Fuel gauge troubleshoot


2006GT

GT Owner
May 13, 2006
11
Using the Workshop Manual to troubleshoot my 2006 FGT fuel level gauge. Page 413-01-21, step B5, results in a reading of 2.4 volts. The instruction for voltage present, which I think means a short to voltage exists, is to repair circuit 900 (BK). Anyone have any experience with repairing circuit 900? I assume that it comprises a black wire, but I'm not sure where to look for a short to voltage.

Thank you in advance for any help.
 

twobjshelbys

GT Owner
Jul 26, 2010
5,341
Las Vegas, NV
I have to ask...

How old is your battery and have you replaced it (or tested the system with a new battery in the circuit)? Many many times people have troubleshot gauges only to find replacing a marginal battery fixed the problem. One characteristic is that the gauge will operate after the vehicle has run for a while (or while running).
 

nota4re

GT Owner
Mark IV Lifetime
Le Mans 2010 Supporter
Feb 15, 2006
3,791
Using the Workshop Manual to troubleshoot my 2006 FGT fuel level gauge. Page 413-01-21, step B5, results in a reading of 2.4 volts. The instruction for voltage present, which I think means a short to voltage exists, is to repair circuit 900 (BK). Anyone have any experience with repairing circuit 900? I assume that it comprises a black wire, but I'm not sure where to look for a short to voltage.
What are the symptoms?
 

2006GT

GT Owner
May 13, 2006
11
Battery: The battery (Optima) is approx. four years old. It was on a smart tender for 2018-2019 when I was out of the country.

Symptoms: The indicator needle is stuck at just over half full, and has not worked for perhaps three years. The voltage at the gauge with engine off, ignition on, is approx. 11.5 volts. Workshop Manual 413-01-21 Step B3 calls for 10 volts or more.

Thank you.
 

twobjshelbys

GT Owner
Jul 26, 2010
5,341
Las Vegas, NV
Battery: The battery (Optima) is approx. four years old. It was on a smart tender for 2018-2019 when I was out of the country.

Symptoms: The indicator needle is stuck at just over half full, and has not worked for perhaps three years. The voltage at the gauge with engine off, ignition on, is approx. 11.5 volts. Workshop Manual 413-01-21 Step B3 calls for 10 volts or more.

Thank you.
Please replace the battery. Its cheaper than anything else you're about to do and the tender on a 4 year old battery is all you're going to get if you never drove it. even if you do nothing more than take a known good new battery from another vehicle it will rell you something. Do not jumper it. Connect the cables to a test battery.
 

gtjoey

Keep Smiling - GT Owner
Mark IV Lifetime
Oct 14, 2005
3,414
Change the battery and clean the contacts, pull the ground wire sand paper the bolt and the frame, others use a larger lock washer on that bolt for better connection. A dot of clear nail polish on the bolt and you should be good.
If not you need a new gage,
gtjoey
 

nota4re

GT Owner
Mark IV Lifetime
Le Mans 2010 Supporter
Feb 15, 2006
3,791
It's not a ground issue.

At every key-on, start sequence, the gauge control module and the gauges do a handshake and check-in. If one or more gauges fail to sign-in during this initialization, those gauges will be completely ignored for that entire run duration. (There are no further attempts to handshake until the next start sequence.)

As many owners have noted, a lower-than-expected-battery-voltage will occasionally disrupt this hand shake and when this occurs, one or more gauges may be "dead" for that entire run duration. In these scenarios (low battery voltage), the GT's alternator will be full-tilt charging the battery. Therefore, turning the car off and then re-starting it will usually remedy the problem.

If your fuel gauge is not working - and it hasn't worked across multiple start sequences (whereas the other gauges have) then the problem is entirely unrelated to a ground issue, it is simply a failed gauge. Call Autometer, order a replacement gauge (~$260), plop it in and you're done.
 

texas mongrel

GT Owner
Mark II Lifetime
May 3, 2009
1,489
Houston Texas
You probably already know this but, before actually cranking the starter, let all gauges complete their initializing sweep around the dials.
 

nota4re

GT Owner
Mark IV Lifetime
Le Mans 2010 Supporter
Feb 15, 2006
3,791
While GT gauges don't sweep, I think it is a best practice to turn the key to on, wait until the odometer & trip meter display, and then pressing the Start button.
 

texas mongrel

GT Owner
Mark II Lifetime
May 3, 2009
1,489
Houston Texas
While GT gauges don't sweep, I think it is a best practice to turn the key to on, wait until the odometer & trip meter display, and then pressing the Start button.
Yup, but I wait til the fuel gauge gauge sweeps up before starting. Guess that would be hard for this guy (!)
 
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2006GT

GT Owner
May 13, 2006
11
Thank you for your replies. I am focusing on the fuel gauge. Does anybody know what exactly fails when the gauge stops working? If something is man-made, it ought to be able to be repaired. Whether the repair is cost-effective is another matter. I'm prepared to disassemble it to examine its workings.

Another gauge problem that recently manifested itself is the speedometer does not accurately indicate vehicle speed. Its response is "lazy" and seems to read about half speed. Any thoughts on this issue?

Thank you, again, to all who replied.
 

twobjshelbys

GT Owner
Jul 26, 2010
5,341
Las Vegas, NV
Thank you for your replies. I am focusing on the fuel gauge. Does anybody know what exactly fails when the gauge stops working? If something is man-made, it ought to be able to be repaired. Whether the repair is cost-effective is another matter. I'm prepared to disassemble it to examine its workings.

Another gauge problem that recently manifested itself is the speedometer does not accurately indicate vehicle speed. Its response is "lazy" and seems to read about half speed. Any thoughts on this issue?

Thank you, again, to all who replied.
Much has been written on the gauge issue. Ford has never disclosed what causes the failure. For them to do that they would have to admit there is a problem and they have never officially done that either. There is lots of speculation here but no definitive root cause.

Your best choice is as was previously mentioned to get a replacement gauge directly from Autometer. The last I checked their web site didn't have the Ford GT gauges so just call them directly. Autometer has replacements for the small gauges only. They use a completely different design and are not prone to failure. Ford replacement parts are the same as the originals and will fail with the same probability as the originals. Given the task of r&r for the gauge bezel some have just replaced all of the small gauges at once. They're not that expensive.

You're between a rock and a hard place on the speedometer. Ford is your only source for the speedo and tach (ie, the large gauges). And they are subject to failing again. Your option here is to replace the entire gauge set and bezel with the Speed Hut setup (search will find that info for you). They don't look original but they are also customizable to a degree.

Good Luck.

PS. I'd still replace the battery or at least try a substitute first. Your saying that the speedo is also involved is another clue.
 

nota4re

GT Owner
Mark IV Lifetime
Le Mans 2010 Supporter
Feb 15, 2006
3,791
Thank you for your replies. I am focusing on the fuel gauge. Does anybody know what exactly fails when the gauge stops working? If something is man-made, it ought to be able to be repaired. Whether the repair is cost-effective is another matter. I'm prepared to disassemble it to examine its workings.

Another gauge problem that recently manifested itself is the speedometer does not accurately indicate vehicle speed. Its response is "lazy" and seems to read about half speed. Any thoughts on this issue?

Thank you, again, to all who replied.
The OEM gauges use an innovative "air-core" motor to drive the needle. The replacement gauges from Autometer (only the smaller gauges are available) look identical to the original gauges but utilize a more conventional stepper motor. You're going to need some good tools and patience if you want to try to open up a gauge - let alone fix it!

I know Tony's comment are well intended - but it is not the battery. A low battery voltage has the potential to inhibit the key-on handshake and can cause a gauge not to operable until the next key-on, but it will not cause bad gauge behavior. To wit, assuming your alternator is working, you 're going to be 13.8-14.2v while running even if the battery is low.

Early symptoms of pending speedometer failure is a sticky needle. Sounds like that one is going on you too.
 

The Grey Ghost

GT Owner
Mar 13, 2009
595
Kansas City
The OEM gauges use an innovative "air-core" motor to drive the needle. The replacement gauges from Autometer (only the smaller gauges are available) look identical to the original gauges but utilize a more conventional stepper motor. You're going to need some good tools and patience if you want to try to open up a gauge - let alone fix it!

I know Tony's comment are well intended - but it is not the battery. A low battery voltage has the potential to inhibit the key-on handshake and can cause a gauge not to operable until the next key-on, but it will not cause bad gauge behavior. To wit, assuming your alternator is working, you 're going to be 13.8-14.2v while running even if the battery is low.

Early symptoms of pending speedometer failure is a sticky needle. Sounds like that one is going on you too.
I had what Kendall speaks of.
Intermittent gauge in-operation, always fixed by a key cycle or 2. Which gauge was acting up varied. No issues starting the car. This was fixed with a new battery.
That would leave sender or gauge as the cause. These guys have been down this road and are right on the money.
 

twobjshelbys

GT Owner
Jul 26, 2010
5,341
Las Vegas, NV
I know Tony's comment are well intended - but it is not the battery. A low battery voltage has the potential to inhibit the key-on handshake and can cause a gauge not to operable until the next key-on, but it will not cause bad gauge behavior. To wit, assuming your alternator is working, you 're going to be 13.8-14.2v while running even if the battery is low.
Well, I can tell you from experience that in my car the boost gauge and the speedo started misbehaving and the battery fixed it... Car ran perfectly but probably never got a chance to top off the battery. The tender alerted me that it wasn't able to fully charge it...

As they say "your mileage may vary".
 

ultrasportracing

GT Owner
Aug 31, 2011
435
Perth Western Australia
Someone on here refits the speedo and tacho around $750 each. Cheers
 

2006GT

GT Owner
May 13, 2006
11
Again, thank you for your helpful responses regarding my fuel gauge troubleshooting. I opened the original gauge using a lathe and a cutoff tool applied at a strategic location on the housing. The air core motor seems intact and the needle shaft rotates freely with some friction. If I can fix the gauge, I will reassemble it by making a few mechanical alterations to the housing so that it can be reinstalled in the instrument panel.

I am now working on bench testing the gauge. As I understand it, the black and red wires provide power, the gray wire is the gauge signal, and the white wire controls the lights. I assume that the signal to the gauge is a resistance that varies with fuel level in the tank. Am I correct?

Also, in my original post, I indicated that I measured a voltage between the gauge signal wire and ground, which is a short to voltage according to the Workshop Manual. The instruction is to repair the connection between the signal wire and the Gauge Control Module. No one commented on this so I wonder if a short to voltage in the gauge signal circuit would really affect the gauge operation.

I realize that it's likely that a new stepper motor gauge from AutoMeter would solve my problem. However, I'm willing to suspend a rational economic analysis of the value of my time to repair my original gauge and potentially understand how to prevent future failures.
 
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nota4re

GT Owner
Mark IV Lifetime
Le Mans 2010 Supporter
Feb 15, 2006
3,791
I assume that the signal to the gauge is a resistance that varies with fuel level in the tank. Am I correct?
That's a good question. Gauge functions are controlled by the Gauge Control Module. It's likely not as simple as resistance - but rather a signal of sorts. I know that when we were having speedos and tachs repaired, the shop (which specializes in gauges) had no means to test/calibrate the gauge. (This is the primary reason we stopped offering the service.) They could somehow force the gauges to sweep, but they had no means to do any more granular testing/calibration. For sure, this would be possible if resistance was the signal.

Cool that you are doing this and sharing your findings!!
 

HighHP

GT Owner
Jun 3, 2019
215
Spokane, WA
Here is a link to Autometer on operation of their aircore. They mention bad grounds as a common problem. Sound familiar? With the gauge connected and powered up, you may want to check the voltage between the ground at the back of gauge connection and a very good car chassis ground. This will be zero or very near zero with good connections at all the various connection points between the gauge and chassis. This assumes the gauge ground uses the chassis as the zero voltage reference ground.

Also included is another link to an air core meter discussion. Seems to be a bit different than Autometer discussion.

Please keep us posted on your progress, very interesting.



 
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HighHP

GT Owner
Jun 3, 2019
215
Spokane, WA
Your gauge may have been reading correctly, given the 2.4 volt signal from the gauge control module (GCM).

Here is a possible assessment of why:

The Ford service manual lists the communication from the GCM to all gauges as a signal and as a PWM.

PWM usually means Pulse Width Modulation (duty cycle), which is a common driver signal to control many components (ex: fuel injector, fuel pump, etc) on an engine. The signal is commonly supplied at a 5 volt square wave frequency. The square wave can vary anywhere from 0% to 100% duty cycle. So if 5V is supplied at 10% of the cycle, the motor sees 0.5V and responds accordingly. If 5V is supplied 50% of the time the motor sees 2.5V and responds accordingly.

If indeed the signal to the gauges is the common 5 volt pulse width modulation, then the 2.4 volts that you read on the signal wire could very well result in a one half full gauge reading. If the signal is a 5 volt square wave frequency at 48% duty cycle, your volt meter would read 2.4 volts.

An oscilloscope connected to the signal line would show what the signal shape is – has anybody done that?
 
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