Lotus Elan

VDO fuel gauge (zetec) reading full when empty

PostPost by: Stratosboy » Sat Feb 27, 2016 9:36 pm

Any suggestions about what I may be doing wrong with my fuel gauge/sender? - Both are VDO units, but when ignition is switched on gauge reads full when empty and when I move the float up gauge moves to empty.
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PostPost by: john.p.clegg » Sat Feb 27, 2016 9:56 pm

Sender fitted upside down?

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PostPost by: Stratosboy » Sat Feb 27, 2016 10:34 pm

no, it can only be fitted one way from top of tank.
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PostPost by: jimj » Sat Feb 27, 2016 10:55 pm

Tank fitted upside down? Ok..yes...but there wasn`t only me in the pub.
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PostPost by: elanfan1 » Sun Feb 28, 2016 12:48 pm

Reverse the wires?
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PostPost by: billwill » Sun Feb 28, 2016 2:08 pm

Is one or both of sender & gauge new?

They seem like incompatible units.
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PostPost by: Stratosboy » Sun Feb 28, 2016 2:56 pm

tried swapping wires round, makes no difference and connector on back of gauge is a plug so can only go on one way. Definitely right plug on loom. Both gauge and sender unit are new VDO matched units.
??
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PostPost by: billwill » Sun Feb 28, 2016 3:03 pm

If inside the sender, the device is a slider over a resistance wire, it may be that the wrong end of the resistance has been connected to the output wire.

But it looks as if your only real recourse is to complain to the seller.


'''''''''''''
There are many different VDO senders.
http://www.vdo-gauges.com/fluid-level-s ... ender.html
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PostPost by: nmauduit » Mon Feb 29, 2016 8:57 am

a coiled resistor behaves just like any resistor, and is not sensitive to current direction : when the resistor is low (slider next to the end connected to the other terminal or ground if grounded) a large current goes through, when the resistor is high a small current goes through. Then this current is used to move the needle of a gauge, and when assembling the two one must make sure that, say, the low fuel corresponds to the current level that gives a low fuel on the gauge.

For accuracy, there is also the question of absolute resistance values (impedance expected by the gauge for the whole range) - in case of mismatch tricks may be played in some instances by adding an external resistance or two, but it is not often easy to get a fully accurate reading then.

If possible I try to do a test with the actual components and circuit (gauge included) for both empty and full levels.
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PostPost by: billwill » Mon Feb 29, 2016 11:58 am

>a coiled resistor behaves just like any resistor, and is not sensitive to current direction


When I said wrong end of the resistor, I was not considering direction of current flow, but the position of the sensor arm.

Connecting to the other end would reverse the full/empty resistance characteristics

eg if it was 0 ohms empty and 140 ohms full
it would now be
140 ohms full and 0 ohms empty.


Howver since VDO make sensors both ways (see that link I gave above) it is far, far more likely that the OP does not have a matched pair of gauge and sensor. i.e that the supplier sent the wrong bits.
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PostPost by: nmauduit » Mon Feb 29, 2016 1:28 pm

billwill wrote:When I said wrong end of the resistor, I was not considering direction of current flow, but the position of the sensor arm.

it was said (post 3) that the sensor/resistor cannot be turned upside down - that would go in the good direction, while necessitating float arm bending

billwill wrote:Connecting to the other end would reverse the full/empty resistance characteristics

eg if it was 0 ohms empty and 140 ohms full
it would now be
140 ohms full and 0 ohms empty.

That would require a 3 terminal (potentiometric) coiled resistor (switching from say, bottom/middle to middle/top connections) - I have never seen such a sensor, though it possibly could be modified (but I personally would be reluctant to do it, since it is inside the tank and not pleasant to do again in case the hack fails prematurely)

billwill wrote:Howver since VDO make sensors both ways (see that link I gave above) it is far, far more likely that the OP does not have a matched pair of gauge and sensor. i.e that the supplier sent the wrong bits.

yes, it is my guess as well : the gauge end is where I would look into (making sure the expected impedance for "low fuel" and "high fuel" levels are matched to the sensor).
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