My gas gauge is possessed!

[deschutestrout]

The gas gauge on my '71 Beetle (I know, tis a bus forum...but hey) is acting up. When I start it registers proper level, then it takes on a mind of its own. Will slowly drop till it's buried on E, then slowly climb part way, then drop, then climb all the way, then drop, etc., etc., really quite humorous to observe. What's up wit dat?

[dtrumbo]

Pop the speedometer out of the dash and make sure the nut that holds the vibrator onto the back of the speedometer is good and snug.

A loose vibrator gave me erratic gauge performance on my '79 bug. It behaved similarly where it would be "normal" for awhile then show completely full, then back to normal, etc.

[deschutestrout]

Thanks! I'll take a look.

[satchmo]

The gas gauge on my '71 Beetle (I know, tis a bus forum...but hey) is acting up. When I start it registers proper level, then it takes on a mind of its own. Will slowly drop till it's buried on E, then slowly climb part way, then drop, then climb all the way, then drop, etc., etc., really quite humorous to observe. What's up wit dat?

I think it is more likely the sender going south. Since you have a beetle it is easy to check. Take the sender out and connect your multimeter to the electrical connectors. If the resistance readings are eratic as you move the float along its usual range of motion, then you have your answer - you need a new sender. If the resistance is nice and linear, look for the problem elsewhere.

Your symptoms are what my bus sender started doing when it went TU.

Tim

[deschutestrout]

How does one remove the sender on a Beetle? Thanks.

[hambone]

I think it's easy on a Beetle, right on top of the tank.

[Westy78]

If it's anything like the bus you just twist and pull out. Kind of like the gas cap works with the tabs. Make sure the rubber O ring is still good.

[MeyerII]

The resistance check is a good one to do. But first, remove the wire from the sender and ground it out on some nearby unpainted surface. Does the needle go all the way to full and stay there? Good. If not, then the probem is a bad connector, break or short in the wire, or problem with the guage.

Now ground it out on the tank. Same thing? Good. If not, then you probably have a ground problem with the tank.

Then you can pull the sender and do the resistance check - but FIRST remove all possibilty of sparking - get that wire the hell away from the tank or disconnect the battery.

When you remove the sender, you might also want to invert it a couple of times like you would a rain-stick. Do you hear the float travelling in there? Does it sound like its moving evenly and freely? That also might tell you something.

But I've only done this on an early Type 3, so there may be differences.

 

[dtrumbo]

How does one remove the sender on a Beetle? Thanks.

It seems it depends on whether you have a standard or a super.

According to Wolfsburg West, a sending unit for a standard Beetle looks like this, which has screws holding it in.



While the sending unit for a Super Beetle looks like this and is the bayonet-type twist mount.



FWIW, I still think the intermittent erroneously high reading points more to a vibrator problem rather than a sender problem, but it's good to check all of it out. Let us know how it goes.

[twinfalls]

I am puzzled by this vibrator.

I think it is a voltage stabilizer because I see it is replaced by a zener diode on later busses.
May be that was a way to stabilize voltage before the zener diode was invented.
Is this correct ?
It gives a chopped 12v giving some lower mean voltage. How come, this mean voltage is more stable than the 12v ?

Another puzzle is ;
How come the dial needle doesn't vibrate. What is the vibrator frequency ? How is it damped at the dial needle ?

[dtrumbo]

I am puzzled by this vibrator.

Here's a great article that explains it all quite well.

http://www.nls.net/mp/volks/htm/fuel_ga.htm

[Amskeptic]

I am puzzled by this vibrator.

No you're not.

I think it is a voltage stabilizer because I see it is replaced by a zener diode on later busses.
May be that was a way to stabilize voltage before the zener diode was invented.
Is this correct ?

Yes

It gives a chopped 12v giving some lower mean voltage. How come, this mean voltage is more stable than the 12v ?

Think of a garden hose that is branched to both the garden and the driveway. The water coming out of the garden sprinkler, let's pretend, must not overshoot the garden wall into the neighbor's yard. But neither can it drop down so low that it, um, hits the electric fence that keeps the deer out because that would crash the entire electric grid of the city.
Still with me?
OK, the problem is that your kid is washing his Citroen 2CV (which you should not do too frequently because it could wash off the entire body of the car) and every time he hits the trigger on the spray nozzle, the water at the garden drops dangerously close to the electric fence, and when he is done hosing off the Citroen (with the roof still open oops), the spike in pressure makes the garden sprinkler shoot up and get too close to the neighbor's yard that is paved with sodium hydrosulfite.
What to do? The kid never listens.
If you restrict the water supply to the sprinkler at the "Y" you will stabilize the pressure to the sprinkler and it will be less apt to react to the on/off of the spray nozzle.
That's the way I see it. . . the gas gauge uses a dopey heater to bend the arm that moves the needle. The voltage must be kept in tight control. I believe, and I have no idea why, that it is choked to 7 volts.

Another puzzle is ;
How come the dial needle doesn't vibrate. What is the vibrator frequency ? How is it damped at the dial needle ?

The electricity is converted into heat at the gauge so it does not react electrically to variations in voltage nor corner-induced sloshing in the tank.
Colin

[hambone]

That is all very weird. Actors behind the scenes.

[twinfalls]

Thanks, a lot has cleared in my mind.
I must read again the hose sprinkler explanation, I got lost but enjoyed the humor.
BTW I did drive a 2CV ( owned by my in law's bros ), I exploded a front axle joint ( a "non constant" CV joint ) and the gear box. I guess I had shifted from 4th to reverse.
I repaired in a short afternoon with boneyard parts.
This 2CV had no roof any more, had holes in the floor and no more hinges at the left door.
This "car" was finished used to carry stones.

From the link posted by dtrumbo

Suppose that battery voltage goes up (as when the generator increases output). The heater in the stabilizer will heat up more rapidly and open the contact points sooner. The result will be shorter pulses of 12V sent to the gauge. The opposite happens when the battery voltage goes lower.

This is, I think this is the right approach to explain stabilization. But this doesn't fully prove it. A 12v increase shortens the heating time, but the mean voltage depends of this time AND the increased 12v. It is not clear that this two conflicting effets will give a very little increase to the gauge.
One needs a correct modelling of this system to find the answer.

[vdubyah73]

I have what looks to be the vibrator in a box. When I was mixing and matching parts from different buses to build Buster, I used a gauge panel from a '76 ( I think ) the vibrator box popped open and lo and behold Mine has what looks like a VW interior light bulb in it. You know the pointy ended tubular light bulb.