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Yukon XL aux. fuel tank operation

56napco

By 56napco
in Ask A GM Technician

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56napco Enthusiast

56napco

Posted
Posted

My 2002 Yukon XL has dual fuel tanks and this is the first truck I've owned with this style of tank setup. I haven't been able to find anything on how these tanks work, there isn't anything in the factory repair manuals. When does the aux. tank pump turn on? Does it continuously fill the primary tank or when the primary tank reaches a specific level? Will the gauge move when the rear pump turns on or does the gauge stay in one spot until the rear tank is empty?

txab Senior Enthusiast

txab

Posted
Posted (edited)

My 2002 Yukon XL has dual fuel tanks and this is the first truck I've owned with this style of tank setup. I haven't been able to find anything on how these tanks work, there isn't anything in the factory repair manuals. When does the aux. tank pump turn on? Does it continuously fill the primary tank or when the primary tank reaches a specific level? Will the gauge move when the rear pump turns on or does the gauge stay in one spot until the rear tank is empty?

 

 

This was answered in your other thread.

 

http://www.gm-trucks...dpost&p=1235387

 

 

On dual tank applications, The secondary fuel pump is located in the secondary fuel tank. The secondary fuel pump is powered by a secondary fuel pump relay when the fuel level drops below a predetermined value. Fuel is transferred from the secondary fuel tank to the primary fuel tank in order to ensure all of the usable fuel volume is available to the primary fuel pump. The secondary fuel pump relay supply voltage is received from the primary fuel pump relay when the primary fuel pump is energized.

 

 

BTW this comes straight from the GM FSM.

Edited by txab
56napco Enthusiast

56napco

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Posted

What is the predetermined level that the primary tank needs to reach before it will turn on the secondary tank pump? Will the secondary pump fill the primary tank fast enough to show on the gauge?

gmtech4 Senior Enthusiast

gmtech4

Posted
Posted (edited)

predetermined means what ever the engineers decided while engineering the system,,,,,

 

you have factory svc manuals ,you need to read the description and operation in the apropiate section to understand the system,,its there ,if not ,let me know,i'll post it for you

Edited by govtech4
56napco Enthusiast

56napco

Posted
  • Author
Posted

Ok, so the manuals say exactly what is already posted. I understand what a predetermined value is... what no one can seem to tell me is what that acutal value is. Is it 1/4 tank, 1/2 tank, empty?

gmtech4 Senior Enthusiast

gmtech4

Posted
Posted (edited)

the answer is no one knows ,!!!

 

from the service manual ....

 

 

 

Secondary Fuel Pump (Dual Tank Applications Only)

 

On dual tank applications, the secondary fuel pump is located in the secondary fuel tank. The secondary fuel pump is powered by a secondary fuel pump relay when the fuel level drops below a predetermined value. Fuel is transferred from the secondary fuel tank to the primary fuel tank in order to ensure all of the usable fuel volume is available to the primary fuel pump. The secondary fuel pump relay supply voltage is received from the primary fuel pump relay when the primary fuel pump is energized.

 

good luck on your search for the answer

Edited by govtech4
txab Senior Enthusiast

txab

Posted
Posted

It's not designed to let the main tank get really low then fill it back up. At the predetermined level the rear tank will begin to fill the main tank, then shut off, until the main tank level drops again. The process begins again. It is designed, as it was explained to me, to keep the main tank at a fairly constant level until the aux tank is empty. Another words you're probably not going to see the gauge drop down and then begin to rise.

 

Why are you needing to know the exact point at which the transfer begins to take place? The systems work great, except in the case of a pump failure

56napco Enthusiast

56napco

Posted
  • Author
Posted

The main reason I wanted to know is so I know my fuel system is working correctly and what to expect. I'm not fully convinced it's working the way it should based on the gauge readings and number of gallons it will take to fill.

gmtech4 Senior Enthusiast

gmtech4

Posted
Posted

im real sure there are tests the system does and when it fails ,it puts on the SES light ,,

 

others can chime in to verify that ,i remember up north a 2 tank system had problems ,cant remember what model

  • 2 weeks later...
testdrive Enthusiast

testdrive

Posted
Posted

I had a dash mounted led light wired so it would turn on when the transfer pump would run. It would first come on about 2/3 tank and run for 5 minutes and then go off. Its time on cycles were between 2/3 and down to 1/2 tank. You never saw the gauge do anything except continue to drop in the normal 8.1L manner. :tear: My rear tank is 11 gallons, front is 26, total of 37. This method is used on all of the two tank Suburbans, Yukon, and medium duty 3500-5500 series trucks.

I studied this in preparation for my transition from the 8.1L to a LBZ Duramax in my 2003 2500 Suburban.

And the system will set a code if the sending units don't report readings properly.

  • 1 year later...
  • 1 year later...
72chevy72 New Member

72chevy72

Posted
Posted

I had a dash mounted led light wired so it would turn on when the transfer pump would run. It would first come on about 2/3 tank and run for 5 minutes and then go off. Its time on cycles were between 2/3 and down to 1/2 tank. You never saw the gauge do anything except continue to drop in the normal 8.1L manner. :tear: My rear tank is 11 gallons, front is 26, total of 37. This method is used on all of the two tank Suburbans, Yukon, and medium duty 3500-5500 series trucks.

I studied this in preparation for my transition from the 8.1L to a LBZ Duramax in my 2003 2500 Suburban.

And the system will set a code if the sending units don't report readings properly.

Old post, but I could use some info on this -

 

I have a similar swap - 2002 Yukon Xl - dual tank - with the LBZ - I have most everything sorted out, but the secondary fuel tank does not transfer to the primary.

 

Quick Question - do you have a wiring diagram for the secondary pump, or do you remember the wire colors for +/- to energize the pump - I will be installing a manual transfer switch.

 

Thanks!

  • Like 1
  • 3 years later...
superdeeduper New Member

superdeeduper

Posted
Posted

so the last post in this thread on 11/19/2014.  did you ever get it figured out? (reviving an old post but this is the problem i'm having.)  i've got an 08 suburban w/LMM.  i'm guessing my secondary tank is not working because my last 2 fill-ups have only taken 24-ish gallons (roughly the size of the primary tank only) AND gas won't run into the filler hole at anything but a painfully slow rate when i'm re-fueling so i'm guessing the secondary tank pump is not activating and the tank is staying full and also preventing normal filling.

  • Like 1
  • 5 weeks later...
swathdiver Senior Enthusiast

swathdiver

Posted
Posted
On 8/17/2018 at 6:43 PM, superdeeduper said:

so the last post in this thread on 11/19/2014.  did you ever get it figured out? (reviving an old post but this is the problem i'm having.)  i've got an 08 suburban w/LMM.  i'm guessing my secondary tank is not working because my last 2 fill-ups have only taken 24-ish gallons (roughly the size of the primary tank only) AND gas won't run into the filler hole at anything but a painfully slow rate when i'm re-fueling so i'm guessing the secondary tank pump is not activating and the tank is staying full and also preventing normal filling.

Is your Suburban on a 1500 or 2500 chassis?  

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