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Fuel Gauge Dead?

cornemuse

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Couldnt find an 'electrics' area, so here goes. '84 Blazer diesel, but that shouldnt matter.
The fuel gauge stopped working. When key is on, gauge goes to 'full', turn off & it creeps towards empty. < Does it need power (un-blown fuse) to do this?

How does one troubleshoot this?

How does one even get to the sender unit?

-c-
 

Will L.

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I am not familiar with this exact rig.
But generally two ways it is done.
Full battery voltage to the gauge in dash through a fuse. The sending unit (inside the tank) has a float attached rheostat that is tied from ground on one side and the gauge on the other. So as the level rises the resistance to ground changes and that voltage differential is what moves the gauge.

so no fuel means barely contact to ground, gauge to ‘E’
Half tank moves rheostat so half the ohms is present , gauge reads half.
Full tank, maximum ohms for the rheostat, tank at “F”
Ground out the wire at the tank bypassing the rheostat and the gauge will go past Full to 3 oClock position. This will not blow the fuse.

Some vehicles actually send power through the rheostat and back to the gauge. The other side of the gauge gets permanent grounding (battery negative). So grounding out this type will instantly blow the fuse and not alter gauge reading. This system was not used much in older rigs because the idea of sending electricity into a fuel tank was frowned upon. Since in tank pumps became normal- this is becoming more common.

hope this helps you.
 

MrMarty51

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The DOT over here has a gauge tester unit. Hook it to the sender wire then adjust the pots to the spec listed on the sheet.
That little critter was a mighty handy unit.
Someone blew one of the dashpots, took a little calculating but the other pots could be adjusted to make up the difference.
 

MrMarty51

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After trying to find a C-3826 electrical tester, I decided to make my own. What I used was a 0-100 ohm variable pot resistor ( about $12 Canadian ) and a solar powered deck light I had kicking around ( I didn't feel like making an extra trip out for a box).After installing the pot resistor into the case ,I used a good ohm meter to set calibration index marks around the face of the case. The paint I used was the same paint I used for the needle pointers on the gauges ( bought from a local dollar store ). I also ran the wiring long enough to sit on the dash while viewing the car gauges
Full =10 ohms
Med =23 ohms
Empty=74 ohms

3671F245-1C28-4A0C-9765-BB13D3981FC5.jpeg

81484DD5-B72E-4BCE-A096-CAC7F45E5D2D.jpeg
 

MrMarty51

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And a little more.
I have used the headlight switch to test fuel, temp and oil pressure gauges. If you hook the B2 terminal on the headlight switch to ground and the I terminal to the sending unit terminal of the gauge or the wire to the sending unit, pull the switch to the park light position and rotate the knob the gauge should go up and down accordingly.
 

Will L.

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Very creative, yes. Overly complicated for fuel gauge testing imo. Ground the tank wire and at worst replace a fuse then a person which system.

Back to this exact rig.

I bumped my head and realized- look up the part. https://www.rockauto.com/en/catalog/chevrolet,1984,k5+blazer,6.2l+379cid+v8+diesel,1050589,fuel+&+air,fuel+sending+unit,4436

YES, key has to on to work.
It is 1 wire grounding unit. If you disconnect the wire that goes to the tank, and ground it out with the key on- the gauge should go to full. So you either have that 1 wire grounding out somewhere, or the sending unit is malfunctioning.

Bad news- drop the tank. Only way to get to it in a blazer. But you can sometimes find the swing arm is stuck and can be cleaned up and get it working. More often the contacts od the rheostat are corroded. You can try cleaning them up, usually the little copper tab that sweeps across the little line contacts is it and just emery cloth kn it does the trick. To test just plug it back in and move float arm while someone watches it. Otherwise a new unit is needed. Yes you can use any potentiometer to the ground wire for different readings to verify the sending unit is bad not the gauge. But really it is like 1 gauge goes bad for every 30 million sending units. Notice all parts stores sell tons of sending units and almost no one replacement gauge clusters...

If not sure and need to return for money- just plug it in before installing in tank and move arm while someone watches gauge. If it doesn’t work then your lottery winning odds says return the sending unit and buy a gauge cluster or a stand alone gauge more likely because of no availability.

If you can’t find the sending unit anywhere- seeing as rock auto above is out of stock- get the one from Leroy. Just plug it in to test with your dash gauge first before cutting anything up.
 

Husker6.5

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I was doing that with the Money Maker the first few months I had it, the fuel gauge always read F when I bought it. After twice running out of fuel when I thought there was more in the tank than there evidently was, I crawled under her and found the frayed end of the sender wire to the gauge hanging and touching the frame.

So, cleaned everything up and spliced in a new section of wire. Hopped in and turned the key to Run to see what happens, as I knew there was between a ½ and ¾ tank of fuel in it as I'd topped it off two days and 180 miles ago. The gauge needle pegged out about a needle width below the E! Ok, sender must be bad. The big chain parts stores didn't show a diesel sender (this was back in 2000) for her, so I went to a locally-owned parts store. They had a listing for a diesel sender for the large capacity side tank for a '94 in their BOOK (NOT computer, like the three Big Chains in town), they could order it in, be there in two days. A little over $200, iirc. Dropped the approx. ⅛ full tank the weekend after the sender came in, swapped in the new one, turned the key, and the needle sat between ⅛ and ¼ on the gauge. Went over to the station to fill it with diesel. Left the key on and driver's door open as it filled, it was so satisfying to see the needle climb as it filled. Solved the paranoia of having to do jobs way out in remote rural areas without a working gauge and trying to guess what kind of MPG I was getting based off of load, weather, roads used, etc. Worth every penny spent.
 

Will L.

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I was doing that with the Money Maker the first few months I had it, the fuel gauge always read F when I bought it. After twice running out of fuel when I thought there was more in the tank than there evidently was, I crawled under her and found the frayed end of the sender wire to the gauge hanging and touching the frame.

So, cleaned everything up and spliced in a new section of wire. Hopped in and turned the key to Run to see what happens, as I knew there was between a ½ and ¾ tank of fuel in it as I'd topped it off two days and 180 miles ago. The gauge needle pegged out about a needle width below the E! Ok, sender must be bad. The big chain parts stores didn't show a diesel sender (this was back in 2000) for her, so I went to a locally-owned parts store. They had a listing for a diesel sender for the large capacity side tank for a '94 in their BOOK (NOT computer, like the three Big Chains in town), they could order it in, be there in two days. A little over $200, iirc. Dropped the approx. ⅛ full tank the weekend after the sender came in, swapped in the new one, turned the key, and the needle sat between ⅛ and ¼ on the gauge. Went over to the station to fill it with diesel. Left the key on and driver's door open as it filled, it was so satisfying to see the needle climb as it filled. Solved the paranoia of having to do jobs way out in remote rural areas without a working gauge and trying to guess what kind of MPG I was getting based off of load, weather, roads used, etc. Worth every penny spent.
Yeah, but I bet you no longer carry a full jerry can like you used to!! Haha
 
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