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Diagnosing dual tank balance module

The rear sender seems to be the same for both gas and diesels. The confusion comes in when you compare single tank systems to dual tank systems, and when people get the years mixxed up. From what I can find, up to 96 they were all 0-90 ohm senders(diesels list as 0-95 ohm, but they're the same thing), starting in 97 GM started going to 39-250 ohm senders for gas, but diesels stayed 0-90 ohm for single tank systems. Cab and chassis trucks seemed to use the gasser senders regardless of engine for the rear tank. They all use the electric transfer pump thats frame mounted to move fuel forward. I had helped somebody else who did away with his balance module, but his gauge wouldn't read right afterwards because of the ohms of the sender. Going through the parts books, it's a crap shoot which sender should be in it. I liked the FORD manual switch setup ALOT better myself. GM tried making it to where there was no driver input needed, and just made it confusing as these balance modules were troublesome to say the least.
 
THEFERMANATOR said:
The rear sender seems to be the same for both gas and diesels. The confusion comes in when you compare single tank systems to dual tank systems, and when people get the years mixxed up. From what I can find, up to 96 they were all 0-90 ohm senders(diesels list as 0-95 ohm, but they're the same thing), starting in 97 GM started going to 39-250 ohm senders for gas, but diesels stayed 0-90 ohm for single tank systems. Cab and chassis trucks seemed to use the gasser senders regardless of engine for the rear tank. They all use the electric transfer pump thats frame mounted to move fuel forward. I had helped somebody else who did away with his balance module, but his gauge wouldn't read right afterwards because of the ohms of the sender. Going through the parts books, it's a crap shoot which sender should be in it. I liked the FORD manual switch setup ALOT better myself. GM tried making it to where there was no driver input needed, and just made it confusing as these balance modules were troublesome to say the least.
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While mine is aftermarket I have two fuel gauges and just a transfer pump from mid chassis tank to rear tank....on the Burb.
 
You cant go by what was "normally done" in cab chassis. Customers would buy trucks all the time that had 1 tank, but want their truck to have 2 tanks.
Salesman might tell them "I'll order a 2 tank truck for you" or "we can add the second tank".

The dealership mechanics rarely do it, it is sent to the truck equipment shops like the one I owned. All the time there would be boxes of parts like secondary tank systems and some of the parts are wrong. "Make it work, this guy needs his truck by Thursday" is what they tell us.

You have to pull info from the ohm spec program, (i could just tell the salseman he has to give me that spec or truck wont run). Then put a meter to the wires. P.i.t.a. I know, but you have to.

GM used this abortion across the board. How much do I I like it? When I add my 2nd tank in my Hummer it will get a lift pump between the tanks and a switch on the dash. The aggrivation of problems and the expense of part failures is not worth it imo.

I feel for you.
 
Will L. said:
You cant go by what was "normally done" in cab chassis. Customers would buy trucks all the time that had 1 tank, but want their truck to have 2 tanks.
Salesman might tell them "I'll order a 2 tank truck for you" or "we can add the second tank".

The dealership mechanics rarely do it, it is sent to the truck equipment shops like the one I owned. All the time there would be boxes of parts like secondary tank systems and some of the parts are wrong. "Make it work, this guy needs his truck by Thursday" is what they tell us.

You have to pull info from the ohm spec program, (i could just tell the salseman he has to give me that spec or truck wont run). Then put a meter to the wires. P.i.t.a. I know, but you have to.

GM used this abortion across the board. How much do I I like it? When I add my 2nd tank in my Hummer it will get a lift pump between the tanks and a switch on the dash. The aggrivation of problems and the expense of part failures is not worth it imo.

I feel for you.
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Well, like I said earlier, it's just never annoyed me enough that I had to fix it. The only time it came close was when it stranded me on the side of the road, out of fuel, with a full rear tank. That was because when I fill both tanks and the gauge goes all the way up, it will sometimes just drop to empty and stay there until the truck is restarted. I believe this is caused by the module shutting down because the inputs are way out of range. That also stops the transfer from front to rear, so on a long trip it I took it just ran the front tank dry and it took several off/wait/on cycles before the system kicked back on again and transferred enough fuel to get to a station just down the road. Ever since then I just carry a hand pump and enough hose to go from one tank straight to the other, in case of emergencies but mostly make it a point to stop for a break a couple of hours after refueling, to let the system reset. Never got stuck again after that.
 
Look underneath the truck just in front of the front tank near the fuel pump, and see if you can find the balance module. This will tell you if it's a GM setup or not.
 
Next step is to ohm the senders and see what you got. Unfortunately all it takes is one little bad spot in the sender resistance strip to disable the whole system. May be worth it to do away with the balance module, and hook your transfer pump to a toggle switch, and just transfer the fuel yourself.
 
IF RA catalog is correct, then your 99 C3500HD SHOULD use the standard 0-95 ohm sender in the front tank, and a 0-100 ohm sender in the rear tank. The kicker is theres 2 different dual tank systems that were used, so you have to hope you have the factpory installed parts still in there, otherwise you're replacing senders and balance module to make sure it's all correct. Otherwise, find the sending unit wire coming out of the front tank, splice it to the purple wire coming out of the balance module, and your gauge will read what the front tank is reading. Hook up a keyed toggle switch to your transfer pump, and do the transfer work yourself.
 
THEFERMANATOR said:
IF RA catalog is correct, then your 99 C3500HD SHOULD use the standard 0-95 ohm sender in the front tank, and a 0-100 ohm sender in the rear tank. The kicker is theres 2 different dual tank systems that were used, so you have to hope you have the factpory installed parts still in there, otherwise you're replacing senders and balance module to make sure it's all correct. Otherwise, find the sending unit wire coming out of the front tank, splice it to the purple wire coming out of the balance module, and your gauge will read what the front tank is reading. Hook up a keyed toggle switch to your transfer pump, and do the transfer work yourself.
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Yeah, like I said, it's never been worth the time or trouble to do it, or hire someone to do it since I'm fully disabled now. As far as I know, and according to the GM history on the truck, the entire fuel system was replaced, under warranty, back about 2002 or so. Everything from the IP back was changed, including the sending units and the wiring harness. Since it was done by the dealer, under warranty, I can only assume that OEM parts were used.
 
Matt Bachand said:
Anyone with dual tanks having problems with fuel gauge or the automated balance flow system this may help.

This module is located by the front (main) lift pump. Slightly toward the rear of vehicle along the frame rail.

On top of this module is a fuel pump balance relay, it turns the rear pump on as needed to transfer fuel.

At the module, you have a harness with 6 wires. Unplug it and check the following with a digital volt/ohm meter

Terminal A- Pink wire, should be 12 volts with key in run position. Fuse is in underhood fuse/relay center marked IGN E 10 AMP

Terminal B- Light Blue wire, this is the signal wire from the secondary (rear) fuel tank, using an ohm meter, measure the resistance from a good chassis ground to this wire and should get a reading between 0 and 90 ohms, 0 is empty tank and 90 is full.

Terminal C- Dark Blue with White tracer wire, this is the signal wire from the primary (front) tank, test is same as for terminal B

Terminal D- Light Green wire, this wire controls the Fuel Pump Transfer Relay, with the ENGINE RUNNING and the front lift pump working, GROUND this wire, the Fuel Pump Transfer Relay should click and then you should be able to feel the rear pump working. If the relay clicked and pump is not working, connect a test light across the rear fuel pump harness connector and see if you have power. If lighted, then replace pump, if not lighted replace relay providing you have 12v at Terminal A.

Terminal E- Black with White tracer wire, This is the ground wire for the control module. Using a test light jump from Terminal A to Terminal E with key on, the test light should be lit.
Terminal F- Purple wire, this wire is the output signal to your fuel guage.
Gound this wire it should read empty, leave this wire open, the fuel guage will go pass full.

The fuel pump relay, which is right above the module, has a wiring harness with 4 wires. They are:

Pink Wire is 12 volt from the same source as Terminal A.

Light Green wire (thicker gage) is the feed to the rear fuel pump.

Light Green wire (thin gage) is the control wire from the module as explained in Terminal D.

Gray wire is 12V from the primary pump circuit to energize the Fuel Pump Transfer Relay.
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I currently am working on a 03 Chevy W4500 Tiltmaster. It has the same problem too. I plan on using this info to trace the problem with it. I have been a mechanic for 45+ years and worked on just about every kind of engine out there. From lawn mowers to jet engines and everything in between.
 
Welcome Larry.
It sounds like you had a fun career. You need to come up with the first lawnmower engine powered jet. Maybe a jet engine powered lawnmower. Now that would cut some grass. Ha ha!!!
Welcome to the forum man.
 
LARRY L DAVIS said:
I currently am working on a 03 Chevy W4500 Tiltmaster. It has the same problem too. I plan on using this info to trace the problem with it. I have been a mechanic for 45+ years and worked on just about every kind of engine out there. From lawn mowers to jet engines and everything in between.
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You're working on a COMPLETELY different animal. 1st thing you need to do is find out if the dual tanks are factory or not. Many of those came with just 1 tank, and upfitters added the 2nd tank and used aftermarket balance modules on them. If it is in fact factory dual tanks, then there is no balance module as it will be ECM controlled. You have to identify what you have to know how to progress with it.
 
Alfred Parkinson said:
Sorry 01 Chevy C3500HD. 8.1L gas
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I honestly don't know if that truck uses a balance module or if the PCM handles it. I would suspect a bad/stuck float in one of the tanks causing it to read the fuel level wrong. Does the fuel guage work? Have you tried a new relay for the transfer pump? It may be sticking on. Without knowing which system the truck uses though, I don't know which way to point you other than basic checks.
 
Ok so it is the transfer module style system.

I filled up the other day and it only took 17 gallons. Fuel gauge was doing some funny things. Would sit on empty and if I rocked the truck it came up???

Drove 100 miles and it cut out. I was able to start my generator so I thought I definitely had some fuel in the front. After tracing the lines it feeds off the back tank. Had it towed home.

I dropped the front tank and changed out the sending unit. The truck started right up. After filling up the gauge read between 3/4 and full. On further inspection I found the fuel transfer pump constantly running. I have ordered a fuel pump control module. I switched out the leveling sensor but the problem continued so I returned it. Praying this control module works. Carrying a 5 gallon can on the truck go now.
 
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