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GM Fuel sending unit conversion 30 ohm

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46K views 28 replies 11 participants last post by  75gmck25  
E
#1 · (Edited)
I'm restoring on older Chevy and needed a new sending unit which is no longer available,I found on the internet an old post where the person said that you can convert a 90 ohm sender by the addition of a 47 ohm resistor put across the terminal where the gauge wire attaches and the ground on the sending unit. Has anyone heard of this? http://www.oldengine.org/unfaq/oddbits.htm

Thanks for any input I haven't pulled the tank down yet for sealing just wanted to see if anyone had heard of doing this
 
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#2 ·
Doc here, :pimp:

Yes, It works quite well..

However, I like to replace the resistor with a 50 Ohm potentiometer, mounted on an "L" bracket aluminum piece , in this way, you can "Fine Tune" the readout by setting the pot for the MOST accurate readings!

The pot needs to be a Linear Taper, 50 Ohms, at about 5 Watt, Can be had at rat shak for about $2 Bucks.

Configure it thus: The lower terminal and Center wiper together, with the sender wire attached there, And the upper terminal to the gauge. Use heat shrink on the terminals so nothing can short them out.

When you have it totally adjusted, melt some hot wax or hot glue on the pot shaft so it won't move until you may want to adjust again, and your good to go.

Doc :pimp:
 
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#3 ·
Doc, Thanks again for the info,going by the description it sound as though
nothing gets grounded. It sounds more as though its almost in series with the gauge terminal on the sender or I'm just reading too much into it. Could you also just put the pot/resistor across the gauge terminals? or does it have to be on the sender. Thanks, Eric
 
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#4 ·
Doc Here, :pimp:

If you put the resistor or pot across the Gauge, It would read Full (or Empty) 24 / 7 / 365....

The pot or resistor goes in the sender wire, Cut it and attach one end to the resistor, and attach the other end to the other end of the resistor or pot..

On a pot, Terminals 1 and 2 tie together with one end of the wire, and terminal 3 gets the other end of the wire..

The Sender is what provides the ground through It's own "Variable Pot" on the float arm, to ground..one side of the sender is on ground the other is the sender information wire (to gauge) You are adding resistance to that wire by adding a resistor or pot..to balance the system to 90 ohms.

BTW , whenever I use a pot, I start with an empty tank, adjust for sub zero indication on the gauge, then add 3 gallons of fuel..then adjust for "Empty". In this manner, you will always have 3 gallons of "headroom" whenever the gauge goes to "Empty".

Doc :pimp:
 
S
#7 ·
docvette said:
Doc here, :pimp:

Yes, It works quite well..

However, I like to replace the resistor with a 50 Ohm potentiometer, mounted on an "L" bracket aluminum piece , in this way, you can "Fine Tune" the readout by setting the pot for the MOST accurate readings!

The pot needs to be a Linear Taper, 50 Ohms, at about 5 Watt, Can be had at rat shak for about $2 Bucks.

Configure it thus: The lower terminal and Center wiper together, with the sender wire attached there, And the upper terminal to the gauge. Use heat shrink on the terminals so nothing can short them out.

When you have it totally adjusted, melt some hot wax or hot glue on the pot shaft so it won't move until you may want to adjust again, and your good to go.


Doc,
The way I read this I can go anywhere in the sending unit wire (where I can access it) and wire it in series, did I read it right!
I am trying to get my gauge to read empty instead of overfull with a full tank and empty at 3/4 of a tank!!
Click to expand...
 
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#8 ·
Doc here, :pimp:

Yes, Provided the wire is NOT part of the problem..(Shorting ect..) You can install it anywhere on the sender wire..

My advise to you is on a "Calibration problem" is use the potentiometer, In stead of a fixed resistance..This way you have the ability to "Adjust" the resistance at full or empty...

And as I said before, when I do It, I start with an empty tank, adjust for sub~zero, then add 3 gallons, and zero the pot for "E"..this way you always have a 3 gallon reserve at empty..

Doc
 
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#11 ·
I didn't use the pot. I just used the 47 ohm resistor and then ran the sender through its full range and it ended up being a 31ohm sending unit,so it does work great,I put it across the sending unit wire terminal and then to where the ground to the frame wire goes in a parallel circuit. I did try before putting it in series with the sending wire that goes to the guage but it didn't work for me.
 
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#13 ·
Doc here, :pimp:

First, you need to know two things..

The resistance of the gauge, and the resistance of the sender you wish to use..

For example, If your gauge is a 90 ohm unit and the only sender you can find is 30 ohm, you'll need to add a 100 ohm pot, and adjust for accuracy..

If the Values are unknown, Get an assortment of 1 watt resistors from Rat Shak, for about $4 bucks, and using the sender wire, substitute each value resistor in the line, to ground, until you get the desired range of meter sweep..starting with about 30 ohms.. (The color code is on the back of the package) . NOTE THAT VALUE.

When you find out what that is, remove and measure your sender on OHMS scale of your DVOM, through the FLOAT range..it should be something like 0 to 30 or 30 to zero..NOTE THAT VALUE.

Next figure the difference Between the sender and the gauge resistance, and obtain a pot (if not the exact value as close , but higher resistance Value as practical..(I.E...If you need 60 ohm, you may only be able to find a 68 ohm pot..that's OK, because you can adjust it down)

Install the pot in series with the sender line , check and calibrate the gauge..once done seal the pot with a hardening, waterproof sealer.

Doc :pimp:
 
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#14 ·
Hey Doc I have a question. I have an antique auto and the sender has been repaired too many times, it's shot. Can I attach a linear pot to the float were it once went through the float sender housing and worked against the curcuit. This would mean that the pot would now work inconjunction with the sender instead of using the destroyed curcuit panel that attaches to the side rim of the sender. It also 6 volt positive ground. If memory severs me correct, I did these years ago with another antique auto and it worked well. Is it dangerous? The pot is on the top of the sender away from the fuel. Should I just by a JC WITNEY sender and use a pot inline. Thanks in advance, Dave
 
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#16 ·
YES, he is missed but his great information and spirit will live on forever here on the web !!!

I could always count on him to set me straight !!
 
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#23 ·
DAVE GIORGIANNI said:
Hi, I have a question, If you just bought "new" gauges and a "new" sender why didn't you buy the sender to match the gauge. There only 20 or 30 dollars? What type or brand of gauge did you use, what type or brand of sender did you use. Sounds like you are going to step over 5 dollar bills to get to a one dollar bill... Dave.
Click to expand...
Hi,

The tank,, a 66 chevy pickup truck had a new sender in it. They dont make a 90 ohm sender for that tank. I did buy the gauge and when it didn't read right I called the manufacturer and they told me it was a 90 ohm gauge,, standard after about 67 on GM cars. They talked about something to convert ohms,, what I think this thread is about but I really need detailed (like part numbers) of what I need.

Thanks!!
 
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