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My previously working fuel gage quit me. With ign switch off the
needle is at rest below zero. With ign switch on the needle moves
up to empty. I've run the diagnostics multiple times and the
results indicate i should have a working gage-What gives??

Pulled spider #27 from gage:
Spider #27 to ground reads 24V.
Spider #27 to gage case reads 24V.

Reconnected spider #27 to gage, disconnected S/U wire #28:
S/U side of gage grounded, gage reads empty.
S/U side of gage open, gage reads full.

Fuel tank @ 85-90% cap. Readings taken with S/U in tank.
Tank to ground zero ohms.
S/U base to ground zero ohms.

#28 disconnected from S/U:
S/U lead-out to ground 20 ohms.

What am i missing here?? Any advice appreciated.

Fuel tank @ 85-90% cap. Readings taken with S/U in tank.
Tank to ground zero ohms.
S/U base to ground zero ohms.

If you had no typo's here then your problem is the sender.

The troubleshooting TB says so:

k. With jumper wire grounded to vehicle (D, fig. 1), the gages should indicate the
following:
Temperature gage Maximum temperature
Oil pressure gage Zero pressure
Fuel gage Empty
l. Remove jumper wire from ground (E, fig. 1), the gages should indicate the following:
Temperature gage Zero temperatureOil pressure gage Maximum pressure
Fuel gage Full

(3) Fuel gage sending unit.
(a) Check ground between fuel tank and vehicle with
an ohmmeter, zero reading should be obtained. If the
fuel tank is not grounded to the vehicle, the sending
unit will not operate.
(b) The ohmmeter should indicate a reading between
zero and 40 ohms, depending on the amount of fuel in
the tank. If the ohmmeter indicates a reading higher
than 40 ohms replace the sending unit.
(c) The ohmmeter should indicate a higher reading
as fuel tank is filled and a lower reading as the fuel is
withdrawn. If the sending unit does not conform to
this test, replace the sending unit.

So an empty tank has a 0 Ohm reading across the sender. Above you siad the Ohm meter reads 0 Ohms and the tank is 85-90% full.

Problem solved. Change sender!

What I was trying to say was the tank and the S/U Base read zero ohms
meaning the tank and S/u base or exterior body @ the mounting screws
were grounded.

With wire 28 disconnected I read 20 ohms @ the sender lead-out with
the tank 80-90 % full.

Now you make a bit more sense. You get a 1/2 tank reading with a full tank and the the gauge shows below empty off and 1/2 on. So if gauge passed the troubleshooting chart then we are still at a defective sender.

With switch off the gage is at rest below empty.
With switch on the gage moves up to empty.

This is while the sender is reading 20 ohms
while the tank is 75%-80% full.

I caught all of that in your 1st post. Let's keep this simple.

You said:

I've run the diagnostics multiple times and the
results indicate i should have a working gage-What gives??

If, indeed your gauge past all the tests in the TB then, by your own Ohm measurments on the sender, the sender is bad.

If, when powered up and sender disconnected from the gauge, your gauge reads empty grounded and full ungrounded then it is a good gauge.

You did say it passed these two gauge tests:

k. With jumper wire grounded to vehicle (D, fig. 1), the gages should indicate the
following:
Temperature gage Maximum temperature
Oil pressure gage Zero pressure
Fuel gage Empty

l. Remove jumper wire from ground (E, fig. 1), the gages should indicate the following:
Temperature gage Zero temperature
Oil pressure gage Maximum pressure
Fuel gage Full

I appreciate the experience, advice and input. Thanks.

I just did some troubleshooting on my gage. It was indicating all over the place. I did notice when I had the ignition on, and tried to ground the gage housing against the dash (connected on and off) that the needle would sometimes sit at 0.
So I made sure the ground between the gage itself and the dash was good. Indication good.
Start up and the needle started to flicker again. This time I noticed that if I put pressure on the transmitter under the seat, the indication would drop to 0. Wiggled the transmitter a bit and everything was normal again.
This is not exactly related to your problem, but maybe it can be of some help.

I intend to pull and bench test the sending unit
per the TM before dropping a hundred bones on a new one.
This ones only been in the tank a year (NOS purchase from a
reliable vendor) altho we know NOS doesn't always mean it
will provide long service.

Bench tested the S/U @ 0-ohms empty, 15-ohms half, 30-ohms full.
Dropped a dowel into the sender hole for a 65% full fuel level so
the 20-ohms I read with S/U in tank makes sense. My earlier fuel
level estimate was way off (peering thru the filler neck).

I must have failed somewhere on the gage testing-back to square 1.
Will also check wire 28 for continuity.

UPDATE-GOT IT!!
I did my diagnostics both gage and sending unit before my 1st
posting and knew, in my mind, that the components were good.
My frustration was born from having what I knew were good
components but no fuel reading.

I found today the wire 28 grommet at the gage had a crack in it
(unnoticeable till I pulled the grommet, bushing and shell apart).
When securing the shell to the gage it was grounding the
W-28 to gage case for 0-ohms and an empty gage reading.

The Devil (that little @$$*^!!) is in the details!!

Thx to Wes and others whose input mattered.

Hope someone else can benefit from my challenge!!

Great, good sleuth work

Glad you solved the issue. Hope everyone shares in your experience. A very good lesson all around when troubleshooting electrical and instrument issues the terminals and plugs can also be a culprit.

The manuals seldom direct you to inspect all connections since that is a known step to all professional mechanics but often overlooked by hobbyist. There is a natural tendency to assume the worse and immediately start with the serious test regimen procedures often overlooking the more simple items we should get to first.