New Sending unit reads 1/2 full
I just replaced my tank and sending unit on my 1966. With the old sending unit I had no problems with the gauge reading. I installed the new one in the new tank and filled it with gas and the gauge reads only 1/2 full. Is it a bad sending unit or bad electrical connection? If I pull the ground the needle does not move. If I pull the connection with the smaller of the two post connections no movement but if I pull the connection with the lagrer post it reads full
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Re: New Sending unit reads 1/2 full
Dave -
Run the diagnostic checks in the article in the link below and it will isolate whether it's the gauge or the sending unit.
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Re: New Sending unit reads 1/2 full
John -
I finally got around to looking at this today too.
My problem began when I pulled the cluster to <attempt to fix> the temperature gauge. Before that I knew it registered empty pretty well and was full at the time of the cluster pull.
After the cluster pull little did I know it wasn't going down and was hung at 3/4 - 7/8.
So checked the tan wire and when the key goes on, the gauge moved down to about 1/2 which is where I think it actually is.
Power wire wants to stay connected, move to the inside for now.
Power to gauge - 12v OK.
Then - instead of checking from ground at sender, I figured to check the gauge ground to IP beam ground. 30ohms resistance...
At the sender, ground connector wants to stay put, so I probed the back of the connector to bumper bolt and there was on 4 ohms resistance.
So I expect that this is a ground issue with the IP wiring...
Ground to the left of the radio looks clean and if a newer nut has been installed - any other ideas on checking out those ground circuits?
BTW -- Did you ever write an article on temp gauges?Comment
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Re: New Sending unit reads 1/2 full
The entire car from the firewall rearward is grounded through ONE wire - the black pigtail that exits the radio connector with a ring terminal on it and is screwed to the bottom of the Z-bar just to the left of the radio. The grounds for everything at the rear (tail/stop/backup/license lights and the fuel tank sending unit) splices to a single 16-ga. black ground wire that comes forward in the main body harness to the multiple connector above the kickpad where it joins the instrument panel harness and ends up at the radio connector.
The temperature gauge may be the only thing I haven't written a feature article or tech column on (148 in the last seven years).
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Re: New Sending unit reads 1/2 full
John, I am pretty sure I have a ground issue somewhere. On the temp gauge, I bench tested a NOS gauge sender together with that and verified that it worked, in car, nothing. I even jumpered out of the car back of gauge directly to the sender to confirm that the green wire was not the problem. Confirmed 12v at the gauge in. Crossed over the base of the sender to battery post etc. to verify that side of the system was grounded, all had zero resistance. Has to be in the dash wiring somewhere cluster to ground.Comment
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Re: New Sending unit reads 1/2 full
John,
Ground to the side of the radio looks good, continuity out of the connector - no resistance from there (double black wire) to the back of the cluster.
The black wire then goes up into the main wire bundle that per the AIM then appears at the junction box/fuse panel.
Advice on next steps to isolate the fuse box?Comment
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Re: New Sending unit reads 1/2 full
John's article is helpful, but there's more information in the GM/Chevy service documentation. The 1963 Corvette Service Manual gives a full system schematic for the fuel gauge (first use of the Whetstone bridge circuit topology), in Section 12-13, Electrical-Body and Chassis.
Then, there's a mechanics troubleshooting chart (Section 12-26) to diagnose faults by gauge symptom. Note, when using this chart for later than '63 cars, you need to translate wire gauge/color to your specific year as wiring color code norms changed over time.
The reason for the unique Whetstone bridge circuit was it made the fuel gauge reading TOTALLY INDEPENDENT of absolute voltage (is alternator charging? what is the current battery voltage level?).
This is based on the fixed resistances of the fuel gauge's internal electro-magnets that drive the pointer needle and the tank sender. The gauge reports what FRACTION of total battery voltage is dropped across the gauge versus the tank sender...
To that end, BOTH the gauge and the tank sender have to be independently grounded. AND, those two grounds have to be the SAME (little/no resistance between them).
Here's what the troubleshooting table lists:
(1) Pointer remains at 1/2 position: Poor connection of terminals in brown wire or break in brown wire. This is the wire that runs from the gauge to the tank sender's variable resistance port.
(2) Pointer does not read below 1/2 position: Poor connection of terminals in light green wire/break in light green wire. The light green wire provides fused power to BOTH the gauge and the tank sender.
(3) Pointer does not read above 1/2 position: Tank sender not grounded.
(4) Pointer remains at full position: Instrument panel/gauge not grounded.
(5) Pointer does not move when ignition is turned on: No current to gauge or poor connection.
Note the distinction between #1 and #3 (pointer needle being PEGGED at 1/2 full vs. being able to vary between zero and half full based on tank sender swing arm position) is something you haven't mentioned...
This post says you're looking for faulty ground(s) in the area of the dash. The troubleshooting symptom list, 4) tells you to expect the gas guage needle to be permanently pegged to the Full position if the instrument cluster (or the gauge in the instrument cluster) isn't properly grounded.
The distinction, paranthesis, is that the gas gauge derives its ground from the instrument cluster via the sheet metal screws that hold the cluster module in as well as the internal ground wire inside the gauge making a good connection to the gauge's internal electro-magnets.
If you're in over your head, remember Mr Goodwrench is STILL in business plus most towns have one or more competent auto electric shops...Last edited by Jack H.; April 4, 2010, 11:54 AM.Comment
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Re: New Sending unit reads 1/2 full
Jack, If I can't figure this out with a little diagnostic help, like that you and John have provided, Mr. Goodwrench certainly can't; much less I'd never entrust a 20 year old kid that has not been within 10 feet of a Corvette.
This can't be that hard, we are close, but the points you mention don't get to the symptom exactly.
After the dash pull to put in the temp gauge, the tank stayed pegged on full, then when the tank was empty it swung to empty and then when full went to full. Stayed there to 3/4 full until this diagnostic - even though the tank is probably 5/8.
So when I performed step one, the gauge should have gone to full. It did not, it went to the 5/8 point, the level in the tank from what I expect.
Int he dash, the black ground that "eventually works its way to the IP beam MASTER ground had a resistance of 30 ohms.
That master ground is very clean, probing the radio connector that is connected so the second wire that exists there and goes over to the fuse box seems to be the wire that is in question. That, the fuse box and cluster.
When I touch the back of the cluster to the master ground there is zero resistance so the issue is either grounding of the gauge or cluster to IP beam or deeper...
How does the gauge ground to the cluster?
How does the cluster ground?Comment
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Re: New Sending unit reads 1/2 full
Ronald,
The cluster grounds through a spade terminal that is secured at one of the top screws. A black wire from the main harness plugs into it and that wire is connected to the radio plug ground then to the birdcage.
Also, FWIW there is a ground strap that grounds the birdcage to the frame near the drivers front splash shield.
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Re: New Sending unit reads 1/2 full
Thanks John, I have that one on good, tested various points to ground all with no smoking gun.
The LH body to frame reads the same 2 ohm resistance as motor to frame.
Back of cluster, back of radio, stud on back of fuel gauge to ground all come up with the same 2 ohms.Comment
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Re: New Sending unit reads 1/2 full
Ronald,
Make sure your negative battery terminal is clean..
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Re: New Sending unit reads 1/2 full
That is all fresh, full charge, etc. Rebuilt starter, clean contacts there too.Comment
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Re: New Sending unit reads 1/2 full
For those interested in fuel senders, here is a quick update.
Recall the original sender is supposed to read 0 ohms at empty and 90 ohms at full. A NOS part from that period measures 0 MT and 90 FULL.
The China? Corv. Central part, 2 at MT and 200 at full OOPS!
The China part Paragon and other sell for about $80 (0 at MT, and 150 at full).
The Delphi part - that used to be made in Flint - 0 to 90, and that one looks the most like the original.
Pray for rain this weekend & I'll have this last and most expensive one inside the tank...
We'll post a pic of the flange, the metal on the China units, while stainless, is real thin. Thanks but no thanks.Comment
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