Fuel Gauge Issues on 1971 early style.

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.

bbrinkley007

bbrinkley007

Joined
Sep 15, 2025
Threads
15
Messages
45
Location
Cotton, GA
I am having a devil of a time figuring out my fuel gauge issue. When I first got this 40, the gauge wasn’t reading. I swapped the wires and it appeared to work, but it constantly read 3/4 or so and never moved from there.

When I got my donor 40, I tried swapping bothe the gauge and the sender, but still nothing. I thought it might be a wiring issue so I ran a wire straight from gauge to tank… still no joy. I even tried bench testing with a dc power supply, but I can’t get anything to work the way it should apparently work. Sometimes I can get the needle to move, but it doesn’t seem to change when I move the sender through its range. I need some help for this old 2 wire gauge and bi-metal (is that the right term) type sender.

Does anyone have any suggestions?
 
I think Coolerman has a good write up on the fuel gauge issue. Don't quote me but I think the ammeter is the 7v power supply for the fuel gauge. So the issue isn't just the fuel gauge and sender.

My 72 which I have had since 82 fuel gauge has never worked correctly. It could be past F for a very long time, 1/2 might be about correct and your are walking at E. Now even with gas up to the cap it, the needle doesn't quite need reach F. IMHO the odometer is a better fuel gauge if it works. Mine odo went earlier last so I'll be fixing that in the spring. I also removed my aux tank, so now I just gas up every trip from town. On a road trip every 250 miles or so. When I put in my Datsun B210 12.5 gallon aux tank I was able to get the gas gauge to read from E to 1/2 which was full by carefully adding some low ohm resistors in wire from its sender.

My oil pressure gauge never work right either. In like the 3rd month of ownership I installed an illuminated direct oil pressure gauge on the steering column. I ran fuel hose over the oil line from the back of the gauge to the engine side of the firewall - that way if the line broke I wouldn't get a face full of hot oil. I have a pipe for the engine port in the ash try to block off the leak so I can keep driving.

My speedo lies too, always has. Ammeter with the battery out read just left of center - 1/2 a needle. Temp gauge works well enough.

Gauges on the tractor are more or less suggestions.
 
If your fuel gauge has 2 terminals its the early 12v one - you also need the early tank sender. These are slightly smaller diameter than later ones. Is there a chance a later tank and sender have been fitted? I think early sender would still fit in a later tank - screw pattern is the same
 
Last edited:
anothernz45 said:
If your fuel gauge has 2 terminals its the early 12v one - you also need the early tank sender. These are slightly smaller diameter than later ones. Is there a chance a later tank and sender have been fitted? I think early sender would still fit in a later tank - screw pattern is the same
Click to expand...
The gauge has 2 wires and the sender is what I believe I've seen called a "bimetal" type? It is the type you can't find if you search for fuel sender for FJ40. I believe it is correct, as my donor 40, one month older on the production line, also has the same two wire gauge and bimetal type sender. I've toyed around with both gauges on both senders and neither give me an expected reading while moving the float through it's range.

I was really hoping there was a 'typical' issue with these that someone had identified. From what I can understand, you can't really use a multimeter to test the sender. I've tried using the process described for the newer style, but all I get is nothing or around 125 ohms (I think). Anyway, thanks for the reply!
 
The bimetal ones cant be checked with ohmmeter but if you remove the sender, hook it back up to the gauge wire and then connect an earth wire to a mount hole you'll see if anything works just by moving the float around. It takes a while to get a reading - leave it in one position for a time.
 
Had a play with an early gauge and sender out of my 69, the gauge would read just under half when right empty in the truck. I replaced gauge and sender with later ones along with temp gauge/sender. With no power gauge reads empty - I hooked the 69 units directly to a battery and they do read, just under half is empty but reads full as full. Mine may just be an adjustment issue, does look like there's holes on the back of the gauge that might be for this but would have to open gauge up to check.

DSC00286.webp


DSC00287.webp
 
Last edited:
anothernz45 said:
Had a play with an early gauge and sender out of my 69, the gauge would read just under half when right empty in the truck. I replaced gauge and sender with later ones along with temp gauge/sender. With no power gauge reads empty - I hooked the 69 units directly to a battery and they do read, just under half is empty but reads full as full. Mine may just be an adjustment issue, does look like there's holes on the back of the gauge that might be for this but would have to open gauge up to check.

View attachment 4060854

View attachment 4060855
Click to expand...
Thanks for the guidance! I went out today and attacked the fuel gauge(s) again. I had movement on my gauge, but it sort of twitched and fell back to empty a couple of times before settling down around a half tank (bench testing with one of my senders). I followed some other suggestions about the adjustments through two holes in the back of the gauge and got it working on the bench.

I installed it in my FJ40 and got what appears to be proper movement. Went to town and filled the tank to find it only read just over half a tank. Pulled gauge again and fiddles with the adjustment on the back until it read full with key on and empty with key off.

Now it’s just a matter of driving it and seeing how it works as the fuel level drops. Fingers crossed I can check one more item off my list.
 
Before you start checking your gauges always disconnect the negative terminal on your battery. I want to say clean your fusible link located near your battery. May not not be the same mine since the one I worked last were newer models, hopefully still the same. I believe they go to your gauges. Chances are if at least one gauge is working the connection should be good and by the way the fuse is 5 amp I think. Remember everything oxidizes over time so you clean your terminals to your circuits (-/+). Sometimes reseating the terminal will reestablish a good connection.

I want say on the last time I messed with a fuel gauge, the fuel sending unit.....and a 79 at that, the ohm readings I got high when empty( around 120 ohms, when full like 16 ohms. They have to be exactly but there should be a transition from empty, half way and full. I have an OEM manual somewhere. It also shows the position where they should be in relation to your ohm readings. It may be different year manual than your model however the theory is the same.

Check the ground at the rear of the gauge cluster. Not sure what color but you can do continuity check with wire in question to ground. Make sure you have a good connection. At each point (tank etc). Same goes to battery ground and chassis. Good objections. Get EOM manual it will be very helpful. It will make things easier and fast to figure out. This site is awesome, i suggest using the search function and see if it can point you to an online oem.
 
fjwagon said:
Before you start checking your gauges always disconnect the negative terminal on your battery. I want to say clean your fusible link located near your battery. May not not be the same mine since the one I worked last were newer models, hopefully still the same. I believe they go to your gauges. Chances are if at least one gauge is working the connection should be good and by the way the fuse is 5 amp I think. Remember everything oxidizes over time so you clean your terminals to your circuits (-/+). Sometimes reseating the terminal will reestablish a good connection.

I want say on the last time I messed with a fuel gauge, the fuel sending unit.....and a 79 at that, the ohm readings I got high when empty( around 120 ohms, when full like 16 ohms. They have to be exactly but there should be a transition from empty, half way and full. I have an OEM manual somewhere. It also shows the position where they should be in relation to your ohm readings. It may be different year manual than your model however the theory is the same.

Check the ground at the rear of the gauge cluster. Not sure what color but you can do continuity check with wire in question to ground. Make sure you have a good connection. At each point (tank etc). Same goes to battery ground and chassis. Good objections. Get EOM manual it will be very helpful. It will make things easier and fast to figure out. This site is awesome, i suggest using the search function and see if it can point you to an online oem.
Click to expand...
Mine is the early bimetal sending unit that can’t be checked with an ohm meter. Thats been my problem from the start. Not being familiar with all the differences between year models I made some useless assumptions right out of the gate. Hopefully I’m on the right path. I’ll have to look into EOM manuals as I only have the Haynes at the moment. Thanks for the input.
 
Sorry for the confusion, I repair electronic equipment, not everything is exactly the same....can be similar....anyhow, just wanted make sure how I check it on a newer model which is a little different. My guess the sendinf unit in the 79 does not have sme readings on a early 70. Anyhow, the theory should be the same. Good luck.

The gauge cannot be checked with ohm meter. You have deflect it with sending unit ot a lamp. I don't remember the amperage on the incandescent lamp. The oem shows how to test it. Gas gauge is notoriously an issue. I usually clean the terminals and they start working again.

If you can get a Simpson meter(or any analog type meter should work), set it on the low ohm setting and ohm out the sending unit. The needle should not jump around. Once confirm it checks good use it to test the gauge.
 
Last edited:
You must log in or register to reply here.

Share this page

Back
Top Bottom