Fuel Tank Gauge Warning Light

[RSOHC]

When I turn on the ignition for my 914 Six, the warning light in the fuel gauge comes on and stays on when there is enough gasoline in the tank. How can I fix this problem.

Lauran

[RSOHC]

The needle on the fuel gauge is reading 2/3 of a tank.

[Dave at Pelican Parts]

First, are you sure there is enough fuel in the tank? The warning light is generally more trustworthy than the gauge reading! Double check that by looking in the tank or dipping it with some kind of stick.

The sender is under the electrical connector on top of the tank, and is held in with four screws or bolts. You will probably have to move the charcoal canister out of the way to properly get to it.

Remove the screws and slide the sender up and out of the tank. You can turn the key on and see what happens to the gauge reading and light. If the reading stays the same and the light is on, the gauge reading is faulty. If the gauge shows zero, it is reading correctly. (Because you pulled the sender out and the float should be at the bottom of the sender.)

Turn the sender upside down and see what happens to the gauge. It should read full, and the light should go out. If the light stays on, that's a problem. If the gauge stays at 2/3 of a tank, that's a problem.

You can check the resistance of the gauge sender--where the green wire plugs in--to ground. It should be around 10 ohms when the float is at the top (sender held upside-down) and 80 ohms or so when it is at the bottom (sender held rightside-up). You can check at the sender or unplug the green wire from the gauge and check there, whichever you prefer. The latter will include any resistance from the wiring and connections along the way.

The sender is not hard to replace, nor is the fuel gauge. You should be able to just remove the fuel gauge module from the back side of the combo gauge with a few screws, though that may not be possible with some early gauges.

--DD

[RSOHC]

there When I removed the fuel tank sending unit I found that there was hardly any gasoline in the fuel tank. When I turned the removed fuel tank sending unit upside down, the gauge in the car showed a full tank. When I turned the removed fuel tank sending unit right side up, the gauge once again showed 2/3 of a tank. I am replacing the fuel tank sending unit. I will also be replacing the strainer in the tank and gaskets when the tank is empty.

[930cabman]

Use a dipstick until such time you are confident the gauge is working correctly. After several tanks of consumption you will know if the gauge is working good. The sending units are very easy to recondition.

[RSOHC]

Pull the sensor from the tank and measure the resistance with an ohm meter. One measurement should be with the sensor in the right side up position. Another measurement should be done with the sensor in the upside down position. The readings should be 0 ohms for one position and 90 ohms for the other position. If not, the sensor needs to be replaced.

Lauran

[RSOHC]

I am continuing to have a problem somewhere with the fuel tank sending unit, wiring, an/or gauge. I have tried two URO fuel tank sending units and one genuine Porsche fuel tank sending unit. The resistance of the original sending unit, the two reproduction sending units, and the genuine Porsche sending unit are as follow. In all cases the pin has been removed from the sending units.

Old
Resistance right side up is 75 ohms and upside down is 15 ohms

URO #1
Resistance right side up is 68 ohms and upside down is 5 ohms

URO#2
Resistance right side up is 66 ohms and upside down is 3 ohms

Genuine Porsche
Resistance right side up is 71 ohms and upside down is 6 ohms.

With 4 gallons of fuel installed in an empty tank, the gauge reads ¾ full rather than ¼ full. When the sensor is pulled up out of the fuel, the gauge reads ¾ fuel with the red light on. With the sensor removed from the tank and turned upside down, the gauge reads greater than full.

What do I need to do to get the sending unit, wiring, and/or gauge to provide the correct amount of fuel in the tank?

Thanks

Lauran

[Dave at Pelican Parts]

The resistance of the sender is changing, which is what it should do.

The red light comes on when the sender is pulled out of the tank and held upright, which is what it should do.

The sender is fine. Double-check the resistance of the sender from the gauge end of the wires. This will let you see if anything funky is going on with the wires.

But this failure mode sounds like the gauge module itself is bad. This exact thing happened to my wife's SC when we were on the way home from buying it; the gauge read about 3/4 and the light came on. We stopped for fuel and filled it up from basically empty. The gauge was the problem, not the sender. I replaced the gauge module (after messing with the sender for a while) and it has worked ever since.

I don't remember if the Sixes had combo gauges with easily-replaced fuel gauge modules. Look for a plate on the back with screws that hold it onto the back of the gauge body. You should be able to use a module from any 914; they are not specific to the Six. Not sure if modules from 911s will work correctly.

--DD

[RSOHC]

what is the best way to source a gauge module?

[RSOHC]

What company can repair the fuel gauge on a 914 Six combination gauge?

[Dave at Pelican Parts]

Gauge modules can be picked up from your favorite sources for used parts. Bruce Stone (Riverside CA) or Garold Shaffer (Portage IN) can often be counted on to have just about anything, and of course there is always High Performance House in Redwood City CA, AA in Atlanta, and other specialists.

Any good VDO shop should be able to repair the gauge. The two that come to mind immediately are Palo Alto Speedometer and North Hollywood Speedometer.

--DD

[RSOHC]

I have sent my gauge and fuel tank sending unit to North Hollywood speedometer. They have determined that my gauge is good. They have told me that that the range for the sending unit should be 1.6 ohms to 67 ohms. They will be calibrating the assembly so that these two items will work together. The range for my genuine Porsche fuel tank sending unit is 6 ohms to 71 ohms. I have also ordered a sending unit to gauge wiring assembly from Restoration Design.

Lauran

[RSOHC]

Should the fuel gauge operate when the ignition key is in the run position or does the engine need to be running?

[Dave at Pelican Parts]

It should work when the key is on. The original ones have very little damping, so they should show a reading pretty much instantly.

--DD

[RSOHC]

When I turn the headlights on the lights in the instrument panel do not come on. I have turned the light switch and it has not helped. Is this a fuse problem? Another problem?

Lauran

[Dave at Pelican Parts]

Could be a fuse problem, could be a problem with the rheostat (variable resistor) in the headlight switch.

Check the fuses first.

--DD

[RSOHC]

Which fuse?

[RSOHC]

I sent my gauge and sending unit to Hollywood Speedometer who told me that the sending unit needed to be calibrated with the gauge when every the sending unit is replaced.
I was told that the gauge was good.
When I received the items back I measured the resistance of the sending unit and found the values to be 2 ohms with the sending unit upside down and 68 ohms with the sending unit right side up.
I checked for a ground fault at the firewall connection and there was none.
The gauge is reading empty under all conditions.
The empty light on the gauge came on when the gauge was right side up.
With the sender installed in the tank and with 4 gallons in my 16.4 gallon tank the resistance is 59.8 ohms.
The generator light is the only light that comes on when the key is turned on.
There are no nighttime lights operational in the instrument panel when the car lights are turned on.
The car was checked for a ground fault at the firewall connection and there was none.
A new wire was run from the green wire connection point on the sending unit to the connection point on the gauge and the gauge reads zero.
The resistance of the green factory wiring from the connection point at the tank to the connection point at the gauge was measured and it was found to be 59.8 ohms with four gallons of fuel in the tank.
Grounding the case does not result in any changes to the empty reading on the gauge with the key in the on position.
When the green wire is removed from the gauge the fuel gauge needle still reads zero with the key in the on position. .
The voltage of the green wire when it is removed from the gauge is zero with the key in the on position.
The voltage of the green wire when it is still connected to the terminal on the gauge is 5 volts with the key in the on position.
The temperature needle is quivering at the low position with the key in the on position.
The voltage across the battery posts is 12.39 volts.
I will be checking the fuses when the rain stops.
Any thoughts would be appreciated.
Lauran

[RSOHC]

(1) On Mar 5, there is continuity between the brown wire and the chassis at the firewall connection and at the gauge connection.
(2) On Mar 5, there is zero voltage on the red and white wires at the gauge connection.
(3) On Mar 5, I checked all the fuses under the dash and they are all good.
(4) When I now (Mar 5) turn the key on the gauge shows more than the full mark on the gauge.

[RSOHC]

I am still looking for a solution to my problem.

A new wiring harness from the sending unit to the gauge made by Restoration Design should be delivered today.

Lauran