Fuel Sender Questions-Wiring Help Needed

[elmariachi]

Fuel Sender Questions-Wiring Diagram Help Needed

Below are two diagrams for main tank fuel senders, early non-USA and late USA. The block diagrams indicate that the switch design has changed. I interpret the earlier on the left as a simple switch and the latter, according to my EWD, as a combination of a variable and a standard resistor (third picture.)

What I am trying to figure out is whether or not these two circuits, though different in design, are accomplishing the same thing and whether or not they would likely be sending the same signal. And does the later design infer that a variable signal is being sent versus the earlier that would only send a finite on/off value? Feedback appreciated.

 

[Tools R Us]

The first drawing is probably just simplified, wrong? All of the tank senders that I have seen have a rheostat. For a better clue look at the testing section on the senders, it should give the resistance values.

 

[elmariachi]

The first drawing is probably just simplified, wrong? All of the tank senders that I have seen have a rheostat. For a better clue look at the testing section on the senders, it should give the resistance values.

The values appear to be the same on the senders. What I am unclear on is the low fuel "switch." From what I can tell, this is the float at the bottom of the unit that sends an empty signal, which I guess is the low fuel light, right? On the earlier non-USA models it is not integrated into the sender, it is a separate three-wire unit in the tank adjacent to the sending unit. On my truck it seems to be integrated all into one, fuel pump, sender and low fuel "switch."

 

[Hants]

The float is part of the fuel level sender (probably the variable resister in the diagrams above).

There is a separate low fuel sensor (simple on/off sensor).

There a picture and some discussion in this thread:

https://forum.ih8mud.com/showthread.php?t=104384&highlight=low+fuel+light+sender

I've not heard of different circuits for different years. I HAVE heard of different configurations of senders/sensors on the sub-tanks (diesel versus petrol).

 

[elmariachi]

The float is part of the fuel level sender (probably the variable resister in the diagrams above).

That is correct.

There is a separate low fuel sensor (simple on/off sensor)

So would you say that the "low fuel sensor" or the "sender" in this pic below (borrowed from that thread) is the "switch" that is noted above in the pre-95 diagram above?

 

[Hants]

My guess would be the "Low Fuel Sensor". The "Fuel Level Sender" would be the variable pot in your diagram.

I suspect the older diagram is a "simplification", as the fuel gauge has to be a variable signal.

 

[Hants]

The sub-tank sender assembly I have here (83320-69215) has a triangular plug on the outside, the float for fuel level, and the low-fuel sensor. Based on looking at the unit, the low fuel sensor switches ground (rather than hot). That also meshes with my experience with the stock fuel tank unit (short to ground in the bulkhead caused low fuel light to be constantly on).

 

[elmariachi]

So from this I take that we believe that the intended functions from the old diagram to the new are the same. In the case of the newer diagram, I do know that the Red wire is the fuel level warning signal (for low fuel light) and the Yellow/Red is the gauge signal. I sure hope those were the same functions on the older model.

 

[Hants]

You might check with CDan-the-man and find out if there are different tank sender part numbers for earlier models. IIRC, the whole assembly comes as one part.

 

[elmariachi]

You might check with CDan-the-man and find out if there are different tank sender part numbers for earlier models. IIRC, the whole assembly comes as one part.

Already emailed him last night.

 

[elmariachi]

EDIT>>. I have figured it out, they are not the same. On Oz trucks with subtanks, they have a separate switch in a second hole in the main tanks that houses a switch that comes on when the tank drops below 3/4's full. That is the signal I need.

 

[TroutRunner]

I just bought the "sender" from CDan today. I don't know that much about them other than I got a new one because I needed a 3wire sender versus a 2wire sender (diesel tank) so I can get the signal from the float that says it is empty or full.

TR

 

[elmariachi]

I just bought the "sender" from CDan today. I don't know that much about them other than I got a new one because I needed a 3wire sender versus a 2wire sender (diesel tank) so I can get the signal from the float that says it is empty or full.

TR

So you mean the subtank sender. The part I now need to complete my subtank install is a totally different animal:

 

[ParadiseCruiser]

J -

I have read this thread three times, and I am still not sure of what you are lacking/missing to complete the install. Why wouldn't the stock main tank sender work for you? (I'm sorry, I know I'm missing something here...).

Or is it that the subtank sender is incorrect? I think I can help, but I'm not sure of the question.

R -

PS:

>> On Oz trucks with subtanks, they have a separate switch in a second hole in the main tanks that houses a switch that comes on when the tank drops below 3/4's full. That is the signal I need.

Why can't you use the signal from the existing low fuel thermistor (the red wire you mentioned above)? I've used that signal to close a relay to start the subtank transfer pump... to automate the fuel transfer process. In my case, however, without the ECU logic, the system is flawed because when the system cycles around a second time, the subtank is now empty and there is nothing to stop the subtank transfer pump from running dry. I would think your ECU would "see" the empty subtank and would prevent that.

 

[elmariachi]

J -

I have read this thread three times, and I am still not sure of what you are lacking/missing to complete the install. Why wouldn't the stock main tank sender work for you? (I'm sorry, I know I'm missing something here...).

Or is it that the subtank sender is incorrect? I think I can help, but I'm not sure of the question.

R -

A lot has happened with this. First off, I obtained some pics from Oz while trying to figure out the plumbing for the subtank to main tank filler. It goes in through the main tank sender bracket/plate. So CDan and I decided I could have my tank shop weld in a new thru-pipe. The new bracket is on its way. The pics I have suggests that there is another wiring conenctor on top of the main sending unit. Hmmm. WTF??? While trying to sort that out I scored some Toyota docs about the subtank assembly which included instruction on how to put the subtank ECU into diagnostic mode, along with the codes.

After all is said and done I have figured out that the Oz 80 Series has a separate "fuel tank switch" mounted in a separate bung adjacent to the tank sender. This switch tells the ECU that the tank level has dropped below 75%. I need that signal, which is represented by the black and white pic above. Now I can "emulate" that by grounding out the Red annd Red/Black wires going into the ECU (which is the same as that switch closing to ground), but....Mr. T is smarter. There is another signal hitting the ECU on another pin that sends voltage to the ECU based on the tank being either full or less than 1/4 tank. That screws me up on grounding the fuel switch signal because if I tell it that its below 75% (by grounding those wires) the other circuit says "Nope, you still have more than a 1/4 tank."

And all this seems to be confirmed by the ECU diagnostics, which is showing an error with the fuel level switch (caused by the conflict I assume.) I might note that the ECU does send the prime signal to the pump when you turn on the key, and the ECU is diagnosing the circuit, so I am really very close to nailing this.

The original post was me trying to figure out of the USA sender had that switch built in. It doesn't.

Are you sorry you asked?????

 

[elmariachi]

And to confirm all this, I have ripped out the driver's side harness at the passenger door sill, and the little white connector under the sill takes 4 signals to the ECU:

R/G = transfer pump 12V+
R/L = solenoid 12V+
R = fuel switch to ECU
S/B = fuel switch to ECU.

They all conveniently plug into that connector so I know they are required.

 

[ParadiseCruiser]

Are you sorry you asked?????

Ahhhh, yep

But you see, as I told you the other day, you are breaking some new ground here. I think I'm going to keep my own counsel, and wait to see your detailed description forthwith

Cheers, R -

 

[elmariachi]

Earlier today I was beginning to think the presence of the ECU connector in these USA trucks was a cruel joke brought about by two old Japanese women sitting in a barn somewhere south of Tokyo drinking sake and whacking out wiring harnesses. But no, its still legit. I may figure out a way to trick the ECU, because I think all my trouble lies in simple continuity/no continuity. But if I can source that switch, its going into the install for posterity sake.

Why can't you use the signal from the existing low fuel thermistor (the red wire you mentioned above)

Okay, so walk through this with me, referring to the black and white diagram above. According to the diagnostic docs, to test the circuits, each wire in the harness should be grounded to check continuity (with the ECU unplugged):

- Both the red AND red/black wires, when grounded are to have no continuity when the tank is full.

- Both the red AND red/black wires, when grounded are to have continuity when the tank is below 3/4 full.

I don't think the Red and Red/Yellow behave that way do they?

 

[ParadiseCruiser]

I don't think the Red and Red/Yellow behave that way do they?

I don't think so.

In our US spec primary tanks, neither the fuel level rheostat nor the thermistor are on-off switches... they are both resistive devices that are creating a variable resistance based upon the fuel level in the tank. You wouldn't see a drop in the resistance ("continuity") in the thermistor (low fuel light) until the fuel level was near empty.

We may be beating a dead horse here -J- because we are applying US spec logic to an Oz spec diagnostic. I'm thinking they are not quite the same.

R -

 

[elmariachi]

We may be beating a dead horse here -J- because we are applying US spec logic to an Oz spec diagnostic. I'm thinking they are not quite the same. R -

That's my take on it.

After looking at all my Oz diagrams again its clear now that the fuel sending unit runs directly to the combination meter (dash gauge) and this PITA switch runs either to the subtank ECU, pre-1995 or to the main ECU (subtank ports) for post-1995.