Testing fuel sending unit on Duel tank?

[leolkfrm]

there is only 1 sending unit on ours, as the tanks draw evenly. can you post a pic so i can compare setups?

[Mrcrowley]

Originally Posted by leolkfrm

there is only 1 sending unit on ours, as the tanks draw evenly. can you post a pic so i can compare setups?

You talking pics of the tanks not a problem ill do that tonight . I have dump body so easy to access . Also there is a sending unit and pump in each tank I know this because when I trouble shot the gauge unhook one showed half tank unhook both empty .Crabtrucks diagram is closest to mine he posted wiring diagram here
and the wires colors are same to tanks but can not find second relay I followed the wires from front to back back to front no deal on finding relay . Only one I have seen is underhood in junction box.

[GilbertC]

Ok here is video of my truck hopefully this will explain a little more .

YouTube - MOV01117.mpg

[leolkfrm]

nice vid!, question, do you have a dash switch for fuel? what was the tube on the rear tank not connected to anything.

[Mrcrowley]

Originally Posted by leolkfrm

nice vid!, question, do you have a dash switch for fuel? what was the tube on the rear tank not connected to anything.

Nope no switch it is all automatic the tube has been that way since I got the truck not sure could be vent it is on both tanks one line is fuel send one is fuel return other it doe's not say.

[crabtruck]

Great video. Always better when we can see real things.
One obvious thing is that you do not have side by side tanks,
that is not the factory norm as far as I know.
You asked before about tank capacity.
New 3500 seem to be 27 main 23 aux.
The capped line from rear tank looked like the vapor tube.
Maybe it's capped because of the other bung in tank to filler ?
Where does the front tank's equal go ?

As best I can see in the video, and I watched several times,
you have only the single 2-wire connector at front, just like rear.
That means ECM is dependent on fuel senders both working
to know when to run rear tank.You said:

I unplugged each tank and the gauge would read half full when one was unplugged
when you unplugged both the fuel gauge would read zero. The rear tank still is full

How full was front at that time? If I followed correctly,
unplugging either, with the alternate one still plugged, read half.
Both unplugged read correctly "empty". This would say senders ok to ECM.
The first time you read a voltage at rear you got 8v, and another day
2.14 volts. Could be you are not getting a good ground for read.
Which makes me wonder: if you only have two wires coming from tank,
one for pump hot, one for sender return, where is the ground ?
Given the tanks location near the "danger zone" of dumping jobs,
could you have merely broke the ground wire loose ? $64 question.

Next step I think would be direct fused suplly to the pump wire
so you know that works. Then the more complicated relay end.
Although it would make sense for the aux relay to be near the relay
that you were showing, I noticed it has the same panel as the 2500burb.
On that truck there are two more fuse boxes: matching one in other door,
and one under driver side dash. Doubt the relay would be there though.

Originally Posted by leolkfrm

there is only 1 sending unit on ours, as the tanks draw evenly.
can you post a pic so i can compare setups?

Does that have diesel style crossover hoses for that ?
If running a pump in tank, ECM would have to know when it is ok to pump.
Perhaps you just have one gauge that shows average fuel like this one.

Just for fun that burb panel:

[leolkfrm]

just a thought, have you traced the fuel lines, it may lend a clue

[Mrcrowley]

Originally Posted by crabtruck

Great video. Always better when we can see real things.
One obvious thing is that you do not have side by side tanks,
that is not the factory norm as far as I know.
You asked before about tank capacity.
New 3500 seem to be 27 main 23 aux.
The capped line from rear tank looked like the vapor tube.
1) Maybe it's capped because of the other bung in tank to filler ?
2) Where does the front tank's equal go ?

As best I can see in the video, and I watched several times,
you have only the single 2-wire connector at front, just like rear.
That means ECM is dependent on fuel senders both working
to know when to run rear tank.You said:
3)How full was front at that time? If I followed correctly,
unplugging either, with the alternate one still plugged, read half.
Both unplugged read correctly "empty". This would say senders ok to ECM.
The first time you read a voltage at rear you got 8v, and another day
2.14 volts. Could be you are not getting a good ground for read.
4) Which makes me wonder: if you only have two wires coming from tank,
one for pump hot, one for sender return, where is the ground ?
Given the tanks location near the "danger zone" of dumping jobs,
5) could you have merely broke the ground wire loose ? $64 question.

Next step I think would be direct fused suplly to the pump wire
so you know that works. Then the more complicated relay end.
Although it would make sense for the aux relay to be near the relay
that you were showing, I noticed it has the same panel as the 2500burb.
On that truck there are two more fuse boxes: matching one in other door,
and one under driver side dash. Doubt the relay would be there though.


Does that have diesel style crossover hoses for that ?
If running a pump in tank, ECM would have to know when it is ok to pump.
Perhaps you just have one gauge that shows average fuel like this one.

Just for fun that burb panel:

1) The vent on second tank is on return line on FSU.interseting thing is the ent on the Primary tank is on the center part of FSU not sure what that is.
2) All lines besides the vent go forward towards engine on primary.The lines return and Fuel from secondary go to overfill hose on Primary tank.
3) The primary was 3/4 full it showed like half between 1/4 tanked used when I disconnected the secondary tank. Second tank is still full it show that it was at half when disconnected the primary tank.
4) Ok both have four wires . Primary tank has Purple wire,Org/blk,grey wire ,black..Second has blue,green,black and org/blck.
5) Tell me which one is the ground and ill make a temp ground to test you know the wire colors are the same as in the earlier wire spec you gave me with the secondary relay.
(IN red) should i use the wire schematic you gave in earlier post? I can make a jumper easy enough.

[Mrcrowley]

Originally Posted by Mrcrowley

1) The vent on second tank is on return line on FSU.interseting thing is the ent on the Primary tank is on the center part of FSU not sure what that is.
2) All lines besides the vent go forward towards engine on primary.The lines return and Fuel from secondary go to overfill hose on Primary tank.
3) The primary was 3/4 full it showed like half between 1/4 tanked used when I disconnected the secondary tank. Second tank is still full it show that it was at half when disconnected the primary tank.
4) Ok both have four wires . Primary tank has Purple wire,Org/blk,grey wire ,black..Second has blue,green,black and org/blck.
5) Tell me which one is the ground and ill make a temp ground to test you know the wire colors are the same as in the earlier wire spec you gave me with the secondary relay.
(IN red) should i use the wire schematic you gave in earlier post? I can make a jumper easy enough.

What about the relay on the outside of the junction box under hood it has no markings and wires colors are different.

[Mrcrowley]

Originally Posted by Mrcrowley

What about the relay on the outside of the junction box under hood it has no markings and wires colors are different.

OK new problem went to start truck no go hooked up code reader and under enhanced reading it says code p1433 fuel lever sensor 2 circuit current to high what is this .........and the gauge will pop to 1/4 tank then to zero in like a few seconds it just turns over which means no fuel