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Old 07-21-2020, 08:57 PM   #1
Copper116
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Fuel gauge problem/question

The fuel gauge in my 72 K5 Blazer was never working right... So I pulled the tank, replaced the sending unit with a new one, installed a new fuel gauge wire lead to the fuse box, made sure the ground was clean and lubed and the fuel gauge still only moves a bit off 1/4 tank.
I checked for power at the fuse box power lug with just a tester light so far and it barely lit up the tester light. I pulled power from another 12v source on the fuse panel for the sending unit and the gauge went to full but back to 1/4 tank when I cut power.
I did replace the resistor on the back of the instrument panel with one (used one) off another cluster I had... and it worked for a while but now it's back to 1/4 tank even though we put in enough gas that should bring it well over 1/2 tank.


ANY IDEAS TO HELP CORRECT THIS PROBLEM WOULD BE APPRECIATED !! THANKS.
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Old 07-21-2020, 10:20 PM   #2
57taskforce
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Re: Fuel gauge problem/question

I bet it’s the resistor again. That’s what mine did when the same when the resistor was bad.
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Old 07-22-2020, 12:29 AM   #3
TBONE1964
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Re: Fuel gauge problem/question

Typically it is the resistor that will cause this,

Truck and Car shop in Orange CA sells new porcelain style resistors.

If you have a temp gauge, you can use the resistor from it to test. I have never had another problem cause the "1/4 tank" issue.
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Old 07-22-2020, 10:55 AM   #4
Copper116
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Re: Fuel gauge problem/question

Thanks all.... I'll look into that resistor.
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Old 07-23-2020, 06:10 PM   #5
Copper116
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Re: Fuel gauge problem/question

I checked with Truck and Car shop in Orange CA. They do not have any in stock; their supplier does not sell them anymore. These resistors are almost as scarce as toilet paper was...
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Old 07-23-2020, 07:59 PM   #6
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Re: Fuel gauge problem/question

Quote:
Originally Posted by Copper116 View Post
I checked with Truck and Car shop in Orange CA. They do not have any in stock; their supplier does not sell them anymore. These resistors are almost as scarce as toilet paper was...
Before you go to all the trouble of tracking down one of those resistors, check the one on your gauge.. The resistor cannot be accurately tested while attached to the gauge.. Doing so, renders a reading that includes some of the "internal" resistance of the gauge also.. Remove the resistor from the gauge and then take your reading.. Should be in the area of 85 - 90 ohms.. Anything else indicates a bad resistor.

I have repaired numerous gauges using a commonly available 85 - 90 ohm 3 watt metal film resistor.. The leads are long enough to fashion loops that fit over the posts of the gauge.. The resistors are available from Mouser electronic supply... Their warehouse is located in the Dallas, Texas area...
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Old 07-23-2020, 09:56 PM   #7
lutronjim
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Re: Fuel gauge problem/question

A question for RustyPile.

Why are the stock resistors apparently wire wound over a flat board?

Thanks
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Old 07-24-2020, 12:40 AM   #8
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Re: Fuel gauge problem/question

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Originally Posted by lutronjim View Post
A question for RustyPile.

Why are the stock resistors apparently wire wound over a flat board?

Thanks
It's designed to specifically fit the gauge. Referred to as a wire wound resistor. The wire is special resistor wire, similar to the resistor wire used in a points type ignition... The resistance is determined by length of the wire..
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Old 07-24-2020, 12:53 AM   #9
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Re: Fuel gauge problem/question

Mouser's website can be difficult to navigate for some people.. Here's a link to the resistor I use:

https://www.mouser.com/datasheet/2/4...fs-1762753.pdf
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Old 07-27-2020, 08:09 AM   #10
Copper116
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Re: Fuel gauge problem/question

Thanks RustyPile. I'll run a test on my resistor. I do have a lead on a new resistorI'm following up on.. but if that falls through and I truly need a new resistor, I'll take your advice on the replacement one.
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Old 08-12-2020, 12:10 AM   #11
Copper116
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Re: Fuel gauge problem/question

The resistor tested fine... but I cleaned it up a bit and reinstalled it... now it seems to be working... go figure! Who knows... as long as it's working, I'm good with that. Thanks all !!!
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Old 08-12-2020, 06:15 AM   #12
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Quote:
Originally Posted by Copper116 View Post
The resistor tested fine... but I cleaned it up a bit and reinstalled it... now it seems to be working... go figure! Who knows... as long as it's working, I'm good with that. Thanks all !!!
A large portion of the electrical problems with these old trucks can be traced to corrosion and bad connections.. The worst thing a person can do is to indiscriminately throw parts at it hoping something will stick.. Job well done Mr. Copper... You used a systematic approach and found the problem...
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Old 08-13-2020, 12:50 AM   #13
Copper116
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Re: Fuel gauge problem/question

I appreciate the pat on the back there RustyPile !! But wouldn't you know it... we go to fill it up and it stays there. A little ways down the road, it goes to full.. then after it sits a bit... and we start it up again... the fuel gauge doesn't register right.. It's got to be a bad connection or, the float I would think may be sticking.. but we replaced the sending unit with a brand new one! We tested it's function before we put it in the tank and it tested fine... We tested a new wire lead to that we bought and it worked just as well as the original. I really don't want to pull the tank again... this is aggravating.. so we'll just keep topping off the tank till time to put it in winter storage. We'll tackle it again next spring once the snow is gone and it warms up. We don't put the top on it... it stays in storage.. it's a double walled top. I have a single wall top but will probably get rid of it. I don't have the time to clean and restore it anyway. Thanks again.
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Old 08-13-2020, 06:55 AM   #14
RustyPile
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Re: Fuel gauge problem/question

Quote:
Originally Posted by Copper116 View Post
I appreciate the pat on the back there RustyPile !! But wouldn't you know it... we go to fill it up and it stays there. A little ways down the road, it goes to full.. then after it sits a bit... and we start it up again... the fuel gauge doesn't register right.. It's got to be a bad connection or, the float I would think may be sticking.. but we replaced the sending unit with a brand new one! We tested it's function before we put it in the tank and it tested fine... We tested a new wire lead to that we bought and it worked just as well as the original. I really don't want to pull the tank again... this is aggravating.. so we'll just keep topping off the tank till time to put it in winter storage. We'll tackle it again next spring once the snow is gone and it warms up. We don't put the top on it... it stays in storage.. it's a double walled top. I have a single wall top but will probably get rid of it. I don't have the time to clean and restore it anyway. Thanks again.
Well shoot!!! It does sound like a loose connection.. Here's something you can do without removing the tank.. Fabricate a ground wire/strap for the tank.. Normally, I don't recommend drilling holes in a gas tank, but this problem requires thinking outside of the box. The tank is made up of 2 pieces welded together.. If you look close, you can see the weld seam near the edge of the tank half flanges.. Drill a small (1/8" or slightly larger) hole in the corner of the tank.. One of the corners is the best place.. Be sure to drill OUTSIDE the weld seam.. Clean the area around the hole.. Make up a ground strap long enough to reach between the hole and the frame.. Either drill a new hole in the frame or use an existing hole. Clean the area area around that hole also. Solder terminal ends on each end of the wire/strap.. Use a sheet metal screw to attach the wire/strap to the tank, and also firmly attach the other end to the frame.. Use star washers on both ends..

If this doesn't correct the balky gauge, check and clean each and every connection associated with the fuel gauge circuit -- EVERY ONE.. Good luck with your troubleshooting.. Post your findings...
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