1970 bay double cab fuel guage issues...or fuel sender issues

[HBvdubber]

Ok this is getting frustrating now. I've read several posts about fuel gauge issues but nothing really addressed my issue. So, my fuel guage reads 3/4 when i fill the tank. Disconnected the fuel sender wire and grounded it out, fuel gauge reads full. Checked ground wire from chassis to sender with ohmmeter and its good. Removed the Brazilian VDO fuel sender and replaced it with a brand new sender from Wolfsburg West. Still reads 3/4 with a full tank. WTF? What else could this be? Is there some kind of fuel guage adjustment? Could the fuel gauge be bad? Other than the guage reading 1/4 lower than it actually is, the guage seems to function properly and the needle moves toward R as I drive and burn gas. Can somebody please give me some idea on what this could be? I appreciate it!

[busdaddy]

How's the short ground wire between the bed crossmember and the sender?, each end freshly cleaned and tightened?

If it's a tube style sender there's no adjustment, weak grounds or internal resistance and sinking floats are the only reasons for error.
_________________
Rust NEVER sleeps and stock never goes out of style.

Please don't PM technical questions, ask your problem in public so everyone can play along. If you think it's too stupid post it here
Stop dead photo links! Post your photos to The Samba Gallery!

Слава Україні!

[aeromech]

Very common issue. More important is how accurate is it when the tank reaches empty? Carry some spare gas and go run it out.
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[kreemoweet]

Evidently your gauge needs "0" ohms resistance (or close to it) on the
ground side of the circuit to register at FULL. Your new and old senders
clearly are not providing that, and I don't think any sender would.
An ohmmeter or good multimeter could measure the actual resistance.

So you could try replacing the gauge. Or you could reduce the resistance
by inserting an external resistor in parallel, from the "G" terminal on the
gauge to a known-good chassis ground. For instance, if the sender has
15 ohms resistance with a full tank, then a 15-ohm resistor in parallel would
make the gauge see a resistance of 7.5 ohms. Ohm sez it's the Law.

But of course that would screw up your reading at the low end. Maybe there's
a happy medium.
_________________
'67 bug: seized by the authorities
'68 bug: seized by the authorities
'71 kombi: not yet seized by the authorities

Stop dead photo links! Post your photos to The Samba Gallery!

[SGKent]

hard to answer your question without more info. You recently replaced the sender. That may be why it reads 3/4.

What year bus and type sender? Is it a tube or a swing arm sender?

If it is a swing arm, the original senders 40 years ago were flat topped and rose higher. The new ones are a round hollow float that has the steel arm wrapped around it. It can't come all the way up. You have to bend the arm a little so it sends that it is higher that it is designed. The way I did mine (a 1977) was to service it when the tank was out. I hooked it up with good jumpers and used a yard stick thru the filler opening to set the empty and full. It is pretty accurate all the way. Earlier bays used a tube sender. They have different issues. The swing arms that are sold don't track like the originals. Frankly I don't like the idea of the steel on the arm rubbing on the tank but that is how it it.
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[busdaddy]

A 70 should have a tube sender, but with some of the aftermarket stuff in play now it's hard to say that's what's in there. Or maybe it's got a later tank installed.

If you go through a German tube sender and solder every single electrical joint it will read 100% full (right on the 1/1 mark), adding resistors to yours will reduce the full reading even further, it's already got too much resistance apparently. I have no experience with the new VDO unit myself (although you say that one was Brazilian), others seem happy with them, I can say first hand the mystery "white box" senders are absolute trash and not worth the effort to even install, I hope the WW unit wasn't one of those.

Since the sender is easy to get out in a double cab you could try operating it out of the tank to see what it does, flip it upside down to read full. Extend the wires with alligator test leads so you aren't farting around with electricity right beside a large open container of gasoline of course.
_________________
Rust NEVER sleeps and stock never goes out of style.

Please don't PM technical questions, ask your problem in public so everyone can play along. If you think it's too stupid post it here
Stop dead photo links! Post your photos to The Samba Gallery!

Слава Україні!

[HBvdubber]

Ok, it's a 1970 Double Cab and it has the tube type fuel sender. The fuel sender I removed was a Brazilian VDO brand, and it was reading 3/4 with a full tank of gas. I replaced that with a Wolfsburg West sender that wasn't branded. Same problem, reads 3/4 with a full tank.
*UPDATE*
Taking Busdaddy's advice, I removed the fuel sender from the tank, connected the ground wire and main wire then flipped it upside down. The fuel reading did increase and read about 1/8 full with a full tank. Could my main wire from the guage to the sender have some resistance in it that's causing this? I'm open to all suggestions, but it seems the problem is NOT the fuel sender since they both read the same. Should I try running a brand new wire from the guage to the sender to see if the fuel gauge reading gets better? Should I try replacing the fuel guage?

[HBvdubber]

Yes, the short ground wire from the sending unit to chassis is good. I actually replaced the wire and terminals and made sure the contact surfaces were clean of paint and rust.

[kreemoweet]

Didn't you say in first post that you grounded the sender wire, and the gauge went all the way to FULL? That would
indicate that both the gauge and the wire from gauge to sender are fine. The problem seems to be that both of your
senders have too much resistance with a full tank, at least as far as your gauge is concerned. Without knowing
what that resistance value is, not much more can be said.
_________________
'67 bug: seized by the authorities
'68 bug: seized by the authorities
'71 kombi: not yet seized by the authorities

Stop dead photo links! Post your photos to The Samba Gallery!

[SGKent]

Take one of your spare senders, disconnect the wire up front, ground the spare, turn it upside down and connect it to 12V at the fuse with a jumper. See if it reads correctly. You might also spin the fuse for it (#9) to be sure that isn't where the resistance is. Using a VOM to measure voltages to see if you are losing any would help answer that question too.
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[HBvdubber]

kreemowest, yes I grounded the sender wire and the gauge went all the way to full. I'm not sure if that would eliminate any small resistance I'm having somewhere in that wire from the fuse?

SGKent, I'm a little confused at what wire you're telling me to disconnect up front. Are you saying put the jumper from the #9 fuse to my sender and ground the sender, then turn upside down ?

[SGKent]

I am saying disconnect the wire to the sender up front. Put the spare sender in place of it all the way up front and see what it does. If it reads full then the problem is the wire going back. You can also run a jumper from the #9 fuse to the gauge 12V side if you think that the supply side from there thru the instrument cluster is the issue. Or for the latter step you could just check for 12V at the one side of the gauge with a VOM. It might also make sense to get a hold of Robbie at Airschooled, or Telford Dorr to see what they think. Robbie has a 1970, and Telford a 1971. They may have seen the issue before.
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[HBvdubber]

ok thanks SGKent I'm gonna try that. I will get back later with my results. On another note, I called Wolfsburg West and ran the problem by them. they suggested to check the resistance reading on the sender to eliminate that. They said it should read 5 ohms full (upside down) and 80 ohms empty (upright). Im gonna remove the sender and try that as well.

[busdaddy]

Exercise caution when fiddling with the gauge connections on the back of the cluster, the plastic discs the terminals are in to insulate them from the metal are super brittle and break out very easily, hold the gauge end of the terminal with needlenose pliers when detaching or attaching.

For testing an alligator lead clipped to the terminal with the bus wire still attached would be sufficient IMO.
_________________
Rust NEVER sleeps and stock never goes out of style.

Please don't PM technical questions, ask your problem in public so everyone can play along. If you think it's too stupid post it here
Stop dead photo links! Post your photos to The Samba Gallery!

Слава Україні!

[HBvdubber]

**UPDATE**
It appears the PO put a late bay window bus fuel gauge in at some point. The fuel gauge that’s in there has a light near the reserve end, which doesn’t belong on a 1970. I believe that belongs on a ‘73 and later bus, which uses a different style fuel sender. I’m guessing the later style fuel sender uses different resistance values for fuel tank readings, which is why mine is not reading correctly. I’m going to try to source a correct 1970 fuel gauge from my used parts go-to guy (Barry’s Boneyard in HB) and see if that fixes my problem.
Fingers crossed

[busdaddy]

[HBvdubber]

Actually, upon closer examination of the fuel gauge, it’s not a light by the reserve part of the gauge, it’s a stop for the needle. Does anyone know what year this gauge would be from? The earlier bay windows fuel gauge doesn’t have a needle stop by the reserve marking.

[kreemoweet]

[HarryFD]

For my 1970 I tried two of the “chinesium” senders before I found a new one from VDO made in Czechoslovakia in 2015. It is still as accurate as ever.

Good info here: https://www.thesamba.com/vw/forum/viewtopic.php?t=...ht=harryfd

And here: https://www.thesamba.com/vw/forum/viewtopic.php?t=651915&highlight=
_________________
1970 Deluxe Sunroof Bay Brilliant Blue/Cloud White
1973.5 911 Targa
2009 MB C300

[igotta40]

Our 1973 has the exact same issue. After 2 gauges and 3 sending units, just to get a working gauge, I think we can live with it. We have to gas up at 1/4 tank anyway.