Testing fuel sending unit on Duel tank?

[Mrcrowley]

Not even funny by the time I am done with this truck I will have to of plowed 6 driveways to equel time and money I have put into it ........@ ME BUT have to fix it first . Oh and today tried to start it no go even when messing with plug l
did buy one plug 36 bucks but it needs new one it's pin inserts are done . I am just surprised no one has same truck or a Chevy tech who would know where that second relay would be . Do you think the relay would be IN wire bundle ? That would suck so bad if it were .

[crabtruck]

Yeah, you have 3 weeks of this so far that I know of...

No, I don't think the relay should be in the bundle/loom, too hot.
Doesn't mean they didn't do it, just not very likely.
It's a long shot, but a quick check, did you look on right frame rail ?
Maybe refit could have left relay there, think you looked before.

chevy parts department and they looked and my vin and it says
there is only one relay for both tanks so it must feed evenly

Another reason to start loom opening at rear, off chance they're right.
In which case you would find the splice rapidly.
I'd still recommend the approach from ten posts back. A quick inspection
of the back of fuse block before tearing in loom.
I attached a "scribble sheet" that might help while de-looming. Maybe print
that and http://www.truckforum.org/forums/att...l-wires-lg.jpg
Scribble sheet does not include main tank pressure sender.

Another odd thought, you may find it useful to re-read this thread from beginning
before attacking search. Might act as focusing, might make you nuts,IDK.

There are couple techs around here but it appears you're modified.
Can't expect those guys to be psychic even if sometimes seems they are.
Looks like you are stuck with loose cannons like me.

[leolkfrm]

have you tried hooking the back unit to the front power source? to see if they will both make the truck run?

[Mrcrowley]

Originally Posted by leolkfrm

have you tried hooking the back unit to the front power source? to see if they will both make the truck run?

Well the primary tank is what makes the truck run it is directly hooked up to the fuel rail the second tank feeds the primary via feeds lines into the overflow hose on side of make fill hose. Still what is strange I tested my VOM meter and it works fine I tested both plugs for power and the only power is on the sensor postive wire of 5.88 something volts yet primary tank still runs the truck when plugged in .

[Mrcrowley]

Hey crabtruck I have a interesting thought what if unplugging the tank tells the ECM it is empty and tells it not to turn pump on ? Thus showing no voltage when testing it...That would explain alot...your thoughts ..AND if so how would I go about tricking the ECM into giving me a reading.. I only say this because I tested with plug out of main tank no voltage yet when I plug it back in the trucks runs ..that would also explain why second tank is not working if the FSU level is broken saying it is empty or just confusing the ECM that is is empty . What we need is a way to fool the ECM if possible that would eliminate having to look for the relay if power comes out to the second tank. AND yes I have pullled the wire bundle away from rail and getting ready to split it ..

[crabtruck]

No.

Because I think some points may be going unsaid or getting lost in shuffle
let me re-hash a little bit of the elementals:

You should never have a mid range voltage at pump connections: 12/0.
A good ground for meter is an ongoing battle unless using an ext to batt.
PCM never knows if the pumps are on.
PCM may detect pressure in main tank if you have 2or3 more wires than basic 4.
Unless PCM thinks tank is empty, it will try to run pump at key on.
PCM will discontinue pumps if engine not running soon.
Wiring for main tank relay is normal pattern, hot switched on by hot PCM signal.
Wiring for Aux tank relay is different, PCM signaling is on the ground side.
Drawing has both pumps grounds sharing a splice at frame.
Drawing has both sender returns spliced before going to PCM.
Drawing has an active main relay supplying both the main pump and
Aux tank relay signal.

When you unplug the main tank three things happen:
You deprive the pump of power, dprive the PCM of reasonable sender info,
and you move the harness possibly changing the integrity.
If you are slow in getting to the plug to take measurement it might be gone.
PCM may attempt to run pump for as little as the first two seconds.
A key turning helper or an extension for meter would be a help,
maybe a test light so you can see it from key.

Presuming the main to work and the Aux to fail:
With truck running on main tank there should be a hot at two points on Aux relay.
One is from fuse and other is from main tank relay when it's active (eng running).
Third connection is to Aux pump hot wire. Fourth is PCM negative signal.
The logic flow for pump signals would then be: Key on,engine starts from main tank;
power is then supplied to pos signal of Aux relay by load side of main relay:
PCM checks main tank for space for more fuel via main sender level:
PCM then checks Aux tank for fuel available to pump via Aux sender;
with space available, PCM sends negative signal to Aux relay;
Aux relay gfoes active and supplies hot to Aux pump.
When either main is full or Aux is empty, PCM disables Aux relay.

Keep an eye on dash fuel levels throughout these next items.

Try this #1: remove Aux plug and start truck,look for a hot on LtGrn
which is for pump. If not, connect a 50 ohm 5w ceramic resistor
across the sender pair and check again. This gives the PCM a definite
signal to work with - not hanging open or empty.

Try this #2: Make connections with key out: bridge the main sender wires
with a second 50ohm 5w ceramic resistor back from tank. Keep plugged in.
This should tell PCM tank is about half empty and needs Aux fuel.
Repeat Try this #1 with this configuration.

Try this #3: This is brute force and if done badly may hurt PCM.
If you are GOOD with connector pins, disengage pin #3 at the PCM,
you may find it easier to cut the yellow wire back 6 or more inches
so you have room to splice it back together later. Either way you are
looking for seperation from the PCM yellow wire. Do not short that wire.
Turn key on, start truck, and look for 12v on the disconnected wire.
If you have 12v, jump it through a 50 ohm to ground and check voltage.
If free end was originally 12v it should now be about 6v.
If it was 12v and is now 6v, short to ground without resistor.
This should activate Aux relay and be giving 12v to Aux pump plug.
You may need to leave resistor on Aux sender as in Try this#1 above,
but I really doubt it, should work immediately. Be very careful in this area.

One other comment on voltage readings. Depending on the chemical make up
of your biology, you can get some strange readings off yourself.
Those 5-7v readings you get bother me - sounds like bad connections.
Doubt your getting tri-state outputs from PCM, definatley not from relays or batt.

This would be sooo much easier to show you in person,
course then there would be two of us to get it done.

[Mrcrowley]

Originally Posted by crabtruck

No.

Because I think some points may be going unsaid or getting lost in shuffle
let me re-hash a little bit of the elementals:

You should never have a mid range voltage at pump connections: 12/0.
A good ground for meter is an ongoing battle unless using an ext to batt.
PCM never knows if the pumps are on.
PCM may detect pressure in main tank if you have 2or3 more wires than basic 4.
Unless PCM thinks tank is empty, it will try to run pump at key on.
PCM will discontinue pumps if engine not running soon.
Wiring for main tank relay is normal pattern, hot switched on by hot PCM signal.
Wiring for Aux tank relay is different, PCM signaling is on the ground side.
Drawing has both pumps grounds sharing a splice at frame.
Drawing has both sender returns spliced before going to PCM.
Drawing has an active main relay supplying both the main pump and
Aux tank relay signal.

When you unplug the main tank three things happen:
You deprive the pump of power, dprive the PCM of reasonable sender info,
and you move the harness possibly changing the integrity.
If you are slow in getting to the plug to take measurement it might be gone.
PCM may attempt to run pump for as little as the first two seconds.
A key turning helper or an extension for meter would be a help,
maybe a test light so you can see it from key.

Presuming the main to work and the Aux to fail:
With truck running on main tank there should be a hot at two points on Aux relay.
One is from fuse and other is from main tank relay when it's active (eng running).
Third connection is to Aux pump hot wire. Fourth is PCM negative signal.
The logic flow for pump signals would then be: Key on,engine starts from main tank;
power is then supplied to pos signal of Aux relay by load side of main relay:
PCM checks main tank for space for more fuel via main sender level:
PCM then checks Aux tank for fuel available to pump via Aux sender;
with space available, PCM sends negative signal to Aux relay;
Aux relay gfoes active and supplies hot to Aux pump.
When either main is full or Aux is empty, PCM disables Aux relay.

Keep an eye on dash fuel levels throughout these next items.

Try this #1: remove Aux plug and start truck,look for a hot on LtGrn
which is for pump. If not, connect a 50 ohm 5w ceramic resistor
across the sender pair and check again. This gives the PCM a definite
signal to work with - not hanging open or empty.

Try this #2: Make connections with key out: bridge the main sender wires
with a second 50ohm 5w ceramic resistor back from tank. Keep plugged in.
This should tell PCM tank is about half empty and needs Aux fuel.
Repeat Try this #1 with this configuration.

Try this #3: This is brute force and if done badly may hurt PCM.
If you are GOOD with connector pins, disengage pin #3 at the PCM,
you may find it easier to cut the yellow wire back 6 or more inches
so you have room to splice it back together later. Either way you are
looking for seperation from the PCM yellow wire. Do not short that wire.
Turn key on, start truck, and look for 12v on the disconnected wire.
If you have 12v, jump it through a 50 ohm to ground and check voltage.
If free end was originally 12v it should now be about 6v.
If it was 12v and is now 6v, short to ground without resistor.
This should activate Aux relay and be giving 12v to Aux pump plug.
You may need to leave resistor on Aux sender as in Try this#1 above,
but I really doubt it, should work immediately. Be very careful in this area.

One other comment on voltage readings. Depending on the chemical make up
of your biology, you can get some strange readings off yourself.
Those 5-7v readings you get bother me - sounds like bad connections.
Doubt your getting tri-state outputs from PCM, definatley not from relays or batt.

This would be sooo much easier to show you in person,
course then there would be two of us to get it done.

One suggestion from friend who came over and help me out was . Take second tank FS assembly out (I have fuel line removal tool) plug it into primary tank plug while out of tank if unit works while moving float up or if fuel guage moves inside cab the Unit should be good if not maybe FSU is bad.? I still get sickly high OHM readings from both tanks even with new leads and I used new plug I got so I got good reading. I am going to do what you suggested with resistance resistor ill go to radio shack today.

[Mrcrowley]

Originally Posted by crabtruck

No.

Because I think some points may be going unsaid or getting lost in shuffle
let me re-hash a little bit of the elementals:

You should never have a mid range voltage at pump connections: 12/0.
A good ground for meter is an ongoing battle unless using an ext to batt.
PCM never knows if the pumps are on.
PCM may detect pressure in main tank if you have 2or3 more wires than basic 4.
Unless PCM thinks tank is empty, it will try to run pump at key on.
PCM will discontinue pumps if engine not running soon.
Wiring for main tank relay is normal pattern, hot switched on by hot PCM signal.
Wiring for Aux tank relay is different, PCM signaling is on the ground side.
Drawing has both pumps grounds sharing a splice at frame.
Drawing has both sender returns spliced before going to PCM.
Drawing has an active main relay supplying both the main pump and
Aux tank relay signal.

When you unplug the main tank three things happen:
You deprive the pump of power, dprive the PCM of reasonable sender info,
and you move the harness possibly changing the integrity.
If you are slow in getting to the plug to take measurement it might be gone.
PCM may attempt to run pump for as little as the first two seconds.
A key turning helper or an extension for meter would be a help,
maybe a test light so you can see it from key.

Presuming the main to work and the Aux to fail:
With truck running on main tank there should be a hot at two points on Aux relay.
One is from fuse and other is from main tank relay when it's active (eng running).
Third connection is to Aux pump hot wire. Fourth is PCM negative signal.
The logic flow for pump signals would then be: Key on,engine starts from main tank;
power is then supplied to pos signal of Aux relay by load side of main relay:
PCM checks main tank for space for more fuel via main sender level:
PCM then checks Aux tank for fuel available to pump via Aux sender;
with space available, PCM sends negative signal to Aux relay;
Aux relay gfoes active and supplies hot to Aux pump.
When either main is full or Aux is empty, PCM disables Aux relay.

Keep an eye on dash fuel levels throughout these next items.

Try this #1: remove Aux plug and start truck,look for a hot on LtGrn
which is for pump. If not, connect a 50 ohm 5w ceramic resistor
across the sender pair and check again. This gives the PCM a definite
signal to work with - not hanging open or empty.

Try this #2: Make connections with key out: bridge the main sender wires
with a second 50ohm 5w ceramic resistor back from tank. Keep plugged in.
This should tell PCM tank is about half empty and needs Aux fuel.
Repeat Try this #1 with this configuration.

Try this #3: This is brute force and if done badly may hurt PCM.
If you are GOOD with connector pins, disengage pin #3 at the PCM,
you may find it easier to cut the yellow wire back 6 or more inches
so you have room to splice it back together later. Either way you are
looking for seperation from the PCM yellow wire. Do not short that wire.
Turn key on, start truck, and look for 12v on the disconnected wire.
If you have 12v, jump it through a 50 ohm to ground and check voltage.
If free end was originally 12v it should now be about 6v.
If it was 12v and is now 6v, short to ground without resistor.
This should activate Aux relay and be giving 12v to Aux pump plug.
You may need to leave resistor on Aux sender as in Try this#1 above,
but I really doubt it, should work immediately. Be very careful in this area.

One other comment on voltage readings. Depending on the chemical make up
of your biology, you can get some strange readings off yourself.
Those 5-7v readings you get bother me - sounds like bad connections.
Doubt your getting tri-state outputs from PCM, definatley not from relays or batt.

This would be sooo much easier to show you in person,
course then there would be two of us to get it done.

Unless PCM thinks tank is empty, it will try to run pump at key on.

Ok when the plug is disconnected would it be showing tank empty to the PCM?

[crabtruck]

First thing you said:

what if unplugging the tank tells the ECM it is empty

I said:

Originally Posted by crabtruck
No.

Then you said:

Ok when the plug is disconnected would it be showing tank empty to the PCM?

Dude, you need coffee.
No, open sender looks like full or broken to PCM, short looks like empty.

Now that I had to read all the junk twice (in case I said something different)
I'm going to go take a nap. And when I wake, I hope you have gotten over
Massive Quoting - I don't want to read all that anymore than anyone else does.

[Mrcrowley]

BUT>>>BUT>>> are we there yet...
I think the rest of my post should be, Is there anyway I can confuse myself and anyone else. If I spent half the time working on what you suggested and less time double questioning what you suggested I would have it fixed by now. Hey one more question 2+2= 4 rite .............LOL . I will not post again till I find something new.....padawan out.....