The Stovebolt Site Forums The Shop Area The Electrical Bay fuel gauge testing

OK, one more trek into this endlessly talked about subject. This is on the '51 Burb with a 12 to 6 vlt step down in the line to the E side of gauge.My guage moves from E to F about a quarter inch when ignition switch is activated with tank full. The tank is well grounded to frame as I checked it from the fuel line at outlet nipple and at the sending unit mount plate. What other tests can be performed without dropping tank and pulling sender (which is what I dread doing) I have a spare gauge, multitester etc.

Burb Stu,

You are right..this subject has been beat to a bloody pulp.

I am by no means a gas gauge guy, but how can one Stu not help another Stu.

I have just read all the posts on fuel gauges, and come to this conclusion. It is either your fuel gauge or your sender..DUH!

If you are getting your reduced voltage of 6 volts to the power terminal of the gauge when the key is turned on, and the gauge is grounded through its case, or a wire to the dash, then it sounds like all would be well on the gauge end of things.

If you can turn the key on, pull the wire from the sending unit, and get the gauge to respond properly, it proves the WIRE to the tank, AND the gauge.

If you pull the wire with the key on, the gauge should show FULL...if you take that same wire and ground it to the body or frame, the gauge should show EMPTY.

This means the wire is good from the gauge, and the gauge works properly.

The only other item in the equation is the sender. This isn't a big deal, UNLESS you are dealing with a burb!

You didn't mention if the gauge goes down as the tank level drops to near empty. If it does, it seems to me that the guilty party is the sending unit. I will bet more info is coming soon!

Stuart

Since we are on this subject, and i'm too lazy to search the previous posts, does anyone know why my 39 chevy pickup, gas gauge (12v converted) continually swings between full and empty, regardless of how much fuel is in the tank... it does swing less when it's full, and doesnt swing as far to the full mark when the fuel is low...

I'm running the original 6 volt sending unit, which is probably the problem. thanks.

atomarc is dead on with his process. Rusty, it sounds like you have a broken or loose wire somewhere. Sometimes the ends touch sometimes they don't causing the swing. The mechanism in the sender could also be the culprit causing the same effect. As atomarc says, when the side of the gauge going to the sender touches ground the reading is empty...when its floating in the breeze (open) it reads full. The sender unit is essentially a variable resistor with a slider. As the gas level drops, the slider moves closer to ground. On a '39 the oscillations are dramatic because it wasn't until later that the gauges were "dampened" to reduce the bobbing of the needle as the gas sloshes in the tank.

Thanks Stu. I'll be underneath the beast soon and praying I don't have to drop the tank AGAIN!

Burb I had the exact same problem a while back auotmarc help me out I'm now an expert on fuel gauges. My problem was I had HOLE in my float which filled up with gas and made the sender register empty. If you do have to take sender out check your float to see if its full of gas!!!

By the way what is a BURB

Petmil,

Almost everything you might want to know about Burbs.

Or, check out Panels \'N Burbs on this same site.

Tim

I've been meaning to ask and while on the subject, the wire on my truck ('66) goes from gauge to fusebox ( gas gauge ext.) would it hurt to just bypass the fusebox and take the wire straight to the sending unit??

sawbuck - the wire to the fuse box is where the circuit gets power [+], the wire to the sender [from the gauge] is the ground [-], flow in the circuit [controlled by the variable resistance to ground] of the sender is reading on the gauge

Bill

Thank you for the info red!!

I troubleshooted mine by removing the sending unit, turning on the ignition, moving the float arm up and down and watching the deflection of the needle. If your sending unit is bad, it will be obvious.

Tab,

If the needle didn't deflect when you did that, how would you know if it was the sending unit or the gauge.

Can you check one unknown with another unknown?


Stuart

Sawbuck, I'm with you, the only wire on my sending unit on the 68 stepside is from the tank to the fuse block?? Not that I would assume that any of the wiring on this truck goes where or is the color that it is suppose to be. But there is only 1 O N E !!! so if this is the pwr to the sender where is the wire that should go to the guage???? only one connection on the sending unit.

The wire from the sending unit should go to the gauge: there is no power connection to the sending unit, the power is to the other terminal on the gauge. The sending unit is grounded to the tank and just supplies a variable resistance to ground to the gauge.

Veeeeerry interesting! I did some snooping, and I think that wire from the sending unit that goes to the fuse block [not powered, just a connection point] continues on through a harness to the ground side of the fuel gauge.

Could the gauge clusters in these trucks be plastic, or mounted in plastic.

I don't have a wiring diagram, or a book, so I might be all wet, but this is how it appears to work.

The connection in the fuse block is not a "hot" connection...just a connection for a secure ground to the gauge.

Stuart

Is someone confusing a terminal block with a fuse block? They are not the same thing.

I just picked up a starter for work from a local auto electric shop. They had a huge library of "Mitchell's Wiring Diagram" books.

I asked to look at one for a '68 chevy pickup. It showed a TAN 20 awg wire going from the sending unit directly to a connector that snapped onto the back of the printed circuit board instrument cluster.

I wonder if the wire is being 'mis-traced' to a fuse block, or terminal block.

Stuart

Stuart, sent you a scan of the 68 fuse block from the manual. I think I have a wiring diagram here somewhere also.


Kirk

Kirk,

I will share what I PM'd you in this forum. I am just a casual observer to this thread, and have no first hand knowledge of this year truck.

The picture of the fuse block does indeed have a tab marked "fuel gauge" on it. It is adjacent to a tab marked "dome".

A picture of the interior connections of the block is included, and it appears that the above mentioned tabs and connected internally.

If the tab for the "fuel gauge" actually means sending unit, and the wire that attaches here actually goes to the sending unit, I would surmise this is actually a connection for a ground.

These two tabs are not fused. Whether Detroit would put ground connections in a "hot" fuse block, I don't know.

I am pretty sure that a non-fused, 12 volt power wire, capable of full current carrying capacity would never be attached to the sending unit.

This is an interesting question, but I just don't see why GM would have changed the way the fuel gauge/sending unit setup worked.

A simple test with a meter would answer a whole bunch of questions.

Stranger things have happened...it will all be revealed.

Stuart

Thanks Stuart, seemed odd to me also. Will keep working the prob.

If the tab is labeled fuel gauge, why wouldn't it be the power to the gauge rather than to the sending unit? Does the schematic show anything other than a single wire from the sending unit? Are you sure the dome light and the fuel gauge don't share a fuse in the fuse block? Usually dome lights are always hot with a controlled ground and are always fused. Sorry to ask so many questions, but I don't have the schematic.

with sw on remove tank wire at tank read gauge then ground wire to tank ground read gauge,,gauge should move full sweep,,if so tank unit bad,,if no wire to tank or dash unit or power to dash unit,,same test on dash unit remove wire read,, ground term on dash unit and read if full sweep wire bad if no dash unit or power supply...doc

I have read most of these posts and everyone is dead on. I don't see any info on what is called a vibrator. In some older vehicles a vibrator (usualy located on the back of the instrument cluster)is used. This little sucker can cause all the afore mentioned problems. Pulsing guages,inopperative guages and incorrect readings. Were they used on these old bolts?
The fuel guage system like the electric fuel pump has me wondering. You see electricity and gasoline don't mix. Why do they not explode? I mean realy an electrical appratus emersed in fuel, some people call this a bomb. But I have never heard of a vehicle explodeing from an electrical failure of a sending unit or an intank fuel pump.

British cars are the only ones I'm familiar with which utilised pulsed regulation of the gauges.

You may be correct on the imports having a vibrator. My first taste of automotive repair was in an import car shop.
But the chrysler/dodge/plymouth cars and trucks also had them in the 60's and 70's. I havent seen them lately.

Not sure if yours is the same or not but on my 68 C-10 the dash unit has a printed circit board.
all wires go to a connector for the board not to each individual idiot light. I grounded the hot side of the guage and got a full sweep to empty the default on the guage is full so must be a bad tank float unit. New one should be here today from American Classic. Will let you know the results.

Ok !!! now the gage works and reads correctly, however now the Generator light comes on (Very Dim) ???? bad ground ?? something else loose ?? If I disconect the gage wire light goes out and gage defalts to over full. This is getting to be a nightmare.
Nice sending unit from American Classic(Chevy Duty) but they changed the fitting size from 1/4 to 3/8's had to put an adapter in the line. Other than that not to bad. Now if I can just get this other problem figured out thing will be good.

OK I just pulled the wire at the sending unit, turned on the ig, and the needle went to full. Then I ground the wire ( I had a good ground), the needle went to empty, for those who know does this mean the sending unit is bad??

most likely bad sender, unless the sender's not getting a good ground

Bill

Thanks (again) Red for your help!! I am fairly sure the sender is ground well, I have 11-12 volts from hot side of gauge all the way to sender ground on cab!

By the way folks..... that little item you refer to as a "vibrator"?? is actually called a "consant voltage regulator".. and Ford has used them since the 40's.... What it does is averages the volage swings as the float in the tank bobs up and down as the car/truck moves and maintains a constant voltage to the gage.... I'm sure you've all watched the gas gage swing as you've gone around corners..... or started/stopped.... Along with Ford, Chrysler products used them as well for many years...but ceased to do so in the eighty's as a cost cutting measure. Although Ford and most Europian makes still use them.

You can use your stock 6v sending unit with a 12v gauge no problem. Just put a voltage reducer inline in your sending unit wire from the gauge to the sending unit. My gauge was reading just above E all the time after installing the guages but once I put a reducer in it worked perfectly. This will keep the gauges working at the 12v they were designed for and keep the sending unit working at the 6v it was designed for. Much easier and cheaper to buy a $1.95 reducer and splice it in

http://www.americanclassic.com/asp/...&iyear=1946&ytype=to&exyear=

zipper, where do you get this reducer? Any idea what the resistance value is? It beats the potentiometer idea I had for sure.
MV

click on the link in my post and scroll about half way down the page. I order a lot of stuff for my truck from them and they are a good company to deal with.

The only item I have not heard mentioned in all of this is the variable resister not making a good connection to the sending arm. The arm is spring loaded on my 53 Chevy truck fuel gauge sending unit. We did all the above testing and found the sending unit to be bad. We took it out of the truck and found out one of the screws that holds the arm to the resister was lose. We pushed it as far foward as possible (to get the best contact) and put it back together. The fuel guage has worked perfectly ever since. rLancia53