Fuel gauge troubleshoot


PeteK

GT Owner
Apr 18, 2014
1,263
Kalama, WA
What’s an “oscilloscope”?

(Pause)

Just joking—I happen to have an olde Heathkit o-scope! Can’t remember the last time I used it though.
 

nota4re

GT Owner
Mark IV Lifetime
Le Mans 2010 Supporter
Feb 15, 2006
3,791
olde Heathkit o-scope!
Thanks a lot, Pete. I actually remember those. Now I'm feeling old!
 

HighHP

GT Owner
Jun 3, 2019
215
Spokane, WA
olde Heathkit o-scope!
That baby should work great. Hook 'er up and let us know what you see.

A lot of those for sale on eBay, the word "vintage" is used in the description on a few. Vacuum tube? Vacuum tubes were hard to find 20-40 years ago. I understand they now are plentiful due to the resurgence in demand for the true sound of music that only vacuum tubes can amplify. Huummm....maybe similar to nota4re's comment on another post regarding speaker cables.
 

PeteK

GT Owner
Apr 18, 2014
1,263
Kalama, WA
Hey, everyone knows speaker cables sound different!

Re the Heathkit scope, it’s buried in a box somewhere. I haven't seen it since I moved from VA to WA late last year.
 
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2006GT

GT Owner
May 13, 2006
11
Thank you very much for the very helpful additional responses. I'm starting to see the way forward in solving the gauge issue, but it is different than I had first imagined. For those who may be interested, here are some recent details of my saga. I have characterized the gauge signal from the gauge control module. It is a pulse-width modulated 5 volt, 1 kHz square wave. Decreasing fuel level correlates to greater off time in the duty cycle. The gauge has a zener diode that limits the voltage to the air core motor. The coils in the air core motor are driven directly by a microprocessor on the gauge circuit board. The driver for one of the coils has failed -- a repair would entail a replacement of the microprocessor. This may be a design deficiency if the failure mode involves overheating of the chip because the coil current is near the maximum capacity of the microprocessor. The microprocessor is readily available from other sources off the shelf, but the code is most certainly proprietary to AutoMeter. I've attempted many times with no success to contact AutoMeter to see if I could obtain only the programmed microprocessor from them. It's a very long shot -- I would be stunned if I were able to buy it from AutoMeter. I could replace the microprocessor, reverse-engineer the code, and perhaps add external current drivers to the microprocessor outputs. Technically feasible, but very far from cost effective. At this point, for me it is enough that I know that the solution exists to repair the original gauge.

I am going to explore several possibilities going forward. One is the feasibility of transplanting a stepper motor into my original gauge housing -- there may be some packaging issues with this idea. Another is the purchase of the AutoMeter replacement if they remain available. A third is adapting the original gauge faceplate to the Speedhut replacement stepper motor gauge and reducing the Speedhut 270 deg. needle sweep to the approx. 120 deg. sweep of the original gauge. A fourth is a full replacement of all the gauges with the Speedhut versions. Maybe I haven't yet discovered the approach I will finally take.

In pursuing this project, I have become more impressed with Speedhut and have become aware of other vendors and suppliers, like Redline Gauge Works in CA, that also could be great resources in solving problems with gauges in the FGT. Any ideas or thoughts?
 
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twobjshelbys

GT Owner
Jul 26, 2010
5,341
Las Vegas, NV
Any ideas or thoughts?
I'd just buy the autometer and move on to enjoy driving the car.

Long term pursuing a solution that allows the larger gauges - speedo and tach - to either be repairable or immune would be a welcome contribution!
 
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HighHP

GT Owner
Jun 3, 2019
215
Spokane, WA
Great analysis and report. Very interesting and informative. Thanks much.
The generally accepted assumption is that this failure is due to some type of rapid voltage fluctuation applied to the gauge. Given your review, do you think this could have caused the failure? Sounds like you think that the driver for the coils is simply undersized and high operating temperature eventually fails the driver.
If this is the case there should be a noticably higher failure rate in the very hotter climates????
Please keep us posted on your progress. Thanks.