Question: What would cause my fule gage to spike full.

[Bolisk]

I recently replaced my incorrect fuel gague with a correct rebuilt one. . .and the gague is spiked over on full? When I first installed it. . .it said empty. So I took the harness off, and re-installed it and it went to full. At the time. . .I had a full tank. Now I'm about 1/2. . .and it still reads full.

Ideas?

-Bolisk

 

[Mr. Chuck]

Your problem is an indication that you have a electrical connection problem, check the connections in the harness connector for broken contact pins or broken wires / strains at the pin causing an open circuit or shorts to an adjacent pin / contact. In technical terms the fuel gage circuit is a DC bridge network, either full or empty reading are signs of a short or open in one of these circuit / bridge legs. The mostly likely point of the problem is at the point where the circuit has been distrubed during the recent repait work; i.e. the harness.

 

[Bolisk]

Could it be that I installed the Seat Belt Warning wire onto the wrong post on the back of the fuel gauge? I have a 1970 vette, and it has the Seat Belt warning lite, and cancelation button on it. On the back of the circut pad that contains the bulb and the cancelation switch, there was a black wire that was attached to one of the posts on the back of the fule gague. Is it possible that I put that wire on the wrong post. . .and would it cause this problem?

If yes, what post is that wire supposed to connect to? Note: the seat belt switch and light are working correctly.

-Bolisk

 

[Mr. Chuck]

I don't know how my 81 wiring compares to your 70; this may help; 81 shop manual shows a black wire (ground) connected to one side of the fuel gage and a "Pink" wire (from the fuel gage sender) connected to the another fuel gage bridge leg & the Low Fuel Warning Lamp. The fuel sender is just a variable resistance to ground (located at "Rear Light ground") for one leg of the gage bridge. While the Black wire connects another leg of the bridge directly to (Instrument Panel ground).
The 20 amp fuse labeled "Gages Fuse" powers the Fuel gage circuit via a "Pinhk.black" wire and also powers the Voltmeter, Coolant Temp gage, Oil Pressure Gage & Generator Warning Lamp. If these gages are working then the fuse is good. With each respective wire disconnected, test using a ohm meter (x1 scale) check the black wire, should read "0" ohms, if not, check ground connections. The Pink wire resistance will read a value greater than Zero, actual value depends on the level of fuel in the tank. Note: Ohm meter needs to be zeroed by shorting the two leads together and adjusting the thumb wheel., assuming you haven't used one before.
Hope this wordy story helps!!

 

[Bolisk]

Thanks for your input. . .I'll try some of your ideas tommrow.

Here is what I learned prior to reading your post. When I got home this evening, I took the dash apart to look at the guage wiring. Low and behold, I didn't have the wiring harnes on the gage correctly. Only the hot side of the wire was connected (pink wire). The black wire was not connected to the other tab. So I reconnected the wire, and the gague came off FULL and droped to empty. The problem is that the tank was about 1/4 full.

So I rechecked all the wiring, then reassembed the dash (I'm getting really good at that now) and went out to fill the tank up. I got gas and started the car. The fuel gauge raised up to 1/4 tank and stoped! So not is kinda working. . .but it reading WAY low.

What gives? The old gague which was from a 74 vette was reading fine. And the sending unit in the tank is only 1 year old.

-Bolisk

 

[Mr. Chuck]

//http://picasaweb.google.com/mrchuck7/81dashlights#5325531701843472818
Hopefully you can access this web site where I have put the Fuel Gage Diagnosis page from my 81 shop manual, it may give you some hints on what to check.
What year is your current fuel gage from? This maybe a case where the gage & Sending Unit needs to be calibrated together; a question for professinal instrument guy.

 

[Mr. Chuck]

Thanks for your input. . .I'll try some of your ideas tommrow.

Here is what I learned prior to reading your post. When I got home this evening, I took the dash apart to look at the guage wiring. Low and behold, I didn't have the wiring harnes on the gage correctly. Only the hot side of the wire was connected (pink wire). The black wire was not connected to the other tab. So I reconnected the wire, and the gague came off FULL and droped to empty. The problem is that the tank was about 1/4 full.

So I rechecked all the wiring, then reassembed the dash (I'm getting really good at that now) and went out to fill the tank up. I got gas and started the car. The fuel gauge raised up to 1/4 tank and stoped! So not is kinda working. . .but it reading WAY low.

What gives? The old gague which was from a 74 vette was reading fine. And the sending unit in the tank is only 1 year old.

-Bolisk

You may have a compatibility problem between the 70 sending unit and the 74 gage, especially if the 70 & 74 Vets fuel tanks capacities where significatly different. May be members with the respective years can confirm what their fuel tanks hold. Quanta Products offer the same gas tank (20 gallons) for 70 - 74, also the their same sending unit is compatible for yrs 68 - 74; so I would expect fuel gage from either year to work without the need for any calibation.
TS'ing wise you should check the ground connection at the sending unit located at the "Rear Light Ground" point, a bad ground or damage wiring can cause a incorrect reading. A faulty sending unit may be the problem if the wiring checks out.