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320 Gas tank Sending Unit Clean Up & Test


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Today spend the morning pulling out the sender on my new gas tank I scored ; )





Took out the 5 screws, took my time, cleaned out heads to not strip them.




Then it pulled right out







As I pulled it out it looked to be in good shape.  No rust or grime but the arm was totally jammed and not moving.  So I opened up the 4 screws to the rheostat and there was the infamous "little wire".  Had to be really careful around this thing.







Then proceeded to clean it out and spray some PD Blaster on it.  Was going to soak in vinegar for a day but I thought might as well start with the PB.  Spent a good 40 minutes cleaning this thing out and the arm started to move.  I dropped two drops of oil inside around the moving hinge and this unlocked it.  It now swings easily and freely!  Cleaned up up inside as best could and let it air/dry out then capped it all back up.










Once I had it cleaned up I did an ohm test to check out functionality


This was my reading for a full tank




And my reading for empty





Then I checked the wires/leads on the truck and got close to 7 volts on there




So went ahead and wired up the sender to do a test.  Before I did I checked the gauge with the wires not connected and it pegged at Full.  I think this means infinite resistance so the gauge goes all the way to Full




When I wired up the sender this is what I got for empty




And when I adjusted the sender arm to full this is what I got




Not sure why the needle is not moving all the way to full.  Wondering if it has to do with only having around 7volts at the sender?  It works which is good but I'm trying to think of why 83ohms would not make the needle move all the way to full.  Anyone have any ideas?












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Full tank, means high resistance, is that correct?

It may be the sender is still not fully travelling all the way to the maximum resistance.  See if there is still something hanging it up.


It is much more important that the fuel gauge reads accurate when the tank nears empty.  If you cannot fix the full tank equals full gauge reading, maybe just live with it.


The fuel gauge on one of my 521 trucks does not go all the way to full.  At about 100 miles of driving the gauge is somewhere close to middle.  At 200 miles of driving, it nears empty.

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10 or under is FULL


80 and over is EMPTY. This is true only for the Datsun  tanks I have seen, '71 and up



But you have a positive ground? Seems like max resistance is FULL  I guess when the ignition is off it reads full.

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To figure out what your gauge actually needs, go get a guitar amp volume knob and wire it into the sending unit harness. Adjust the knob to get the desired reading, then check the resistance across the volume knob with an ohm meter. If you still can't replicate the needed ohm range with your sender, try wiring in a resistor.


I've had to do this a few times and it works great, if you have the patience to do the diagnostics, which it sounds like you do.

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I just connect the wires to the sender in the truck and move the float arm to empty, half, full and if it reads correctly I call it good.

The 320 sender and the very early 1500 roadsters are different than all the rest of the senders made after them, if you connect a 521 sender to the 320 wires in a 320 truck the gauge reads the opposite it should, empty is full and full is empty, only a 320 and early 1500 roadster sender will work properly in a 320.

The fuel gauge in a 320 shows empty when the ignition is off, but the temp gauge shows full hot when the ignition is off.

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At this point its all working so I'll call it good enough for now.  Like DanielC mentioned really the more important reading is the empty mark, not that big of a deal if my full level sits a bit above halfway.  My hunch is that 83ohms is not quite enough (needs to be 90) to get the needle to move all the way full.  I may hack away some more at it tomorrow.  Thanks for insights - next up is taking my tank to get cleaned up.

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Yes, it will. I was sharing a method I use for diagnosing. Sometimes the resistor works, sometimes not.


But the guitar amp volume knob is a trick I am especially proud of coming up with. It helps diagnose all kinds of senders and sensors. I used it in place of an EFI temp switch once while diagnosing the problem and I could affect the tuning just with the turn of the knob. Maybe I geek out a bit on that stuff...

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Nice clean up job Eddy. Glad the sender was repairable. Tank inside as clean as  it seemed?

The early 60's temp and fuel gauges are adjustable from the back.

You can adjust the needle to sit at the full mark when the tank is full.

Wow great tip. I've got a 62 and I'd love to ditch my aftermarket temp gauge. I take it you adjust it to cold when its cold?

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Here is a write up for the procedure on NICO.


There are two adjustment holes on each gauge. The one indicated adjusts the needle in the hot position.

What you are turning inside the gauge is like a rheostat to change resistance.

Same concept applies to the gas gauge.

The other hole is to adjust the set point where the needle rests when you turn the key off. Really of not much concern and I have not tried it.

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I took a 320 temp gauge apart. The adjusting wheels were stuck and I was unable to move them from the back. I had to spray some lubricant on them from the inside and grab with pliers and rock until they were free. I imagine this will be a common problem after 50 years without movement. The adjustments are mechanical, not resistive. Adding resistance to the circuit will also affect the reading, but would affect the whole range and is not preferred. 
I have a good picture to post, but still can't figure out how to post here. I am going to send it to Wayne and ask him to do it. He seems to know the secret.


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Unfortunately I do not have a photo hosting service Steve, I was talking about the senders on another Datsun site when I posted the other photos for you, that site does photos a different way and I posted the photos of the senders there as it was relevant to the topic, and then I copied them and posted them here, but we are not talking about the gauges over there so there is nothing to copy to post here.

Last time it just worked out, not so lucky this time. 


So I joined Postimage and now have the photo to post here.


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I decided to get more info on the fuel gauge. The fuel gauge adjustments are a little different than the temp. They have little sliders that move after loosening up the nuts, instead of the stars like the temp gauge has. I proceeded to loosen them and see how they affected the high and low reading. I got them so screwed up that the gauge didn't work anymore. After playing with them for a long time and getting nowhere, I decided that I had nothing to lose so took the gauge apart. It turns out that they control the position of two electromagnets. I never did get them in exactly the right spot to work right. My advice is not to monkey with the adjustments. Here are some pictures.

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