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Author
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Topic: fuel gauge inaccuracy
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mikel3891
Gearhead Posts: 111
From: philippines
Registered: Sep 2005
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posted 10-20-2005 05:04 AM
 
when i put a full tank of gas on my mustang my fuel gauge reads 3/4 only. how do i fix the problem?
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kinger44
Gearhead Posts: 376
From: Waterloo, Ontario, Canada
Registered: Sep 2004
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posted 10-20-2005 09:21 AM
Hey,When I first got my car the guage didn't work at all. I had the sending unit pulled from the car and found that the float didn't float anymore. They rigged something up and then it worked a little. On this year's upgrades I replaced the tank and sending unit. Mine now acts like yours - 3/4 tank when it's full. So my thought is that it's electrical from the sending unit up to the guages. Not much help but maybe the others can finish the puzzle for us. Cheers, Gregg ------------------
68 coupe, red on black. http://mustangsandmore.50megs.com/MembersPics/kinger44.html [This message has been edited by kinger44 (edited 10-20-2005).]
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adragon8u
Gearhead Posts: 5867
From: Oceano, Ca. member# 2895
Registered: Mar 2003
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posted 10-20-2005 09:23 AM
Mines empty at a 1/4 tank 
I Plan on replacing the sending unit soon.
I'm assuming it's the sending unit on mine because the gauge reads empty when the cars shut off so it can't be the gauge.------------------
"It's better to have loved a short girl,
Then never to have loved a tall"
http://mustangsandmore.50megs.com/MembersPics/adragon8u.html
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ddenton749
unregistered
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posted 10-20-2005 09:32 AM
My first thought is the sending unit. My second thought is ground connections.------------------
'73 Convertible, 351C 2V
'91 LX Convertible 5.0
Member: Valley Forge Mustang Club
"The best things in life are free, but the really kick-a** things cost a fortune."
My Site
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buening
Gearhead Posts: 317
From: Decatur, IL
Registered: Mar 2005
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posted 10-20-2005 10:44 AM
The problem may be with the constant voltage regulator on the back of the instrument cluster. It's a little box that maintains constant voltage and when it is out of whack the guages will read funny. It can be adjusted with the little metal pin. Whenever you are positive that it is at 3/4 or 1/2 tank, turn the pin CCW or CW for adjustment. Trial and error at this point. The temp guage, oil pressure, and fuel guage are all regulated by this little box and the adjustment will affect all three guages.------------------
1970 Grabber Blue Mach 1 H-code
1970 Fastback
2003 Torch Red Mustang
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V8 Thumper
Gearhead Posts: 4777
From: Phoenix, Arizona
Registered: Dec 2001
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posted 10-20-2005 12:08 PM
Exactally what buening said.
I'd only add that the sending unit operates off of resistance or ohms; 40-some years of crud can accumulate inside the sending unit (which is constantly submerged in gasoline) and inhibit the unit's accuracy. To remove it you need to drain the tank completely and rotate the locking ring. The rubber gasket will not survive  but they're cheap.Sending units are not concidered servicable but the the repop unit I run in my car (3/8" pickup  )is of good quality and accurate  ------------------
1965 GT coupe, 333ci aluminum headed/solid cammed stroker, four speed, 3.70:1 9" 12.58/110 on street tires, more to come;) All Blue Oval, no blue bottle
http://mustangsandmore.50megs.com/V8Thumper.html
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mellowyellow
Gearhead Posts: 8198
From: So. Fl.
Registered: Aug 2000
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posted 10-20-2005 12:59 PM
Live with it. Know that 3/4 is full and go from there. My 65 GT was like that and was sure I could correct it by bending the pickup, but didn't want to deal with it. The guy I sold it to was told about it and he's yet to have a prob.
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Clark
Gearhead Posts: 749
From: Rowlett,Texas
Registered: Aug 99
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posted 10-20-2005 02:31 PM
Mellow hit on the head,On original sending units it is common to see the metal float have a small leak keeping it from reaching the full mark. On aftermarket sending units the accuracy is hit or miss. After I installed my unit, I also had it reading low. I then ran it down to empty and drained the tank, pulled the sending unit out and bent the float arm up. Reinstalled and the filled with 5 gallons of gas checked for accuracy and then had to repeat. This is a pain but now when I fill it up it goes to full and when I am on "E" I know I have about 3 gallons left. Not real accurate down low but this will keep me from running out. On a side note watch out for those plastic floats mine leaked right after install and I went back with a metal one. ------------------
69 393W Sportsroof Deluxe
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Scott H
Gearhead Posts: 1480
From: Chicago area
Registered: Mar 2005
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posted 10-20-2005 02:51 PM
The sending unit is a resistance to ground.If you pull the yellow wire off from under the tank and ground it straight to the chassis, the gauge should immediately swing to FULL if you have the key "on". Don't leave it like this or you could damage something. If it works, then you can focus your efforts on a better ground for the gas tank, a better connection between the sending unit and the tank, or possibly the sending unit itself. If you take the sending unit out, you can test it through its full range of motion by reconnecting the yellow wire and then grounding the unit to the chassis. Move the float up and down and see how your gauge reacts.
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Fastymz
Moderator Posts: 22791
From: Reno Nv M&M #1240
Registered: Apr 2001
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posted 10-20-2005 02:58 PM
I never let my 65 go below 1/4 tank. But I do that with all my cars. ------------------
Mustangsandmore member #351w
oddly obsessed with big scoops on little Mustangs 65 coupe BIG Boss hood scoop My Pics "Politics is the art of looking for trouble, finding it, misdiagnosing it and then misapplying the wrong remedies." Groucho Marx
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Mooney
Gearhead Posts: 2357
From: Marietta, Ga
Registered: Oct 2003
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posted 10-20-2005 05:49 PM
quote:
Originally posted by Fastymz:
I never let my 65 go below 1/4 tank. But I do that with all my cars.
Geez.. I wish I was like that.. I'm too lazy to stop off for gas sometimes so I'll park it at the house on E.
Bad though when I forget I parked the Mustang on E and then I get the Datsun to E and then its time to goto work OOPS!
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buening
Gearhead Posts: 317
From: Decatur, IL
Registered: Mar 2005
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posted 10-20-2005 06:59 PM
quote:
Originally posted by Fastymz:
I never let my 65 go below 1/4 tank. But I do that with all my cars.
This is also a must for fuel injection vehicles with in-tank fuel pumps. The fuel keeps the fuel pumps cool and if you run your tank near empty enough it will burn up a fuel pump. Let me tell you, changing one of those pumps is a PIA! Had to do that on a ford Probe, those fuel tanks are braced enough that those tanks aren't gonna fall out, not to mention getting to all of the hidden bolts.
------------------
1970 Grabber Blue Mach 1 H-code
1970 Fastback
2003 Torch Red Mustang
[This message has been edited by buening (edited 10-20-2005).]
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68mustang351w
Gearhead Posts: 558
From: San Jose, Ca
Registered: Sep 2004
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posted 10-20-2005 09:56 PM
This isnt going to help but my 68 acts the same way. Even after i cahnged the fuel tank (along with new sender & wire) it still shows that 3/4 is full. I tested the guage it was was accurate as well. To test it i pulled the gauges out wired a couple of batteries to one side (giving it about 5vdc) then on the other side of the gauge i had a wire connected to a variable resistor. I think 0 ohms is full, 90 ohms is empty. After i did all this, the gauges acted normal, so i slapped everything back together and it still showed 3/4 to be half. I gave up on it... David F.
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mikel3891
Gearhead Posts: 111
From: philippines
Registered: Sep 2005
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posted 10-20-2005 10:40 PM
quote:
Originally posted by buening:
The problem may be with the constant voltage regulator on the back of the instrument cluster. It's a little box that maintains constant voltage and when it is out of whack the guages will read funny. It can be adjusted with the little metal pin. Whenever you are positive that it is at 3/4 or 1/2 tank, turn the pin CCW or CW for adjustment. Trial and error at this point. The temp guage, oil pressure, and fuel guage are all regulated by this little box and the adjustment will affect all three guages.
My constant voltage reg is new. which one do i adjust? where is the adjustment located? thanks.
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exlocal
Gearhead Posts: 1552
From: hacienda hts., CA, USA
Registered: Dec 2004
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posted 10-20-2005 11:38 PM
quote:
Originally posted by Fastymz:
I never let my 65 go below 1/4 tank. But I do that with all my cars.
I'm so paranoid, my cars visit the gas station at the half tank empty/full level. Running out of gas in rush hour in the fast lane leads to this type of behavior. ------------------
reliving youth
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buening
Gearhead Posts: 317
From: Decatur, IL
Registered: Mar 2005
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posted 10-21-2005 09:11 AM
quote:
Originally posted by mikel3891:
My constant voltage reg is new. which one do i adjust? where is the adjustment located? thanks.
If you look at the regulator, there should be a + and - connections , which on a 69/70 appear like a 9v battery connection and on the earlier cars are male spade connectors. Right below one of the spade connectors is a little metal pin that is flat on the end. That is the adjustment screw. Attached is an Ebay auction that has a picture for you to reference.
http://cgi.ebay.com/ebaymotors/FORD-LINCOLN-MERCURY-VOLTAGE-REGULATOR-67-68-MUSTANG_W0QQcmdZViewItemQQcategoryZ10076QQitemZ4583566765QQrdZ1QQsspagenameZWDVW
A few reference posts regarding this issue
http://forums.vintage-mustang.com/forums/showflat.php?Cat=0&Board=forum&Number=1282528&Searchpage=1&Main=1282434&Words=instrument+regulator+Midlife&topic=&Search=true#Post1282528
http://forums.vintage-mustang.com/forums/showflat.php?Cat=0&Board=forum&Number=947670&Searchpage=1&Main=947399&Words=constant+voltage+regulator+Midlife&topic=&Search=true#Post94767 0
Midlife is the guru on adjusting them, if anything try emailing him for help. Good luck! Oh, what does your stock temp and oil pressure guages read when the motor is at normal operating temps? ------------------
1970 Grabber Blue Mach 1 H-code
1970 Fastback
2003 Torch Red Mustang
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mikel3891
Gearhead Posts: 111
From: philippines
Registered: Sep 2005
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posted 10-21-2005 09:50 PM
quote:
Originally posted by buening:
If you look at the regulator, there should be a + and - connections , which on a 69/70 appear like a 9v battery connection and on the earlier cars are male spade connectors. Right below one of the spade connectors is a little metal pin that is flat on the end. That is the adjustment screw. Attached is an Ebay auction that has a picture for you to reference.
http://cgi.ebay.co m/ebaymotors/FORD-LINCOLN-MERCURY-VOLTAGE-REGULATOR-67-68-MUSTANG_W0QQcmdZViewItemQQcategoryZ10076QQitemZ4583566765QQrdZ1QQsspagenameZWDVW
A few reference posts regarding this issue
[URL=http://forums.vintage-mustang.com/forums/showflat.php?Cat=0&Board=forum&Number=1282528&Searchpage=1&Main=1282434&Words=instrument+regulator+Midlife&topic=&Search=true#Post1282 528]http://forums.vintage-mustang.com/forums/showflat.php?Cat=0&Board=forum&Number=1282528&Searchpage=1&Main=1282434&Words=instrument+regulator+Midlife&topic=&Search=true#Post12825 28[/URL]
[URL=http://forums.vintage-mustang.com/forums/showflat.php?Cat=0&Board=forum&Number=947670&Searchpage=1&Main=947399&Words=constant+voltage+regulator+Midlife&topic=&Search=true#Post 94767]http://forums.vintage-mustang.com/forums/showflat.php?Cat=0&Board=forum&Number=947670&Searchpage=1&Main=947399&Words=constant+voltage+regulator+Midlife&topic=&Search=true#Pos t94767[/URL] 0
Midlife is the guru on adjusting them, if anything try emailing him for help. Good luck! Oh, what does your stock temp and oil pressure guages read when the motor is at normal operating temps?
my temp gauge reads at 1/4 and sometimes close to 1/2, it is unstable. my oil pressure is the one that has the prob, it just moves a little bit and in the low position but the engine runs great.
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68mustang351w
Gearhead Posts: 558
From: San Jose, Ca
Registered: Sep 2004
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posted 10-22-2005 04:40 AM
I would put a set of mechanical gauges on it so you actually know how your temp and pressure are running because as you have noticed, the stock gauges dont really work very well. Summitracing.com has a set of three gauges (water temp, oil pressure & voltmeter) for 25 bucks
http://store.summitracing.com/partdetail.asp?autofilter=1&part=SUM%2DG2923%2DV&N=400304+302223+4294859916&autoview=sku
Just my 2 cents... David F.
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buening
Gearhead Posts: 317
From: Decatur, IL
Registered: Mar 2005
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posted 10-22-2005 12:46 PM
quote:
Originally posted by mikel3891:
my temp gauge reads at 1/4 and sometimes close to 1/2, it is unstable. my oil pressure is the one that has the prob, it just moves a little bit and in the low position but the engine runs great.
I think we just found your problem. The fuel gauge reads low, temp gauge can read low at times, and the oil pressure reads low. This isn't a fuel sender problem, it's the CVR on the instrument cluster. By adjusting the CVR, the gauge will either read lower than previously or higher. You want it to read higher. I'd adjust it when you know how much gas you have in your tank. Keep logs of your fuel mileage and if they stay pretty steady, fill the car completely and keep track of how many miles you put on that tank. You know the capacity of your tank, the mpg the car gets, the miles you put on a full tank. From there you can tell how much gas you've consumed and how much is left. I'd adjust the guages when you have 1/4 tank of gas. As stated above, the CVR controlls all three mentioned gauges which all read low. Good luck! ------------------
1970 Grabber Blue Mach 1 H-code
1970 Fastback
2003 Torch Red Mustang
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mikel3891
Gearhead Posts: 111
From: philippines
Registered: Sep 2005
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posted 10-25-2005 01:35 AM
quote:
Originally posted by buening:
I think we just found your problem. The fuel gauge reads low, temp gauge can read low at times, and the oil pressure reads low. This isn't a fuel sender problem, it's the CVR on the instrument cluster. By adjusting the CVR, the gauge will either read lower than previously or higher. You want it to read higher. I'd adjust it when you know how much gas you have in your tank. Keep logs of your fuel mileage and if they stay pretty steady, fill the car completely and keep track of how many miles you put on that tank. You know the capacity of your tank, the mpg the car gets, the miles you put on a full tank. From there you can tell how much gas you've consumed and how much is left. I'd adjust the guages when you have 1/4 tank of gas. As stated above, the CVR controlls all three mentioned gauges which all read low. Good luck!
I just recently put a full tank of gas and I already adjusted it to full on the gauge. I think I still have a good reading on my temp but my oil pressure gauge is really unstable(it is actually flunctuating). I guess the fuel gauge is already fixed but I think the oil gauge is now the problem, maybe the oil gauge is the one that's broken. Thanks for the fix on the fuel gauge.
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