68 fuel gauge

[pwdcougar]

Sorry if this is a repeat of a very common problem. I've searched the and read a ton of fixes and diagnostic info but I'm still in the dark here.

The gauge in my 68 is reading at the 3 O'Clock position. From what I've read this means the wire from the tank is grounded somewhere.

I can see brown and tan wires going from the back of the fuse block into the wiring that goes to the back of the truck. Is anything supposed to be plugged into the spade connector on the front of the fuse block?

Also if that terminal is grounded to the frame the fuel gauge should swing all the way to empty correct?

Or have I got this all bass ackward?

[vectorit]

There is only a single lite brown wire that runs front to back for the sending unit, it has a clip style connector that connects to the sending unit and has a plastic cover. Often times this connector loses it's connection, and can cause wacky behavior on the gauge.
There is a black wire that comes off the sending unit, and that one is bolted to the chassis for ground.

For the front fuse block, I don't remember anything had to be connected. Since it is all handled by the harness that comes down the A pillar, and has a big plug that connects to the fuse block.

Have someone watch the gauge as you move the wires on the sending unit with a wooden dowel, you can do this from the passenger rear wheel well.
Careful of the brown wire connection, like I said it's kind of a fragile connector.

[LockDoc]

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You might try testing the continuity in the light brown wire from the under dash connector to the connector by the drivers side tail light. If your truck has interior panels you will have to remove the rear drivers side one to get to the connector... Also check where the wire loom goes around the emergency brake assembly under the dash. My emergency brake ratchet plate was rubbing on my wiring. (this won't apply if you are talking about the '68)

LockDoc

[pwdcougar]

Weird

Got an email saying there was a response to my post. The email says this "The light brown wire Chris mentions plugs directly into the "FUEL" spade
terminal on the fuse block. The wire end has a Black plastic cover on it."

Your response above says something different!

On my truck the spade terminal on the fuse block is empty on the side facing me with the fuses exposed. The only light brown wire is bundled into the wiring harness coming from the back of the truck!

I'll pull the rear wiring harness connector and check for continuity tonight. Hopefully it will stay warm out.

Thanks for the help Chris and Leon

[71K20chevy]

I think if it goes to the 3 o clock position, that it's an open circuit, meaning the connector probably came off.

[pwdcougar]

In this picture posted to the forum it shows a tan wire plugged into the fuel spade on the fuse block. Do all all trucks have this? The wire doesn't look like it is part of the regular harness.

http://67-72chevytrucks.com/vboard/a...1&d=1386117752

Paul

[LockDoc]

Originally Posted by pwdcougar

Weird

Got an email saying there was a response to my post. The email says this "The light brown wire Chris mentions plugs directly into the "FUEL" spade
terminal on the fuse block. The wire end has a Black plastic cover on it."

Your response above says something different!

On my truck the spade terminal on the fuse block is empty on the side facing me with the fuses exposed. The only light brown wire is bundled into the wiring harness coming from the back of the truck!

I'll pull the rear wiring harness connector and check for continuity tonight. Hopefully it will stay warm out.

Thanks for the help Chris and Leon

The reason being, I had a brain f@%t and edited my post..... It finally dawned on me that the burb fuel gauge wire does not connect to that terminal, it runs in the wire loom as you stated... Oh well, that's what happens when you get old.

The picture you posted is for pickups.

LockDoc

[pwdcougar]

Ohhh brain f@%t. Very familiar with them.

Speaking of brains not working I'm still not getting this. Fuel gauge in the 3:00 position.
Open circuit or grounded circuit?

I hate winter. It is too cold and too dark to go out and tinker with the truck and it won't fit in the garage!

Paul

[TBONE1964]

Hi Paul,

Maybe I can help. The gauge goes to overfull (3:00) when there is an open in the tan wire that comes from the sending unit or the sending unit is bad.

The thing to remember is the circuit works on the resistance of the ground signal through the sending unit. When the sending unit moves up and down it creates more or less ground resistance to the gauge.Because there is an open in the ground, the gauge thinks the sending unit is at no resistance and reads past full.

If you can get under and see where the gauge is, there are two studs. one is the hot from ignition and one is the ground to the sending unit. The one that is most near the center is the ground (tan wire) from the connector. What I would try is taking a new piece of wire and running it from that stud on the back of the gauge to the lug on the sending unit and see what happens.

If you are sure you have a good connection with a new piece of wire and there is good 12 volts from the ignition, then the gauge should read at the tank level. If it does not, I would suspect a bad sending unit.

I hope this is not to confusing and helps you,
Keep us posted,
Tom

[pwdcougar]

Got it Tom. Thanks. Thanks to everyone who responded also. If the weather cooperates I'll give it a go tomorrow.

Thanks again!

Paul

[LockDoc]

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If you don't have a problem cutting an access hole in your floor above the tank read this thread...

http://67-72chevytrucks.com/vboard/s....php?p=5233558

LockDoc

[pwdcougar]

Great thread Leon. It has the measurements and everything! Good idea to use a circular saw too and not a jig saw.

Paul

[pwdcougar]

Found the break in the tan wire back in the taillight area. I fixed the wire and the gauge goes to almost empty. Then I unplugged the wire and connected an ohm meter and the resistance went to 5.7 ohms. I bounced on the tailgate figuring that the sloshing gas should change the resistance and it didn't. Then I put in 5 gallons of gas and again it didn't change the reading.

Am I correct in assuming that the sender is toast?

Thanks

Paul

[TBONE1964]

Originally Posted by pwdcougar

Found the break in the tan wire back in the taillight area. I fixed the wire and the gauge goes to almost empty. Then I unplugged the wire and connected an ohm meter and the resistance went to 5.7 ohms. I bounced on the tailgate figuring that the sloshing gas should change the resistance and it didn't. Then I put in 5 gallons of gas and again it didn't change the reading.

Am I correct in assuming that the sender is toast?

Thanks

Paul

Hey Paul,

Can you get the sender out without dropping the tank? I think you are on the right track and the broken tan wire explains the 3:00 position. If the gauge moves at all, that eliminates the gauge being bad for the most part. It also eliminates the power from the ignition switch being a problem.

If you can get the sender out and do the same ohm test while moving the float up and down, it is by process of elimination the sender.

Keep us posted
Tom

[pwdcougar]

Tom I'm going to try Leon's trick for cutting the bed floor and pull it out. Oh one more thing. With my test light I checked the pink wire on the back of the sender. When I touched the tan wire on the sender I had partial voltage. Not bright like the hot side but it did glow. It had the same glow at the end of the tan wire in the back of the truck.

If I had to estimate, I'd say 3 maybe 4 volts. Maybe I should get my butt off the couch and get the multi meter out again.

Paul

[TBONE1964]

Originally Posted by pwdcougar

Tom I'm going to try Leon's trick for cutting the bed floor and pull it out. Oh one more thing. With my test light I checked the pink wire on the back of the sender. When I touched the tan wire on the sender I had partial voltage. Not bright like the hot side but it did glow. It had the same glow at the end of the tan wire in the back of the truck.

If I had to estimate, I'd say 3 maybe 4 volts. Maybe I should get my butt off the couch and get the multi meter out again.

Paul

When you had it on the pink wire it should have been bright as in 12V bright or whatever battery voltage was at the time you checked it. The dim on the tan is what it should be for the position the sender is in. If you had the sender out and it was working correctly, the light on the test light would glow brighter or dimmer depending on which way you swept the float. Does that make sense? Gas gauge sending units work the same way as the dimmer switch for your dash lights. It increases or decreases the amount of ground resistance in the circuit.

[pwdcougar]

Makes perfect sense. Thanks!

Paul

[LockDoc]

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Keep at it you'll get it figured out...

LockDoc

[pwdcougar]

I'll take some measurements and compare them to yours before I cut the hole. The tank in this burb is different from the 70 4WD. It is smaller, has a concave curve in one side and has only one strap holding it in.

Paul

[67 Burb]

Originally Posted by pwdcougar

Ohhh brain f@%t. Very familiar with them.

Speaking of brains not working I'm still not getting this. Fuel gauge in the 3:00 position.
Open circuit or grounded circuit?

I hate winter. It is too cold and too dark to go out and tinker with the truck and it won't fit in the garage!

Paul

Hey, Paul, for your info, have a look at my post from last January on a similar issue.

http://67-72chevytrucks.com/vboard/s...7&postcount=11

Bob

[pwdcougar]

Excellent explanation and pictures.

Thanks Bob.

Paul

[TBONE1964]

Yes that is good information Bob posted and you can see what changing the resistance does to the position of the gauge. It is a good simulation of what the sending unit does.

Excellent,
Tom

[pwdcougar]

I belong to a Mercury Cougar forum and there is a section that has a special section for troubleshooting common things like this. For the Cougars they have a section on troubleshooting vacuum operated hidden headlights and another one for the screwy sequential taillights. Do we have something like that here?

[AzDon]

I just went through this a couple of months ago...... Here's what you do to isolate the sender as the problem....
That brown wire has a connector that can be unplugged.......
Disconnected puts the needle at one end of the gauge and grounding the wire puts the needle at the other end (I've forgotten which function is which end of the gauge)
If this test produces these results then the problem is either the sender itself, or the last piece of brown wire going to it..... The problem on mine was that the float had a hole in it and sank ($6 from LMC)
As for the hole in the floor, I would NOT use a circular saw!! The PO of my burb had used a circular saw and the hole looked like crap because he had run past the corners some and then resorted to chiseling to cut all the way to the corners because the circular blade wouldnt go all the way to the corners underneath without WAY overrunning the corners on top.... Obviously you cant use a sawzall or sabre saw because youd damage stuff underneath....
The correct tool for the job is a router with a quarter inch straight blade and a proper frame (fence) fastened around it. This solves both the depth issue and the corner issue. Just drill a hole where you want to start the cut, set the depth, and away you go!
!

Good Luck!

[LockDoc]

Originally Posted by AzDon

I just went through this a couple of months ago...... Here's what you do to isolate the sender as the problem....
That brown wire has a connector that can be unplugged.......
Disconnected puts the needle at one end of the gauge and grounding the wire puts the needle at the other end (I've forgotten which function is which end of the gauge)
If this test produces these results then the problem is either the sender itself, or the last piece of brown wire going to it..... The problem on mine was that the float had a hole in it and sank ($6 from LMC)
As for the hole in the floor, I would NOT use a circular saw!! The PO of my burb had used a circular saw and the hole looked like crap because he had run past the corners some and then resorted to chiseling to cut all the way to the corners because the circular blade wouldnt go all the way to the corners underneath without WAY overrunning the corners on top.... Obviously you cant use a sawzall or sabre saw because youd damage stuff underneath....
The correct tool for the job is a router with a quarter inch straight blade and a proper frame (fence) fastened around it. This solves both the depth issue and the corner issue. Just drill a hole where you want to start the cut, set the depth, and away you go!


Good Luck!

As noted in the thread I posted the link to, I used my small tool box saw to finish the corners out and it worked very well. I went a little past the corners with the skill saw but not enough to hurt anything. If a person was concerned about it all they would have to do is put a little filler in it and touch it up with some flat black paint...

I don't think I would tell people not to use a certain tool because someone that obviously didn't know what they were doing messed something up...

Yes, the router would work very well for that and I have several, but most people do not have a router unless they do a lot of woodworking. That is why I chose to do the how to with a tool that almost every homeowner owns. The circular saw worked very well.

LockDoc