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Diagnosing dual tank transfer problems

L

leather guy

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hey guys new guy here I'm working on a 2004 5500 and duel tank xfer problem but thats not the hole problem thinking xfer problem is the balance mod called dealer they needed vin # they say the mod is in tank but what i see for wires coming out of tank tells me it the same as you guys are talking about. two wires going into tank light blue and black..so I'm sure it is the one you guys are talking about but the primary pump and balance mod and relay are no wheres to be found any ideas
 
Depends on he truck. GM stopped using the balnce module with the gmt-400 trucks, and went to ECm controls for the dual tank setup with the new body style. So your 2004 will NOT have a factory balance module. Where it gets trick is if it was upfitted by another company. Many of them DID use a balance module to run a transfer pump to move tank from the auxilary to the drivers side tank, and control the fuel level reading to the ECM. So on a MD truck, all bets are off. What all are the details on the truck in question? Once I know all the details, I will be moving this to it's own forum as your 04 shares nothing in common with the trucks that had a balance module in them.
 
wow thanks thefermanator well that makes me all warm and fuzzy.ok well deal is he has had this problem for a while and every time he is on the road with a 40f horse trailer it breaks and he is on the side of road.last time they changed his ecm fixed it before that they changed a sending unit in front tank. so if it only has two wires going into the tanks does it have a pump and a sending unit my 17 years with an aerospace eng. deg says it can't unless there is another wire or the hole unit in take has a ground strap lol.it nuts i was just heading out door to go fix dam thing thinking i had a game plan. ok so seeing that we can't find the balance mod anywhere comp must be controlling it
 
The transfer pump is external of the tabk with GM. They don't use intank pumps with diesel. The dual tank system if it's stock is a picky system. If theres a problem with either sender, it's disabled. If fuel runs from the transfer tank to the main tank uncommanded, it's disabled. When you say hes broke down, you mean he runs out of fuel with fuel still in the transfer/auxilary tank?
 
yes, just come in from working on it tuck sending unit out of back tank ohms out 45and 250 that sounds like a gas sending unit so all wires check good but there is no power to pump. we think front is about half full so when i rase the back sending unit higher then half with truck running pump should come on right? now I'm sure the pump voltage has to still come from a relay and not computer what and where would that relay be. and thanks for the help
 
leather guy said:
yes, just come in from working on it tuck sending unit out of back tank ohms out 45and 250 that sounds like a gas sending unit so all wires check good but there is no power to pump. we think front is about half full so when i rase the back sending unit higher then half with truck running pump should come on right? now I'm sure the pump voltage has to still come from a relay and not computer what and where would that relay be. and thanks for the help
Click to expand...
GM went to 39-250 ohm sending units starting in 98 in everything but single tank diesel trucks(dual tanks got the 39-250 ohm senders). You cannot get the system to work like you are doing, the ECM is to smart if it is in fact using the GM setup. Don't use ANYTHING you've read in here, NONE of it applies to the ECM controlled systems.
 
Posts moved from 6.5l balance tank module thread to its own here as it has nothing in common with that system other than 2 tanks and a transfer pump.
 
Leatherguy, welcome to the site.

I owned a truck equipment shop many moons ago. Once you get into medium duty, all bets are off. Once you deal with anything "made" instead of specific from GM (read as dealer option or dealer upgrades) all bets are off. Shop a might have real engineers on staff, and shop b might alter it with a roll of bailing wire.
All you can do is test each component, but double check it is the factory component, not an aftermarket part installed.

It sounds like your chasing the signals and about to test the big electron box from happy land, if so- yes thats your next step.

Trace the harness and ensure the didnt add anything along the way. Pan in the butt, but required. Harry the Hack might have thought you need an inline fuse just because...

Trust your training, you are far more educated than most that would have laid a wrench to it...
 
The system if it's GM's is fairly simple. The engine runs off of the drivers side tank iirc, and the right side tank gets pumped over into the left side tank. Once the level in the left tank reaches roughly 3/4-7/8 of a tank, the ECM will trigger a transfer pump to move fuel from the auxilary tank to the primary tank. It will stop the pump once the level goes above the 7/8 mark roughly. The ECM reads the level sensor from each tank, controls the transfer pump via a relay mounted in the underhood fuse panel(this is how pickups are done, BURB's have an auxilary relay outside the box for it), and it also outputs a fuel level amount over the class 2 j1850 data bus for the cluster to then read and display. If there are any problems in the system, the ECM will default the gauge to empty, and not run the transfer pump. Faults that can disable the system are a level unit that stays when the ECM expects movement, if it detects fuel being transferred when it shouldn't(try filling one with the engine running, the gauge will plummet to empty and stay until you shut it off and restart it), or eratic readings from a level unit.

As far as I know, GM only used left and right saddle tanks. You say rear tank and no mention of the gauge going to empty, sovthis is sounding like an added on system, in which case you're pretty well stuck trying to find where somebody put it, and how all it works.
 
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