70 Fj40 fuel gauge issue...

[MattieP]

Hi guys my fj40 was a trail rig when i 1st got it. i had no real Toyota wiring so been going through and wiring up all the factory gauges. All work well except the fuel gauge. when the truck has power ( i have a kill switch) the gauge always reads a half of tank. even when tanks empty and trucks running. If i kill all the power it goes down to empty. If powers on but trucks not running still reads half of a tank. today. i drove the truck around for couple minutes. and the gauge moved up past half a tank n kept going til i parked it. knowing there's only a 1/4 tank of less fuel in there. is it possible i routed the power wire to the wrong side? the sending unit is brand new. got it from Cruiser Corps. Tank is a oem tank. gauges are oem 70 fj40 gauges. I saw other posts about resisters etc. haven't messed with that yet since everyone seemed to be having issues with gauge no moving. i have 12v going to the gauges from fuse box. any thoughts on the subject would be greatly appreciated.

 

[65swb45]

I was not aware that cruisercorps offered the pre-73 fuel sender. Are you sure the guage and sender are compatible?

 

[alan m]

Hello
From what I've read the fuel gauge has to match the sending unit.
On my 79 the instrument cluster was an assemblage of parts from 72 thru 79, it was a mess to sort out
Alan

 

[Cruiser Corps]

Our sending unit covers years 72-78 (clearly indicated in product description). We cannot guarantee reliability for other years and models. Sorry. We accept exchanges or offer refunds, but only on unused products. Very sorry.

 

[MattieP]

I was not aware that cruisercorps offered the pre-73 fuel sender. Are you sure the guage and sender are compatible?

yea the truck is lil mismatched. the tank is from a 78 fj40. n that's why i got a cruiser corps sending unit. could i fix the issue with a resister something of that nature?

 

[MattieP]

Our sending unit covers years 72-78 (clearly indicated in product description). We cannot guarantee reliability for other years and models. Sorry. We accept exchanges or offer refunds, but only on unused products. Very sorry.

not your guys fualt at all, my truck just has a ton of mismatched parts from when i got it. just figuring out all the differences.

 

[MattieP]

i know the ohms for the 72-78 is as follows:

Tank Full 2.1 ohms
Tank 1/2 Full 4.5 ohms
Tank Empty 6.5

most of my truck is from may of '70.....i'm finding things here and there are newer and older. i heard some say on here that some of the fuel gauges in earlier years were 6 volt rather then 12v's?

 

[FJ45Dude]

MattieP,

If your gauge is indeed original from 1970, you will need to match that to the appropriate sender from that vintage. Good news is that these are still available brand new.

However... I do not know if that correct sender for your 1970 gauge will fit a 1978 fuel tank.

That said...

Be careful with the early gauge itself as they are becoming difficult to find in good working condition from that era. Always disconnect the battery before taking out the instrument cluster to avoid shorting anything out.

I went through a couple combinations of used senders (and finally new one) as well as several used gauges to find a working match that allowed me to retain my original 1966 fuel tank with no modifications to the wiring or instrument cluster.




FJ45Dude

 

[alan m]

Hello Mattiep
You could bench test the fuel gauge withits sending unit.
The gauge should consist of two wound wire resistors and the sending unit should also consist of two identical wound wire resistors. These four resistors make a bridge circuit, one of them is variable, the one in the sending unit.
you can build a resistor substitution box to place in the circuit to get the right value needed to be added to the circuit then you should be all set.
The resistor that you add cant be bought at radio shack, you will have to make it from the wire out of some other sending unit or gauge.
Good Luck
Alan

 

[mumphra]

easiest way to get around this is to take out the original fuel sending unit and fab a new one on for new cluster I did it and it took me like an hour and I got cool all new gauges that work good.

 

[bj40green]

Hi Mattie,

Just saw this thread. Don't know if you solved the problem.
Sept. '72 was a change date in clusters and senders.
See also: https://forum.ih8mud.com/40-55-series-tech/544543-clusters-gauges-odo-meters.html
If you have a pre '72 cluster (with the turn indicators in the dash) you need the sender #83320-35020 Empty:6.5Ω Full 2.1Ω
If you have a '72 - '78 cluster (with the turn indicators in the speedo face plate) you need the sender 83320-60050 Empty: 120Ω Full 17Ω

Good luck,

Rudi

 

[mdman]

I know this thread is really old but for future ih8mudders, this is a good link for fixing fuel gauges:

Electric Fuel Gauges

 

[Edwardsrealtor]

I am needing to resurrect this old thread. Working on a 73 FJ that has had allot of random hodgepodging to it. Currently the tank will read just below half when bone dry. It has the gauges that have the turn signal on the dash, so it leads me to believe the gauges would be from a pre-72. I read the ohms from the tank and I am at 33 ohm. I took off the wiring and put inline a random 6ohm resistor and it pushes the gauge FULL. Any help would be appreciated. Thank you!

 

[charliemeyer007]

My 72's gauge has never worked right. Read past F when full for a long time and on E I'm walking. Sometimes 1/2 to E goes real fast. I put a 12.5 gal aux tank rom a Datsun B210 about 1987. The Chilton book had the ohm range for LC and I looked up the data for the 210. I bought an assortment of small 2% resistors from radio shack. I was able to get the Datsun tank to read on the Toyota gauge from E to 1/2 which was full, and it work for like 35 years.

I don't remember the mechanics of the electrical, and my aux tank has issues and has been pulled.

So resistor in series add IIRC, you went from 30 to 36 ohm and the gauge went from 1/2 to full - I would try removing the 6 ohm and add what ever it takes in parallel to get 24 ohm and see what happens. You should first check the tank sender by pulling it and check the numbers at like F, 1/2 and E. The gauge is powered by a voltage in the amp meter. There are threads about around here.

A working odometer is the best gas gauge IMHO.

I'll find a book and a calculator and see if I can figure out the magic ohm. You really need red line 2% resistors or perhaps some sort of potentiometer. I only had a year of Electrical Engineering and like 1.5 years of Physics, I'm happier in Chemistry & Thermo

120ohm

 

[Engineer8000]

The 73 has the blinkers in the cluster so you may have an early cluster.
The pre 72 sender is not a resistor. It is a points type regulator that works the same as the oil pressure sender. The gauge and sender need to match.

 

[anothernz45]

Just spent a few hours down this rabbithole. My 69 FJ45 had no temp sensor, with an auxillary ether gauge fitted. Put a sensor from a 75 in and hooked up wiring to dash - nothing. Figured it needed a gauge to match so fitted a later gauge into the 69 cluster, everything was good until temp soared past boiling as water just started to warm. Googled madly, found later temp gauges run off 5volt supply fed from a voltage regulator built into the later fuel gauges. Added a later fuel gauge by drilling a 3rd hole in cluster case for 3rd terminal on later gauge and ran a shunt from this terminal to temp gauge - now it reads as you would expect. A bonus was my fuel gauge now reads empty when the tank is empty instead of reading 1/2 full - guessing the sender in my tank was from a later truck too.
Lot more work than I was hoping but now everything reads as it should. Will post a couple of pics later if anyone else goes down this track.

 

[anothernz45]

I am needing to resurrect this old thread. Working on a 73 FJ that has had allot of random hodgepodging to it. Currently the tank will read just below half when bone dry. It has the gauges that have the turn signal on the dash, so it leads me to believe the gauges would be from a pre-72. I read the ohms from the tank and I am at 33 ohm. I took off the wiring and put inline a random 6ohm resistor and it pushes the gauge FULL. Any help would be appreciated. Than

You need the early fuel tank sender

 

[Edwardsrealtor]

An update, I started with the basics and checked my ground and found i was getting resistance in that. I cleaned it up and the entire things working great!

 

[anothernz45]

I insulated the later gauges with a butynol washer with an o ring dropped over the posts to keep them centred. Then the 69 plastic washers and nuts. If you fit the later fuel gauge there is a copper grounding tab that must contact the case of the cluster, that is why there is a strip cut out of the butynol - this is the earth for the 5v regulator. I went down this route as I have plenty of later spares from decomposing FJs but this is the only F powered 3 speed. The second pic shows the shunt feeding the temp gauge - tape the original feed off.