HELP - Removal / testing fuel sender unit

[nick_the_fish]

OK so on my 105 I have the Toyota installed sub tank with a switch that changes between tanks, and which should change the fuel gauge to show the level of the tank you are running from. However this has never worked (since I got it anyway). I took the switch out and tested it and sure enough there doesn't seem to be any current from (I presume) the sub-tank sender unit.

So after removing ALL of the built in expedition set-up and loads of 'ing around I have got to, and removed the cover to access the sender unit. I have alto managed to tease out the old rust cover screws which was a bit of a win.

HOWEVER here I am stuck. I can't remove the sender unit from the tank because there isn't enough slack in the cable. I don't want to just pull it. Photo of the sender unit below

Should that blue cover come off the top of the sender? If so how (I tried wiggling it but there is no play). Or is there a way of getting more slack out of the cable?

Any thoughts anyone????

N

 

[chapel gate]

The sender unit will plug into a connector to the main vehicle loom. This should be sitting on top of the tank. Get to it from under the vehicle. Its held in place with this;

 

[clivehorridge]

The sender unit will plug into a connector to the main vehicle loom. This should be sitting on top of the tank. Get to it from under the vehicle. Its held in place with this;
View attachment 149706

And on receipt of that post, nick_the_fish jumped off the bridge...

 

[nick_the_fish]

Thanks @chapel gate thats a really useful photo - now I can see the set-up I reckon I can confidently work back to find the connector without dropping the tank which I am really trying to avoid if possible. I'll let you know how I get on

 

[chapel gate]

And on receipt of that post, nick_the_fish jumped off the bridge...

He sounds like a good swimmer clive..

 

[chapel gate]

Thanks @chapel gate thats a really useful photo - now I can see the set-up I reckon I can confidently work back to find the connector without dropping the tank which I am really trying to avoid if possible. I'll let you know how I get on

Please do.
There is a good chance it will be trapped. You may be able to lower the tank just enough with out completely removing the nuts.

 

[nick_the_fish]

Ended up dropping the tank which wasn't as much of pain as I had assumed it would be.

Tested the sender unit which I think is fine - it read between 9 and 109 ohms depending on where the float was. I also tested the earth which seemed fine.

So..... either the problem is with the switch, the gauge, or the cable is broken somewhere.

The question now is how does the rest of this system work at the gauge/ switch end? My assumption is that there is only one cable to and from the switch and the gauge, and that it is the switch that changes between the input feeds from the 2 sender units depending on which one it is pumping fuel from. If that is correct, then I know the gauge works because I get a correct reading when its on the main tank.

Therefore its the switch, or the wire broken somewhere. I can easily test the continuity of the cable, if only I knew which of the many connectors within the switch goes to the sub-tank sender unit?

Also, is there any way to test the switch?

Any thoughts anyone? @StarCruiser Rich I know this is a bit cheeky but can I pull you in at this point (in case you hadn't seen it). I've yet to see an electrical problem you can't advise on

Thanks

 

[nick_the_fish]

As an afterthought, the engine can run from both tanks, so the solenoid is engaging, so the switch is doing its job there, BUT when switching between tanks the gauge always shows the level of the main tank, i.e. it doesn't drop to nothing when the sub-tank is engages. I'm wondering if the switch is somehow sending the signal from the main tank sender to the gauge even when the sub-tank is engaged??

 

[StarCruiser]

Hi Nick,

Excuse my ignorance here but what makes you think the sender units are switched to the same gauge? I say this because the 80s (I know yours is a 105 so forgive the lack of specific knowledge) often have a ‘sub tank’ gauge. Is this not the case with the 105?

Just a preliminary thought.

 

[nick_the_fish]

hummm a good thought. I can't remember how or why I thought that but some googling seems to confirm my thoughts that the 1 gauge does both tanks. A light is supposed to come on when the sub-tank is being used but again I haven't seen this. I also haven't come across a second gauge.

 

[chapel gate]

i can only speak for the 80, the dash switch isn't connected to the sender unit. the sender unit on a diesel has two wires, an earth and a signal wire that goes to the sub fuel gauge. the petrol is a different story...
the switch once pressed energises the solenoids. when the ignition is switched off the solenoids switch back.
the 100 series sub tank wiring diagram would answer all your questions, it is a pretty simple system.
I would guess your presumption is correct. once the dash switch is pressed, the sender unit signal wire from the main tank is switched over to the sub signal wire. which then shows the sub tank level on your dash fuel gauge.
just a presumption, I have no experience with the 105 series set up.

 

[nick_the_fish]

interestingly the sender unit has 3 wires, and I can't find anything on the internet to explain why....

Had the dash switch out this evening. I get a completed circuit from the sender unit connector to each of the 4 contacts in the dash (yes all 4 of them) so working on the basis for now that the wires aren't broken somewhere between here and there.

Testing the switch itself provides an equally confusing array completed circuits depending on wether the switch is in the main or sub position. I did note that without the dash switch connected and the ignition on the fuel gauge is dead. This is expected but it add weight to my early thought that the switch seems to be sending the main tank feed to the gauge whichever tank it being used.

Think your right CG I need a sub-tank wiring diagram

 

[Chris]

Isn't one of the wires the low level indicator? On mine that's what the third wire is. I connected it all to a separate gauge with a low level light too.

 

[chapel gate]

as chris states, the 100 sub tank sender must have a level float then. hence the third wire. this will send a signal to your fuel gauge that lights up when the fuel level is low. same as the main does.
the 80 diesel sub gauge doesn't have a low level warning light. if your sub sender unit has three wires it does have a level float but its for a petrol. the petrol sub gauge also doesn't have a low level warning light. its purpose is to stop the transfer of fuel from the sub tank to the main tank.

 

[StarCruiser]

When you do your continuity tests Nick, it’s important to disconnect the plugs as much as possible from the senders, solenoids, the switch, gauge, anything really. Make a drawing noting wire colours. You can also test through the switch in both positions. Just pick a pin (a) and find what it’s connected to (b) then operate the switch and test from (a) again to the other pins. If you get nothing, test from (b) to see if there’s a second terminal (2 way) or if there isn’t then it’s just the two terminals (1 way). Make a drawing with the connections as they appear on the switch and draw solid lines for switch position 1 and dotted for switch position 2. From there you can fill in lines to your power, gauge, solenoids and senders.

You can then work out which wire does what from each unit that’s plugged into the loom. Put it all on the drawing and all will become clear.

 

[nick_the_fish]

OK thanks guys. Well outside my comfort and knowledge zone here but surprisingly enjoying the challenge.

There isn't a separate sender for the low fuel light, so are you saying that the bulb would be triggered when the float gets to its lowest position? If this is the case wouldn't you expect there to be a completed circuit between the low light terminal and the earth terminal (on the sender) when the float was in the lowest position? Because I just tested this and didn't get a circuit.

Also, taking Rich's advise I have just been probing the switch methodically. Its has 4 contacts in a row, labelled usefully enough 1,2,3 and 4. Now 1 and 2 always make a circuit weather the switch is in the main or sub position (and the resistance is the same in both positions). Contacts 3 and 4 only make a circuit when when in the sub position. There are no other circuits. Drawing below. Does this seem weird?

 

[StarCruiser]

Does the switch have a light on it Nick? Either when activated or with the headlights for nighttime illumination? It looks like the switch may operate a relay to change things over as there’s not enough contacts on it to do the job.

 

[nick_the_fish]

Yep it lights up with the headlights. Looks like i'm playing find the relay. Its not under the bonnet but hopefully i'll get lucky and it'll be in one of the footwells...

 

[StarCruiser]

Yep it lights up with the headlights. Looks like i'm playing find the relay. Its not under the bonnet but hopefully i'll get lucky and it'll be in one of the footwells...

One way to find it is to disconnect your solenoids on their plugs so they make no sound and then get someone to flick the switch on off on off while you try and trace where the clicks come from. Crude but it often works. If there’s more than one relay, pull each out in turn until the clicks stop.

 

[chapel gate]

There are a few clues in this thread. It sounds like relays and maybe even a ecu are involved.
the 80 system is, lets say, analogue..
i would still be looking for a 105 series Australian FSM.
https://forum.ih8mud.com/threads/subtank-wiring-oem-100-series-style.768140/