ok - what am i doing wrong? (fuel sender/gauge q)

The color drawing appears to be correct....   If the tank is full, then a 4/4+ reading may be "accurate"....  If I understand what you plan to do with the potentometer, you will parralell the resistance of the sender....  This will cause the circuit to see a reduction in overall resistance.....   The "POT" center leg is normally the "input".... A drawing of the intended circuit would help....   Does your sender have more or less resistance at full tank????

Is the sender new? I bought a new sender about a year ago and the coil was broken from the tab it was supposed to be soldered to. The sender was a bit different (quality wise) than the sender I removed so I searched and found the same sender that I pulled out on the Samba, made in Germany and it works like it is supposed to - not sure why you would need a potentiometer.

Some of the "new" products these days are just poorly manufactured with cheap materials.

If the sender isn't new and had been working up until today, check it with an ohm meter to see if it possibly has broken the circuit somewhere along the coil. I'm pretty sure this sender is the same one I'm running now: http://www.thesamba.com/vw/cla...etail.php?id=1465496

that is the same sender i have.  not sure if it was German.  seem to recall it being ~$35, 5 years ago.

how do i check the resistance of it without pulling it out? (my tank is full to the top of gas)

cause i am a visual dood:

how does the potentometer wire in?

On the top of the sender, measure the resistance between the ground terminal and the terminal connected to the gauge wire. Disconnect the gauge wire first as this can change your reading if you leave it connected.

The pot replaces the sender entirely. The sender needs to be disconnected from the gauge and the pot wired in its place. The point of using it is to test that your gauge responds as expected to changing ohm inputs and possibly to measure the ohms the gauge needs at both empty and full in case you do not have the specs for the gauge. Which, given that your gauge read backwards before, you likely do need to know because you probably don't have the right sender for your gauge to begin with. Otherwise, your picture is correct, ground one outside leg of the pot and connect the center leg to the gauge, leave the other leg unconnected.

Connecting it in parallel with the sender like your picture shows will make the gauge give non-linear readings and is of no use for testing.

Why do you think there is a problem if the tank is full and your gauge reads full? You need to have more data points than that before we can say if there might be a problem. Unless this is different than expected? It's a bit unclear what exactly you changed in your setup, if anything. If the gauge showed empty when the tank was full with that sender before, the gauge reading full now could indicate the sender wire is disconnected/broken, the sender itself has an internal open connection, the gauge terminal has an open connection, or the sender ground is bad.

But it's certainly much easier to test things with the sender out of the tank.

The circuit with the POT won't allow the sender to send the correct voltage / current through the tank sender....  The circuit is a parralell resistive one and actually reduces over all resistance of the circuit as drawn....  Not what you want under any circumstances.....

So the pot problem looks solved. Any chance he needs an exciter in the circuit? the VWs had them plugged into the back of the gauge. Without it mine read all over the place.

that potentometer is a pretty cool little thing.

i wired it up in place of the sender and adjusted it for empty and full - according to my gauge.

resistance at empty:4.3 Ohms (0.3 volts)

resistance at full :40 Ohms (2.3 volts)

now - this is where i am confused still.  Do i take the readings that the potentometer resistance was for full and empty reading on the gauge and find a suitable sender?

So, technically, i would need to find a sender that works with that, right?

i went onto CIP's site and see that they are 180 degrees different.  higher resistance at empty and lower at full.  this would explain why my gauge reads backwards.  why is this?

and i couldnt find a sender that was 40 Ohms.

http://www.cip1.ca/ProductDeta...ductCode=VDO-221-012

Empty = 80 Ohms
Full = 8 Ohms

What is everyone else using that has these 356-stykle VDO gauges?

Nothing. Watch it bounce like a superball and fill up every two hundred miles.

I had North Hollywood speedometer rebuild my gauges. I told them I was using a VW sender. It was not an issue for them to convert it. 

Thanks, Justin H! I guess I didn't get my point across very well.

Mango, with that particular sender you need to bend the wire 180 degrees to make the hinge work opposite for full and empty, then it will read a bit better for you.

hmmm, good idea danny.  what about the resistance difference?

although reassuring to hear, you guys must know i'm a dog with a bone on problem solving.

If you get a resistor of say 10 ohms and put it in parallel with your sender(if you have the 80 ohm sender), it should get you close. 80 + 10 is 90/2 is 45 ohms at full. 10 + 8 = 18/2 is 9 at empty. If you can live with that, it should be pretty close to working correctly for you. Get a 10 ohm ceramic wirewound resistor and call it a day. Heat shrink it right there by your sender or attach one leg to the combi gauge sender input and the other leg to ground.

http://www.radioshack.com/product/index.jsp?productId=2062291&utm_source=Google&utm_medium=PPC&utm_term=2710132&utm_content=Exact&utm_campaign=PLA&cagpspn=pla&gclid=CPCsqOf6_rYCFUXQ4AodzWAAAA&gclsrc=ds

i freaking love learning things - thank you Danny for explaining it to me.  much appreciated.

just so i am clear - it gets wired like this:

... or, you could just make it into a switch, and use the gauge space for something useful.

Just sayin'....

Thanks Justin, again. I forgot most of the electronics stuff that used to be in my head......

OK, I'll ask.

Why is having a gas gauge so important? As I said in my first post, the resistance range of the sender is wrong for the gauge. You can add a resistor to make the gauge read empty when it's empty, or full when it's full... but adding a resistor isn't going to change the range of the sender. You could lengthen the arm of the sender, and reduce the range, but then you'd have to bend it to the point you wanted it to start, and add the appropriate resistor, etc.

I understand that it's aggravating to have a gauge that's useless-- that's why I advocate for a useful gauge. If I had a choice between an imperfect useful gauge (oil pressure), or a perfect secondary gauge (gas level), I'd always pick the useful gauge.

As long as you have a functional trip odometer (as Terry advocated), you have what you need. Fill up at 200 miles (275 or so if you've got a big-boy tank), and you're golden.

Oh Stan, we get you. Some guys just like to have the gas gauge. And guys like Mango, things they can't get right will NOT beat them. Period!

As a fail-safe for a bad sender, or miscalculating your mileage (variance from driving habit or road topography) It would be cool to have the little 'reserve tank' lever on the floorboard, like the pre-60s VWs had! 

But I'll bet that Mango(Paul) will figure out that if he carefully takes apart his sending unit and re-wraps the resistance wire, he can get it perfect. Like half the wire, Paul, and double the space in between the wraps to get it to approx. 40 ohms for full.

Knock yourselves out. I'll be over here doing useful stuff... like tilting windmills.

Me I don't care, my gauge is wrong anyway. It does read full though! Just trying to help the guy is all.

You tilt windmills, I am in search of a vertical horizon!

fair enough Stan.  i know you converted yours to a low gas light and are happy with it.

that solution will not work for me.

like Danny said, it's not so much that i care about the gauge, it's that it doesnt work they way it should - or the way i expect it should.  so, i *will* fix it.

i have tons of gauges, fuel pressure, oil pressure, oil temp, air suspeion (tank/F/B/S/S), air/fuel mixture - all where i like them and find them useful.

this is just one of those things that i am slowly uncovering that everyone just lives with.  maybe i can help with me fixing it.

next thing is for me to stop at a specialty electronics shop to ask their opinion.

cool idea danny - about rewiring the sender.  that is an option.

as you guys can probably tell, electronics are not my strong point, so please keep the feedback and ideas coming.  i will do my part with legwork and measuring whatever is needed.

thanks again.

Pretty subtle, no?

If it's your white whale, I can understand that. I'm out on the edge of sanity in my own way, so I get it (if not in this particular, then generally).

A good part of what I do for a living involves electro-mechanical controls. If you have a sender with an 80 ohm range and a gauge with a 40 ohm range, no amount of resistors on the ground side of this circuit is going to correct that.

The easiest solution would be to modify the sender somehow to only travel through half the arc to read from 0 to 40 ohms. If that's not possible, rewinding the coil in the sender with half the wire might be.

Another solution might be to reduce the voltage to the hot side of the gauge by some predetermined amount. Perhaps reducing the voltage to 6v with one of the big resisters sold as voltage reducers for accessory motors in 6/12v conversions would work. I don't know.

I do know this: whatever you do, I'd still keep a close eye on the trip odometer, and reset it when you fill up.

sometimes i feel like i am building this car completely by myself (ie:literally no help at all).  please note - this is not pointed at you guys on SOC.  this site (and TheSamba) is my only source of external information.  i truly appreciate your feedback and comments.

update: this morning, i stopped at a specialty electronics place this am.  TONS and TONS of little do-dads and electrical whachamadingers.

the dude at the counter (owner) listened very carefully to me explain what i was trying to do/solve.  noding his head the whole time.  then i ask, what parts he would suggest.  he says, go to the internet and print out a schematic and bring it to him. 

i said....ummm ok.

then i asked him if he understood the issue.  he said yes. 

then i asked him if he had the parts to resolve it. he said yes.

then i asked him to point me to those parts. he said no.  it was against company policy.

g'ah.  

the thing is, this only adds fire in my belly.  i guess it's time to figure this out.

Electronic part stores are kinda like auto part stores:

The young kids (anyone under 50) don't know diddly about electronics - all they can do is look stuff up on the computer to see if it's in stock.

The old farts have been doing this since they were kids, long before computers were around and they just KNOW what needs to be done.

It's kinda like the difference between a Technician and an Engineer:  Looking at the same problem, the Engineer with analyze what's happening, figure out what's causing it, do a root cause analysis of the problem and failure mechanism and then recommend a solution.  While he was analyzing what's happening, the Technician shot-gunned it and fixed it and went to lunch.

Hope I didn't offend any Engineers.....I'm one, too.

this dude was no young-en.  and what p'd me off is he completely knew what i had to do but would tell me.

i think i have mentally figured it out.

http://www.vernco.com/Sparks/id697.htm

now, just need some time to get smelling like gas in the garage

Tom:

For the second half of my career (as a successful matrix manager of new product introductions in the computer biz) I had two pictures on the wall of my office.  One was a simple statement that said: "There comes a point in time in any project when you have to shoot the engineers and go into production".

The other one was a picture I took in what was then the store of the back lot tour of the Disney/MGM studios (now Disney Hollywood) with a bunch of cartoon-faced balloons holding a sign that said:  "Welcome to the Looney Bin!"

Yup, that was my office

Mango-dude:  You're on the right track, just don't use Danny P's version of "Ohm's Law".  Use Justin's and you'll be fine.  This ain't Rocket Science.  All it is, is a variable resistor driving a meter.

And Stan reminds me of the NASA project engineers (OK, they were from Hughes Industries, or Rockwell or Lockheed, but we called them all NASA guys just to piss them off) during the Apollo days.  I can still hear this wormey-looking guy saying: "Our astronauts will be extremely busy every moment they're in the spacecraft.  They should never see any annunciator (status light, to you and me) that doesn't need them to do something, and they should only see those that they need to correct a problem and only when they need it".  Kinda sounds like Stan's low-gas light.

To this day, somewhere on the Moon, is the ALSEP package that was left behind after Apollo 17 with a secretly-included led (green) showing system status to absolutely no one (after the lab test phase).  We added it just to see if it would be removed by the Hughes Worm and it wasn't.  I wonder if it still lights?  It's part of an atmospheric experiment and ran on solar panels so....?  They supposedly powered down all of the ALSEP's in 1977 but Ham operators were still hearing signals into the 1990's.  The ALSEP control room was eventually re-tasked for the Skylab missions.  Poor little ALSEP bugger might have been calling home with news of water on the Moon and getting no answer................

Geez Gordon, haven't you ever forgotten something? I like to live by Albert Einstein, who said something to the effect of if you can look it up, why bother wasting brain power to remember it. Now I could look up the quote, but I just don't feel like it!

I ain't 50 yet, but have been playing with electronics since I was ten. My Dad(deceased this past December) taught me to solder at ten. So maybe I am an older than you think dude, but younger than y'all.... I think I have forgotten way more about this stuff than most people ever cared to know. Everybody forgets.

Easiest and most practical solution for Paul is to match the sender and gauge resistance requirements. Cheapest is to modify the existing sender to match the gauge. Modifying gauge = spending money, modifying sender = way less money. You have perhaps motivated me to make mine work right, it reads full when full, but I NEED to get gas right around 1/4 tank(accompanied by a jumping needle). Close enough for jazz I say, but I just might muck around with it.

"Peace out", to quote my friend MusbJim.

Signed,

JAFT(Just Another &^% Technician)

I have to be the guy who asks this.... and I have no electrical right to ask it in such a knowledge encrusted thread... Is it not as simple as using a working gauge with a stock VW unit ? This sounds incredibly complicated for what it isn't.... Or, is the NHS retro'd aftermarket 356 gauge and my VW sender never going to work? 

It's basically simple, made difficult by different specs for the gauge head and the possible senders available because these are NOT Porsche or VW gauge heads, they are (sometimes poor) reproductions, made to whatever spec they chose, often because the gauge house was also building repro sensors and simply matched what they had, whether or not it corresponded to the original gauge specs..

Some gauges want to see zero resistance for "full" scale on the gauge, while others want to see a higher resistance (like 80 ohms, say).  

Some senders show zero at "full" while others are the opposite (not to mention differing resistance ranges across their working operating range).  

Then you have the issue of the mechanical range of the sender and in which direction the float drives the sender (does it move the sender to higher or lower resistance when it rises to "full"?)  That's why many of us, with our Brazilian repro gauges, found an easy fix in simply bending the stock, VW float rod in the opposite direction, making the sender work backwards, for what our repro gauge head wanted to see.  Some actually had Brazilian gauge heads that work properly with VW senders with no mods at all.  Others, with different gauges (like some of the Chinese or Taiwanese versions) weren't so lucky and had to shop for different senders.

So while that outlines the scope of the problem, it doesn't yet answer your question, which is "Shouldn't my NHS rebuilt 356 gauge and my VW sender both work?"

And the answer is:  "Maybe".  VW used VDO for their senders and there were several different ones used, depending on car model and year.  For example, some were swing-arm floats, while others had a cylinder in which the float moved and all were in "stock" VW applications.  

Try yours on the bench (they're easy to set up) and see if they are compatible.  If they are, you win.  If they are not, then re-read the thread above and use it to figure out what type of sender you need or have to modify.

Hey Mango.... 

Is the gauge linear though with the resistor?  I had ...and still have linearity problems with the fuel gauge, and it accurately representing what is in the tank. 

Mike

Mike.... I don't know of a single gas guage that is "linear", except the old "T-model" dip stick... Or the sight tubes seen on stationary industrial engines from time to time....  

Hahahha!!!  Ya... I guess you are right.

Mike