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Diagnosing dual tank balance module

How much does the ohm reading change? Most likely when the key is on the module is activated and both tanks are connected as one circuit, when it's off they are separated again.
I have my rear tank on a toggle now and it works great that way, I have it hooked through the factory wiring though so if the engine isn't running then it will only run for a short time. I figure that is safer than someone forgetting the switch is on and having it run until the pump burns out.
 
I'm wrong, its when the balance module is plugged in it drops by about 35ohms, this is measuring the senders wires themselves. When I unplug the balance module the senders readings raise by about 35.

This is measuring the senders wires directly, but somereason when bal module is plugged into harness the readings lower even on both empty tanks.

Re-read pages 2 and 3 of this thread I go into it a bit more in detail.
 
My '99 GMC C3500 C&C DRW utility truck has had similar gauge issues since I bought it two years ago. If I fill both tanks the gauge will read about 1/4" past the full mark, and doesn't hit exactly on full until about 130-140 miles later, at which point the tanks will take about 12 gallons of fuel.

I've also had an intermittent problem with the gauge sometimes dropping to "E" for no apparent reason, and not going back up until the truck is shut down and restarted, and not always then. Mostly this happens when both tanks are full, which lead me to believe that the problem is likely that the sending units are misadjusted (causing the high gauge reading) and that after driving a bit, with the fuel sloshing around, it eventually causes the balance module to either shut down, or the signal from the balance module to the PCM is so far off the scale that the PCM itself shuts down the balance module. That sorta makes sense to me, but I would think that something like that would set a code, and I've never had any code set on the truck so this is strictly speculation on my part.

That latter problem, with the gauge dropping to "E", never really worried me because the gauge was so inaccurate that I just got into the habit of using the trip odometer and fueling up every 300 miles or so, but this past Sunday it became a major PITA and moved to the #1 slot on my "Things To Fix" list.

My wife is currently out of state, working on her master's degree at the Naval Postgraduate School in West Virginia. Since it's only about 150 miles from home I drove up there Sunday to visit and have dinner with her. I had topped the truck off the day before leaving, and shortly after getting on the road the gauge went to "E" and stayed there he whole trip up. With about 37 (I'd done some running around town the day before) gallons in the tanks I didn't even really think about it, just ignored it and kept on driving.

Coming back though it kind of surprised me that the gauge was still on "E". I'd burned off enough fuel on the way up that it should have started working again, but since the trip odometer was showing only 190 miles I again didn't give it much thought, knowing that the truck has a total range on full tanks of about 430 miles.

About 35 miles from home, in the dead of night, I suddenly started getting some bucking and sputtering, and then it just quit on me. Fortunately, I was able to coast it into a gas station that was closed and proceeded to try and diagnose the problem. I'd had a PMD failure on the truck the previous winter, and the symptoms were very similar, so I knew it was a fuel issue. The trip meter was sitting right at about 270 miles, and a little thought at the back of my brain started telling me I had an empty tank and fuel wasn't getting transferred, but I went and checked the PMD connections first, just in case. They were fine, so the next step was to check the filter for a blockage. When I cracked the bleeder open, and got just a little puff of air, that little voice started sounding a lot louder. Sure enough, the filter housing was almost bone dry.

Still believing that the whole problem was a stuck float that was causing the PCM to shut down the balance module I got out my tire club and crawled underneath to give the tanks a couple of whacks. The truck rides rough when empty, so I knew it was probably futile, if the bumps and bounces of driving hadn't jarred the floats down then banging on the tanks with a stick wasn't gonna have much effect, but it felt good to whack something at that point. Nope, no luck. The gauge was still on "E" and would crank but not start, but I had confirmed that the front tank was empty, and the rear was pretty full.

Letting it sit for awhile while I took a walk around and had a smoke seemed like a reasonable idea at that point so that's what I did. Getting back to the truck I decided to give it one more try, then see if I could figure out a way to manually transfer fuel to the front tank. Lo' and behold, when I cranked it the gauge came up to just under 1/2 and a couple seconds later it fired right up. I knew I was only about 3 miles from the next place I could get diesel that time of night, so I wasted no time getting it in gear and back on the road, but as soon as I got out of the driveway the gauge went back to "E" again, and less than a half mile later it died again. By then I was up to 55mph though, and had just topped a hill, so I put it in neutral and tried cycling the key a couple of times to see if the gauge came back up. No luck with that, but when I coasted to a stop on the shoulder at the bottom of the hill I was directly across the highway from a 24hr truck stop with diesel. Perfect! I keep a 5 gallon fuel can (empty) in one of the side boxes on the truck, so I figured I'd just walk across with the can, get a few gallons to put in the front tank, start it up and then pull a U-turn down the road and pull in to fill up. The first problem with that idea was that, even though it was pretty late at night, this four lane road (two each direction) was pretty heavily traveled. Even with the truck sitting there lit up like God's own Christmas tree (it's equipped with front and rear light bars, arrowstick, and multiple high intensity strobes), and me in my self-illuminating Class III safety vest, crossing the road was going to be hazardous, especially coming back and lugging the full can. I caught a break in traffic though, so I crossed to the divider, where I found my next hurdle... a very deep ditch running down the center of the median strip, with steep banks on both sides. I'd done some serious damage to my left knee a couple of years back, and there was no way I could cross that ditch... back to the truck and move on to idea #2.

Idea #2 was to get out the siphon I keep in the truck, siphon fuel into the can then pour it into the front tank. Which probably would have worked quite well, if I'd actually returned the siphon to the truck after using it to transfer kerosene into my garage heater last winter. It was still hanging up on the hook in the garage at home. Okay, what hose on the truck can I remove, use as a siphon, then reinstall? Couldn't find a single one that would work. Time for plan #3.

Plan #3 was me trying to attract the attention of one of the truckers sitting across the highway in the truck stop. Now, this plan should have been darn near foolproof. The truck had enough lights to be seen by the International Space Station up on orbit, I had on the previously mentioned Class III illuminated safety vest, and the truck is equipped with both a very good CB and a 2 meter ham radio, and I had another ham radio (2 meter and 70cm) on my belt. Nope, no luck at all. No one saw me waving that I could tell, the CB was dead quiet on both 19 and 9, and the only station I could reach with the ham gear was two states away. Sigh...

So, moving on to plan #4, I got out my cell phone and attempted to call AAA. I know they will deliver gasoline but not diesel, but I figured that whoever they sent out would have a siphon hose, or at least be willing to give me a lift to the other side of the road and back with my can. And, of course, I was in a cell dead spot. No bars, not even a flicker of a bar. Time for plan #5...

Plan #5 was real simple... Get in the truck, cycle the ignition a few times, try cranking a bit, and pray real hard.

Plan #5 also actually worked! After about the fifth or sixth cycling of the key the gauge came up to just below 1/2 again, and I was able to start the truck a few moments later. I let it sit there and idle a bit, with the gauge still reading 1/2, then got it back on the road and over to the station to fill up. $70 later, the front tank had taken 18.5 gallons, and the rear had taken about 4 gallons. The gauge was back up to 1/4" past "F" and it stayed right there the rest of the way home. Indeed, the gauge has not dropped to "E" since then.

The next day, Monday, the very first thing I did was go out and buy a siphon hose for the truck that's actually long enough to allow me to siphon directly from the rear tank to the front, or just to the can if that doesn't work. That night I also printed out all the fuel system wiring diagrams for the truck to see if I could figure out the failure point (ignition switch, connector, etc.) and, while that let me eliminate a few things, like the ignition switch (same terminal on the switch also controls the turn signals, etc, which work fine) the only two points of failure I can see are the common ground point and the PCM. At that point I also did a Google search for more info, which is how I found this this thread and this forum, so I signed up and here I am.

Now, reading through this thread has given me some diagnostic procedures to try, and I have a good idea of what others have found and tried, so that will save me some time and trouble as I continue to troubleshoot this. And, of course, I plan to share my results with the rest of you, especially if I do locate the actual cause. One thing I especially want to look at is the data signals on that line from the balance module to the PCM. It's supposed to be a variable amplitude square wave, so I plan to borrow a scope from a ham buddy in the near future and take a close look at that signal to see if it really is a square wave, or if some component might be failing and causing the wave to change shape, possibly to a sine wave, thus "confusing" the PCM, or see if anything might be causing interference or noise on that signal.
 
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You can just hook the sending unit directly to the dash gauge, I did that and it works just fine, although the dealership said it wouldn't. I used a two position toggle to choose which tank to read from and put my transfer module on a toggle as well, way cheaper than a new balance module, and it works better too.
 
Ah, but I'm one of those oddball characters who likes things to work the way they were intended to work, so I plan to dig prety deep into this problem and determine exactly what is happening, and find a way to get it back to working properly.
 
I know what you mean, I just decided to change mine because I thought the way the stock system worked was kind of retarded. I hated having to top off both tanks every fill up and when the balance module crapped out I end up being stranded somewhere not realizing that I was out of fuel due to the module averaging both tanks.
 
Bernie said:
Ah, but I'm one of those oddball characters who likes things to work the way they were intended to work, so I plan to dig prety deep into this problem and determine exactly what is happening, and find a way to get it back to working properly.
Click to expand...

Hey Bernie, first off, Welcome to "TheTruckStop"...(aka DTR = DieselTowingResourse)...Secondly, you very much remind me of one of our alumni members (who is currently in seclusion, working on a top secret projet for DARPA and also the secret for turning water into Diesel fuel, amongst other things) that has been MIA for the better part of a year, he is missed, to say the least.

You wouldn't happen to know a fellow in Texas by the name of J.D.......would ya' :eek: :rolleyes5: :)

I'm sure there is a member here by the name of Matt Bachand who would be very interested in your quest for the "correct" operation in the repair of the "Dual tank balance module"

Anywho, by the description and sarcasm of your previous post....I'd say your gonna' fit in just fine round' here.....Welcome home!:thumbsup:
 
Brooklyn Tow said:
Hey Bernie, first off, Welcome to "TheTruckStop"...(aka DTR = DieselTowingResourse)...Secondly, you very much remind me of one of our alumni members (who is currently in seclusion, working on a top secret projet for DARPA and also the secret for turning water into Diesel fuel, amongst other things) that has been MIA for the better part of a year, he is missed, to say the least.

You wouldn't happen to know a fellow in Texas by the name of J.D.......would ya' :eek: :rolleyes5: :)

I'm sure there is a member here by the name of Matt Bachand who would be very interested in your quest for the "correct" operation in the repair of the "Dual tank balance module"

Anywho, by the description and sarcasm of your previous post....I'd say your gonna' fit in just fine round' here.....Welcome home!:thumbsup:
Click to expand...

Yeah, when a new member shows up wearing a squirrel suit you kinda "know" he's going to be a bit of a character. ;)

And, while I know quite a few Texans, even a couple with those initials, I've never been to the state myself, though I was born and raised in Brooklyn, and lived there until 1999. You and I might even have some mutual friends or business associates, as my last job in NYC was managing a 3 bay shop in Williamsburg.

I'd be interested in that water to diesel project though. I seem to have managed to do it the other direction, based on what I've had to drain out once or twice, but to go the water to diesel route I suppose I'd have to fill the tanks with water and I don't feel like trying that yet. :)

On the subject of the dual tank balance module and transfer pump issues, I have a ham radio buddy here who works for the county down in Richmond. He's asking around in the shop down there to see if any of his fellow mechanics have any info on this problem, and also checking with the dealers the county buys their trucks from to see what info they might have. Their trucks are all single tank though, so he doesn't know if he'll find out anything.
 
Not so far. I was a bit out of it this past week due to a case of the flu and two abscessed teeth that had to be pulled Thursday. Trust me when I say I was in no shape to be crawling around under the truck.
 
This post has just solved a problem I have had with my truck since day 1.

The truck I own started life as a cab and chasis which was turned into a tow truck.
Now it is a general purpose trailer towing rig.
Ever since I have owned it the fuel gauge would only read for about 10 seconds at start up and then go to empty until the next key cycle,and it would never read above 1/2 a tank even when the tank was full.
After comming across this thread I went out and identified an unknown harness that was laid across the crossmember,like you would imagine it was for the second fuel tank that was removed to install the wheel lift when it was a tow truck.

I jumped the sending unit plug with a variable resistor and low and behold the gauge works perfectly.

The next step will be to test the fuel pump circuit and if the module is working well I hope to find a 90 ohm sending unit that will fit my 50 gallon saddle tanks and let it control my transfer pump that is currently run by a toggle switch.
Otherwise I will just bypass the module and connect directly to the sending unit and add an auxiliary gauge and sending unit to the saddle tanks and leave the toggle as is.
Thanks for all the information contained here,this has been driving me nuts for about 5 years and the only information I have gotten before this thread was to diagnose the wiring problem the tryuck must have.
 
This is a very long thread. I have a 1997 Diesel with dual tanks. The fuel gauge is sitting straight down - it is interferring with the oil pressure guage. It "flutters" - I have only been filling the front tank. What should i do?
 
Good gawd, I'd completely forgotten about this thread! Well, just to update y'all, I never did find a solution to my problem, and I tried everything I could think of, including looking at the signal going to the PCM with a scope. I also found out that this has been a problem on this truck since day one, and that the GM dealer it was bought from replaced the entire fuel system, from tanks to injector pump... twice. And that includes the wiring harnesses too. About the only thing I can think at this point is that it's an oddity in the PCM, and I don't have pockets deep enough to replace that these days, not for a minor problem like this.

As "insurance" though, I've taken to keeping a full 5 gallon can of diesel in one of the side boxes, along with a siphon. So far I've never had to use either.
 
Racer 55: ak diesel driver directed me to this thread. My 2000 K3500 is the exact same scenario as your truck. i.e. began life as a cab & chassis. My trouble began after I lost my AUX tank sender to rust and as a "band-aid" (till I could get a new one) disconnected the AUX tank from the circuit (in much the same way as your AUX tank was removed to make way for a wheel-lift)

My gauge DID work, however, after the utility body was removed. It began acting like yours a year later after I un-plugged the sender. I received my new sender and installed it but the gauge is doing the "10-second dive" thing also.

GM Guy suggested I disconnect the batteries to re-boot the brain, which I'm going out to the garage to do right now.
 
I would also use some electrical cleaner on the contacts in the plug.

Sounds like a ground may have been disturbed as well-forget if the sending unit harness or balance module controls that?
 
Hi I'm new here to the site. I'm a GM Tech who use to work for the dealership but I'm on my own now. I am at a shop We have a 1998 C3500HD 6.5TD Auto with dual tanks It's a Repo body tow truck. We have had many problems over the years with this truck. Right now and has been for a wile the truck is doing the same thing with the fuel gauge when you start it up it will go to half take for a minute then drop down to "E" every time you start the truck no matter how filled the tanks are. We have replaced both sending units they are not GM but I believe Spectra brand both lift pumps work we got a new updated balance module from the dealer changed out the relay above the module I checked connections below the hydro boost swapped out gauge clusters. I do not drive this truck everyday but I'm the one trying to solve this problem. Yes it's run out of fuel plenty of times. We are not looking to add switches or modify it just trying to get it to work like it should. It use to lol. I know the Gas trucks very well 5.7L no problem but this is a different story I also own my own Tech 2 GM scanner.
 
I would say you are lacking a connection-porobably ground for one of the sending units.

If you have a variable resistor or even a static resistor between 10 and 90 ohms you could put in the circuit in place of one sending unit at a time to discover which one is the problem.

When you install say a 50 ohm resistor in place of a sending unit and the gauge gives a constant reading-then taht is the sending unit with a connection problem.
Or if you put it in place and the aguge only reads 1/4 of a tank for 10 seconds and drops you know it's the other sending unit causing trouble.
 
That's good advice. I know for a fact the ground is perfect on the rear sending unit Have to check the one on the front. But I have gone over all the connections on the truck inside and out and used dielectric grease on all connections through out the truck. I also have at work a old school GM dealer tool that is used to check fuel gauge operation in old cars where you hook the tester to back then the one wire at the sender and ground and has a dial on it to change the resistance to make sure the gauge is working and wiring. I will hook that in line as the reading going to the balance module to see if it corrects the gauge on the dash I been thinking it's one of the aftermarket sending units giving a faulty reading causing this. I will test this when I get the truck in will be on the lift soon we have another rebuilt 4L80E to install as this one the reverse is almost gone. Thank you for your input racer55 How did you make out with your gauge problem? I read your post you said your truck was also a wrecker and they removed the rear fuel tank did you or someone before you install another tank when the body was removed?
 
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