Why does the wire in my fuel sender keeping breaking?

wshawn · Post by **wshawn** » Tue May 06, 2008 5:42 am

I have just fitted another fuel sender unit into the tank on my 412 and it worked after a good clean up of the connections.

Went for a drive this morning, it showed over full as the tank is full but after about 5 minutes it stopped working.

When I got home I popped the unit out and as I suspected the resistance wire that runs through the inside of the cylinder has snapped.

This is the third unit this has happened on and I'm either very unlucky or there is another problem that I am missing somewhere that is breaking the wire.

Anyone know what might be the problem? Could it be a short circuit in the fuel gauge to sender wiring that is using the resistance wire as fuse or is that impossible? I'm stumped...

albert · Post by **albert** » Tue May 06, 2008 9:09 am

wsham,,, you are sure your + wire is on the good contact,,, the broun wire is not the ground,, look in your hayns book ,, albert

wshawn · Post by **wshawn** » Tue May 06, 2008 10:02 am

The sender unit end just has the one wire, will check the back of the gauge tomorrow and see what the wires are hooked up to. The wiring in the Haynes is a bit vague and only shows 411, would the 412 be the same?

I did think that earthing the wire at the sender unit to the car bodywork to test the gauge (the needle shoots up to the full doing this) would only work if the wires were all hooked up correctly but at this point I'm prepared to look at anything as I'm running out of sender units

raygreenwood · Post by **raygreenwood** » Tue May 06, 2008 10:40 am

There is a ground wire straight to the back of the instrument cluster itself. It is a brown wire and a ground.

I beleive what Albert issaying is that the ground wire at the fuel gauge is actually positive. Botice that its wire connector is cast into a plastic lug...that isolates it from the body of the sending unit....if memory serves.
Make sure teh ground to teh instrument is very tight...and that the other end of that ground to the metal under the dash is tight. Ray

albert · Post by **albert** » Tue May 06, 2008 3:35 pm

yes ,,RAY,, you have the good point,, if you look in the ,,haynes book ,, page 168,,169,,0r 167 ,, they said the wire for the sender is ,,br,,broun,, and many persons thing ,,the broun wire is for a ground ,,, not in this application,, and on the fuel gauge we have 2 wire ,, the broun from the sender and the black ( can bee différent color ) from the positive line ,, on this contact we can doo error,,plus make sure if the sender have a good contact with the gas tank,, on that you have 2 slut plug-in ,,one in the white plastic is for the broun from the gauge and one for the body ground ,, ( this is from my 412-73 ) there too if you mix the wire you have a bad reading on your guage,, if your connection are ok,, you have a bad ground on the thank,, clean this contact ,, they are outside of the car and the rain (wather ) can doo the rust ,, albert

albert · Post by **albert** » Tue May 06, 2008 3:52 pm

oh,,wshawn,, i have see only now ,, your car is in england,, and we geeve you ,, nta,, north of américa,,, information,, they can bee différent for you,, make attention with haynes book,, you have ,,européan section and ,,usa ,, section,, albert

Lars S · Post by **Lars S** » Wed May 07, 2008 1:36 am

I would guess that if the resistance wire in the sender unit keeps burning it is more likely due to a too high voltage than due to bad connection, possibly the two wires at the back of the gauge are mixed up.

/Lars

albert · Post by **albert** » Wed May 07, 2008 3:58 am

we are correct ,,lars,s,,, the tech. is that,,, the + line start from the guage, the power pass in the needle résistance and go on the sender ( with the broun ) to the sender + ( in the white plastic contact ) and the power pass in the sender réhostat ( résistances ) and return by the negative ground on the body,, and sometime, i have see that , if the carbon résistances in the sender is broke in some place , that geeve ,,on and off power on the needle guage ,,we have the same problem with the carbon résistor in the air flow swith ,,if you want to test your sender ,, install the 2 wire from your ,,vohm,, on the 2 sender contact ( the testor on holms x100) and moove the car trunk ,up and down ,, with that you can see your testor needle in moovement ,,oh disconnect the 2 wire from the car before,, and if you remoove the sender from the car ,,attention with the fire,, no cigarettes,, and no électric light,, you doo that outside of your home ,,albert

raygreenwood · Post by **raygreenwood** » Wed May 07, 2008 4:19 am

Remember...that there can be resistance anywhere in the loop...from the rheostat in the fuel sending unti, to the ground point for teh gauge itself. If its not just defective or old sending untits...it may very well be the ground at the gauge, or a high resistance crimp or connection. If this resistance changes rapidly with vibration...it coul be current spikes killing thesending unit. Ray

Lars S · Post by **Lars S** » Wed May 07, 2008 5:18 am

Yes, you are right, it is a coil in the gauge that could help to cause some spikes at bad connection possibly...

Lars

Lars S · Post by **Lars S** » Wed May 07, 2008 8:15 am

...however I still believe that wshawn should check that the socket with two black wires in it is conected to the terminal at the gauge marked "+" (+ on my gauge, but "15" in the diagrams).
The other wire, carrying "earth" through the sender to the gauge should be on "G" at the gauge.
Possibly "G" stands for "geber" (sensor) in german or ground in english, who knows...

Lars

wshawn · Post by **wshawn** » Thu Jun 12, 2008 12:57 pm

Well guys I have just recieved another unit BUT need to check things before I fit it.

It has been suggested that the guage may not be correct for the sender unit. The sender, as already said is the tubular kind.

The guage seems to have the vibrator/voltage regulator fitted but according to some sources this type of sender unit is not normally connected to one.

Can someone please confirm what guage should go with the tubular sender.

raygreenwood · Post by **raygreenwood** » Fri Jun 13, 2008 4:44 am

Ok...the sender you have is early (tubular)...411 and early 412.

As far as I know it will not work with the late gauge. Decsribe your car.

What month?
Does it have aluminum bumpers with the shocks underneath?
Describe what lights your gauge has in it....ie: blinker indicators (green single or double light), generator, high beam etc.

Are teh markings on the fuel gauge safety orange or brick red.
The last characteristic and the most telltale of the late versus early gauges....is the fact that the late gauges have quartz clocks and tehearly are mechanical. Ray

albert · Post by **albert** » Fri Jun 13, 2008 4:59 am

,, shawn,, take the date on your fuel gadge ,, print on the back,, and if ,, you never know if the first guy d,t change the gas tank with the fuel sender ,,or something,,albert

wshawn · Post by **wshawn** » Fri Jun 13, 2008 6:18 am

Firstly I need to point out my car is a UK spec right hand drive 412 LE Variant first registered in the UK in October 1972. This, I think makes it a 1973 model. Not sure if this affects anything or not but ...

The bumpers are chromed and I'm not sure if the UK ever had the aluminium ones with the shocks you mention Ray.

The dash only has the one green flashing warning light for the indicators and the low fuel level on the guage is orange.

As a picture speaks a thousand words...

I haven't had chance to pull out the guage an see if it is dated as of yet and I am awaiting confirmation of the cars exact date of manufacture from Wolfsburg. The guage has never worked properly since I've had the car so a bit longer won't hurt.

Once again thanks for all your help guys, it is truly appreciated

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