[djfirebird93]

So on my 99 V6, the fuel gauge has never worked. Except when I am parked uphill. Everyone says fuel sending unit, but when I grounded the connector to test the gauge, it stayed on full, and when I took the ground away, it stayed on full for a bit, then dropped to empty, like it normally does. This procedure was given to me by shoebox.Here's my question. What is actually bad when the "Sending unit" goes bad, thus causing the erratic fuel gauge behavior? How come it would work when parked uphill only? I want to maybe cut the hatch and fix the sending unit if possible. Supposedly the 99+ fuel pump is different and weird all on its own. Just looking for a bit of info/advice. Thanks all!

 

[djfirebird93]

Also, my gas gauge wonders all over the place when when the key is out of the ignition. Why is this? It will go to halfway. move around a bit, then come back down to 1/4. It happens over hours of sitting, not instantaneous. Everyone else's that I have seen stays at the empty mark

 

[joesperformance]

sounds like bad wiring to me.....

and as far as the sending unit in tank.... its basically metal on metal... hard to explain, but let me find the video and that explains it......

start watching at 2 min mark

http://www.youtube.c...feature=related

 

[Injuneer]

The "sending unit" is basically the assembly that holds the pump, the float level sensor, all the wiring and the tubing for the pump. The float level sensor is a rheostat (variable resistance) attached to the assembly with 2 rivets. Like any electrical component, it can corrode, wires can fray, etc. The rheostat has a long arm with a float at the end of it, so it moves with the surface level of the fuel. The level sensor has two spade lug terminals. There is a purple wire and a black ground wire that have spade lug connectors for the level sensor, with the ground being common to both the level sensor and the pump.

 

[Injuneer]

This is for a 94, but with regard to the level float it should be the same as your:

 

[joesperformance]

The "sending unit" is basically the assembly that holds the pump, the float level sensor, all the wiring and the tubing for the pump. The float level sensor is a rheostat (variable resistance) attached to the assembly with 2 rivets. Like any electrical component, it can corrode, wires can fray, etc. The rheostat has a long arm with a float at the end of it, so it moves with the surface level of the fuel. The level sensor has two spade lug terminals. There is a purple wire and a black ground wire that have spade lug connectors for the level sensor, with the ground being common to both the level sensor and the pump.

Thanks Fred, I couldnt figure out a way to word it..... Hopefully between what u said and that video I found, he can get it fixed or at least know the problem.....

 

[djfirebird93]

Alright, I understand everything you said and thanks for the great info. But what would cause the level sensor to only be correct when parked on a hill? Any idea? And the gauge likes to "hunt" for a spot after the car is off, usually ending up sitting at the half mark. All others i have seen stay at "Empty" mark.

 

[Injuneer]

If the level sensor resistance is correct for all tank levels (how many points did you check it at?), then it's an intermittent wiring problem or a gauge problem. The sensor can develop sections that don't work because of corrosion, leading to erratic readings, I would think. Did you check the contacts in the harness connector over the axle for corrosion? Is there any evidence of the insulation being worn away on the purple wire?

Just guessing here. I had a fuel level gauge that used to stop working when I stood on the brakes, and only start working when I nailed the accel pedal. This was an older vehicle. Turned out the nuts the held the gauge in the panel were loose, and it would loose contact under heavy braking, and return to contact under heavy acceleration.

 

[GaryDoug]

The "shbox" procedure is correct in the description of testing the fuel gauge circuit. What you have done in your test is to eliminate the fuel level sensor in the tank as a cause. Grounding the purple wire (at the connector near the rear axle) simulated the sensor at tank empty, Leaving the wire open (disconnected) simulates it being at full. If you grounded the purple wire and got anything but an empty reading, the fault is with the gauge, or the PCM, or wiring in between the three locations. Most likely fault is with the gauge itself. I believe your instrument panel has a Class 2 data-linked module driving some of the gauges (fuel level, coolant temp, and tach). So if your coolant temp gauge and tach are working ok, you can rule out any problem with the PCM-to-IP circuit. Other than the gauge itself, that just leaves the wiring from the rear connector to the PCM (to C2.69) or the PCM itself (not likely).

FYI, in case anyone is interested. The PCM delivers 5vdc through an internal ~250 ohm resistor to the sensor when the ignition is on, but only 1vdc when the ignition is off. Just in case that tidbit is helpful.

 

[djfirebird93]

Perfect. Thanks Fred and Joe and Gary. VERY useful information here. I did test that purple wire from my connector, grounded it to the frame. I checked it only at one time, when the tank should be about half full. The wire and the connector isn't corroded. The gauge didn't respond to being grounded/not grounded. I will probably pull the gauge and check it. I do remember trying another gauge cluster from a 99 camaro, and the gauge on that one did the same thing. I am pretty sure I tested everything correctly, so I'm thinking there is a loose wire on the connector to the fuel pump itself, or the problem is in the fuel pump/tank. Thank you for helping me understand this with wonderful information.

 

[GaryDoug]

OK, if you are still not convinced the sender unit is ok, here is how to be sure:
Get a wire tap-in http://www.radioshac...oductId=2103500 ...and about 6-8 feet of small gauge wire (anything 18-22 AWG is ok). Go under the car and use the wire-tap-in to attach the end of this test wire to the purple wire http://shbox.com/1/f..._connector.jpg. In the attached photo the tap is not clamped closed yet (and is the wrong size/color). All you need is a pair of pliers to do this.
Route the test wire around the tank and up the fuel filler pipe and out the fuel filler hole/door in the quarter panel, then up and into the hatch area. Temporarily tape it to the filler pipe and to the quarter inner rear trim panel near the speaker. Make sure the wire's end is not touching any metal. Now you have a place to monitor the voltage from the level sensor.
Now any time you want to check the gauge sensor, measure the voltage from the end of the wire to chassis ground. One place for a ground is at the screws holding the fuel filler hole cover (see attached photo), or choose another if you can find one. The voltage should be 0-1.4vdc, with the ignition ON, depending on the level of fuel (1.4vdc at full, 0vdc at empty., and proportional in between). If the ignition is OFF, the readings will be 1/5th of those described here, because there is then only 1vdc instead of 5vdc out of the PCM.
Once you are done with the test wire, cut it off close to the tap-in and remove it.

 

[djfirebird93]

Ok need some help again! I climbed under the axle and unplugged the connector for the fuel pump. The Gas gauge is supposed to stay at "Full" with the connector unplugged, correct? Well, after 1 minute, the gauge dropped to empty. it was exactly 1 minute, I timed it two or three times and always dropped to empty after 1 minute. Where should I look next? I tried a different cluster, and it did the same thing.

 

[GaryDoug]

I'm not sure anyone here will have an answer for you. It seems like your car is different from the pre-99 ones, since you have the "smart" instrument controller. I can tell you how it woks on a 98 or earlier, but those have the "dumb" control circuit. The 1 minute time interval may be the update cycle of the PCM. I suppose it is entirely possible that the 99 and later work backwards to the method in the earlier cars. It would make more sense to design a circuit that reads worst-case (empty) if the circuit is open due to a defect. Your results may actually be normal for a 99.

 

[djfirebird93]

I'm not sure anyone here will have an answer for you. It seems like your car is different from the pre-99 ones, since you have the "smart" instrument controller. I can tell you how it woks on a 98 or earlier, but those have the "dumb" control circuit. The 1 minute time interval may be the update cycle of the PCM. I suppose it is entirely possible that the 99 and later work backwards to the method in the earlier cars. It would make more sense to design a circuit that reads worst-case (empty) if the circuit is open due to a defect. Your results may actually be normal for a 99.

It happens no matter what. If the car sits for 1 minute without moving, whether it's started or not, it will drop to empty. If I sit at a stoplight for a minute, pops to empty. Sometimes, very rarely, it will slowly creep back up to full. BUT the kicker is, if it's parked on an uphill incline, it will never drop to empty. Almost like it only gets a signal on an incline... Can you find me something that explains how my "smart cluster" works? Or if you know any info, that would be great. Thanks

 

[GaryDoug]

I don't have much detail on this but this is a description of your car's fuel gauge operation:

"Fuel Gauge
The fuel gauge is controlled by the PCM. The PCM monitors the fuel level sensor and sends the Class 2 information to the instrument cluster through circuit 1036. The fuel gauge displays the fuel level based on the Class 2 information it receives from the PCM. Refer to Engine Controls.

When the fuel level sensor reads the fuel level at less than 1/16 full, the PCM sends a Class 2 message to the instrument cluster through circuit 1036 to turn on the check gauges indicator."

Most of the other gauges in the panel have similar descriptions. They also have a diagram that shows the gauge controller in the cluster as a black box type connected to the one-wire Class 2 bus. I. e. It is connected to the bus in the same manner as the BCM, brake control module, air bag module, etc.

This all came from the Power Library Remote Access site for which I posted the link that is in the Tech Article section. http://www.firebirdn...to-repair-info/

When I was looking at signals on the 1998 bench mock-up I built, I saw that the PCM generated a variable duty cycle output to the gauge who's "on time" varied with the voltage from the fuel level sensor in the tank. In other words, the pulse width changes with the input voltage, lower voltage was a narrower pulse width and higher voltage was a wider pulse width. On a dc voltage display on a multimeter this equated to zero volts at minimum voltage input and about 3-4 volts (average) at maximum voltage input. It is possible that when they switched to using a smart controller, they kept that format(?).

 

[djfirebird93]

I'm not gunna lie, I have no idea as to the last question you asked. All I know is that the "Check gauges" light comes on before the gauge even reads empty. Like, the Check Gauges light pops on a millisecond before it starts to move to empty. The information you are telling me is making me think the computer is sending weird signals to the gauge, rather than the sending unit/gauge itself having problems. Agree? And where's the relay that you are talking about? The 'black box' one wire thing?

 

[GaryDoug]

Wasn't really a question. more like rhetorical, or asking myself.
The cluster control module?? I don't know, probably on or near the back of the cluster. I have never seen one and until a couple of days ago while reading the thing I posted, I didn't even know your car had a Class2-connected controller for the cluster.

 

[djfirebird93]

Can you help me understand what a Class 2 bus is? It's all foreign language to me.

 

[GaryDoug]

Can you help me understand what a Class 2 bus is? It's all foreign language to me.

Sure, I'll try. The PCM and other modules, like the BCM, brake module, air bag module and any others you may have are connected together for communication with each other. This is done with a single wire to a common (star) point in the middle somewhere. Or it might be daisy chained from one to another. That wire also comes out to the "Datalink" connector under the dash that you use for the scanner. Your car also has one for the instrument panel, so it is also connected to the PCM and others. I am not sure why they call it Class 2, perhaps because it came after the ALDL bus and before the CAN bus. Some cars actually had a mix of the ALDL and Class2 systems in the 90's. You can tell by looking at the wiring diagrams to see how many circuits there are for the varoius modules.
They get away with a single wire for communications because serial data is used (like the old phone modem and the newer cable modems). Software routines in the computers in each module take care of keeping order in the communications so that one module's messages do not conflict with the others.
Too much?
Think of it like a system of switches wired to control one light in your house.

 

[djfirebird93]

That's actually understandable. The next thing I need to check is the link between the gauge and the computer. Any idea where this "black box" would be? I checked under the dash and it's not near the cluster. I just wanna get this gas gauge workin. I just find it too coincidental that the gauge would drop at the 1 minute mark. I just don't get what could be wrong to cause this. I wanna check out this box between the computer and the gauge