I've read a number of threads about troubleshooting the fuel gauge. Mine has been broken since I got the car.
I pulled the actual sender unit out of the gas tank and checked the resistance which seems close enough to factory specs so I think the unit is OK.
I checked the voltage coming from the wires which go into the sending unit and am seeing 3V with the ignition on.
I looked at the FSM wiring so I think I know where the harness from the rear enters the hood area.

Is there a way for me to send a signal through the wires from the tank to see if the gauge works?
If it's simply the wiring between the dash gauge and the sender unit, HOW do I check it to see if it's intact?
As you've probably guessed, I'm not great with electrical stuff!

I should add that the clock has never worked. Is it a common power supply to the fuel gauge????

 

Options 1:

Since you say the sending level ohms are correct, putting gas in the tank should make the gause work. If it doesn't, work you'll need to ensure the wiring is good to the gauge. This will require taking the cluster out and measuring the resistance on the wires that supply the ohms to the gauge.

Options 2:
I would just buy a small potentiometer that can do at least 125 ohms. Put a couple of leads on it and attach it to the tank leads that go the the gauage. Very the pot and see if the gauge works. The ignition need to be on for this to work.

Look at page 15:5 in the 1979 factory service manual. They have a procedure for doing this test using a mazda test box. The pot would duplicate the box essentially.

Full = 3 +2 -3 ohms (0 to 5 ohms)
Empty 110 +11 to -7 ohms (103 to 121 ohms)

Take an ohm meter and mark where these values are on the pot as you vary it.

 

Thank you. I figured the next step was to pull the dash and check resistance going to the actual gauge. I guess is the signal isn't making it to the gauge, I could simply run a new wire.
I've been reading up on pulling the cluster out,....hopefully the FSM and Hayes are accurate!

 

Cluster removal:
-St Wheel need to come off: 22mm, then rock back & forth at 3 and 9 o'clock positions to work off the spline shaft. Make sure car is aimed straight so you are remounting the wheel correctly!
-front shroud insert that surrounds the front of the panel needs to come off: 2 screws under the upper lip, 2 clips at bottom edge (pic) facing inward, ea about 3" from outer edge.
then: (pic)
-once panel is free of 4 screws you can slide it forward towards you so you can reach your hand behind and pull off the 2 wire cluster plugs. The column surround might need to come off to make this easier (if so, the HEADLIGHT switch-end just pulls off!)


Stu Aull
80GS
AZ

 

Wow, thanks Stu. Nice graphics!
It’s next on the to do list.

 

Make sure the grounding of the sender via its chassis and screws into the tank are good. This can cause weak or no level indication. I actually fashioned a ground wire from the outside base of the sender and terminated it under one of the mounting screws. My sender works flawlessly now and has for years.

Also, you do not have to remove the steering wheel to get the console out, just carefully tilt and rotate to get it out and back in. Be careful to not let the headlight stalk scrape the clear plastic cover over the console.

 

KC is correct about the pot. In my case I used fixed resistors with 100 and 10Ω values. For full I shorted the leads to be 0Ω at the tank sender and for empty I put 110Ω in series.

Is there no movement at all in the gauge? Is there some but not registering completely full after a fillup?

t_g_farrel is spot on about the importance of a good ground.

If you do remove the steering wheel, make sure when you replace it you are careful about the two plastic pins that engage the turn signal reset. It's easy to break those but not if you are reasonably careful (not trying to scare you away from it, just need to look out).

 

Another tip is to remove the speedo cable at the trans. This will allow you to pull the cluster out without having to reach behind the cluster to disconnect it.

 

OK, that's a *really good* tip.

Is there a face palm icon? I need one of those.

 

OMG, I figured out why the fuel gauge has always been on empty.

I mentioned earlier that I had removed the fuel sender unit and that the resistance was within specs for empty and full. Then I decided to take the sender out again, hook up the wires, and then move the sensor arm back and forth. To my surprise, the fuel gauge registered properly. So I put the sensor back in the tank and once again it was stuck on empty. So I checked the resistance and it did indeed correlate to empty (about 120).

Hmmmmm. Why does the sensor work outside of the tank but no inside?

I figured the float was hitting something in the tank so I turned it 180 degrees and refitted it and voila the gauge works!!!!!!!

That gauge was installed backwards years ago before I acquired the car. LOL.

I am in need of a gasket for the sender unit. Anyone know of any old stock. I can't find it anywhere online. I figured I can just cut one from cork or rubber.

THANK YOU for all of the input. This was not what I was expecting.

Can anyone confirm if this is the correct orientation?????