Saab 900 fuel gauge erratic. I measure 5 volts on the sender bus going into the tank. This should be 12 surely. The pump has 12 and works fine. My 9 3has 12 volts to the sender .Is the 900 diferent or am I loosing voltage. The question is where to look. I took the sender out of the tank and it measured 30 to 350 ohms so nothing wrong with the sender. Should I takre out the dash panel and look for bad contacts here

Question edited to add year make and model.

The information is here by pinout, (pin 12)

http://orangecowboyhat.com/s10.html

https://www.google.com/search?q=98+s10+instrument+cluster+wiring+diagram

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As the fuel pump and gauge sender unit are in close proximity to each other I would be checking a wire connection is not loose or an earth wire left off at the tank

The plug you are referring to is probably the fuel tank pressure sender not the fuel pump. It would be plugged into a black sensor about 2 inches long pushed into a rubber grommet on the fuel bung. The fuel pump connector would have more that 2 wires going to it. most likely a heavy purple wire would be the power to the pump. It has to have 12 volts to it to work. The sender may have 3 wires, 5 volts, ground, and sense. the pump connector would have 12 volts, ground and 5 volts for the fuel gauge sender down in the tank.
If the 12 volts is not there, check the fuel pump relay. it is usually up on the drivers side firewall.
With the key in the on position without starting the engine, you should have 12 volts to the pump for 3 seconds then it will click off. Without cranking the engine you should be able to locate the relay by the clicking sound.
There is also the obvious, like checking the fuel pump fuse.

Buick dealers have a diagnostic box that checks gauge sweep and range, an 0-100 Ohm variable potentiometer with key on can check this gauge however you must tap into the wire going for the sending unit at the tank. With a voltage meter check the four wire connection (only one)connector going to the top of the tank, find a good ground and check for voltage one will be 12 volts with KEY ON ENGINE OFF look for a 12 volt reading for 5 seconds and then it will stop this is for priming fuel rail and is to fuel pump, you want the ground side of the gauge the other two will be grounds, fuel pump and gauge. The grounds may be near the wiring harness going to the frame. In either case they will be grounds you are looking for the one that is not either ground or power, most likely it will be either a flashing light or very low voltage. Attach the pot to the gauge wire and have someone watch the fuel gauge if it is erratic the gauge is the problem. If the gauge sweeps slow and steady the problem is in the sending until in the tank. AND HAS TO BE DROPPED! Run the tank down on fuel as low as you dare and drop tank. Be carefull working with fuel can be very DANGEROUS if you are not experienced working around fuel DON'T Attempt!

There is a TSB on high sulfur fuel usage, and how it impacts the fuel level resistance.
If you change fuel, it might start working again. The TSB is : ERRATIC FUEL GAUGE OPERATION TSB 08-2-9

Publication Date: January 22, 2008
FORD: 2004-2008 F-150
LINCOLN: 2006-2008 Mark LT


ISSUE:
Some 2004-2008 F-150 and 2006-2008 Mark LT vehicles, built before 12/3/2007, may exhibit erratic fuel gauge operation or illuminated malfunction indicator lamp (MIL) with diagnostic trouble codes (DTCs) P0460 or P0463, and/or Instrument Cluster Module DTC B1201. This may be caused by sulfur contamination in the fuel causing an open or high resistance in the fuel level sender.

ACTION:
Follow the Service Procedure steps to correct the condition.

SERVICE PROCEDURE

Install a new fuel level sender following the service procedure instruction sheet provided in the fuel sender kit.

PART NUMBER PART NAME
8L3Z-9A213-E Kit - Fuel Sender (2006 starting 12/8/2005 through 2008 - FFV-26 Gal Tank - 126/132" WB )
8L3Z-9A213-F Kit - Fuel Sender (2006 starting 12/8/2005 through 2008 - FFV-30 Gal Tank - 139" WB)
8L3Z-9A213-G Kit - Fuel Sender (2006 starting 12/8/2005 through 2008 - FFV-27 Gal Tank - 145/151/163" WB
8L3Z-9A213-H Kit - Fuel Sender (2006 starting 12/8/2005 through 2008 - FFV-36 Gal Tank - 145/151/163" WB
8L3Z-9A213-A Kit - Fuel Sender (2004 through 2008 - Gas-26 Gal Tank - 126/132" WB
8L3Z-9A213-B Kit - Fuel Sender (2004 through 2008 - Gas-30 Gal Tank - 139" WB
8L3Z-9A213-C Kit - Fuel Sender (2004 through 2008 - Gas-27 Gal Tank - 145/151/163" WB
8L3Z-9A213-D Kit - Fuel Sender (2004 through 2008 - Gas-36 Gal Tank - 145/151/163"WB



WARRANTY STATUS:
Eligible Under Provisions Of New Vehicle Limited Warranty Coverage And Emissions Warranty Coverage
IMPORTANT: Warranty coverage limits/policies are not altered by a TSB. Warranty coverage limits are determined by the identified causal part.

OPERATION DESCRIPTION TIME
080209A 2004-2008 F-150, 2006-2008 Mark LT: Install New Fuel Level Sender, Includes Time To Remove And Install Fuel Tank (Do Not Use With 9002A, 9002A4, 9002A6) 1.9 Hrs.

DEALER CODING
BASIC PART NO. CONDITION CODE
9A299 42

You Probably need a new fuel pump(comes with fuel sending unit). But you'll want to proof it out before you start buying expensive parts. If you access the fuel pump, remove it and inspect it. You will narrow it down. The float could have fallen off or failed, the sending unit could be damaged (use a multimeter to test), or the wiring or plug to the sending unit could be corroded or broken.

Replace the fuel pump. First though make sure your are getting power to your fuel pump. You should have at minimum 12 volts DC. If you have volts then it would be a sending unit problem if no volts then the pump has gone bad and needs replaced

Sorry if this gets posted twice. I have a 95 Saab 900, which I just acquired. I can get only about 4 or 5 gallons in at a time. The best analysis I found was this one where the man pinpoints a stuck valve in the gas tank, and tells of what he did to address it.: http://www.quasimotors.com/9_5_gas_fill_prob.htm

You will need to replace the sending unit, that is really the only thing that will cause this issue

You don;t say whether the fuel gauge is reading accurately, or whether it's always on empty. If the gauge is always showing empty, it's most likely a problem with the sending unit in the fuel tank. Check the resistance across the two terminals of the sending unit.

On a C900, the fuel sender level unit is separate from the fuel pump and located on the right side top of the tank, accessible by removing the trunk floor lift up panel (the pump is located on the left side top of the tank.
Possible problems are: electrical line off or broken, float not working (it's fairly delicate); or gauge not working.

Test the resistance of the sending unit with the lines disconnected, the resistance should be 2.6 to 3.8 ohms full tank, 63.3 to 67.5 ohms empty tank; and test for voltage across the top of the sender unit, if no voltage, that's likely the problem!

If the gauge is otherwise reading accurately (showing 'full' for a full tank, etc) then the problem is likely a gauge malfunction and can be simply replaced.

Good luck!

Saabreur