5 replies to this topic

[RsSpyder]

Hi guys,

 

I'm after some advice. When I was restoring my car I fitted a second hand fuel tank with sender. The tank is the smaller type which I think was originally fitted to 60s cars. However the fuel gauge never worked. I finally got round to sorting it, removed the old sender and found that it was extremely corroded and had siezed up, hence not supplying a reading (float wouldnt move).

 

To check it was the sender or the gauge itself, i touched the two wires going to the sender together and the gauge goes straight to full. Sounds good.

 

Ordered a new sender from Minispares, fitted it, and still no reading! Double checked the wiring to the gauge again and it's definietly still fine.

 

So then I removed the sender from the tank to try to test it manually. With it plugged in, moving the float by hand, the gauge doesnt move (stays at empty). Then I got a multimeter and put it across the two terminals on the sender and I can't get any reading! There is no continuity across the terminals, let alone giving any resistance reading.

 

Does this sound like the new sender is duff? Or is there a better way I can test it? I tried the same test with the old sender (continuity across the terminals) and it also shows no continuity, so I'm not sure if thats right or not! Thinking logically there should be continuity as it's just a variable resistor but I'm not really sure!

 

Sorry for the long post, any help would be greatly appreciated.

 

Thanks,

 

Mike.

 

[dklawson]

You have done a very good job troubleshooting this.  You are correct, with the meter set to measure resistance and its probes connected to the new sending unit's terminals you should have seen the value change.   There are different types of meters though so I must some cautionary question concerning how you made your tests.  Please don't be insulted.  1) Did you have the probes in the correct sockets on the meter (one will be marked "com" or "common", the other will likely say "Volt/Ohm" or similar), 2) If your meter is "manual" (as opposed to auto-ranging) did you have it set for a low enough resistance range?  3) Does the meter have a fresh battery?

 

Resistance measurements are made using some of the power from the battery in the meter so the meter battery has to be good.  If you had your probes in the Amps & Common sockets or any other combination you won't get a resistance reading.  Finally, if the meter is set to some huge range (like set for measuring 2-Mega-ohm full scale) you won't see the changes on the sending unit as they are such a small fraction of  what the full-scale reading can be.  Set it for no more than 2000 (2k) Ohms full scale.

 

With the meter properly configured and probes attached to the sending unit you should see about 33 Ohms = Full (arm up) and around 270 Ohms = Empty (arm down).  This is for the common sending unit designs introduced after 1965.  If you are dealing with an early tank and bolt-on sender from before 1965... that's totally different.

[RsSpyder]

Thanks for such a comprehensive reply.

 

I definitely had the multimeter set up correctly with the leads in the correct sockets and I was using the lowest range possible (around 200 ohms), i tried several other settings too just in case.

 

Am I correct in thinking there should be continuity across the two terminals on the sender? When touching the multimeter probes together it shows continuity so that seems OK. But when trying it across the terminals there is no reading of continuity at all.

 

Just for clarity - it's the later style tank and sender with the locking ring, its not a bolt on jobbie.

 

Would you still expect the gauge to move with the sender removed from the tank and the float being moved by hand? It doesn't earth to the tank or anything? I assume not as one of the leads that connects to it is an earth (I checked this was earthing with the multimeter connected to the battery earth).

 

Im suspecting a dodgy sender but I dont want to send it back without being sure!

 

Thanks a lot for the help.

 

Mike.

[dklawson]

You are correct.   There should be continuity across the sending unit terminals.  In post #2 above I mentioned what the resistance values should be with the float arm up (full) and the arm down (empty). 

 

You asked about earthing the sending unit during the tests and testing the sending unit outside the tank.  There should be two wires on the sender, black and green/black.  The black wire is an earth ground connection.  The green/black wire goes to the fuel gauge.  As long as those wires are intact and connected properly at each end (black to the car's chassis for earth and green/black to the gauge) then you should be able to operate the sending unit outside of the tank.

 

If you found continuity across your joined meter probes but did not find measurable resistance across the sending unit terminals, you have a problem sending unit that should be replaced.

[RsSpyder]

Thanks again for the advice. Seems like the only option is to send it back and get a replacement sent out.

 

I will report back when I get a new part - there's nothing worse than leaving threads like these with no conclusion!

 

Cheers.

[RsSpyder]

Just to update and close off this thread, I got a new part through and fitted it on sunday...and voila! Fuel gauge works like a dream! Never expected a new part to be faulty but thats exactly what it was!

 

Very pleased, thanks as always for the fantastic help.