Thread: how to diagnose and repair fuel level gauge issues 2001 grand prix se 3.1

Today i fixed my fuel level gauge issues on my 2001 Grand Prix se 3.1 after a year of headaches and i felt i should share my findings because i never found a final fix on any forums and hopefully i can help save somebody alot of time and money.

my issue was that when i first started my car, my fuel gauge would swing to full and after about 10 seconds it would plummet down to empty as long as it was in neutral. in gear the gauge would drop much slower but it still would never indicate the proper fuel level. very frustrating.

first i will say that the final fix was to add a new chassis ground to the vehicle with just a short section of 4 gauge power wire i got from my old speaker system. it might be overkill but it was cheap.
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now this in the end fixed my specific issue but here is some information for those who this does not resolve the problem.

first, you will need a multimeter that has a resistance setting as well as voltage.

the first step i would recommend before anything is to check the quality of the body ground. all i did to check this was to connect my multimeter to the positive side of the battery and the negative (should read 12+ volts with the car off) just to make sure the multimeter is operating correctly and that i have it on the proper setting. next i took the negative probe of the multimeter and stuck it to the bolt that ties the chassis ground cable to the body. it should again read 12 or more volts. in my situation it read 5 volts. obviously an issue. so instead of removing the factory chassis cable i just disconnected the bolt to the negative terminal of the battery and connected the ring terminal on the end of a section of 4 gauge wire and screwed it back down. the other end i attached to the inner wheel well after grinding the paint away and applying dielectric grease to ensure it keeps a good ground for a while.

now if this doesnt fix your issue well then we can move on to the next step.

On my car (not sure if its the same on all grand prix) but for me I was lucky enough to have the GM gods looking out for me and there was an access panel to the fuel pump. To get to the panel you pull up the carpet closest to the rear seats in the trunk





Take off the flimsy little screws on the silver cover and you should now have access to your fuel pump.



Now if you look at the square white connector there are 4 wires going in to it. The ones we will be focusing on is the purple wire (that is your reference wire), and the black wire with the white stripe (that is your fuel level sensor ground wire). Now if your probe those 2 wires with your multimeter set to volts ac you should get a reading of 5 volts. If not then there is most likely a power supply issue or a ground issue. We can test the same wires for resistance (ohms). The normal range is from 40 ohms(empty) to 250 ohms(full) anywhere in between and you are ok. If you try this test and you dont get any reading (my multimeter just displays the number 1) it would be an indication of an open circuit. The first step in that situation would be to replace the fuel level sensor (I went to ROCKAUTO.COM to get the sensor by itself for $40 instead of a whole pump for around $100 for an off brand) all you do is remove the fuel pump wiring and fuel lines (not as simple as it sounds) and remove the ring that holds the pump in the tank and remove and replace the fuel level sensor which is simple to replace just the part of getting to it is a pain. Before replacing just check the board that the float arm is attached to (sorry dont have pictures of this part) and see it there are any of the "fingers" still contacting the board, if not then you know where part of the problem is. Now if you replace this sensor and still have an issue then now we will proceed to the next step

To the right of this access panel there is a connector that goes through the floor of the trunk that connects all of the fuel pump wiring to the interior of the vehicle and runs back to the pcm


If we look at the wires at that connector we will again have a purple wire (fuel level sensor 5v reference) stick the positive probe of the multimeter (set to volts ac) in the purple wire and I just touched the other probe to a solid body ground point. If you get a reading of 5 volts then you now know there is an issue somewhere between that connector and the fuel pump. Inspect the wiring for damage. If you did not get a reading of 5 volts then we will check the wiring harness under the dashboard on the passenger side of the vehicle. The harness in my situation was located just below the bottom of the glove box near the front of the passenger side door. There will again be a purple wire. Check the wiring again and see if you have 5 volts. If not then you will need to check closer to the pcm.

Now on my car I did some research and found out that there was a TSB indicating that the pcm wire harness would come unhooked from its fastener on the transfer case and chafe against a part of the ac system (I believe its the dryer but im not sure) check to see if you are experiencing this issue. This harness is on the drivers side of the vehicle just under the air box. Theres a large bundle of wires that will be running there, just make sure they arent contacting anything. If not we can move on. Now I checked the purple wire again at the wire loom on top of the strut tower on the drivers side against ground. Again should be 5 volts. If not then your issue may be closer to the PCM. The pcm on these cars are inside the air box assembly. First remove the black support bar going from the radiator support to the strut tower. Next remove the intake tube going from the throttle body to the mass air flow sensor and from the mass air flow sensor to the air box. Next remove the air filter cover by releasing the 2 clips. After this, you will remove the 2 bolts facing forward on the air box. After removing these bolts you should see your PCM. There are 2 computers there. The one that I needed to inspect was on the bottom but it may be different in someone else's situation. First before disconnecting either of the computers you should disconnect your battery so you do not damage the pcm. Now there is a small screw in the middle of the connector that holds it to the pcm. Remove that bolt. Next remove the gray cover that goes over all of the wires. This gray cover is zip tied on and it also has small clips that hold it to the connector. Be very careful with this because you do not want to damage any wires at the pcm. Now under that gray cover you will see all the wires going in to the socket for the pcm, all of these wires have a number stamped next to them on the connector. Look to ensure that in the slot numbered 69 that there is a purple wire. If not then you are on the wrong harness and need to check the other one instead. If all of the other tests have indicated no voltage in the purple wire then what you will do now is either remove the clear cover on the bottom of the socket very delicately or you can probe through the wires on the opposite side but either way you do it you will be checking the purple wire in spot 69 against the black wire in spot 35. Check the resistance (ohms) between these wires. Again it should be between 40 ohms empty and 250 ohms if the tank is full. If you do not get a reading then try and check all of the ground points for the wires from the fuel pump. If you get a reading between 40 ohms and 250 ohms then you may have an issue with your pcm and you may need to replace it. Before we give up all hope though we need to check the signal wire to the gauge cluster. Re-connect the PCM wiring and re-connect the battery. Now the fuel level gauge wire will be an orange wire that will again be in the bundle of wires on top of the drivers side strut tower. Check to ensure that this wire has power when you turn the key on. Watch the voltage reading, if the voltage doesn't drop but the gauge does then you may have an issue with your gauge cluster and you should try replacing it to see if this fixes the problem. If you check the voltage in this orange wire and it drops at the same time the fuel gauge drops then again it leads back to the pcm possibly being the culprit. If everythign leads you back to the pcm being bad then I would suggest that you write down the munbers on the side of your pcm and go to a junk yard and try to find a pcm marked with the same numbers to see if you can get everything working again. If you really want to try and buy a pcm aftermarket or from the dealer you could attempt that as well but I would suggest trying a junk yard first because if the pcm isnt bad, you dont want to spend an arm and a leg to get a new one. If you try and put in a new pcm and you still have the same issues i would suggest starting over the inspection process to ensure that you didnt miss a step.

hopefully this post will help someone out that is scratching their head trying to figure this out.

best of luck everyone.

I know the level sensor itself is a high wear piece but at least there's a chance of replacing only the sensor. If you have any questions I can try to help

What codes are you getting? Mine indicated quite a few unrelated issues

Well I can tell you that the fuel level sensor circuit high voltage and the fuel tank pressure sensor low voltage were both codes I fixed by resolving the ground issue. The MIL issue I have but that's because I pulled my check engine light bulb. The MAF could potentially just be that the car needs to re-learn all the sensor info but I'm not positive on that one. I'm going to try and work on uploading a full pcm wiring diagram on these cars so hopefully that will help you and anyone else with these issues

Look at the first picture. Went to my local audio shop and got about a foot of 4 gauge primary wire and attached 2 ring terminals (also from the audio shop) and attached one end under the bolt on the battery negative side and attached the other end to a new ground location I prepped on the passenger side fender. I ground the metal down and applied dielectric grease to the location to prevent rust. And shazam my fuel level and tank pressure issues were resolved. May not always be that simple but for about $10 it fixed my particular issue

There is supposed to be a ground location on the body where the sensor grounds connect but I tried to hunt it down with no luck. Instead of cutting the original body ground from the battery I just basically added a second ground for it just in case. I actually got a solid reading at the sensor yet the issue still related to the body ground. It may also be the black ground wire on pin 35 of the pcm so it wouldn't hurt to check resistance of the wire against the body to see if there is a break in the wire

Correct. 35 is supposed to be a ground. If you check 69 to 35 you should see a resistance in ohms. If not there is an issue

You would insert the probes into the wires for 35 and 69 and read the resistance with the multimeter set to ohms

So in theory you have probably about 1/8 to 1/4 tank. Does that sound correct?

You can test by rocking the car and checking for a variation in resistance