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testing a fuel tank sender unit off the car
http://international2cvfriends.com/forum/viewtopic.php?f=7&t=2220		 Page 1 of 2
Author:  Neil [ September 22nd, 2010, 7:14 pm ]
Post subject:  testing a fuel tank sender unit off the car
Just a quickie; I've got a second hand sender unit here, how can i test that it works before dropping the tank (again)?

fanks.
Author:  Nelsthebass [ September 22nd, 2010, 7:20 pm ]
Post subject:  Re: testing a fuel tank sender unit off the car
Firstly, visually check both the rubbing strip to see if it still has a contact point on it - these quite often fall off with age and the brass strip then rubs up and down the windings - also check the windings for damage or breakage.

Then, using a voltmeter and a battery connected to the contatcts, just watch what the voltage does when you move the arm up and down - if it goes down as the arm goes up, or visa versa, its fine - if you dont have a circuit at all at any point in the arc of the arm, then its buggered.
Author:  toomany2cvs [ September 22nd, 2010, 7:43 pm ]
Post subject:  Re: testing a fuel tank sender unit off the car
Nelsthebass wrote:
Then, using a voltmeter and a battery connected to the contatcts, just watch what the voltage does when you move the arm up and down - if it goes down as the arm goes up, or visa versa, its fine - if you dont have a circuit at all at any point in the arc of the arm, then its buggered.


Probably easiest to just use the multimeter on a resistance range.

As Nels suggests, you want a nice smooth change in resistance as you move it up and down. The float also needs to live up to it's name.
Author:  Devils Advocate [ September 22nd, 2010, 9:15 pm ]
Post subject:  Re: testing a fuel tank sender unit off the car
Nelsthebass wrote:
Then, using a voltmeter and a battery connected to the contatcts, just watch what the voltage does when you move the arm up and down - if it goes down as the arm goes up, or visa versa, its fine - if you dont have a circuit at all at any point in the arc of the arm, then its buggered.


CAUTION! Make sure you know what you are doing before you test anything. If you connect a car battery directly across the sender's two terminals, for example, you'll likely end up with a melted sender. Connecting a voltmeter there won't help either 'cos (a) it's not the right way to connect it, and (b) you won't see the display anyway because of the smoke :P

I don't understand how you can connect just a voltmeter and battery to the sender to test it - another component is required where the voltage can be taken.

I guess one way could be to add a 12V bulb such as a 5W sidelamp bulb into the circuit. The battery, sender and bulb would all have to be joined in SERIES - ie: so's the current has to pass through each one to get to the other (one end of the battery goes to one end of the sender. The other end of the sender goes to one end of the bulb. The other end of the bulb goes back to the other end of the battery...)

If you try this, I think the lamp should give a good indication of whether the sender is working ok - I'd expect the lamp brightness to vary as you move the float arm.


(I had a faulty sender on the Charlie I bought last year. What I found was that the sender's fuel delivery pipe had made a bad connection with the top cover/flange where it passed through. It needs to be a good electrical connection as this is where the earth returns. (A bit of 'gentle' manipulating showed that the pipe was actually loose where it passed through. It took a bit of cleaning of both surfaces before I was able to place a bead of solder around the join - I think I needed a blow torch on gentle 'pencil' flame to provide enough heat.)
Author:  Devils Advocate [ September 22nd, 2010, 9:20 pm ]
Post subject:  Re: testing a fuel tank sender unit off the car
As toomany says, if you have a multimeter with a low enough resistance setting - I think it would have to read down to a couple of hundred ohms - then connecting this directly across the sender's terminals, with no battery involved, should do the trick. I don't know what range of readings you should expect, but I suspect it would be in the order of 30 ohms changing to 150ohms as you move the arm. What ever the actual readings, you are looking for a smoothish transition, with no 'dropouts' - where the meter indicates 'open circuit'.
Author:  Xmas [ September 22nd, 2010, 9:27 pm ]
Post subject:  Re: testing a fuel tank sender unit off the car
Couldn't you just connect it to the fuel sender wiring and an earth on a 2cv and see if moving it makes the gauge work correctly? :?
Or is that just silly?
Author:  rebel2cv [ September 22nd, 2010, 9:59 pm ]
Post subject:  Re: testing a fuel tank sender unit off the car
Just have a look at the contacts inside, some of the new ones are now plastic so they can't be opened up, you need to make sure that it is all clean inside, the sweep touches all the way through but it can't have too much resistance or it won't work properly, to be honest I tested mine using the fuel gauge in the car and as I moved the sweep up and down the needle moved, fool proof... Trouble is a van tank is a lot more accessable, sorry if I've repeated someone else, I dodnt actually read some of the other posts... Sorry!
Author:  toomany2cvs [ September 22nd, 2010, 10:21 pm ]
Post subject:  Re: testing a fuel tank sender unit off the car
Xmas wrote:
Couldn't you just connect it to the fuel sender wiring and an earth on a 2cv and see if moving it makes the gauge work correctly? :?
Or is that just silly?


Go away with your logic and common sense. It has no place here.
Author:  ken [ September 22nd, 2010, 10:30 pm ]
Post subject:  Re: testing a fuel tank sender unit off the car
On the other hand, testing the sender off the vehicle with a multimeter bypasses any doubts there may be about the function of the gauge and its circuit. ;)

http://www.flickr.com/photos/30132857@N06/5015827034/
&
http://www.flickr.com/photos/30132857@N06/5015827202/

I'd be wary of applying full battery voltage across a sender, even with a 5W bulb in series.

That would probably only introduce a slight delay in the onset of smoke production. :lol:

ken
Author:  Devils Advocate [ September 22nd, 2010, 10:54 pm ]
Post subject:  Re: testing a fuel tank sender unit off the car
toomany2cvs wrote:

Go away with your logic and common sense. It has no place here.
:D

Is there a push connector on the sender wire accessible from inside the car? That would make that solution even more obviously the best one.

Oi, Ken! As you know, a 5W bulb would pass only four hundred and sixteen point six six six six ad infinitum milliamps which should protect the sender quite nicely...
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