Fuel Guage Problems

[67-72lover]

My fuel guage does not work I have replaced the fuel sending unit and the circuit board on the back of the cluster plus both resistors (which I think there bad) for the fuel and temp guage. If i pull the sending unit wire off it pegs out pass full. If I pull the sending unit out of the tank and hook a jumper wire off the negative cable of my battery to the sending unit and hook the sending the unit wire up to the sending unit and move the float it only goes to the 1/4 mark on the guage no farther I tryed this with the old and the new sending unit same thing. Can anyone give me some imput as what to try im at a total loss with it.

Thanks
Eric

[VetteVet]

The most common cause for this problem is the fuel gauge resistor on the back of the gauge is bad.

http://67-72chevytrucks.com/vboard/s...ly+reads+1%2F4

If you contact Tom T-Bone, he has some resistors

[HO455]

Here is a link to help you with troubleshooting.

http://67-72chevytrucks.com/vboard/s...d.php?t=640615

[67-72lover]

Originally Posted by VetteVet

The most common cause for this problem is the fuel gauge resistor on the back of the gauge is bad.

http://67-72chevytrucks.com/vboard/s...ly+reads+1%2F4

If you contact Tom T-Bone, he has some resistors

Ive replaced the resistor with a new one from the Car and truck shop out of California. Is there a special way they go in place? My originals were just slipped on the studs then nuts then circuit board over it then the nuts to tightin it all down that's the way I put the new ones back on.

Thanks

[Coley]

Did you ever have a dual (saddle) tank set up in the truck?
If so, there is usually a mechanical and sometimes and electrical switch to control the fuel flow.
Some of those were tied into the fuel gauge wiring so if you had the switch in the wrong direction the gauge would respond.

I had this happen on mine. I have a saddle tank that is long redundant and is no longer part of the fuel system on the truck but the electrical switch is still tied to the gauge.
If I flick that switch the gauge drops to zero......damn confusing until I figured this out.

Coley

[VetteVet]

Originally Posted by 67-72lover

My fuel guage does not work I have replaced the fuel sending unit and the circuit board on the back of the cluster plus both resistors (which I think there bad) for the fuel and temp guage. If i pull the sending unit wire off it pegs out pass full. If I pull the sending unit out of the tank and hook a jumper wire off the negative cable of my battery to the sending unit and hook the sending the unit wire up to the sending unit and move the float it only goes to the 1/4 mark on the guage no farther I tryed this with the old and the new sending unit same thing. Can anyone give me some imput as what to try im at a total loss with it.

Thanks
Eric

Sorry I didn't read your post good enough and I see that you have alreaedy changed the resistor. To answer your question, it doesn't matter which way the resistor goes.

The fuel gauge reading is based on the resistance of the gauge to ground.
A high resistance gives a full reading and a low resistance gives an empty reading. 0 to 90 ohms is the range.

What this means is that you are not getting a high enough resistance in the sending unit wire. You should ground the fuel terminal on the fuse panel and look for the gauge to read empty. If it does then unplug the ground and you should get about a three o'clock reading on the gauge (high resistance).

If you get these readings the gauge is OK, (unless the resistor is bad)and you may have a grounded tan sending unit wire from the sender to the fuse panel someplace. The most common place is under the drivers sill plate where it can chafe the insulation and touch the truck cab or the sill plate.

I would first try a new jumper wire from the fuse panel to the sender and check the reading on the gauge. You can recheck the sending unit out of the tank like you have already and you should get the full range on the gauge.

If you do then the original wire may be partially grounded like I described. If none of this helps, then you may just have a bad gauge.

"INFO
I just got my new sending unit yesterday and it was chromed. It is so pretty I hate to put it in the tank LOL.
I checked the readings and it was dead on, 0 empty, 90 full. $34.00 plus $14 shipping and they sent a new lock ring and O-ring. Ordered from PartsGeek.com. I also ordered the filler grommet for $21.00.

I didn't realize it but the sending units come in two different sizes. 5/16ths
for the inline sixes and 3/8ths for the v-8s. I had a 5/16ths because my truck was originally a six cylinder. It has had a v-8 in it from the PO and it ran alright so I don't know how much difference that makes.

[davepl]

VetteVet, is there any chance that someone could be working under the dash and accidentally blow themselves up by sending 12V down the fuel sender wire, causing a spark in the tank?

I've NEVER heard of it happening, but it seems like it shoudl be pretty easily to do if you're not careful. Like you go to hook up the grey dome light and you connect it to the brown fuel sender wire instead...

I imagine under normal operation there is very little current sent down the sender.

[Coley]

Originally Posted by davepl

VetteVet, is there any chance that someone could be working under the dash and accidentally blow themselves up by sending 12V down the fuel sender wire, causing a spark in the tank?

I've NEVER heard of it happening, but it seems like it shoudl be pretty easily to do if you're not careful. Like you go to hook up the grey dome light and you connect it to the brown fuel sender wire instead...

I imagine under normal operation there is very little current sent down the sender.

Very good question.
....I'd be curious to know about this as well.

Coley

[dmjlambert]

It sounds like a good time for an experiment and try it. If you don't hear back from me in 5 or 10 minutes, you can deduce the answer.

[dmjlambert]

I couldn't work up the nerve, sorry I chickened out. I'm sure somebody else on the forum is brave.

[VetteVet]

There is cautionary advice on most of the major websites like ours that say not to do it but I don't think they say why. Anytime positive voltage is applied through a resistance to ground it is done to accomplish some element of work. Example a light. This current flow through the resistance always generates heat and in the case of a light it will produce a glow.
In the case of our fuel gauge the positive current runs through the gauge resistor first and then through the sending unit resistor. This reduces the amount of current in the sender resistance enough that almost no current is allowed through to ground.

If you bypass the gauge resistor, which is what I think you are asking, then you are applying voltage to just the tank resistor which is going to generate more heat. How much depends on the resistance of the sender and as we all know can be anywhere from
0 to 90 ohms. At 0 this would be a tank with little gas and a lot of fumes. It also means that there would be almost a direct short to ground which would create a lot of heat and maybe a spark.

The small amount of current passed by 18 or 20 gauge wire would not be a problem if the resistance was high, such as the gray dash light wire that was mentioned. But a jumper wire of 16 or 12 gauge wire could be enough to generate a lot of heat even on a 90 ohm resistor.

The resistor in our sending unit is a band type which is inside a small container but it is open to fumes or gas. Lets say that enough heat is generated to burn the band in two and possibly create a spark at the moment it separates. It also uses a wiper type contact to conduct the resistance to ground variance and this could heat enough to pull away from the band and cause a spark.

Have I convinced anyone yet.

[67-72lover]

Originally Posted by VetteVet

Sorry I didn't read your post good enough and I see that you have alreaedy changed the resistor. To answer your question, it doesn't matter which way the resistor goes.

The fuel gauge reading is based on the resistance of the gauge to ground.
A high resistance gives a full reading and a low resistance gives an empty reading. 0 to 90 ohms is the range.

What this means is that you are not getting a high enough resistance in the sending unit wire. You should ground the fuel terminal on the fuse panel and look for the gauge to read empty. If it does then unplug the ground and you should get about a three o'clock reading on the gauge (high resistance).

If you get these readings the gauge is OK, (unless the resistor is bad)and you may have a grounded tan sending unit wire from the sender to the fuse panel someplace. The most common place is under the drivers sill plate where it can chafe the insulation and touch the truck cab or the sill plate.

I would first try a new jumper wire from the fuse panel to the sender and check the reading on the gauge. You can recheck the sending unit out of the tank like you have already and you should get the full range on the gauge.

If you do then the original wire may be partially grounded like I described. If none of this helps, then you may just have a bad gauge.

"INFO
I just got my new sending unit yesterday and it was chromed. It is so pretty I hate to put it in the tank LOL.
I checked the readings and it was dead on, 0 empty, 90 full. $34.00 plus $14 shipping and they sent a new lock ring and O-ring. Ordered from PartsGeek.com. I also ordered the filler grommet for $21.00.

I didn't realize it but the sending units come in two different sizes. 5/16ths
for the inline sixes and 3/8ths for the v-8s. I had a 5/16ths because my truck was originally a six cylinder. It has had a v-8 in it from the PO and it ran alright so I don't know how much difference that makes.

I got my guage to work good by hand lifting the float. Put the sending unit in the tank hooked it all back up turned the key guage went up to about where the gas was. Grabed a gas can topped it off to full. Looked good like it was all working correct. But then about two hours later I decided to take the truck to the store about five miles I put on it got to the store shut the key off. Came out of the store and guage has dropped down to just a hair over 1/4 of a tank when its full and now the guage is stuck. Pull the sending unit wire and pegs out then hook it back up and it drops. Any idea what has caused this?

[Coley]

Have you checked the float bulb to see if there is a pinhole leak in it?
I've had this happen on several of these in the past.
If it doesn't float then you are going to get a low/zero fuel reading.
Note; the float bulb is removeable but you have to remove the sending unit from the tank to inspect it.
You will be able to tell simply by its weight and giving it a shake once you have unclipped it from the hanger.

my two bits.
Coley

[67-72lover]

Originally Posted by Coley

Have you checked the float bulb to see if there is a pinhole leak in it?
I've had this happen on several of these in the past.
If it doesn't float then you are going to get a low/zero fuel reading.
Note; the float bulb is removeable but you have to remove the sending unit from the tank to inspect it.
You will be able to tell simply by its weight and giving it a shake once you have unclipped it from the hanger.

my two bits.
Coley

Coley,

The guage was working for the five miles I put on it. When I shut the truck off and came back out is when it wasn't working. If the float had a whole on it wouldn't the guage read zero instead of just over a 1/4?

Thanks
Eric

[dmjlambert]

It depends on where the hole is. You may not be leaking 100% of the air out of the float, some of the air could still be trapped in it.