agreen92

Hey Guys,

Hoping that maybe someone out there has has this happen to them and can tell me how to go about fixing the problem. I own a 92 with the LT1 motor. Although the motor has had major work done to it, I don't think that has anything to do with the problem. Problem was present before motor work began. Anyway the problem would be that the fuel gauge has stopped reading correctly. When the stock sending unit started screwing up I bought one from one of the corvette grave yards out there and it worked fine for about 8 months or so. Then it started reading incorrectly as well. I have had 4 brand new ac delco sending units installed in the vehicle and none of them have ever worked right. Meaning, while the gauge in the car is showing half a tank of gas, there's actually only about a quarter tank or lower amount of fuel actually in the tank. The dealership i was taking it to said they have absolutely no idea why thats happening. In a desparate attempt to fix the problem, I had the vechiles ECU rebuilt. But unfortnately to no avail. Problem still exsists and I'm having to watch the odometer to know when to fill up again. If anyone has ever experienced this or just have heard of someone else having the same problem, please by all means let me know what the fix is. I'm completely out of ideas and unfortunately i haven't been able to find an aftermarket supplier for the sending units. Again, any and all help with this matter will be greatly appreciated. Thanks in advance.

pcolt94

You have to actual find out what is wrong. You need to actually measure the resistance of the unit as you move the wiper (float). According to specs the float unit has a range of 0 to 90 ohms. I measured 20 to 110 ohms on my float unit. The 20 ohms representing the empty end and the 110 representing the full side. I would prefer to use an analog meter to see if the resistance changes smoothly. You can do a back up with a digital meter (DVM).

FYI - In the older cars a low reading would indicate a full tank. But in the corvette and digital world it is set up where a low reading indicates an empty tank. So you would think a high reading would make the gauge read high. But what really actually happens is if the reading is to HIGH and out of the normal resistance range, the circuits (in CCM) don’t work right and the reading just goes to some arbitrary mid range reading on the gauge and hangs there.

If the resistance is not in that range than it needs to be repaired and cleaned till it's works properly. Then while unit is out of tank connect the electrical connector and move the float and check the reading on your fuel gauge. You can pull the fuse for the fuel pump for safety. At this point you will have a good idea of what's going on depending on your results.

If the sending unit works OK as far as resistance goes, then the CCM could be the problem which is what it connects to. From there it drives the dash LEDs. You would also consider any bad connection in a connector from the tank unit to the CCM could cause this problem.

You say you had the ECU rebuilt. Are you actually referring to the ECM? Because if you are, that has nothing to do with the fuel gauge. The ECM controls primarily the engine. The CCM basically controls the chassis electronics. If the dealership told you to rebuild the ECM, I think you're in the wrong place trying to get it fixed.

Having an FSM to troubleshoot this problem would be a great help.

redvetttes

PC94
What happen when you loose the ground?
Dose it go hi or low?

pcolt94

Firstly I would have to know what ground. But if you are speaking of the ground at the sensor it would be the same as the wiper of the sensor having high resistance or open. The net effect would be the gauge would just flounder around. That happened to me and my gauge read around 1/2 to 1/3 of a tank. But there is no definite answer and that’s just on a sample of one occurrence.

If you fill the tank and it goes to full chances are you did not lose the ground or the hot or wiper wire for the sensor. If you lost the ground, it would never do anything different when you put fuel in it.

These sensors are known to have a problem with corrosion and many need to be cleaned. Don’t know if there is something in your tank to cause an oxidation. Seems like you have been thru several sending units and not quite sure what the real root of the problem is yet.

redvetttes

The sensor ground
My c3 goes way past full when it fell off
Just bought a 86 for the wife and its getting the once over
Do you meen corresion at the plug ?

rodj

http://forums.corvetteforum.com/c4-t...h-article.html

pcolt94

I have seen this thread. It has good information and great pictures.

Why do I remember it….Because of the neat and organized looking garage, which is nothing like mine.


Here is another post if you want to go the distance and have a problem with accuracy. It may help some of the cars.

http://forums.corvetteforum.com/c4-t...-and-more.html

agreen92

Thanks to everyone who has posted responses to my problem. Thanks for all of the great info pcolt94. I have several digital meters so I will do my best to get some readings this weekend to see what it is doing. I have read thru the two links you posted pcolt94 and again great info in those. Although I have had 4 brand new, straight out of the box, ac delco units installed in the vehicle. I find it hard to believe that I could have gotten 4 bad new units in a row. Although I guess that wouldn't really surprise me if thats what the problem actually is. Just gotta love factory replacement parts, lol. But like pcolt94 sugguested, could my ccm be the x factor in all of this. If thats the case, I haven't noticed any other strange electrical problems with the car. But again, I will do my best to get some readings from the sending unit to see what its doing. Will post results when I have them. Again, a very big thank you to everyone who has responded.

agreen92

Something I forgot to add in my last response. All of the new sending units that have been installed right from the git go acted like what the original unit did when it started going bad. When I fill up the tank the gauge shows full. But it takes a couple of days of driving the car for the gauge to show any decrease in level. I did read in one of the links that pcolt94 posted, that someone reversed the sending unit assembly on their car and it started working fine. Should i try to attempt this to see if maybe the unit thats currently in the car could have been put together wrong?

pcolt94

Not sure what you mean but;

You cannot reverse the wires because of how it is constructed physically and grounded thru the sending unit. But if you did and isolated the wires nothing would change because there is no polarity for resistance and the CCM would see the same net effect.

If you physically turned the sending unit upside down it then would read in reverse but that is not helpful because the gauge would then also read in reverse.

FYI - If you are not aware of this but the it is characteristic for the C4 to have this problem where the gauge stays full to long of time. The thing never seems to drop fast enough and shows to much fuel. This is why I wrote the post to improve the accuracy of the system.

I just don’t think personally that the engineering was done correctly to drive the gas gauge. I think to fix it engineering wise, you would have to change the resistance of the sending unit or change the software (or firmware) in the CCM to interpret the resistance differently.

LannyL81

I have been fighting this with my 95 this past summer. The gauge worked fine for a long time and then after running out of fuel twice...I had to use the trip meter. Trouble is...if you pull the battery cable...you lose the trip meter....did not know that...hence why I ran out of fuel the second time.

I changed out the fuel sender with a used on after checking its resistance range...checked about the same as the one I took out. But right from the start the fuel level was way off.

So I have been thinking that the CCM has gone bad and was going to try to get another one...but never got any further. Anyone ever look into the CCM itself and determine if this can be fixed this way? Any schematics available for the CCM?

pcolt94

Do not have a schematic for a CCM and several members have been looking for years. It's protected information from GM I believe.

I believe the CCM could be the root of the problem also. Something could be changing over time. But until I get one to experiment on or take a look at I don’t see how to figure it out. I am not going to pull the CCM out of my car just to play with when it is all working OK.

rons85

Not really relevant to This discussion, but food for thought...

On my '85 (no stand alone CCM, the fuel gage electronics are contained within the LCD dash electronics "module"), there is also an issue with fuel level accuracy and the responce or lag of the display.

While playing with the sending unit - removed from the tank but plugged in to the wiring harness - it is obvious, watching the dash display, that there is a "lag" or delayed responce to changes in the position of the sending unit.

The displayed level Eventually catches up to changes in the sending unit, but it takes some time, up to several minutes. Interesting to note that the displayed level will Instantly show the "correct" amount if the ignition is turned off and then back on.

You can easily see this happen while filling the tank - if you fill it with the ignition On the display will Not be showing full when the tank is topped off, and will take a Long time to do so. However if you fill it with the ignition Off the display immediatly shows full when the ignition is turned On after the tank is full.

This behaviour leads me to believe that there is an intentional delay designed into the display circuitry, perhaps to "dampen" the displayed level to compensate for fuel slosh in the tank? It also makes be believe that this intentional delay is somehow increasing - something dying in the electric circuitry (such as a failing capacitor). In my case this is 26 year old electronics, after all.

LannyL81

Yep, do not want to take the CCM out and experiment with it. Just did a check on ebay and found that there is a business that rebuilds them...but not cheap.
I am going to do some additional searching, but I am going to try your resistor in parallel fix....just to see what it does....as soon as it warms-up a bit.

Mo_Bandy

If you can get to the sensor wire in the back facial where it plugs in there is a plug with a greay, black and purple wire.

If you have some electronics experience, take a 100 ohm resistor and a couple clip leads connect them to the black and purple wire, you should see the fuel guage go to full. You may need to cycle the ignition key to see the change.

This will let you know if it is the CCM or something forward of the sensor.

I would suspect that it is perhaps just a bad ground. If the gauge works properly doing the above I would suspect a ground since the chances of having so many sending units being bad is "fairly" remote.

Hope this helps,

MO

agreen92

Thanks for all of the responses to my problem. I guess its nice knowing that I'm not the only one out there haveing issuses. Although, the sending unit that the car rolled off of the assembly line with always worked perfectly fine. Never had any issuses with the gauge reading slow or anything of that nature. But again, the car is now going on 18 yrs old, so who knows what could be going wrong. . Haven't had a chance to get out and pull the unit out to get ohm readings off of it yet. But it is on the to do list for this weekend. Again, will post the results when I have them. Again, thanks to everyone for all of the great info. I am keeping track of all the help.

LannyL81

I have the orignal fuel sensor on my work bench with my DMM connected across the black and purple contacts. I also have the float arm off.
What I am seeing is the resistance range is good, until the wiper is wiggled around...like what would occur due to fuel sloshing. The resistance value greatly varies and even with the wiper at the position that would represent empty...I can get a resistance reading over 100 ohms. The problem appears to be in the rotating contact. I have tried to clean with carb cleaner but made no difference.

Have not been able to find a 180 ohm resistor....150 is as close as Radio Shack had.

I am going to connect my DMM across the black and purple terminals of the fuel sender that is in the car and then bounce the car and see what happens to the resistance value. I have constructed a 90 ohm resistor to put across the CCM side of the connector to see what the gauge reads and I can change to two values.

pcolt94

Aside from cleaning the main wiper contact that rides across the coil wires, there are 2 other contacts which make the ground connection. The 2 other contacts need to be cleaned also. The surface that they ride on and the contact themselves. There is much room to work on it and it is a delicate assembly. Make sure not to bend anything to much in fear of breakage. I used some very fine emery paper, not emery cloth (to thick), took me a long time to clean it up. The ground return connection is as important as the main wiper itself.

When I initially got my car the gauge did not work at all. I think I spent more time on the ground wiper connections than the main one.

LannyL81

I have worked on the ground connection quite a bit as well. It is a simple push-on spade terminal. I have filed the piece of metal that the terminal slides onto and have cleaned/filed the spade terminal itself as best as I can. Resistance readings show a good solid connection.

I have cleaned/filed the wiper contact and cleaned the sesor windings (carefully). But the problem is still in the movement of the float arm contact. No way that I can see to take it apart, nor no way to reduce the "wiggle" that is has.

My gauge works, it is just nowhere near accurate. Right now I have 280 miles on this tank and the gauge shows I have 3/4 of a tank yet. I know I do not..more like 1/3 left. As both sender units showed the same, I still am suspecting the CCM has a problem. Hope to try different resistance values across the terminals today on the car to see what the gauge shows.

pcolt94

This was my exact situation also after I got mine working. I don’t think you will ever get the resistance reading to be perfectly smooth. But I don’t think it is all that important because you are not driving an analog gauge as in the cars of the 60s. As long as the resistance does not just jump to high or open, I think it is OK and here is my reasoning.

Each bar of the fuel gauge represents a specific resistance range to turn that bar on. Whether it is a few ohm higher or lower those bars will be on. Or as an example, each bar represents a 10 ohm range. So 50-60 ohms might indicates a half tank.

So aside from having a high resistance wiring problem from the tanks sending unit back to the CCM, the CCM does not seem to interpret the resistance properly to display an accurate indication on the fuel gauge.

Because of where the CCM is located, I have never had the connectors off to make a complete resistance path reading from tank to CCM.