Fuel Level Sending Unit

[01yellerCobra]

Before I embark on the joys of troubleshooting wiring with redfirepearlGT I thought I would post up about this issue to see if anyone else has found a fix that was out of the box. Plus I'll try to keep the thread updated on the steps taken and results. Unless I "accidentally" set the car on fire before it's fixed.

The car is a 2006 GT. 99.9% stock. Stock fuel system, stock wiring, and a stock tune. It has the issue where the fuel gauge will drop to empty and the 0 miles to empty warning comes on. The real issue is that this causes the CEL. Which is a problem since I need to get the car smogged soon.

When the problem first happened I picked up a new sending unit for the drivers side and wired it in. Then the problem seemed to only happen between a full and half tank. At that point I swapped the entire sending module on the passenger side. The problem persisted and slowly got worse. In a 2 mile drive it would beep 5 times. The fuel level seemed to stop mattering. So last weekend I picked up a full module from Amazon. It was a Spectre brand, but as I wasn't sure it would fix the problem I was leery of spending $300+ on a Motorcraft unit. I installed the module and all seemed well for 4 days. Then yesterday (Thursday) the wife comes home and tells me the problem was back. Yesterday I pulled the driver side rear interior panel to check the wiring. I first checked the wiring around the plug for obvious damage. I didn't see any so I did a continuity check between the terminal about an inch back on the wire. No breaks found. Today I went as far as plugging the old module back in and manually worked the arm and watched the fuel gauge follow the arm. For the most part. A couple times it seemed to get stuck around 1/4 tank. Even though I know there's 1/2 tank in there. I plugged the module that's currently in the tank back in and messed with the arm and had the same results.

Something else is that the car would sputter and buck a little when the fuel level would drop. Since everything comes from the same plug that's what had me thinking a possible wire issue. I pulled the plug apart and cleaned and pinched the terminals for a better contact. I must've done something right because my wife said when the fuel level dropped this last time the car didn't sputter or buck.

After I got tired of it I put the lower seat back in then turned the key to on without the engine running. The gauge showed half full. Cool. But then after about a minute it went back down to quarter tank. I also checked for codes and got two body codes. B2627 and B2879. From my quick search I believe those have to do with the VCT and I'm assuming it's something to do with a test as that hasn't been a problem. Yet.

We've had the typical SJB issues in the past with it getting wet. But pulling it apart and drying it fixed the issues at the time. I'm assuming the gauges run through the SJB. I'm really hoping it's not the issue.

I think that's everything up to this point.

 

[46addict]

When you say module are you talking about the fpdm? If I'm understanding the fuel gauge system right, the level sensor/sending unit is where the measurements start so it may help to inspect that area first. Make sure the level sensor arm is not binding and there is no debris caught on the plastic plate where the arm "ratches" on.

 

[01yellerCobra]

By module I mean the piece that has the pump and sending unit mounted to it in the tank. I've triple checked the arm and everything is moving freely. I even zip tied a couple wires back just in case.

 

[86GT351]

By module I mean the piece that has the pump and sending unit mounted to it in the tank. I've triple checked the arm and everything is moving freely. I even zip tied a couple wires back just in case.

Something to do is remove this from the tank. Let the float swing freely. With the car key in the on position go ahead and slowly move the float upward and see if the gas gauge responds to the movement.

 

[redfirepearlgt]

The variable resistor on that float switch in the tank should vary smoothly from 15 ohms to 160 ohms. Came straight from the diagnostics section on the tank. You may wish to make sure that is good even on the replacement unit.

Most of that diagnostic btw deals with an interface device communicating with the SJB. There are a scad of connections between the tank float and where the signals finally land on the SJB so it may be time to revisit your corrosion control learnin! LOL! (USN reference for those who never served)

BTW your codes are related to the fuel system. The diagnostics steps you through. B2627 start at step A13, for B2829 start at step A17. I am attaching the on line manual for this procedure.begin on page 413-01-11. Step A2 in the right column lists your codes and the step to begin at.

http://iihs.net/fsm/?d=343&f=Instrument Cluster.pdf&p=2

 

[01yellerCobra]

Something to do is remove this from the tank. Let the float swing freely. With the car key in the on position go ahead and slowly move the float upward and see if the gas gauge responds to the movement.

I did that. The gauge responded for the most part. It did stick, but resetting the key got it moving again. I checked both the old and new units.

Redfire....that was my concern. Trying to follow these wires isn't going to be fun. It probably is a bad connection somewhere.

 

[redfirepearlgt]

No problem. We'll do our best to work through each point. Let me know when you are ready. I just got back from the golf course. Rolled my right ankle and it is swelled up like a grape fruit. So I have plenty of time on my hands.

C2280c pin 31 at the SJB (light blue/ yellow stripe) feeds allthe way back to the fuel tank float sensor. That is the signal return side to the SJB. C2280c pin 44 at theSJB (light green/violet stripe) provides the voltage reference from the SJB to the float. the first thin I would do if I were you would be to disconnect C433 at the fuel assembly and C2280c at the SJB. This way the wire paths are isolated. Jumper Pin 2 and pin 4 together at C433 then at C2280c check for continuity (less than 2 ohms) between pins 31 and 44 at C2280c at the SJB.

Between these two connectors are two other connector points. C423 and C431. C431 is a round 8 pin connector. C423 is a rectangular 14 pin connector. Each has a mating connector and does not connect into a device. I would disconnect and clean these connectors even if you get a good reading. I would also wiggle them and the associated wiring harness around to see if you can simulate an intermittent open or short which is what it sounds like you may have.

Below is the location for C431 and C423. Hope this helps. I'll focus on the sender side of the circuit if this all checks out and cleaning the connector points doesn't help.

 

[01yellerCobra]

Well feller's, looks like I won't be much help in the troubleshooting department. After a long talk with the wife we decided it was time to move on from the Mustang. Between the sending unit, using oil randomly, a battery drain suddenly appearing, and being worried about it killing another phaser she was tired of being worried about it. She said she never really enjoyed driving it anymore. So we're trying our hand at being German car owners for a bit.