I'm sorry if this has been asked before, but I searched and didn't find anything.. I put an aftermarket Mustang tank in my '46 Chevy so I could hold more fuel for long trips. I put a new Stewart Warner gas gauge under the dash with the correct Ford ohms. Well, when I fill up the tank, it will be pegged way past full on the gauge. As the tank gets lower, the needle starts bouncing around, enough to where I cant tell how much gas I have. It drives me nuts! Do you guys with Mustang gas tanks have bouncy fuel gauges or is something wrong with mine? A bad gauge? Bad sending unit? I have both the gauge and tank grounded.
Mine works great. I guess start with trying another guage if you have one laying around. Over full usually indicates bad ground but you say you did that.
Maybe I could recheck the grounds and try to make them better. Would it be a bad ground in the tank or the gauge that would cause this?
Sounds much like a ground problem. Did you insulate between the body and the tank. Did you insulate between the mount and the body. Possible that if you did you don't have a good enough ground. I'm sure you set up the float arm like the instructions said. Made sure the float was turned the right direction.
I'd call S/W to find if a voltage limiter is needed for your gauge to drop from 12 to 5-6 volts. The OEMs used one. I also used a 60s ford tank in my 49 ford coupe and used a 56- 67 ford pickup limiter [harvested at my local U-pull-it] in the fuel gauge circuit...my gauge worked perfectly.
On a Ford circuit when you lose ground it stays at E. Short to ground it will peg to F. I would ground the sender wire & check to see if the dash gage goes to F or past. If it does problem is the sender. If not, wiring or dash gage.
Just an aside; Grounding the gauge won't do a damn thing for you it is completly un- nessary. Grounding the tank is important. Normal Norman
If you put in a repop sending unit along with the tank swap, they are not compatable with some aftermarket gauges (Autometer has a tech alert on their website). I had a similar problem and had to install a good used Ford OEM sender which corrected it.
I didnt insulate between the tank and trunk. I actually ground the tank and trunk to bare metal where they meet to ensure a good ground. I didn't have to set up the float arm, the new sending unit was already installed in the tank when I bought it. In the gauge there is '12V' stamped into the case. Is it possible that the sending unit is 6V? Okay, I will try that out. Thanks! I didnt ground the gauge at first because I figured it would ground itself. I went ahead and grounded it just in case when I started having problems.. I was worried about it just being a cheap sending unit. I put a new sending unit in another car once and it didnt work from the get go. I might have to just try an OEM sender if none of these other suggestions work.
Here is (possibly) the reason why your needle bounces. First, on the OEM Ford unit, the gauge is operated not by constant voltage, but by alternating 12v and 0v, to average near 6 volts. Don't get ahead, senders care naught of voltage. Second, the OEM gauge unit itself is, for lack of a better term, an 'averaging' gauge, meant for the needle to move (somewhat) slowly, and 'average' the reading. Third, the sender is simply a rheostat, nothing more, or less. Your gauge, OTOH, is NOT an 'averaging' gauge, it is an 'instant read' gauge. Meaning that it is designed to read instantaneously what the sender indicates. The sender, BTW, is moving nearly all the time, as gas sloshes (I know, you knew that). Means that the gauge fluctutates with the sender. In other words, Ford knew what they were doing with that weird constant voltage unit, the gauges work well in their original environment. BTW, the 'constant voltage unit' is not really, it is a r*e*a*l*l*y slow flasher. Sadly, I know not of a cure. Kinda like the Veglia (Vague-lia) gauges on my early Ducatis, you, yourself, need to 'average' the readings. Cosmo
Cosmo, I had a friend tell me that. Do you know if there is any way to get a gauge that is an 'averaging' gauge? Do any companies sell these? I tend to think youre right, because the needle only bounces as I go down the road, not when I'm parked or at a stop light... Hmmmm. It doesnt move when its full. Maybe because there is not enough room in the tank for the gas to slosh, making the sending unit move around.. and as the gas dwindles (sadly a little too quickly) it gives the sender room to move up and down.... therefore making the gauge bounce. I got the cheapest gauge I could find, maybe an expensive unit would have the 'averaging' quality?
The tank is out of a Falcon/Comet. I got a new SW gauge with the Ford Ohms Unless the aftermarket tanks come with different ohms it should be the same.
No, because the CV unit gauges all run on a nominal 6 volts. Possibly you could put a damper on the float's movement. Just FWIW, Motor manuals have a great section on gauges and the function thereof. Worth a look-see, if you have access (library??) Cosmo P.S. If the ohms were different, the gauge would read wrong, unlike your problem, which is gauge reads correct, but needle bounces. P.P.S. I have a Falcon (not my first, or second), quite familiar with how these gauges/sending units operate.
I'm going to buy a more expensive auto meter gauge and install it and see if it fixes the problem. If it doesnt fix it I can always return it. I'll quadruple check the grounds, just in case. The only thing that concerns me is that instead of reading "Full", the needle points way past full, as far over to the right as it can go.
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