Fuel gauge issues

[joecool911]

Need to do a write up on this along with a material and bolts list.

Yeah, I know you've offered to give me the material list, but would be cool if everyone had the info.

[Mailboxperry]

We'll I have another dumb question. How can you tell if the sensor ground is grounding on the PCM?

[mopartechnician]

Ohm the wire to ground

[Mailboxperry]

This is what I have found now, if I unhook the negative side of both batteries and hook them back up the gauge will move to full with the key on. After about 90 seconds the gauge will slowly go back to empty. Does anybody have any theories on this? I'm trying to wait until all else to just buy a new PCM and hope that works.

[Mailboxperry]

I did ohm the ground and signal wire and it's good from the sending unit to the PCM. Is it possible to just ground the ground wire from outside the PCM for testing purposes or am I risking damage?

[JAG1]

I will get the material/ bolt list done this weekend. It is a somewhat limited way in which you can do this mod, so I'll post up what I can on it in a few days.

 

Comment on the fuel module....It goes to the full mark when the ground is complete (no resistance full connection).

 

Fuel modules send a graduated scale of resistance so empty would be a fully open circuit like shutting off a light switch but with graduated/ varying degrees of being more or less connected in between the off and on. Think it's called a potentiometer or rheostat.

 

You said you got the gauge to show full with a full connection to ground. This tells me the wire system is okay, gauge is okay but still have a bad sending unit

 

. Did you see the old one when they changed it out? They are supposed to save the old parts. It's good business to do that for the customer. Especially when you could easily then test the old one to see if they hit the mark on what was wrong.

[Mailboxperry]

We'll it's been awhile since I've had time to work on this. I ohm the sending unit from the top of the tank and get 0.0. So for dodge to warranty the part they have to diagnose the sending unit themselves. Took it to dodge and they say the sender is good and have a bad PCM. They also said the PCM isn't sending any power to the sending unit. This doesn't make sense to me. If the sending unit doesn't show any resistance then the sending unit is bad? And the unit runs off a ground? So they will never get power from the PCM to the sender. Please help

[Ilikeoldfords]

I would still think that the sending unit is bad. If you are getting a perfect 0 ohm when you tested it, it tells me that there is a short somewhere inside. There should be SOME resistance in the sending unit. That is how they work after all. Power comes in, goes through basically a variable resister (hooked to the float inside the tank), comes back out, and heads to ground. A perfect 0 ohm tells me there is a short to ground somewhere in the sending unit.

[Mailboxperry]

From the wiring diagram it shows a negative wire coming from the PCM. The dodge dealer say they are looking for power in that wire. So does power really go through the negative wire. I just want all my bases covered when I go to get my truck.

[MnTom]

If you want to check it you need to pull it. I favor lifting the bed for this, but that is up to you. Pull the fuel tank module leaving the wiring in place then move the float while observing your fuel guage (with the key on). Like Ilikeoldfords said, there has to be some power travlling through the sender to get a reading.

[Mailboxperry]

I will pull it tonight!

[Mailboxperry]

We'll pulled the sender and moved the arm up and down with no movement on the fuel guage. What kind of ohm should I get from the sender with no wires hooked up? Fuel guage is still stuck way below e.

[Ilikeoldfords]

I am not sure on the specific resistances but if the tank is full, there should be little resistance. The farther down the level in the tank goes, the more resistance you should get. At least, this is how most senders work. I haven't ever actually gotten to play around with the senders in these tanks.

[Mailboxperry]

We'll I put in a new sending unit and have the same results. The dodge dealer here claims the PCM sends a 5v signal to the sending unit. So my question is can you instal a 12vdc to 5vdc converter and bypass the PCM? Found some online really cheap. I really don't want to spend 500 for a new PCM just for the fuel gauge to work.

[MnTom]

Did you actually check for voltage at the sender and also check for a ground? If either wire has an open spot it won't work. That could be anything from a connector to a chafed wire to a bad ground.

[Mailboxperry]

Where am I suppose to find voltage? There is only 2 wires coming from the sender. From the diagrams I've found the black one is a ground that runs to the PCM and the other is the signal going to the PCM also. Sorry if I am making this hard

[Mopar1973Man]

One wire should be +5 volt and the other sound be a signal ground.

[Mailboxperry]

We'll I put 5v to the sending unit and the gauge reads 1/4 tank. I just filled up yesterday and only have 30 miles on the tank. If this is correct sounds like I have the float stuck.

[MnTom]

Entirely possible. Did the module happen to rotate any when you tightened it up?

[Mailboxperry]

I made sure it didn't rotate when installing but I am going to check. I did notice when I put 5v to the wire the gauge first went to full and then slowly dropped to 1/4 where it stayed. Won't know anything until I lift box again tonight. I am getting pretty good at now. Lol