replacement sending unit inaccurate

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JonDangerous

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1971 Fj 40. I had to replace my gas tank. Bought an aftermarket tank. Came with an aftermarket sending unit. Testing sending unit (before installing it) shows that when float is totally extended and gauge should read empty, the gauge shows still half full. Moving float up to about half makes the gauge show full. The OLD sending unit works perfectly. But, the old sending unit is slightly too big around the top of the neck (see photo) to fit into aftermarket tank. I could use dremel tool to widen the throat of the tank (about 1/32 inch) and it should fit, but would rather not do that. The aftermarket sending unit shows 120 ohms when fully extended and about 20 ohms when in "full" position. ANYWAY: Has anyone run into this issue? If so, what did you do? Any ideas? Would one of the "generic" sending units you can buy (with 180 ohms when in empty position work? Meaning, would a generic unit have the attachment holes lined up right?

any thoughts appreciated!

SENDING UNIT
 
B y r o n

B y r o n

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So if the original sending unit works, how do the ohm values compare to the aftermarket unit? If they are not the same the aftermarket sender will not work with the factory gauge. If the values are the same then maybe you need to bend the arm on the aftermarket sender so it is in the right position at full and empty.
 
Engineer8000

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A 71 should use the 83320-35020 pre 72 unit that should read 3 ohms full, 110 ohms empty. The 10/72 + senders are 120 ohms empty, 17 ohms full. It sounds as if your sender is the later style.
 
JonDangerous

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A 71 should use the 83320-35020 pre 72 unit that should read 3 ohms full, 110 ohms empty. The 10/72 + senders are 120 ohms empty, 17 ohms full. It sounds as if your sender is the later style.
Thanks for that part number. That does look exactly like the old one I pulled out. So, I'd have to modify the opening in the aftermarket tank a bit for it to fit, but at least I'd be putting in a new sender.
 
JonDangerous

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So if the original sending unit works, how do the ohm values compare to the aftermarket unit? If they are not the same the aftermarket sender will not work with the factory gauge. If the values are the same then maybe you need to bend the arm on the aftermarket sender so it is in the right position at full and empty.
It's hard to tell really, when ohm meter I'm using jumps around a lot so hard to get a firm reading on either the replacement or the original. Why does the ohm meter jump around even when the float arm is held absolutely still?
 
Engineer8000

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The resistance wire gets oxides on it, the ohm meter uses very little current to "burn" through the noise. Just my guess on what is going on. Which version are you trying to measure?
 
Engineer8000

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I believe the 83320-35020 version has a different operating mechanism than the later one. The 10/72 + version is a wiper on resistance wire. I believe the early one has a points type voltage regulator inside it similar to the oil pressure sender, so measuring the resistance doesn't make any sense to me for that one. I'm not sure where the 3-110 ohm numbers came from, I've seen them listed on here in other threads.
 
JonDangerous

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The resistance wire gets oxides on it, the ohm meter uses very little current to "burn" through the noise. Just my guess on what is going on. Which version are you trying to measure?

Oh! good stuff, that would explain it. I tried measuring the replacement sending unit AND the original. In both cases very difficult to figure out what the ohms were.
 
JonDangerous

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I believe the 83320-35020 version has a different operating mechanism than the later one. The 10/72 + version is a wiper on resistance wire. I believe the early one has a points type voltage regulator inside it similar to the oil pressure sender, so measuring the resistance doesn't make any sense to me for that one. I'm not sure where the 3-110 ohm numbers came from, I've seen them listed on here in other threads.
Yes, the original version I have is definitely a different operating mechanism, it is NOT a wiper on resistance wire. It is a mystery how it works, but you're suggesting points? If like points in a voltage regulator, then the points are opening and closing extremely fast. I suppose the lever pulls the points further apart, or closer together, changing the voltage? The old one I pulled out was disastrously rusty, a horrible mess, but it still works perfectly. (The picture is after it had been submerged in white vinegar for a week, the rust mostly came off easily after that.) The design is such that I can't see the points. Learning a lot from you guys, thanks, if you can think of anything else about this system, I'd appreciate hearing it! I'm leaning toward widening the opening on the aftermarket tank so I can use the original Toyota sending unit.
 
Engineer8000

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This is the inside of the oil pressure sender, the diaphragm moves the points closer together as pressure increases. Same sort of thing happening in the older fuel senders.

1641658696033
 
JonDangerous

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Makes sense, I'm out of town now, but hope to get back to this next week.
Updating on what I did. I tried several ways to manage the voltage with the "wiper" type new sending unit the tank supplier sent me by putting a variable resistor in series, but not good: what I could do was set it to show empty when it would be empty, but then when float was moved to full it would only show 1/4 full... I suppose I could have lived with that, but decided to do the work to re-install the old sending unit. For those who've purchased, or are planning to purchase an aftermarket tank, the neck is just a little too small (maybe 1/32 inch) for the old sending unit to go in. Decided to use a dremel tool to widen it slightly. very carefully went round and round, trying to be sure I didn't take too much off anywhere. It worked. HINT: If you are concerned (as I was) that the shavings of steel would drop into tank and be a bummer, I just reversed the hose on my shop vac and put it into the tank fill tube. That meant MUCH air was blowing upward out of the sending unit opening as I was doing the grinding work, I doubt ANYTHING dropped in. OH: And of course, never try anything that could cause a spark unless tank is totally empty, I mean totally empty, no gas, no fumes. Mine had never had any gas in it, so completely safe. Happy camper.
 
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Racer65

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Over the years I've had a couple of people tell me a brand new 83320-35020 OEM sender (for '66 to '72 FJ40) I sold them is defective because it doesn't have resistance between 3 ohms and 110 ohms. I've seen this range cited a few times on IH8MUD; however, I've never seen such spec published in the FSM.

What I can confirm is that if I measure any new OEM 83320-35020 sender I have in stock, the resistance will range from 0 to around 22 ohms as I move the sender's float from one end to the other. All units measure the same.
 
Downey

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Toyota (TMS) originally gave me the math on the 3-110 and 17-120 ohms values, but the pre 9-72 also works on a 7 volt drop, I believe the 7 volt drop is at the dash though, not at the sender ???
 

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