Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Thanks Vic,
I'll see if I can find the resistor. You said they are available on this site. Are you talking about 67-72chevytrucks.com? I don't see anywhere to buy them.

Thanks again.
Ron

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Does anyone have a source for the fuel gauge resistor on a 72 C`10?
Thanks,
Ron

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Hi Ron,

Send me a PM with your address information and I will send you a good resistor.

Typically the main problem when the resistor goes bad is you will never register more than 1/4 tank no matter how much fuel you have.

I don't see what photos you are refering to but the stock fuel gauge resistors will read around 90-100 ohms. The 23.3 ohms you have would only get your gauge up to 1/4 tank based on 90 ohms at full, 45 ohms at half and 22.5 ohms at 1/4 tank.

Hope this helps, let me know,

Tom

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Hi guys, this is my first post and the fuel gauge in my blazer is driving me nuts.

I purchased a new gauge and sender for the blazer and did all the testing shown on page 1 of this post. Everything was spot on with the readings so I decided to go ahead and hook everything up while on my bench. at .8 ohm's the tank read empty. as the ohm's increased the needle on the gauge started to move, but once I raised the float arm to 1/4 or 22.5 ohm's the needle was already on full. If I lift the float arm anymore than 1/4 tank the needle continues past full. The sender reads true and so does the gauge. The other bummer is I had to install everything in the blazer so I could not only use the vehicle but know when to get gas. when the needle starts to drop below full, I find a gas station quick. any ideas would be great.

Mark

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Hey, Mark, what is the resistance of the sender with the float all the way up? (As in full tank.) Is it around 90 Ohms?

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Hi Bob, yes float all the way up reads 90 ohm's.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

That sender will work properly with a GM gauge. What gauge did you buy? It sounds like it is not a match with your sender.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

i purchased a stock replacement gauge for my 72 blazer.

Mark

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

I'll set up a fuel gauge on the bench and see if I can duplicate your readings.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

When you say stock replacement, are you talking used OE, NOS or new aftermarket. Most of the new aftermarket gauges do not require the stock style porcelain resistors. If you are using the stock resistor on an aftermarket gauge, that might be your problem.

Depending on where you bought the new fuel gauge, was the needle removed for paint and not put back on in the correct position?

Just some thoughts.
Tom

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Tom, I purchased the new fuel gauge from Brothers. Part #GGE1001 nothing was removed to my knowledge. I will try removing the resistor and let you know.

Mark

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

2 thoughts (ok maybe more than 2),

Contact brothers and ask if it requires an external resistor or if it is built into the gauge.

I can sell you a decent used fuel gauge for $25.00 shipped that would be tested and working. Or I would check with Brothers and see if they can send you a new one. These are not complicated. They need battery voltage, ground and a signal from the fuel sender to work. It sounds like you have done your home work and possibly just got a bad gauge from Brothers.

Hope this helps.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Mark, I tried everything, but could not duplicate the readings you are getting. I agree with Tom that you could have a bum gauge.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Thank you Tom, removed the resistor and the gauge now reads 1/4 with 5 gallons of gas in the tank. i will go top off the tank later today and see what happens.

Mark

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Thank you Bob for taking time out of your day to help with this issue.

Mark

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Not a problem, Mark. Glad to help. Let us know how it turns out.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Works perfect now. gauge read 1/4, added 17 gallons, now reads full.

Mark

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Good stuff here. Glad We are able to help each other get through these gremlins. ;)

Take care,
Tom

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Sure helped me out! Thank You cluster king!

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

This is a great write up however I am having a slightly different problem. I know my sender works as it is new and tested. My fuel gauge just moves erratically, it goes from being correct to way past full for a few minutes then back to correct and it may stay there or it may go down to empty then back to past full to correct again. There is no rhyme or reason to this, it just moves all over the place. is it a bad ground? Where should I ground the gauge to make sure I have a good ground?

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

How is the sender grounded? It sounds like it momentarily loses ground and the gauge goes past full, then it finds ground and reads correct. If it is using the tank for ground, that may be the problem. It needs a ground wire from the sender to the truck frame.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

I have a good ground on the sender so I believe the problem is a bad ground on the gauge. I'm just not sure how the gauge should be grounded, from one of gauge terminals?

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

The gauge is grounded to the cluster with the lower terminal on the gauge. If it was bad the gauge would give a non reading rather than what you are seeing. Concentrate on what Burb is saying and check the tan wire connection on the fuse panel marked FUEL. The other possibility is the no. 4 terminal on the cluster plug connection. It is the return path to ground for the gauge.

Attachment 1742085

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

If you want to add a ground, you can use the center lower terminal on the fuel gauge itself. Do you have any issues with any other gauges/lights? Also check the ground screws on the circuit board, one by the fuel gauge and one by the speedo. The cluster gets its ground through the connector and through the circuit board. very common burn spot on the circuit board is near the connector plug in. Make sure the circuit board does not have any burn spots and the grounds are tight. Also if the cluster tins have been painted, it can cause a ground issue between the fuel gauge tin and the main tin.

Check all the 5/16 nuts on the back and make sure all are tight. It sounds like a loose ground or bad connection to me.

Share what you find here please.

Thank you,
Tom

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Subscribing

Curious about the common burn spot near the connector on the printed circuit, What could cause that spot to burn out?
Posted via Mobile Device

Edit: reason I ask is that my fuel gauge (71 Blazer) is stuck below empty, I noticed printed Circuit was burned out on ground circuit next to plug in. Replaced circuit and it burnt out again.
Turn signals are also permanently lit..
Grateful for any advice I’m stumped.
Ben

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

I am also a bit curious but my thought is it is the closest to the connector but weaker than the wiring in the connector. If any circuit gets shorted to ground, that spot seems to be the weak link in the flow.

While doing my testing, I have fried a few by shorts and that is the spot the burn through.

Maybe some of the others have some ideas. I know it is hard to keep good ones around for long ;)

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Thanks Tom, looks like I’m on the hunt for a short to ground somewhere, I’ll report back once I find out where it is..hoping it will fix this fuel gauge stuck on empty situation.
Ben

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Thank you everyone for your input! It turns out I too have a burned out circuit board. When I reached up to feel what looked like a bad spot, I heard an arc so the battery is disconnected until the new circuit board is installed.
Posted via Mobile Device

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Ok, so I figured out my electrical gremlin causing stuck fuel gauge below empty. I was having issues with all dash lights (turn signals solid, high beam indicator lit, everything that could light up on dash)to go on when ignition on and as I said above, fuel gauge was stuck below zero.
After checking the whole harness, replacing circuit board (only to fry the ground connection again) and hours of scratching my head..I realized the factory tach hot wire was running from fuse box ‘ignition unfused’ port to the upper center post on the tach. Problem is, that upper post is the ground post..Face palm...I attached it to ground and boom, everything works.
Fuel gauge is now working perfectly. I’m now on a mission to find correct wiring for factory tach.
This thread really helped me tackle all this and get it solved, Thanks!
Ben

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Hi Ben,

Did you get the tach figured out? Pretty simple to wire up. Let me know if you need help with it.

With face in palm, looking at the back, 12v is on the right, coil on the left and ground is the center stud which would only be used if the tach is not getting a good ground through the tin(s).

Tom

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Yes, from the previous tach wiring debacle, I fried one of the grounds in the printed circuit so decided to add a ground from the center stud at the tach to dash and all works great. Thanks for your help!

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Just read this entire thing, plus a few other threads on fuel gauge stuff. Followed the steps for troubleshooting and narrowed it down to a sunk float on my sending unit. Everything is working as it should now. Thank you everyone for the awesome information. Saved me a lot of time and headache.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

That is awesome and if this helps just one person it was worth the time.

Yes, thank you everyone who adds there experience here.

Take care,
Tom

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

I know this is an old post but I hope someone will direct me a little bit. Not being great at electrical work I have read the previous threads and checked some things out with my gas gauge. Let me start and say that the gauged worked fine before I removed the cluster to change the printed circuit on the back in order to get my dash lights working. When I reinstalled it the gauge was reading at three o'clock. I then reached up behind the dash and found out that I haven't tighten the nuts down on the gauge terminals. I don't know if I had the key on or not but as I was tightening the nuts the gauge went from 3 o'clock to 3/4 full and hasn't moved since. I checked the resistance at the tank and got 68 which is what I think is correct for the gas I think is in the tank. When I remove the tan wire from the fuse box nothing happens to the gauge. When I grounded the terminal nothing happens with the gauge. Did I blow the resistor when I was tightening the nuts up I'm stumped

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

It sounds like you have lost the power to the gauge. My first question is does the temp gauge work? Make sure the temp gauge works before anything else is done.

If the temp gauge does not work, check for battery voltage at the #3 terminal of the cluster connector. You should have voltage with key on. If not, check the gauge fuse.

Also it is critical that the circuit board tangs where the connector plugs in have not moved and caused a short. Over years they do come loose. Yu can repair with a dab of super glue if needed.

If you suspect the resistor is bad, remove it from the gauge and test it with an ohm meter from one side to the other. You should have approx 85 ohms.

The gauge will set at the 3 oclock poistion when it looses ground or the tan wire is not connected, which is the signal wire from the sender.

The gauge will stay where ever it stopped after ign is shut off or the gauge looses battery voltage.

Hope this helps,
Tom

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

I do have power at the gauge and the temp gauge works. What I did was run a ground wire to the bottom terminal and went down and put some gas in it the gauge is now reading a bit over full not at the 3oclock position. not sure if the ground fixed it or not yet I'll have to take it for a ride and see if it moves before I move to the next step. thanks you are a great help I'll let you know if the ground worked thanks again

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

;)

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

I may need therapy before this task is accomplished...
Gauge is at 3 o'clock position..
new tank/sending unit/ wires go all they way into the cab..proper ground.(LMC under bed tank)
pull the brown wire clip out of the fuse block and it reads 92 ohms with a full tank of gas. plug the clip back into the fuse block an check it while plugged in..32 ohms..back feed? I have 12.6 volts at the #3 on the plug with the key on. the battery and temperature gauge is disconnected. after market gauges installed oil/volts/temp.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

I pulled my gauge cluster out during my resto and I replaced the circuit strip thing on the back because my temp gauge wasn't working. When I put it back together the gas gauge was at 3'oclock and before that it worked fine. I found out by reaching up behind the dash that I forgot to tighten the two nuts that hold the circuit strip to the gas gauge. As I was tightening the nuts the gauge move to 3/4 full and did not move so I attached a ground wire to the bottom terminal of the gauge and went down and put some gas in it then the gauge move to a bit over full. I did drive it a little bit the gauge is now at the full mark so it did move just not sure if its accurate yet. going to have to drive it a little more and carry a gallon with me.