low fuel relay fixable?

[FY1Cuda]

The "low fuel" light on the ceiling console is always on, so I thought I would investigate a little.  The relay is getting power, but beyond that I have no idea what's happening inside the box.  Are those things reparable, or do you (A) disconnect it or (B) suck it up and buy one on ebay for $150?

[the_engineers]

Check to see if the relay is stuck on...remove the power from the relay and see if you hear a click. If it does, it should be working and is therefore getting a false trigger.

[FY1Cuda]

Thanks. I can try that.  Isn't the "trigger" the blue wire from the gas gauge?  If so, any ideas how much voltage needs to come through to turn off the (normally closed?) light circuit?  Also, what really confuses me is the circuitry  for the relay, and the factory wiring diagram doesn't completely clear it up.  On the back of the instrument cluster the gauge is wired like shown in the first picture with the blue wire (I presume) coming from the fuel sender (G4-18DBL) and the black wire from the voltage limiter.
The second and third pictures are from the accessory diagram which show a blue wire going from the gauge to the relay (G4A-18DBL) and another blue wire (G4-18DBL) ALSO connected to the gauge.  If you have a low fuel indicator, are you supposed to replace the black wire on the gauge with another blue wire to the relay?
I'm guessing that if I didn't have the wire from the gauge going to the relay-the trigger-then the relay would constantly signal low fuel, but I don't think I have anymore blue wires lurking in there to plug in, and I wouldn't know where to put them.
Any help appreciated.

[sublime r/t se]

I have poked around back there and have had to replace the low fuel relay in order to get the low fuel warning lamp to light.  I know that there are two dark blue wires connected to the two separate terminals on the fuel gauge. 

[FY1Cuda]

Thanks. I pulled the radio and actually did spot an extra blue wire (with a right-angle rubber plug).  The problem is that it's pinched between the cluster and the dash frame, so I'm going to have to take a few things off to get it out.
I still don't understand why I wouldn't need the feed from the voltage limiter on the gauge (and why the wire from the relay replaces it). 

[FY1Cuda]

I got that blue wire free and replaced the black wire (from the voltage limiter) with it.  The gas gauge didn't function and the "low fuel" light was still on.  Reversed the blue wires--nada.
The gas gauge only works with the black voltage limiter wire attached.
Any thoughts?

[dodj]

Check to see if the relay is stuck on...

I'm thinking your relay is (at least part of)the problem. It looks like the relay and light work by the fuel sending unit resistance to ground changing as fuel goes down. Once your resistance is low enough, the relay will flow enough current to pull in the coil of the relay, close the contact to the G7-20 wire and illuminate the low fuel light. Would seem to indicate that your relay is stuck on.

The diagram does show the 5v limiter wire being replaced by the G4A 18 wire. Not exactly clear to me how that would still allow the gauge to function  Maybe Goody or Rob will be able to shed more light. no pun intended..

[willard]

with LOW FUEL relay you dont use dash 5V. gauge in the cluster gets voltage from the relay. I tried to repair my unit but when the resistance wire is burnt it's hard to solder it - even small difference in resistance results in bad gauge/lamp behavior. So I bought a nos one at ebag ;/

[FY1Cuda]

Thanks for the info.  When time permits, I'll try to play with it some more.  What are the two pictures on your post?

[dodj]

Looks like the innards of a low fuel relay. I was wondering if the relay was a form of limiter as well for the gauge. The resistance wire Willard meentioned must be part of a voltage divider inside the relay to cut the 12v for the gauge down to 5v while the contacts for the light can still supply 12v

[spamtank]

I'm having the same problem.  I tried replacing the relay with an E-bay purchased unit.  And still the light remains on.  When I disassembled my original, it appeared physically to be okay. 

I haven't had time to debug this further, but I have another piece of evidence that may or may not be relevant.  I discovered that with an empty tank of gas, the gauge reads 1/8 full, and AAA take 1 hour to show up with 4 gallons of gas.     So I don't know if the sending unit is hanging up, if it's just inaccurate, or if there is a bad ground connection that may be disrupting the function of the gauge and the LFI light.  The gauge will indicate a full tank of gas, while the LFI light is still on. 

At least now I have those nice LED bulbs installed, so they aren't creating too much heat when it's on for long periods of time.

 

[sublime r/t se]

I'm having the same problem.  I tried replacing the relay with an E-bay purchased unit.  And still the light remains on.  When I disassembled my original, it appeared physically to be okay. 

I haven't had time to debug this further, but I have another piece of evidence that may or may not be relevant.  I discovered that with an empty tank of gas, the gauge reads 1/8 full, and AAA take 1 hour to show up with 4 gallons of gas.     So I don't know if the sending unit is hanging up, if it's just inaccurate, or if there is a bad ground connection that may be disrupting the function of the gauge and the LFI light.  The gauge will indicate a full tank of gas, while the LFI light is still on. 

At least now I have those nice LED bulbs installed, so they aren't creating too much heat when it's on for long periods of time.

 

I think you would have to make sure the sending unit was working correctly, and also make sure that the low fuel relay is well grounded.   Before my low fuel relay was replaced, it would not read full.  After I replaced it, it reads perfectly.   

Does the lamp illuminate when the connections are removed from the relay?  Could there be some direct short to ground or something?   I know that the wiring in the roof of my car was fried.  I had to replace some of it. Total PITA.

[spamtank]

I think you would have to make sure the sending unit was working correctly, and also make sure that the low fuel relay is well grounded.   Before my low fuel relay was replaced, it would not read full.  After I replaced it, it reads perfectly.   

Does the lamp illuminate when the connections are removed from the relay?  Could there be some direct short to ground or something?   I know that the wiring in the roof of my car was fried.  I had to replace some of it. Total PITA.

Humm, I'll have to check to see what happens with the low fuel relay removed.  Hope I don't need to replace any of the wiring in the roof.  The other indicator lights work fine.  I definitely check the ground. 

Thanks

[willard]

First check all grounds - at sender, at realy - this is crucial. If your sender is giving improper resistance it wont work well. My light comes on when I have bout 1 gallon in the tank. Indeed the relay incorporates both the relay for the light, as well as 5V source for the gauge.

Read this and you know everything:
http://rt-eng.com/mediawiki/index.php/RTE_Gauge_Faq#Information_about_how_the_Mopar_RT.2FSE_and_Grand_Coupe_Low_fuel_light_relay_.232983828_works

[FY1Cuda]

Thanks for the RTE information.  I just yesterday tried poking around again and still had no luck with the relay and so I gave up and simply disconnected the relay.  Now I see the picture and explanation and am already to tear into it again.  I had difficult time finding #1892 bulbs and ended up having to buy a box of ten of them when I was reinstalling the console last month. 
I'll let you know if and when I'm successful.