VDO Fuel Gauge

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Hersir
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Location: Virginia

VDO Fuel Gauge

Post by Hersir »

I recently replaced my fuel tank sending unit--thanks to the forum for some great guidance. The fuel gauge is now reading properly but when I turn off the truck the needle only falls back to 1/4 or 1/3 instead of all the way to Empty. Is this normal, or should it return to Empty when off? I had hoped the fuel level readings would be fixed with replacing the sending unit only. Thanks.
1975 710M (Swiss)
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rmel
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Re: VDO Fuel Gauge

Post by rmel »

The normal reading with power off is "R" (e.g. reserve, commonly refereed to as empty).
A needle position noticeably above R indicates there is current flowing through the gauge
which either means power is not off or you have something wrong with your gauge or wiring.

You could try removing the purple wire from the gauge again and see if the needle drops
or hangs in there. Shorting that terminal on the gauge to ground should peg the reading
to just over full when power is on, as another test.
Puller: 71' 710K 2.7L EFI aka Mozo
Follower: Sankey MK 3, 3/4 Tonne
Rescue Pinz: 73' 712MK

Driver: Ron // KO0Q
Hersir
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Posts: 57
Joined: Fri Jul 24, 2009 12:41 am
Location: Virginia

Re: VDO Fuel Gauge

Post by Hersir »

Thank you, I'll give it a try.
1975 710M (Swiss)
Hersir
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Posts: 57
Joined: Fri Jul 24, 2009 12:41 am
Location: Virginia

Re: VDO Fuel Gauge - UPDATE

Post by Hersir »

I finally found some time to take a look at the original fuel gauge. The purple wire and two black wires were secure but somehow the two brown wires became detached when I loosened the bracket for the gauge behind the dash and now I'm not sure where they connect. The gauge is original with a 1973 date of manufacture and uses screw attachments on the back of the gauge. See below. The purple wire is attached to one and both black wires are attached to the other. Where do I re-attach the two brown wires?


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The potential replacement gauge is a newer one made in the 1990s with blade attachments, but I'd rather not needlessly replace the original if I don't have to.


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Thanks!
1975 710M (Swiss)
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rmel
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Re: VDO Fuel Gauge

Post by rmel »

Brown = Ground, needed for both the Gauge to operate and the illumination Bulb.
Just having the Gauge mounted may provide a path to ground for the meter part
but not necessarily a reliable one.
Puller: 71' 710K 2.7L EFI aka Mozo
Follower: Sankey MK 3, 3/4 Tonne
Rescue Pinz: 73' 712MK

Driver: Ron // KO0Q
Hersir
United States of America
Posts: 57
Joined: Fri Jul 24, 2009 12:41 am
Location: Virginia

Re: VDO Fuel Gauge

Post by Hersir »

Well, I'm at a complete loss. I added ring connectors to the two brown grounding wires and secured them to one of the threaded posts on the old gauge. When everything was connected it registered the correct fuel level but would barely move when power was cut. Double checked everything, same result.

I then decided to just replace the old gauge with a new one from Expedition Imports. Same gauge except it had quick disconnect attachments and had the letter L instead of R on the gauge faceplate. I added female quick disconnect recepticles to the purple wire, twisted the two power wires together and added a female quick disconnct recepticle and the same with the brown grounding wires. Hooked everythign up--purple to G, power to + and brown to the ground symbol, and checked the other end of the grounding wires attached to the frame and checked the gauge. The new gauage only registered 1/3 of a tank of gas instead of the 3/4 tank I had, but the needle correctly went all the way to the left to L when power was cut off.

I thought to myself, maybe I had less gas in the tank than I remembered. I added 5 gallons and could see the tank was almost full. I turned the power on and now the new gauge registered almost empty instead of 1/3 of a tank. Essentially, the gauge is reading backwards at this point.

It might be time to just go for a drive.

**Afternoon update**

I took out the new sending unit from SAV and compared it with the old sending unit. The new Brazilian-made unit was about almost an inch longer than the old one (made in Germany). After some more online reading it seems to me that I may have a mismatched sending unit for the replacement VDO gauge that EI sent me. I put the old sending unit back in and the gauge worked perfectly---for a bit, but then the needle began to settle back to the 1/4 indication (headlights on) and 1/3 indication (headlights off). Back to the drawing board.
1975 710M (Swiss)
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rmel
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Re: VDO Fuel Gauge

Post by rmel »

The stock Pinz fuel Gauge and sender are VDO. The sender is a resistive element with a
internal float -- Full is low resistance <10 Ohms, empty is high resistance ~90 ohms. The
gauge is nothing but a calibrated ammeter.

VDO tubular senders work as mentioned above, Low R when full, however Lever arm senders
are typically the opposite, high resistance when full. The proper gauge needs to paired with
the sender.

You can quickly check your gauge by shorting the "G" terminal to ground, the meter should peg
to FULL if it's the proper meter. You can also measure the senders resistance with a Multi-meter.
If you don't have one HF has inexpensive meters that are good enough for this purpose.

With your tank full, you should be reading somewhat less than 10 Ohms if the sender is good. BTW
the Ground connection for the sending unit relies on screws to chassis, and is notorious for loosing
a good ground over time as the cork gasket settles. Make sure the old sender has a clean surface
for a decent low resistance connection.

The gauge reading change with headlights On/Off suggests a voltage drop somewhere. Check
your 24V lights On/Off and ground connections.
Puller: 71' 710K 2.7L EFI aka Mozo
Follower: Sankey MK 3, 3/4 Tonne
Rescue Pinz: 73' 712MK

Driver: Ron // KO0Q
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