Fuel gauge school needed

[jkl72-40]

My fuel gauge has never worked on my 71 fj55
I do know that some one cut the wire under the hood next to the fuse box.
so the question is...... what is the bunny trail that I should take to find out what is the problem and how do I fix it?
thanks

 

[jkl72-40]

anybody..?.?? Bueler...Bueler????

 

[Tobugrynbak]

According to my FSM,,,

Having had a look at the FSM for FJ55 (USA & Canada) - 1979 model. Also the diagram for non USA & Canada models (no date.)

The wiring diagram says that the wire is Yellow with a red stripe which runs from the instrument cluster to the sender and another wire which is White with a Black stripe goes from the sender to ground.

On the back of the FJ55 Temp/Fuel/Oil cluster, as well as the FJ55 Temp/Amp/Fuel/Oil cluster, the Yellow/Red wire should connect to pin 8 which is the top left pin.

I'll list the pins, purpose and wire colours below.

Pin .. Purpose .............................Wire Colour
1 ......To oil Pressure Sender ........Yellow/Black
2 ......From ignition power feed .....Light Blue/Red
3 ......Ground ..............................White/Black
4 ......Dash illumination ................Red/Black
5...... Park brake light ..................Red/White
6 ......Charge light .......................(can't find it)
7 ......Temp Gauge sender ...........Yellow (or Yellow Green)
8 ......Fuel Gauge sender .............Yellow/Red

Hope that helps.
Cheers
Perry

 

[FJROSS]

Just to rule out another possible problem, check the circuit board on the back of the gauge cluster and make sure there's no damage to the copper tape. This area is supsetable to damage when removing and installing the gauge cluster with the steering wheel in position. After checking my grounds and replacing the fuel sender, my last resort was to replace the gauge. That's when I found that the copper tape on the rear of the circuit board must have been damaged and a half-*** repair done by a previous owner (the butcher) with some solder, which didn't work. I ended up switching gauge clusters.

Dave

 

[Tobugrynbak]

Some quick basic checks you can do to see where the problem lies.

Disconnect the wire that goes to the sender and connect it to ground. Now turn on the ignition. If the wire to the sender is intact, and the gauge is working then the needle will swing to the Full mark. If that's the case, then the problem is in the sender/ ground wire.

If you have a multimeter, check that the sender body is grounded by measuring the impeadence between the sender body and any handy chassis/body point. i.e. seat mount bolt. The impeadence should be very low, under 1 ohm.
If you have an open circuit, then the ground wire is not attached to a ground point, could be faulty or missing.

The fuel sender has a total impeadence (resistance) of 120 ohms +/- 6.5 ohms when the tank is empty, and 17 ohms +/- 2.1 ohms when full. It should have something like 40 ohms +/- 4.5 when the tank is half full. If you don't get any measurement with your multimeter then the sender is faulty.

See how you go with that lot.

Cheers
Perry

 

[FishNinJay]

Some quick basic checks you can do to see where the problem lies.

Disconnect the wire that goes to the sender and connect it to ground. Now turn on the ignition. If the wire to the sender is intact, and the gauge is working then the needle will swing to the Full mark. If that's the case, then the problem is in the sender/ ground wire.

If you have a multimeter, check that the sender body is grounded by measuring the impeadence between the sender body and any handy chassis/body point. i.e. seat mount bolt. The impeadence should be very low, under 1 ohm.
If you have an open circuit, then the ground wire is not attached to a ground point, could be faulty or missing.

The fuel sender has a total impeadence (resistance) of 120 ohms +/- 6.5 ohms when the tank is empty, and 17 ohms +/- 2.1 ohms when full. It should have something like 40 ohms +/- 4.5 when the tank is half full. If you don't get any measurement with your multimeter then the sender is faulty.

See how you go with that lot.

Cheers
Perry

Makes sense.. but here's my situation. I want to drop my aux tank and clean it out. While dropped, I want to install a sender.

I have a custom rear aux tank, 7" deep.

I have a stock tank under the passenger seat with working sender going to a Sunpro gauge. I have no idea what kind of sender it is, but I assume it's stock Toyota?

It looks like the Sunpro gauges that I found on google all say 240-33ohm with 240=empty, and 33=full

The Autometer gauges seem to all say 0-90ohm, with 0=empty, and 90=full.

I found universal adjustable fuel senders with both ranges. Since mine is a Sunpro gauge, should I use the 240-33ohm sender? What happens if they don't match, and I use a 0-90 sender to a 240-33 gauge? I don't know for sure what the requirements are for my exact Sunpro fuel gauge, but my google search seemed to show most list 240-33. That doesn't seem to correlate with your numbers. From your numbers above, it sounds like the stock Toyota sender is probably a 0-90ohm device? Is my logic wrong here?

Finally.. any idiot-proof tips on wiring this?
I have a AUX dash switch that activates an electric fuel switch bolted to the frame below. 2 wires go towards the switch. One is power from the switch, and the other is the sender wire from the passenger main tank, also travels towards the fuel switch and then continues up to the fuel gauge. Obviously, what I'd like is for the dash switch to BOTH, activate the fuel switch to open up the aux fuel line, AND switch from the MAIN sender to the AUX sender.

 

[Tobugrynbak]

The stock sender is 17-120 ohms, with the 120 ohms at empty and 17 when full.

You need to use the matched sender to the gauge. Otherwise it will either not work in the right direction(showing full when empty), or not have the correct span of indication (showing 1/2 when your really empty).

There are other options involving bespoke electronics to adapt between senders and gauges. But I don't know if there is anything off the shelf available to you.

 

[Tobugrynbak]

As for;
"Finally.. any idiot-proof tips on wiring this?
I have a AUX dash switch that activates an electric fuel switch bolted to the frame below. 2 wires go towards the switch. One is power from the switch, and the other is the sender wire from the passenger main tank, also travels towards the fuel switch and then continues up to the fuel gauge. Obviously, what I'd like is for the dash switch to BOTH, activate the fuel switch to open up the aux fuel line, AND switch from the MAIN sender to the AUX sender."

Ok you will need a DPDT (double pole double throw) switch and if you want to get sneaky get one that is center off (I'll explain later). Preferably this switch will have the current capacity to either run the pump directly or (better option) switch a relay to run the pump.


The switch has 2 poles which are electrically isolated from each other. One pole will switch between Main and Aux Fuel tank senders and the other pole will switch between the Main and Aux senders. I'll try to show the position of the wiring on the switch below.
..............Normally closed.... Common..... Normally Open

..................................To Fuel Gauge
Pole 1... Main Tank sender ............... Aux Tank sender

Pole 2 .. Main Tank Pump.................. Aux Tank Pump
.................................12v+ Ign Feed

(ignore the ... )

As for the pump wiring you could make the switch turn relays on to drive the pumps. That would be my preferred way of doing it. Don't forget to add a fuse in the power circuit before the switch.

If you use a switch with the center off it will give you an extra stumbling block for car thieves if you remember to switch it to the "Off" position when you park the vehicle

 

[FishNinJay]

As for;

If you use a switch with the center off it will give you an extra stumbling block for car thieves if you remember to switch it to the "Off" position when you park the vehicle

In the center "OFF" position, how is this a fuel kill switch? The solenoid fuel selector valve defaults to the MAIN tank when not energized. I don't think the FSV has a setting where BOTH fuel lines are closed off, does it? It's either ON=AUX, or OFF=MAIN.. as far as I know??