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Old 05-05-2013, 05:50 AM   #1 (permalink)
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Date registered: Apr 2013
Vehicle: 1972 MOG 404 / 1963 VOLVO C202 LAPLANDER
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Posts: 22
My 404 fuel gauge is pegged full all the time???

Cant figure out whats wrong with it. You turn on the key, and it reads full. I pulled both leads off the tank and it reads the same until I turn off the key. Traced the wiring for a pinch ground, but nothing. I can however pull off one of the 3 wires at the gauge and it will go to empty. What gives???
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Old 05-05-2013, 06:36 AM   #2 (permalink)
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Did the gauge work before? If not, are you positive your reading the gauge right?
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Old 05-05-2013, 06:57 AM   #3 (permalink)
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Date registered: Jan 2011
Vehicle: 1978 406 w/ Eller hardtop
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I have the same problem on my 406.

I eliminated the gauge as cause by putting resistors in place of the sender. A 100 ohm resistor should yield a reading of 1/2 full on the gauge,

If I does, then you have eliminated the gauge and wiring harness.

That leaves the sending unit and its connections as suspects. That's where I am now.

Best of luck. I'll report more as I learn it.

Schroeck


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Old 05-05-2013, 07:33 AM   #4 (permalink)
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Ya'all are on the right track. The full-tank gauge reading is caused by an open circuit at or to the sender. The sender is about 200 Ohms so may be checked with an Ohm meter. Remember to compensate for however much gas is in it. Any reading up to 200 Ohms is probably good.

One of the wires at the sender has to be to ground. If ground is lost the gauge goes high. Clean up all the screws that hold the sender into the tank (to ground the sender to the tank) and also verify the ground wire is clean on it's screw.

If the gauge has never worked then it's possible the two wires got switched sometime. Same results, full reading. I had a 404 wired that way once. The brown wire should be the ground if I remember right.

After all that, if the gauge still does not work then put a variable wire-wound pot across the two sender wires. The test with the 100 Ohms is a great test but it's kinda nice to see the gauge move from full to empty to verify the dash meter is ok.

Another hint. The sender can be taken apart and the resistive element repaired and/or cleaned. Have to drill out some rivets. If the gauge is working sometimes but jumps to full as the fuel sloshes, there is a dirty spot on the resistive element.

Bob
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Old 05-05-2013, 11:41 AM   #5 (permalink)
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Yah, I pulled both wires off the sender and its still full unless I turn off the ignition. I have cleaned all the connections. I will ohm it out. Thanks for the info.
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Old 05-14-2013, 08:30 PM   #6 (permalink)
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Fixed my fuel gauge-and here's how

After two years of checking my fuel level by looking in the tank...and by the even more definitive method I call "running out of fuel", I have made the fuel gauge work.

After carefully opening up the sending unit to expose the wirewound potentiometer inside, I re-soldered the connection of the super-thin potentiometer wire to the blue wire in photo.

Then I soldered the other end of the blue wire to the terminal at the top of the sending unit.

Last, I cleaned the contact surfaces of the potentiometer and the contact on the float arm with a spray electronics cleaner, and carefully re-assembled the sending unit.

Luckily for me, the one I have was held together by bent metal tabs--but yours may have rivets like hammogger mentioned.

Now, it reads 7/8 when it is COMPLETELY full, and reads 1/8 when it is 1/4 full. A bit pessimistic, but it totally works for me.

It might be possible to get more accuracy with cleaner terminals, or with some sort of tuning pot added to the system...but I don't think I'll bother.

Schroeck
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Old 05-14-2013, 09:25 PM   #7 (permalink)
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Vehicle: 1970 Unimog 406 was VF3 Snowcutter
Location: Ventura County
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Funny I got up the gumption to check mine out this weekend too unfortunately I wasn't so lucky/persistant. The ultra fine wire was broken in two places on mine right where the wiper goes which explains why mine read full then started working at half tank. Since the phenolic/fiber insulator is swaged in there and I didn't have any ultra fine wire around I honestly gave up, and just ordered a new one. See pics for what can go wrong.
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My 404 fuel gauge is pegged full all the time???-tn_img_4670.jpg   My 404 fuel gauge is pegged full all the time???-tn_img_4671.jpg   My 404 fuel gauge is pegged full all the time???-tn_img_4672.jpg   My 404 fuel gauge is pegged full all the time???-tn_img_4673.jpg   My 404 fuel gauge is pegged full all the time???-tn_img_4674.jpg  

My 404 fuel gauge is pegged full all the time???-tn_img_4675.jpg   My 404 fuel gauge is pegged full all the time???-tn_img_4676.jpg   My 404 fuel gauge is pegged full all the time???-tn_img_4677.jpg   My 404 fuel gauge is pegged full all the time???-tn_img_4679.jpg   My 404 fuel gauge is pegged full all the time???-tn_img_4678.jpg  

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Old 05-14-2013, 09:26 PM   #8 (permalink)
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Couple more
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My 404 fuel gauge is pegged full all the time???-tn_img_4680.jpg   My 404 fuel gauge is pegged full all the time???-tn_img_4681.jpg  
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Old 05-15-2013, 05:46 PM   #9 (permalink)
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Date registered: Nov 2004
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Excellent photo documentation, thanks!

Bob

PS, the ni-chrome wire can be soldered with some types of 'exotic' solder. I have some stuff called Eutectic Rod that adheres to the wire. Looks like that latter example was beyond hope.
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Old 05-15-2013, 09:37 PM   #10 (permalink)
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Vehicle: 1972 MOG 404 / 1963 VOLVO C202 LAPLANDER
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Im taking mine apart!
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