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Discussion Starter #1
62 Nova vert, new gas tank and sending unit. Gauge shows 1/4 tank even after I filled it up and topped it off yesterday.
When I bought the car the gauge showed 3/4 or a little more when filled up but would run out of gas with gauge still showing 1/4 tank or a little less.
What should I check?
 

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Hey Winch,

I have to clarify, did you just buy that new sender and tank or were they new on the car when you bought it?

Either way you likely have a grounding problem to chase but there's good details inside this Section on how to quickly troubleshoot the circuit - and verify your gauge as well, before dropping the tank.

Under this Section you'll find at least 3 different Fuel Gauge threads you can review, consider what applies and approach for your next move. It might help to first determine the operating resistance of your gauge so you know what to expect. I believe 63 vintage gauges use high resistance for empty and low Res. for full but that detail is available within the threads.

Custom Jim and others offer great troubleshooting advice and insight throughout these threads.

Let us know what you find, please.
 

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The first thing I would check would be the ohm range of the new sender vs the ohm range of the gauge. They must match for the gauge read accurately. Second would be the float on the sender. It must be free and have a full range of motion.
 

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Discussion Starter #4
To clarify I installed a new tank and sender when I restored the trunk and floor pans which included fabricating and welding on the last 20 inches or so of the frame rails. As best as I remember now I got the sending unit from Rock Auto. A stock replacement.

I'm not sure how to check the ohm range as suggested. Are you saying to check what the manufacturer says the range is or actually do a test with a meter? It's been a couple of years now since I put the tank in and yesterday was the first time I filled it up.
 

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Discussion Starter #7
Found it. I bought it from Advance Auto. It is a Spectra Premium FG90A which is 0-35 ohms.
 

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Could be the problem. As suggested above a sure way to tell is drop the tank , pull the sender and move it while watching the gauge. Being that your gauge is reading then I would think it is ok.
 

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I might think that after dropping the tank you could measure the depth of the tank and contrast that to the drop of the float and bend the arm accordingly.
 

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When I first started getting into knowing how a fuel gauge system worked, I did the two basic tests of disconnecting the sender wire in the trunk and turning on the ignition and the gauge should swing to full or above. The second test required leaving the key on and then on the wire end that went towards the gauge and NOT the tank, if this was grounded, then the gas gauges needle should go to empty or below.
These two tests were the only thing I though was needed to verify if I had a gauge issue and/or a sender issue but have since found out these two test can give you some false positives.
I never knew years ago that the fuel gauge has what they call a shunt across the back terminals of the gauge and if this shunt is bad or has issues, then the basic tests of disconnecting the sender wire and then grounding it could show the gauge working right but in actuality you could then put in a sender that does work and perform right but still have an issue of the gauge reading wrong.

SOOOOOO, with all that said, ideally get some resistors to mimic the sender values it can output and then try these and if the gauge performs as it should with these resistor tests then chances are it's from your test point back to and through the sender and where it is grounded at.

On a 0-30 ohm setup. shorting out or grounding out the sender wire to the gauge will be 0 ohms. Now if one put in a 30 ohm resistor between the sender wire going to the gauge and the other end of the resistor to ground the gauge will be seeing 30 ohms of resistance and should read Full. Now do the math and if we put in a 15 ohm resistor in place then the gauge should read 1/2 full. By doing more math a 7.5 ohm resistor would or should get the gauge to read 1/4 full and guess what,if we series the 7.5 with the 15, the total ohms would be 22.5 and then if inserted would get the gauge to read 3/4 full.

Gotta test things fully and do more than just guessing as to the issue and adding more tests than just the two of disconnecting the sender wire and then grounding it.

Jim
 

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Discussion Starter #11
OK I'm going to show my total lack of understanding of all things electrical when I ask is there a current flow in that wire that runs from the gauge to the sending unit? How else would it be able to measure ohms of resistance? Isn't it the ohmmeter itself that provides the current via the probes when testing for resistance?
 

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I'm just going to add that Custom Jim inserted a video within a thread in this section that helps understand the testing procedure he uses.
Find it on Page 4 of this Section in a thread by Ruperflood entitled "Another Fuel Gauge ??? Help Requested".
(Sorry, my browser will not allow linking within this forum)

I used Jim's method and saved dropping the tank and 'enjoying' (NOT!) a gas bath, as sanitizing as they are. :mad:

Yes, there is a very small current 'flow' in essence but in this type of circuitry, current can cause an electrical arc (spark) and not really what you want near gasoline vapours. Think of it more as 'information' the sender is providing to the gauge and your anxiety should subside enough to drive again.
And yes, an Ohmmeter has an internal battery to allow measurement of resistance but again, don't get hung on current. It's important, yes but not what causes these problems. A different 'current' value is a symptom of the problem but a change in resistance is a cause and an absence of sender voltage is another. Is that change caused by a high resistance grounding point (bad ground) or a bad wiper on a sending unit, a bad gauge, an open wire or a stuck float?

Consider there are two circuits involved:

Circuit #1 is Switched (Keyed) Bat+ to the gauge and a ground wire at the gauge (provides lighting (often) and gauge operation).

Circuit #2 is the Sender connection point on the gauge with a single wire to the Fuel Sender at the tank. Electrically speaking, the sender wire is the moveable 'pointer' on the Fuel Sender's wiper inside the tank. The float physically moves this pointer across the wiper.
There is also a ground wire from the Fuel Sender to an appropriate ground point at or near the tank. (BTW - That may have been damaged when you did your Trunk work)

The ground wire at the tank is, electrically speaking, connected to one end of the Fuel Senders wiper. Think of the wiper as a fixed point. As the gas level changes, the float moves up and down moving the pointer on the wiper lengthening or shortening the electrical path from the pointer through the wiper to ground (varying resistance). This "Sensor info" based on the position of the pointer on the wiper is sent to the gauge where that Sender value is interpreted and reflected as a visual representation of that varying resistance - like an Ohmmeter. Circuitry inside the gauge dampens the Sender info values preventing the needle from bouncing as the gas sloshes about inside the tank.

Note: In my limited experience, I have found a Fuel Gauge needle that doesn't change (stuck) even with key removed has been resolved by repairing the ground point connection at the Tank.

If anyone finds errors within the above, please feel free to point it/them-out for all to benefit.
 

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Discussion Starter #13
Wow, thanks for pointing me to that video. I've been reading a lot of threads but hadn't got to that one yet.

I suspect you are right about a ground problem. I think I see how to check it out now. I hope I can find some resistors if needed. All our Radio Shacks went out of business around here.
 

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Weren't they the best? (RS).
Not sure where you are in the world - other than in the US, but try a known local Industrial Electrical Supply business or failing that, search online for "electronic components stores near me".
 

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Hey guys, I just dealt with this issue just 1 month ago.... not that I am expert/nor electrically adapt, but it is easy to go around and around on this, which of course, I did. In the end, I had a old sending unit/float, bad wires (or grounding), and a faulty gauge. I bought the new sending unit from Classic Industries to ensure the type was correct for car. Also, the 62/63 shop manual talks about the types of faults and that the gauge type is "floating". Therein, there are gauge 'floating' symptoms and gauge 'indicates full' symptoms. The incorrect sending unit type/oms is not talked about as its assumed 'you get the right one'. :) I recommend to check to ensure its compatible with your car. Here is the one, Sending Unit, to check against as I bought it and installed, it works.

Importantly, I experienced both 'symptoms' from 'floating' and 'indicates full'. I thought the same that 'the sending unit was of wrong type'... it was correct type. I also thought to test the oms or to manually move the float to judge full/empty. But the manual does not suggest this, it basically says: if its 'floating' its a wiring issue, if it 'indicates full' its a short, if reading is incorrect its the sending unit. (yes, just replace the old unit and it threw me as, isn't a short a wiring issue? :) )

I think here, we talking about a 'floating' symptom. I also started with a floating gauge and also that it somewhat indicated something. So, 'some' reading is not a good path to solving in my opinion, again, just ensure the part is correct (!). After that, the manual says: When the key is 'on' and the gauge 'floats', it seemed to say "this is a wiring issue" which I tackled.

Here is what I did,
A. Symptom 'floating' gauge with key 'on'.
1. Replaced the old tank sending unit
2. Replaced the wires
3. Cleaned grounding contact (near tank)
3.1. Tested ground by connecting to Neg battery terminal directly with a long wire. (test failed, but was just to ensure the non-functioning gauge was not result of bad sending unit grounding)

B Symptom changed to 'indicates full' with key 'on' (normally wired with new sending unit)
--- I was a bit confused as "what else is there?" ---- 1. Sending Unit (check), 2. Wiring (check). 3. The Gauge itself (?).
4. Pulled Instrument Cluster
5. Removed fuel gauge (very carefully - the resistor is wire coiled and exposed - here is the same type but not the same gauge as Nova). if broken, that is expensive, but my thinking was 'its broken already'
6. Disconnect the parts, very carefully, clean with copper brush. Clean very sensitive parts with only a small amount of spray cleaner. (I used electrical spray cleaner)
7. Reassemble Gauge into Instrument Cluster
8. Clean Instrument Cluster grounding
9. Lean the Cluster into place and test.

Cleaning the gauge itself worked for me, works perfectly. Old Gauges can be the issue. Most times guys replace the entire instrument cluster. The fuel gauge seems 'solid state' so the contacts (screws, nuts and copper contacts) were old and dirty. Brushed clean and spray cleaning the rest (very carefully!).

.02 instramentcluster2.jpg
 

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I hope I can find some resistors if needed. All our Radio Shacks went out of business around here.
See if you have a local electronics repair place. If not, maybe mail order. You only need 3 (7.5 ohm,15 ohm, and 30 ohm). These values do not have to be exact to where if you find a 5 ohm, that would be fine. The same amount of tolerance on the others is fine.

Looking at service parts here at the shop we have resistors in ohm values of 7.5, 15, 27, and 33 and I would think a better electronics repair business would also stock these values.

Jim
 

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Discussion Starter #17
I fiddled with it a little this morning and my results are not what I expected. Here is what I did:

1. Disconnect sending wire at sending unit
2. Turn ignition on - needle still shows 1/ 4 tank and hasn't moved.
3. Unplugged double connector (pink and brown) on back of fuel gauge while still in car
4. Checked voltage on pink wire - shows 12v
5. Rigged up jumper to connect pink wire to it's spade on the fuel gauge so as to leave the brown sending wire off .
6. Turn ignition on - needle still shows 1/ 4 tank and hasn't moved.
7. Rigged up jumper to attach sending wire terminal to ground
8. Turn ignition on - needle still shows 1/ 4 tank and hasn't moved.
9. Tried various ways to make insure cluster is grounded but no difference. Looks like a 62 fuel gauge does not have a dedicated ground pole so it must ground through the cluster. All other lights and gauges in cluster work OK so I assume I have a good enough ground there.

What's next? Pull fuel gauge out of cluster? Maybe check continuity of sending wire to insure it isn't shorting out on it's way back to the tank?
 

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Discussion Starter #18
Uh oh. I'm starting to get a sick feeling. I haven't found a fuel gauge at Classic Industries or Ecklers. Are they not repopped?
 

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I would not freak out yet. The shop manual, pictured, shows it could just be the electric connections. (Gauge does not register correctly, hoping its not just bent!) B. C. and D (high resistance, partial short, loose connection) seem to say to me, its dirty. That was it for me. Pull the gauge, clean it (I mean pull the gauge carefully, the coil of wire is exposed.)

I think Custom Jim is correct too. if the oms are wrong, then it could be reading 1/4 tank when empty (I don't know really). I think he is saying' if the gauge is getting oms but wrong amount, then its the oms. I'm tactical, I couldn't figure out which setting on the tool was oms! So, went with option B, just clean it.

I think many people just replace the entire instrument cluster for 600+ dollars (too much for me), there are guys on the site who will get a used one for much cheaper. 20200513_125904.jpg
 

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This may not be an issue but these "rigged" up jumpers, could those be an issue ?. I know I have grabbed what I "thought" were good alligator clip ended jumpers only to find out they had an open between one end and the others or I've shoved a stripped wire end into a connector only to have it contact with rust, paint, or oxidation preventing a good connection.

If all else fails, get the gauge out, look over things real good, clean connection posts if needed, and then bench check things.

I know on an old test I did on a factory indash tachometer for my 74, there were push in connections between the case of the housing and the circuit board and maybe yours is assembled the same way ? and corrosion and poor connections are buried inside ?.

Below is the inside of the tachometer housing with the female connectors inside that then male connectors off of the circuit board push onto when things get assembled.





You also stated:
4. Checked voltage on pink wire - shows 12v
5. Rigged up jumper to connect pink wire to it's spade on the fuel gauge so as to leave the brown sending wire off .
9. Tried various ways to make insure cluster is grounded but no difference. Looks like a 62 fuel gauge does not have a dedicated ground pole so it must ground through the cluster. All other lights and gauges in cluster work OK so I assume I have a good enough ground there.

Now did you check to see if you had 12V on the pink wire still at the terminal of the gauge ?. Reason I'm asking is a meter needs a VERY small amount of current/amperage to measure voltage and if there is a poor power connection tow the gauge, when the gauge is then powered up and in the circuit and working, if the voltage goes away, then it's a power delivery issue. Sorta like checking a battery. It might show 12 volts while sitting there but will then not crank over a motor BUT if you measure the voltage WHILE trying to crank over the motor and the voltage drops to 6V or less, then it's a battery issue.

Jim
 
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