Installed '90 GM Factory EFI tank, '72 Blazer Factory gas gauge is pegged past full?

[Okie528]

Installed '90 GM Factory EFI tank, '72 Blazer Factory gas gauge is pegged past full? I know the ohms are the same, just curious if anyone else has run into this problem, and how I can fix it. All my wiring harnesses were replaced before the tank swap and gauge was working properly before. Wire is grounded well. Tested the sending unit with a multimeter it works properly. Assuming I need to do something to get the newer style sending unit to work with the factory fuel gauge. I did the swap so I could use an in-tank pump for my Holley Sniper EFI. I have posted this in the Blazer Forum and the ls swap forum with no responses, hoping this is the right forum to find my answer.

Thanks in advance.

[damnyankee36]

When a gauge pegs full there is an open circuit between the gauge and sender on the tan wire or even an open at the gauge or sender.

[lsversaw]

Thanks to TBONE1964 and his post http://67-72chevytrucks.com/vboard/s...d.php?t=640615 , I may have this figured out. I hope so, because my '72 C20 has the same problem. Here goes...

One of these things has probably happened:

1) The gauge has failed.

2) The tan wire is not connected at the gauge, or the connection is corroded.

3) The tan wire is open, that is, it has a break in it. You had to handle it while installing the new tank, so this is a likely possibility.

You can check for all of the above by measuring voltage to ground at the terminal on the sender that's connected to the tan wire (measure with the engine running). I'm not sure what the voltage should be, but if it's more than zero then none of the above are the cause of the problem.

4) The tan wire is disconnected from the sender. You can check this visually, but just to make sure there is no corrosion on the tan wire terminal, clean the terminal. The best fast way is just remove and replace the tan wire connector from it's tab several times.

5) If the gauge needle still reads 3:00, it's almost certain that the ground wire from the tank sender to sheet metal is the problem. Run a aligator-clip jumper from the ground tab on the sender to a clean, paint- and rust-free spot on the sheet metal. If the needle drops down to a reasonable level, your ground wire is the problem; repair / replace / re-terminate / whatever as required. This is the most likely problem, because grounds are almost always a problem.

Good luck and let us know what you find, especially if my suggestios don't lead to a solution.

[Okie528]

Thanks for the response, I'm going to start with the ground, although I cleaned the frame with a wire wheel, I may just not have a good ground on the frame. I'll let you know how I progress.

Thanks,

Andy

[Andy4639]

Tank the sendor out of the tank and jumper a ground wire to it and move it by hand and see if the gauge works. If so it's a ground issue. If not then it's the sendor are wiring.

[Okie528]

Ok, ground is good. My sending unit is a four wire (Gray wire - Fuel pump power, Purple - Sending unit, Black w/white stripe-I assume ground, then another black wire-ground. I have the both the black and black with white stripe grounded to the same spot. Does this shed any light to my problem?

[damnyankee36]

Sounds right to me. The two grounds are for the sender and fuel pump.

What happens to the gauge when you ground the purple wire?

[Okie528]

Originally Posted by damnyankee36

Sounds right to me. The two grounds are for the sender and fuel pump.

What happens to the gauge when you ground the purple wire?

Nothing, gauge does not move. I have even replaced the gauge, and the printed circuit just in case, but nothing changed. As soon as I turned the key it pegged just like the original.

[VetteVet]

Find the FUEL terminal on the fuse panel, It's on the end in the center. Unplug any wire on it and with the key on run a ground wire from a good ground to the terminal. Your gauge should read EMPTY. If it does then your gauge and dash wiring are good.

Check the continuity looking for Ohms between the ground wires and the fuel tank only, not the frame. You should get a resistance reading on both wires but the purple wire is the one you need 0 to 90 ohms on.

Check between the gray wire and the black/white stripe wire for fuel pump resistance.
The solid black wire may also be a common ground from the truck for both the pump and the sender and it would need to be grounded to the truck at a good ground point before the sender would work. Check for resistance between the solid black and the B/S wire to determine this.

[lsversaw]

VetteVet, thanks for chiming in. I've learned a lot from your posts. I'm following along here because I have the same problem.

I haven't understood what the FUEL terminal on the fuse panel is connected to. From your description, it sounds like the back of that terminal is connected to the sender terminal on the fuel gauge, and the tan wire from the sender plugs into the terminal on the front side.

Is that correct?

[damnyankee36]

Hope you don't mind me stepping in; I'm stuck at home so I can answer that right now for VetteVet.

Yes, the the FUEL terminal is connected to the tan wire. I had forgotten about that terminal. Grounding that helps isolate the dash harness along with the gauge from the rest of the circuit back to the sender.

[Okie528]

Originally Posted by damnyankee36

Hope you don't mind me stepping in; I'm stuck at home so I can answer that right now for VetteVet.

Yes, the the FUEL terminal is connected to the tan wire. I had forgotten about that terminal. Grounding that helps isolate the dash harness along with the gauge from the rest of the circuit back to the sender.

I apologize for my ignorance on the electrical system. But, so I'm clear, I'm sticking a grounded wire into the fuel terminal on the front of my fuse block after disconnecting the tan wire. I thought there was power there? Sorry, I'm just nervous of misunderstanding and causing a dead short. All my wiring is new, due to a small electrical fire about a year ago, so I want to be very careful not to smoke any wires or burn up my printed circuit.

Thanks in advance!

[damnyankee36]

Yes, you ground that terminal. There is "power" there, but it is after the coil in the gauge. It's the same thing as having 12 volts on one end of a relay coil and grounding the other end to energize it.

Look at the attached pic. You can see how each end of both coils are connected to 12 volts. The one on the left is already grounded. The right one is attached to the sender which varies the resistance from 0-90 ohms. 0 ohms being "ground."

[Okie528]

Originally Posted by damnyankee36

Yes, you ground that terminal. There is "power" there, but it is after the coil in the gauge. It's the same thing as having 12 volts on one end of a relay coil and grounding the other end to energize it.

Look at the attached pic. You can see how each end of both coils are connected to 12 volts. The one on the left is already grounded. The right one is attached to the sender which varies the resistance from 0-90 ohms. 0 ohms being "ground."

Great! Thanks. I'll try it tonight and see if my gauge goes to Empty.

[Okie528]

Originally Posted by damnyankee36

Yes, you ground that terminal. There is "power" there, but it is after the coil in the gauge. It's the same thing as having 12 volts on one end of a relay coil and grounding the other end to energize it.

Look at the attached pic. You can see how each end of both coils are connected to 12 volts. The one on the left is already grounded. The right one is attached to the sender which varies the resistance from 0-90 ohms. 0 ohms being "ground."

OK. I disconnected the tan wire from the fuel terminal on the fuse block, alligator clipped a ground wire to the fuel terminal and the Gauge dropped to Empty, removed the temporary ground and it jumped right back past full. Is that what it should do?

Assuming this means the gauge and cluster are good, I ran a new wire back to the sending unit, gauge does not move, still pegged, so my tan wire is not the problem. So now that leaves me back to the sending unit which is grounded to the frame which should be a good ground. Starting to think this modern "High Temp" sending unit just wont work with the factory gauge. I got a little lost on how to check the Ohms on the sending unit, but I'll lean on my buddy Whitt to help me figure out how to check that in a few weeks.

[damnyankee36]

Yep, the gauge reacted properly. Yeah, it looks like that sender might be causing the trouble.

Never heard of the term "High Temp" sender. At the moment, I don't know if the ohm range is 0-90 ohms on a 1990 model.

[Okie528]

Originally Posted by damnyankee36

Yep, the gauge reacted properly. Yeah, it looks like that sender might be causing the trouble.

Never heard of the term "High Temp" sender. At the moment, I don't know if the ohm range is 0-90 ohms on a 1990 model.

I wasn't familiar with the term either. Here's the description of the fuel sending unit that fits the GM14G Tank I'm running.
DORMAN 692055 {#25028501} Steel Fuel Tank; Number of Outlets: 4; OHMS: 0-95 Info

31 Gallon Tank; w/ 4 Outlets; w/High Temperature Fuel Sender; 135.5" Wheelbase; 159.5" Wheelbase

[VetteVet]

Originally Posted by lsversaw

VetteVet, thanks for chiming in. I've learned a lot from your posts. I'm following along here because I have the same problem.

I haven't understood what the FUEL terminal on the fuse panel is connected to. From your description, it sounds like the back of that terminal is connected to the sender terminal on the fuel gauge, and the tan wire from the sender plugs into the terminal on the front side.

Is that correct?

Yes the back of that terminal is the connection to the fuel gauge sender terminal. It is the tan wire to the no.4 terminal on the cluster plug. Here are the rest of the cluster connections for the gauge clusters.




The wires for the light cluster with warning lights is different but I think the FUEL sender wire still goes to no. 4 terminal.

Originally Posted by damnyankee36

Hope you don't mind me stepping in; I'm stuck at home so I can answer that right now for VetteVet.

Yes, the the FUEL terminal is connected to the tan wire. I had forgotten about that terminal. Grounding that helps isolate the dash harness along with the gauge from the rest of the circuit back to the sender.

Thanks for the help! VV

[lsversaw]

Thanks VetteVet. The ground jumper test is much simpler than the voltage test I suggested. That's the kind of quick-and-easy troubleshooting I need to learn about.

damnyankee36, thanks for your answer to my question as well.

Now we'll wait and see what the OP finds out about the sender...

[Okie528]

Finally success! I used the plug from a 3 wire non-high temp sending unit off an earlier model on the 4 wire high temp sending unit that fit the tank I bought and voila, fuel gauge dropped to empty . It was just a hunch, but it worked! It was a bit of a pain removing the wire pin's and carefully removing the pressed in plug to the top of the sending unit, but worth it to have a functioning fuel gauge!

Thanks to all that responded to the thread helping me troubleshoot this issue.

I love this message board!

[VetteVet]