the coil, when empty it is at the bottom of the coil. 7V is applied to the top end of the coil, the other end is insulated, the wiper is connected to ground.

20211220_155524.jpg


20211220_160551.jpg



This arrangement makes sense if you look at the gauge. This is one I tore apart that had a failed voltage regulator a few years ago (to see how it works of course).

20211220_155600.jpg


The needle is moved by a bi-metallic strip that bends when it is heated. The more current, the farther it bends, hence the lower resistance of the sender to ground when the tank is full. The coil on the right is the 7v point type regulator. This is if your cluster rear looks like this, you can see where the voltage regulator grounds to the back of the cluster (little square area) and the three terminal gauge.

1640036220226.png



Based on what I could find on the S10 sender, it reads 0 ohms empty, 90 ohms full. That is in line with what you said about the gauge when connected to the S10 sender. That is the problem. The resistor on the S10 sender is wired backwards.

image_2021-12-20_163011.png


It looks like the blue wire is your sender, you would have to find a way of reversing the action of that sender resistor and add some external resistance so it isn't siting on 0 and pegging your fuel meter when full. Does someone make a sender that fits your s10 tank that has adjustable resistance?
 
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According to the great @bj40green write up, 9/72 to 9/73 cluster still had a 7v regulator in the fuel gauge, and used the 9/72 to 1/79 sender, 83320-60050. This is what I have in my 10/72 truck. This sender reads 120 ohms empty and 17 ohms full. I tested a spare sender and these numbers are correct. I also recorded how the sender is wired. The level gauge is basically a wiper that moves up and down a coil of resistance wire as level changes. When full, the wiper is at the top of the coil, when empty it is at the bottom of the coil. 7V is applied to the top end of the coil, the other end is insulated, the wiper is connected to ground.

View attachment 2870614

View attachment 2870615


This arrangement makes sense if you look at the gauge. This is one I tore apart that had a failed voltage regulator a few years ago (to see how it works of course).

View attachment 2870618

The needle is moved by a bi-metallic strip that bends when it is heated. The more current, the farther it bends, hence the lower resistance of the sender to ground when the tank is full. The coil on the right is the 7v point type regulator. This is if your cluster rear looks like this, you can see where the voltage regulator grounds to the back of the cluster (little square area) and the three terminal gauge.

View attachment 2870627


Based on what I could find on the S10 sender, it reads 0 ohms empty, 90 ohms full. That is in line with what you said about the gauge when connected to the S10 sender. That is the problem. The resistor on the S10 sender is wired backwards.

View attachment 2870625

It looks like the blue wire is your sender, you would have to find a way of reversing the action of that sender resistor and add some external resistance so it isn't siting on 0 and pegging your fuel meter when full. Does someone make a sender that fits your s10 tank that has adjustable resistance?
Wow, Thanks for taking the time to put this together. I will have to take some time to sift through it. I appreciate you!!!
 

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