Sending unit? Gauge? Fuel needle confusion...

[RickTheEngineer]

Hey all, got another one for ya.

So in my M1009, the fuel needle very recently started reading inaccurate readings. The current situation is when the tank is full my gauge reads 3/4. It also goes to E properly when empty. Here is a small list of what I have done.

-Cleaned ground by rear drivers wheel. Was already clean, but sanded it down anyways.
-Inspected the push on pink wire on the sender, VERY shiny and clean... Wiped it down anyways.
-Checked resistance of the sender at the sender, reading 87 OHM when "full".
-Checked resistance at the cluster, reading 87 OHM (wiring must be good)
-When keyed up I am getting 12.4V to the gauge
-Resistor on back of gauge is 50 OHM

Does this all sound in spec? I know the sender should be 90 when full, so it seems really close. The needle only started doing this very recently. Any other grounds or things to check? Could it be the gauge itself?

 

[porkysplace]

Rust on the sender in the tank keeping it from going to "full" is a posiibility.

 

[RickTheEngineer]

That is true. Just so odd to me that this problem kind of started overnight without warning.

 

[Mainsail]

The TM has an excellent troubleshooting process for this issue. I followed it and found I needed a new gauge.

 

[cucvrus]

I see you have an M1009. I know it is a lot of work but it is also an opportunity for improvements and upgrades. I would start by dropping the fuel tank. Clean all the connections up to and including the ground on the sending unit. I had one in the past that has 1 strand of the ground wire attached to the sending unit. I soldered the ground again and it worked. The upgrade is you now can clean wash and repaint the entire rear lower frame and floor area. It needs it after over 30 years of sludge and dirt build up. Bonus would be to remove the sending unit and check and see if it works with the dash unit. I do it this way as I have the tank out and want to assure everything is operational. Now is the time to clean paint and undercoat the frame, fuel tank and skid plate. I will be teaching a class on this in a few weeks. I have one coming in that needs a new fuel tank. The Joy was it was here last week and filled to the brim with diesel. I said NO way am I dropping that tank. It has a damp spot at the straps and needs new straps also. Not bad for one I sold 20 years ago and has 185K on it. But I have a few dash sending units if that helps you out. Let me know. Good Luck.

 

[acthomp781]

When I replaced the printed circuit board on my M1009 the fuel gauge starting working properly. I did not replace the printed circuit board just because the fuel gauge was wrong, I was having Gen 2 issues. Maybe the connections for the gauge into the circuit board/instrument cluster are in need of cleaning.

 

[cucvrus]

When I replaced the printed circuit board on my M1009 the fuel gauge starting working properly. I did not replace the printed circuit board just because the fuel gauge was wrong, I was having Gen 2 issues. Maybe the connections for the gauge into the circuit board/instrument cluster are in need of cleaning.

Excellent point. The circuit board contacts from the board to the fuel gauge studs are steel clips that get rusty and do make poor contact after the rust. Excellent idea. I like to explore and check things out vs reading a TM. I learned more that way. My motto IF all else fails read the instructions. The book of LIFE. IF is the middle word.

 

[RickTheEngineer]

Here are some updates fellas. When full, sensing terminal on sender to ground was 90 ohms. Removing ground, needs reads full, removing sensing wire, needle reads full, grounding sending wire it reads empty. So the gauge seems to be working pretty well. Cleaned grounds again, and they are all nice and shiny, no changes. I cleaned the sensing wire and terminal, no change. (needle reads about 1/2 but can sometimes float around between 1/2 to 3/4). Voltage on terminals behind gauge is 12.6. Resistance was around 34ohms, which might explain the lower than full reading. I cleaned the traces in the back of the cluster housing, and even removed it to clean the housing where it touches the traces, all clean and no changes. Resistor on the back of the gauge looks good. After a long drive and yanking out the fuel gauge I notice it was pretty warm actually. Fuse is good, fuse terminals are good. I traced the sensing wire as far as I could and there was no damage on the insulation, in fact it was in great shape. Factory splice near rear axle was perfect and shiny. I am just at a loss here

 

[porkysplace]

Here are some updates fellas. When full, sensing terminal on sender to ground was 90 ohms. Removing ground, needs reads full, removing sensing wire, needle reads full, grounding sending wire it reads empty. So the gauge seems to be working pretty well. Cleaned grounds again, and they are all nice and shiny, no changes. I cleaned the sensing wire and terminal, no change. (needle reads about 1/2 but can sometimes float around between 1/2 to 3/4). Voltage on terminals behind gauge is 12.6. Resistance was around 34ohms, which might explain the lower than full reading. I cleaned the traces in the back of the cluster housing, and even removed it to clean the housing where it touches the traces, all clean and no changes. Resistor on the back of the gauge looks good. After a long drive and yanking out the fuel gauge I notice it was pretty warm actually. Fuse is good, fuse terminals are good. I traced the sensing wire as far as I could and there was no damage on the insulation, in fact it was in great shape. Factory splice near rear axle was perfect and shiny. I am just at a loss here

You may need to check the resistance of each wire from connection to connection , it could be a broken or corroded wire under they insulation , just because the wire looks good doesn't mean it is .

 

[cucvmule]

Excellent point porkysplace. It does often happen at connection points, splicing, and any place there is a bare wire or crack, split in the wire. Corrosion under the insulation will most certainly give a week ohm reading, but at that point it is about to fail. Heat in a wire is a sign that there is resistance somewhere or overload of that circuit.

That is why the ground connections are so important. I was working on my 73 Monte Carlo and was relocating the battery to the trunk and a ground to the chassis. I did no other ground and the starter was moaning bad. I had leaned on the steel brake lines at the master cylinder and they were red hot, making the ground there to the engine. Cure was full cable ground to the frame also. As cucvrus has said learning curve then instructions.

Now there has to be an electrician here who can explain it better than I.