928 Fuel Tank Sender
08-31-2020, 08:20 AM
#1
Instructor
Thread Starter
928 Fuel Tank Sender
Finally after persevering without a working fuel gauge I decided to pull the sender unit out for a look. The thin wire that sensors the level was broken so I soldered about 20mm of thin copper wire to it and then soldered that to the terminal and now it’s all working as it should. I see this as a temporary repair as I’m sure the joint will fail in time. What I am wanting to know is if I replace the wire completely what can I use in its place as I assume it has to be a certain resistance. Anyone ever done this?
08-31-2020, 03:24 PM
#2
Rennlist Member
Just measured my spare and the wire is 60cm with a total resistance of 80ohms, so I make that 133ohms per metre resistance wire.
I think the resistance changed slightly when they went to the digital dash and lost the seperate circuit for the reserve light, the above was taken from the new style with only 2 active pins.
Obviously the replacement wire needs to be ok sitting in fuel.
I think the resistance changed slightly when they went to the digital dash and lost the seperate circuit for the reserve light, the above was taken from the new style with only 2 active pins.
Obviously the replacement wire needs to be ok sitting in fuel.
Last edited by gazfish; 08-31-2020 at 03:26 PM.
08-31-2020, 10:59 PM
#4
Racer
I tried this and had no luck BUT I didnt understand the physics of it,,, the electrical aspect.
Wishing you the best and will be paying attention!
09-01-2020, 12:47 AM
#5
Archive Gatekeeper
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Rennlist Member
The fuel level senders are strung with 0.09 mm Nichrome wire, which has a resistance of about 50 ohms per foot. The level sender float varies the length of wire the gauge 'sees', so with an empty tank the gauge sees about 80 ohms, a full tank registers 5 to 8 ohms. For '78-88 the sender is rebuildable as long as it's not so badly corroded that the ground wire can no longer be attached to the center post. No such ground connector on the '89-95 senders, they're almost always rebuildable. Almost.
Here is the guts of an '89+ sender, to illustrate.
Here is the guts of an '89+ sender, to illustrate.
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dzaprev (09-02-2020)
09-02-2020, 07:57 PM
#6
Burning Brakes
The fuel level senders are strung with 0.09 mm Nichrome wire, which has a resistance of about 50 ohms per foot. The level sender float varies the length of wire the gauge 'sees', so with an empty tank the gauge sees about 80 ohms, a full tank registers 5 to 8 ohms. For '78-88 the sender is rebuildable as long as it's not so badly corroded that the ground wire can no longer be attached to the center post. No such ground connector on the '89-95 senders, they're almost always rebuildable. Almost.
Here is the guts of an '89+ sender, to illustrate.
Here is the guts of an '89+ sender, to illustrate.
09-02-2020, 08:07 PM
#7
Archive Gatekeeper
Rennlist Member
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1) I took a multimeter to the terminals of a working fuel level sender and moved the float, and realized it varied the resistance.
2) I Googled "fuel level sender" and "resistance" and found a couple of sites discussing nichrome wire as a resistance element. (The VW site thesamba.com was very helpful)
3) I got out my micrometer and measured the wire to be 0.09 mm
4). I discovered that vaping pens use 0.09 mm Nichrome wire as the heating elements
5) I bought a lifetime supply on Ebay for about $4.
6) Using my awesome soldering skills, I re-strung the sender and low and behold it worked.
I have tons of free time.
2) I Googled "fuel level sender" and "resistance" and found a couple of sites discussing nichrome wire as a resistance element. (The VW site thesamba.com was very helpful)
3) I got out my micrometer and measured the wire to be 0.09 mm
4). I discovered that vaping pens use 0.09 mm Nichrome wire as the heating elements
5) I bought a lifetime supply on Ebay for about $4.
6) Using my awesome soldering skills, I re-strung the sender and low and behold it worked.
I have tons of free time.
Last edited by Rob Edwards; 09-02-2020 at 08:09 PM.
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Hey_Allen (09-04-2020)
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09-04-2020, 10:15 AM
#8
Nordschleife Master
10-07-2023, 05:20 PM
#9
Rennlist Member
The fuel level senders are strung with 0.09 mm Nichrome wire, which has a resistance of about 50 ohms per foot. The level sender float varies the length of wire the gauge 'sees', so with an empty tank the gauge sees about 80 ohms, a full tank registers 5 to 8 ohms. For '78-88 the sender is rebuildable as long as it's not so badly corroded that the ground wire can no longer be attached to the center post. No such ground connector on the '89-95 senders, they're almost always rebuildable. Almost.
Here is the guts of an '89+ sender, to illustrate.
Here is the guts of an '89+ sender, to illustrate.
How does one take the sender apart? I've removed the cup on the bottom. But, am perplexed on how to separate the black plastic from the metal tube.
Kevin
10-07-2023, 05:56 PM
#10
Archive Gatekeeper
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Under the anti-slosh cap thingee, there's another M3 nut that holds on the black plastic lower cap. The nut is the only thing that holds it on. (It may be stuck if there was varnish in the bottom of the sender, but it'll pry out, just work it back and forth with some channellock pliers. )
10-07-2023, 06:09 PM
#11
Rennlist Member
Under the anti-slosh cap thingee, there's another M3 nut that holds on the black plastic lower cap. The nut is the only thing that holds it on. (It may be stuck if there was varnish in the bottom of the sender, but it'll pry out, just work it back and forth with some channellock pliers. )
Kevin
10-07-2023, 09:30 PM
#12
Team Owner
NOTE on an old sender make sure to inspect the return line as it could be full of rust this will be rinsed into the bottom of the tank once the part is put back into service.
I recommend that this part be replaced if any corrosion is found in the tube.
NOTE if using the 944 sender it has two ports one for the return from the engine and the other a vent port.
The vent port is usually capped with a rubber plug.
NOTE make sure to properly cap off the return port with a clamped hose.
Please understand that the sender is sometimes supplied with a rubber dust cap over this return port,
this is not a safe cap off for this port.
NOTE when the tank is full the sender is fully submerged. if the rubber cap should be pushed off,
a full tank will drain about 2.5 gallons of fuel out, this could cause a garage to burn down.
I recommend that this part be replaced if any corrosion is found in the tube.
NOTE if using the 944 sender it has two ports one for the return from the engine and the other a vent port.
The vent port is usually capped with a rubber plug.
NOTE make sure to properly cap off the return port with a clamped hose.
Please understand that the sender is sometimes supplied with a rubber dust cap over this return port,
this is not a safe cap off for this port.
NOTE when the tank is full the sender is fully submerged. if the rubber cap should be pushed off,
a full tank will drain about 2.5 gallons of fuel out, this could cause a garage to burn down.
Last edited by Mrmerlin; 10-07-2023 at 09:36 PM.
10-08-2023, 04:45 AM
#13
Rennlist Member
I bought a genuine Porsche VDO 944 sender and the low level light wouldn't work event though the circuit tested fine on the car's side in my 85.
The new 928 sender I had in my 81 worked perfectly when I installed it in the 85 of course
. YMMV with the 944 sender. I may have just had bad luck though.
The new 928 sender I had in my 81 worked perfectly when I installed it in the 85 of course
. YMMV with the 944 sender. I may have just had bad luck though.
10-08-2023, 05:55 AM
#14
Team Owner
Note the 944 sender is not shipped with a float stop like the 928 sender is this float stop will prevent the float from damaging the wire inside the sender from rough handling
The stop must be removed prior to installation of a new 928 sender it’s a wire clip that must be pulled free from the tube
The 944 sender doesn’t have this stop and as such it’s possible the wire was broken from rough handling. I found one new 944 sender that had this issue
The stop must be removed prior to installation of a new 928 sender it’s a wire clip that must be pulled free from the tube
The 944 sender doesn’t have this stop and as such it’s possible the wire was broken from rough handling. I found one new 944 sender that had this issue
10-09-2023, 12:02 AM
#15
Rennlist Member
Note the 944 sender is not shipped with a float stop like the 928 sender is this float stop will prevent the float from damaging the wire inside the sender from rough handling
The stop must be removed prior to installation of a new 928 sender it’s a wire clip that must be pulled free from the tube
The 944 sender doesn’t have this stop and as such it’s possible the wire was broken from rough handling. I found one new 944 sender that had this issue
The stop must be removed prior to installation of a new 928 sender it’s a wire clip that must be pulled free from the tube
The 944 sender doesn’t have this stop and as such it’s possible the wire was broken from rough handling. I found one new 944 sender that had this issue
Just odd that the level itself was fine and reported accurately (as accurate as a 928 sender allows) but the light wouldn't come on when low.