? for all of you

[Andy4639]

I have been working on these trucks for years and I saw something yesterday I never saw before.
A friend of mine has just bought a 70 truck. Very nice truck and he has already joined this site because of it. We were trying to find out what was wrong with the fuel gauge, why it wouldn't register. With the truck running it showed empty. I hooked my test light to the wire connection that hooks to the tank and runs to the fuse block. I then touched it to the tank and it read full which is where it should have been.
My ? for all of you is would this work as it suppose to if you added a little light to the wire or even a resistor inline so it would read right or would it read full all the time?

[cdowns]

did you try to just install a ground wire from the sending unit to the body?

[big_al_71]

Originally Posted by cdowns

did you try to just install a ground wire from the sending unit to the body?

+1

[Andy4639]

NO,
I didn't really know where to start.
The tank should be already grounded to the truck since it's bolted in right. Is there a wire suppose to be on it someplace?

[cdowns]

tank may be grounded sending unit not because sometimes the rubbeer oring inteferes with ground // i dont know if its"supposed"to b grounded i always ground them and my gauge wworks when i do

[PanelDeland]

^ What cdowns said.^ I have had a couple that didn't work and grounding them always solved the problem.
If you put a resistor or a light bulb in line then the resistance that causes 'graduation' in the guage would change.It might be a fix if the guage were for 90 ohms and the sending unit for 60 ohms,if the resistance changed by 30 ohms.IIRC these were the two stock resistances on these trucks.Even this may not be accurate since the sending unit is basically a variable resistor and adding resistance may cause it to read off unless completely full or completely empty,I don't remember which end is the higher resistance.This info is simply my opinion since it has been years since I learned/used any electrical thoery and I suppose it doesn't really apply but I typed it so I'm leaving it.

[Green Machine]

If you unhook the brown wire at the sender, gauge should go past full, ground that wire and it should read empty, this verifys the gauge and wire, switch needs to be on of course.

[Andy4639]

With truck running and wire loose the gauge went to way past full. When you hook it up it goes to empty and stay's.

[Longhorn Man]

sounds like a bad float in the sending unit.

[mr48chev]

Originally Posted by Longhorn Man

sounds like a bad float in the sending unit.

Or a bad sending unit.

Make sure that the fuel level is plenty low in the tank and pull the unit and check the float but also check the resistor in the sending unit with an ohm meter.

[Andy4639]

I have my old sending unit that we checked it out and it seems to be working. I ran out of time to install it for him so we plan to do it another day. My ? is though would the small light bulb work in a pinch if need be. Say until you could get a new sendor. Just wondering!

[mr48chev]

I'm not a fan of the light bulb idea to check the resistance. You can buy a multimeter for around 10 bucks at Sears most of the time or about 4 bucks at Harbor Freight. The inexpensive ones will tell what you need to know and if you have one you will find yourself reaching for it more often than a test light.
I almost always have a good test light in my truck but more often than not reach for the multimeter first.

[Andy4639]

I have about a $100.00 in my meter. I use it for trouble shooting electric golf carts all the time.


I was asking if the light bulb resistants would make the gauge work properly.
It made it read full right but will it read as the level in the tank goes down or will it stay at full? This is my ?

[DZL1]

I had the very same problem. Everything checked out. It turned out to be the sending unit itself. you can pick them up for about $30.00.

DZL1.

[VetteVet]

Originally Posted by Andy4639

With truck running and wire loose the gauge went to way past full. When you hook it up it goes to empty and stay's.

If I've read your posts correctly then your fuel gauge is working properly but the quantity never varies unless you disconnect the tan wire from the sender and then it goes over past full. This verifies that the resistance from the sender is not changing. The sending unit is just a variable resistor and it has to move up or down to change the resistance. If you plug in a 30 ohm light all it will do is move the gauge reading up to a little past 1/4 tank. Just hook the light leads in series with the tan wire and the fuse block where the tan wire connects and it should bear this out.
The only problem you have is that there is no way to vary the light resistance, so the gauge will read constant.Your test light must be close to 90 oms for the gauge to read full when you connected it to the brown wire. Below is a picture of how the sender varies the resistance.

[Andy4639]

Brown wire connected to the fuse block and to the sendor. I could disconnect either end and place my test light on either end and it would show full. Would it drop down while the level drops?
Also what do you think we need to do about his. I have my old one we plan to install.

[VetteVet]

Originally Posted by Andy4639

Brown wire connected to the fuse block and to the sendor. I could disconnect either end and place my test light on either end and it would show full. Would it drop down while the level drops?------- It won't matter unless the float drops and lowers the resistance at the sender.
Also what do you think we need to do about his. I have my old one we plan to install.

If you have your old sender handy, Hook one lead of your meter to the terminal where the tan wire connects and the other lead to the sender body ( flange). Set the meter on Ohms and then move the float arm up and down. If your readings vary between 90 and 0 then your sender is good and you can install it in his tank, providing it's for his vehicle too.
Pull his sender and repeat the test and if it checks good then you need to determine why the float arm is not moving in his tank. I think that is what the other guys have been trying to tell you also.
At any rate do not put any positive voltage on the tan wire while it's connected to the sender in the tank. It could cause a fire and burn down the truck or worse.

[cparman]

IF, everything is good from the sender to the gauge, you should be able to touch a grounded test light to the sending unit wire and IF you look carefully, you will see a very dim light, that will pulse. If you see this the all should be good to the front. Been beat up with a lousy ground at the tank. Clean the ground and reattach. Someone else posted that this might be a ground issue.

[Mach1]

Today I grounded the sending unit with no luck. Then hooked a meter up on Ohms. 2.1 ohms. Inserted a metal rod into the tank,moved float up and down, Ohms and gas gauge followed. It is definetly the float/sending unit that is bad. Thanks for everyones input Andy made the initial post as I'm a new user. Thanks Mach1

[Andy4639]

Cool Mach1. Good deal, we just need to get the old sendor out of mine and install it. See you at work tonight. I will get it out and have it ready to do whenever.

Mach1 is a friend of mine who also works with me. Hope to see him on here a lot more in the future. He bought a 1969 Chevy 1/2 ton short bed. Very nice truck! I'll let him give all the details when he is ready. I like the color though it's the one in his Avator. You going to drive it tonight.