Hi Folks,

Other fuel gauge threads got me motivated and I haven't seen my particular situation addressed in the many threads I've read.

83 FB, no electrical mods or 3rd party gauges.

The behavior is that the fuel gauge never goes above 3/4 tank. I suspect the fuel gauge itself because:

• The sender tests good for electrical (it shows 2 to 112 Ω over full range of motion) and it moves perfectly freely when I pulled it from the tank.
  
• The ground strap from the body to the muffler clamp is clean and intact.
  
• The temperature gauge seems to work just fine (goes to about middle once warmed up, stays there).
  
• I tested the sender connection to the gauge using resistances from 0 to ~100 Ω and the gauge never reached Full.

So now I've really torn into it, removed the gauge cluster and am trying to decipher the wiring diagram to see if I can bench test the fuel gauge. My intent is to supply 12V to the right place, and provide those same resistances for full to empty and see how the gauge behaves.

Have any of you done this before?

I really don't want to blow up the cluster. Here's the wiring diagram and also the back side of the cluster. I am trying to see how the two relate, and am not getting the schematic to physical relationships for specific conductors.



On the cluster I am looking for the right place to input 12 V +/- and also to attach the test resistor to the proper pins.



Thanks!

Tom

 

On page 16 : 26 of the 1983 wiring diagram book, there is a pin layout for X-05. GY (green-yellow) supplies the 12V. You'll also want to supply your own ground since the cluster is off the car.

 

Thanks for the reply KC, especially the +12V GY bit. And just to confirm, B is the X05 Ground and it has two connections, Pins 4 and 8, either one of which works for the negative of a bench pwr supply?

For other folks interested in the cluster, I related the wiring colors with the functions that I could find, and also translated the wire color codes into words, and confirmed them against the reality of the colors in my connectors in their described locations on the connectors. It's an 83 FB GSL.

Here's my plan, and please fact check me if you have the motivation (thanks):

• Connect a 12V pwr supply ground to X05 Pin 4 Color B (or Pin 8 as the both are labeled B) and +12V to X05 Pin 9 Color GY
  
• Connect resistances of ~1 to ~100 Ω between Ground and X05 Pin 10 Color Y, Fuel Gauge.
  
• See what the fuel gauge does.

So here are some annotated photos to demystify this stuff for you. They are huge so you can see the details of the pin numbers etc etc.

This is the back of the instrument cluster, showing the location of the circular plastic connectors, X05 and X06


This is a shot of X05 including pin diagram annotated with the devices connections that I could figure out from the wiring manual.


Same for X06, it was there so I figured it would help to show it, even though I am not testing anything on X06.

 

So I bench tested the fuel gauge using the method below. It seems the range of the gauge movement is limited. Here are some shots illustrating the fuel gauge behavior and the test setup. The inset shows connector X05 on the back of the cluster.

I think the voltage regulator is fine, I get 7V solid at various test points. Anybody ever "calibrate" a fuel gauge?

Hey, at least the voltage meter works .

At direct short to GND, the meter ought to show above full.


My sender puts out 112 Ω at empty, and this is only 100, but it shows completely empty, should be a bit higher.

 

I think your problem is neither the sender or the gauge but the grounding of the sender to
the tank. Its first issue that happens and is the easiest to fix.

 

tg, I hear you! And I sincerely wish that was the case. Your threads on the subject have been hugely helpful. This is one of those things that you start on a Sunday afternoon, you know, just to check it out, and now there's a pile (an organized pile mind you) of parts in the hatch, one of which is the steering wheel.

To sum up:

Your suggestion above regarding the ground strap set me on the course of testing and verifying the ground part of the circuit, tl;dr = .2 Ω between a good frame ground and pin 4 (B) in connector X05.

So I put a resistor in place of the sender to check the wiring from the front to the back of car, tested perfect.

I had tested the sender out of the tank and measured a good range of resistance, no skips and drops.

Voltage Regulator? Check.

Aaaaaand this is when I became (more) obsessed with the gauge.

What I've learned is that the gauge is actuated by a U-shaped bimetallic strip, the top of which is wrapped with super fine resistance wire. I just love the ingenuity and creativity of the people who engineered this. Here is a borescope video of the guts (I know, a bit much):

https://drive.google.com/file/d/0B-l...ew?usp=sharing


As I said in the thread above, it looked like the gauge itself was consistently low, but I could not be sure without exercising it with different resistances. So I set it up on the bench, and resistors from dead short, 10, 46, 100, 110 Ω between pin 10 (Y) and ground to my 12V pwr supply. Doing that confirmed that it was consistently low. So this why I admire those engineers even more



After a while fiddling with it, I figured out how to calibrate the gauge, at least for my 83 GSL cluster.

Note: This calibration is the last resort after testing every other part of the system.

My theory is that after all of this time, the bimetallic strip had lost some strength. Anyway, I'll see how this behaves in the coming months and report back if it all goes wrong.

1. Take a picture of the calibration mechanism as it is, before you eff it up too much. You'll need to go back to "base" as you figure out how the needle moves with the left set and right set of teeth.
   
2. Connect 12V +/- and jumper to pin 10 on X05 as described above.
   
3. Short the fuel gauge lead (Y) to ground, wait for the needle to max out. Then tweak the teeth to move the needle above full.
   
4. Put a small resistor between the fuel gauge and ground, say 10 Ω. Make sure the need drops to something akin to dead nuts full. Tweak as required.
   
5. Now a 100 to 110 Ω resistance and make sure the needle rises just a hint from the lower stop, it will take some time as there is such low heat going to the bimetallic strip. If it does not rise, use lower and lower resistances until you see the needle move. Depending on how little resistance actually makes the needle move will tell you how far off you are. Not to belabor the this but Empty happens at ~100 and Full between 0 and 10 Ω, which is what the manual says.
   
6. Then go back and do a sanity check. If it's all hopelessly weird, go back to the settings in your picture from step 1 and start over.

Here is the shot of my gauge's adjuster after the calibration, you can see it is very different from the base settings in the pic above.

 

Ok then, you convinced me and in the process contributed something that none of us knew
about. Interesting that you can adjust the guage on an FB. Now I have to wonder if its the
same on an SA.

I think this should be archived or make a new thread about adjusting the guage and then
archive that.

Good work Tom.

 

Awesome thread. My gauge only goes up to halfway - even when the tank is full. But once the tank is about half full-empty it starts to drop on the gauge for what seems like a normal rate. I'll have to test some of this

Archive would be good.. But I wish people could post comments on the threads in there for various reasons. Update dead links (lots of them!!), new/better sources or products to use, questions or just someone who wants to post pics of how they did it which could come in handy for others etc..

 

Thanks for the vote of confidence! This was a fun one, in most senses of the word. I was pretty chuffed that it only took me a minute to find my steering wheel puller from circa 1987, which was the last time I used it (on my black 83 GS). Been moving that thing for ~30 years. Pack rat FTW!

So far so good, I got the cluster back into the dash. May I just say...what a bitch that is getting the &*^$)#!ing cluster back.

The fuel level seems appropriate for 48 miles into a full tank. We'll see with time.

For anyone who thinks they need to adjust their gauge as above, look at the photo below and think again. Really makes you want to check that ground strap first, doesn't it?

 

Update, a month has gone by and the system works solidly.

Finally drove the car enough to get the fuel level low and to check the gauge for accuracy toward empty. With the needle hovering just above the E indicator by about the thickness of the needle, the tank took 14 gallons to fill.