Diagnosing dual tank balance module

[Hydrauk]

Hello Fellas...

I have been really busy with other things. I want to keep this thread somewhat alive. Especially with results after tests. I will do my best to proceed with getting to the bottom of this, over the weekend.

With that being said. Test results and some pictures are comming soon.

Happy Thanksgiving!!

Cheers!
David

 

[GM Guy]

guess i better check in too.

Sorry matt, i haven't been much help. the 98 is down right now, and dad hasn't taken the 2000 anywhere to suck the fuel down, so the tank is sitting full. so until dad does something with the trucks, im not much help.

good luck to you though, i hope you get the old girl de-sassed ):h

 

[Hydrauk]

Here are some results...hope it helps..

Hello fellas! Well Yep..I did some research..here is what I found....

First off I must say...I believe my Dual Tank Module IS WORKING the way it should.

A few weeks back, I noted that when my truck would reach a 1/4 tank of fuel..It would act up. It would occassionally Stall. Upon pulling over...and cranking it for several seconds it would start.

I then noted then the REAR tank was still full, and I could only put fuel in the FRONT tank. I was WRONG!


With the gauge reading 1/2 tank I took measurements:

Checked REAR Tank Ohms: 9.2 Ohms was the reading. (Tank was darn near empty I suppose.)

Checked FRONT Tank Ohms: 43.6 Ohms was the reading.

Went to BP for Fuel:

With Gauge reading 1/2 Tank:

REAR Tank took: 16.675 Gallons.

FRONT Tank took: 13.656 Gallons.

After filling Both tanks up to the top....The Dash Gauge now reads 1/4 PAST the "F" mark.

Checked REAR Tank Ohms: 99.5 Ohms is the reading now. (Tank Full)

Checked FRONT Tank Ohms: 92.1 Ohms is the reading now. (Tank Full)

OK Soooo..It seems that the REAR tank was indeed empty, as it should be first to empty.

In the next post i have some pictures. See what you think.

 

[Hydrauk]

This is the Gauge reading, with truck running, BEFORE I added Fuel.


This is the Gauge reading, with truck running, AFTER I added Fuel.
REAR Tank took: 16.675 Gallons
FRONT Tank took: 13 Gallons.


Needle is Removed from it's Pin on the cluster.


Back shot of Needle, Note..it CAN be Inserted back onto the pin in ANY posistion!!!


I re-Installed the Needle CAREFULLY with the truck running. Remember..I have both tanks full of fuel.
Remember the Gauge HAD a reading of 1/4 PAST the "F" mark.


I replaced this Fuel Gauge a few weeks back. It was indeed Faulty. It would always "PIN" itself WAY over the "F" mark. No matter how much Fuel was in the tanks!!

I think what MAY have happened, after I replaced the Gauge I installed the needle back onto the cluster it did not have a "master spline". So that means the needle can be installed pointing any direction.

This in turn MAY be why everytime my Gauge was around the 1/4 tank mark..I was running out of Fuel? So really instead of having a 1/4 tank, I was actually almost on "E"

It just seemed odd, that after I filled my truck up all they way...the Gauge was now 1/4 PAST "F".

Hence the needle was inserted back on incorrectly by a 1/4.

I dunno...makes sense?? I hope it's not too confusing.

Tell me what ya think....Cheers!!:smile5:

 

[Hydrauk]

By the way...How much Fuel does each tank hold anyway??:mad2:

 

[Matt Bachand]

Front 22, rear 18 on mine anyhow.

Thanks for the input! Where these numbers with the balance module plugged in, or did you read the tank ohms via the harness meaning the module is unplugged?

My readings are way high unplugged, then drop down about 35ohms when the harness is plugged back in. Does yours do the same?

Great input on that fuel gauge needle!

 

[GM Guy]

22 and 18 is right. I do believe that every 96-00 C&C had it this way.

 

[tinman]

I'm glad I found this thread

I just spent 9 hours stranded on the highway after the truck died, and after spending $1000 at one supposedly expert diesel shop for an FSD module and almost another $4000 at a dealer for an injection pump, I luckily decided to just tow the truck home and fiddle with it myself.

Well, low and behold my front tank was empty and I managed to track it down to a bad transfer module, with help from this forum.


I will add my own experience as well, a few days before the truck died my fuel gauge decided to read empty all the time and apparently when that happens the transfer module will shut down.

As a test I pulled my cluster apart and if I moved the needle up to half or so with my finger the transfer pump would start up.

I have no idea how that works, but I know that the gauge itself feeds a signal to the trucks PCM and I'm wondering of when the gauge reads empty if the PCM shuts down power to the transfer system for some reason?

Maybe a safety backup to keep from spilling fuel due to accidents or sending unit malfunctions? Kind of makes diagnosis a little tougher, especially when even the dealers have no idea how the system works.

 

[Turbine Doc]

:welcome5: to the site Tinman and we are so very glad we were able to help you sort out your issue. When you get a breather, update your signature line, just below the main header right side you'll see a button called user CP toggle it and you can edit your signature to include truck info.

You stumbled IMO upon the best 6.5 site on the web, 6.5 guys who have a passion to keep them alive, and you'll soon find out sort of an extended family.

 

[tinman]

Well, I just decided to get creative with my old transfer module. Managed to open it up and solder on a short length of trailer wire to the B,C,D,and F terminals, and ran that to my dash. A few minutes work and now I have both fuel tank senders wired through a 3 way switch and the transfer pump controlled by it's own toggle.
I could have just wired in directly, but I figured if this didn't work, or if at some time I wanted to go back to the transfer module, I can just unplug my mods and plug a new module in.
At least now I can use the tanks separately, I am a total GM guy, but this dual tank system is retarded.

 

[Matt Bachand]

Well, I just decided to get creative with my old transfer module. Managed to open it up and solder on a short length of trailer wire to the B,C,D,and F terminals, and ran that to my dash. A few minutes work and now I have both fuel tank senders wired through a 3 way switch and the transfer pump controlled by it's own toggle.
I could have just wired in directly, but I figured if this didn't work, or if at some time I wanted to go back to the transfer module, I can just unplug my mods and plug a new module in.
At least now I can use the tanks separately, I am a total GM guy, but this dual tank system is retarded.

Welcome! I know there isn't much on the WEB about this issue, and whatever usefulness is out there I tried to bring into this thread.

Good Idea about your Soldering.

Is your fuel gauge still Fubar?

Your backfeeding comment may help explain why one time during my testing I came out to my idling truck and it was constantly overfilling my main tank, the Bal module was pumping while front tank was full. Since that mess on the ground, I have disconnected my balance module, and havn't used my rear tank since.

10,000 lb truck running on a 22 gallon tank without baffles that shows OOF symptoms aroun 18 gallons deep is not fun. I get about a 220-230 range before fuel ups.

I have had factory fuel gauge issues for a long time now, and gave up with it completely and went aftermarket fuel gauge. Still yet to install a toggle and a rear tank guage.

Thanks for your added input here. We are the few, the unproud, the balance module suckers. We must stick together.

 

[tinman]

Not just the WEB, but even the dealers don't have a clue how this works. I thought my fuel gauge was no good, so I bought another cluster from a wrecker, but it turned out to be the transfer module. What got me thinking of rewiring the way I did was that the new cluster came from a single tank truck but the gauge was the same, so I figured at least I would have the front tank working.
The only issue now is the rear sender is flaky, so I am hoping to get away with cleaning it, I don't want to dump another $200 for a sender. The rear tank has a leak on top somewhere, and I'm hoping it is just the seal on the sender and not where the steel ring attaches to the plastic tank.
It sure is nice being able to just flip a switch to see what each tank is holding and be able to choose when to empty the rear.
220-230? I'm assuming that is miles? I get around 400 km on the front tank, but my truck is 15,000lbs.
That's half the reason I didn't like the system the way it was, my deck is 12 feet long and it was hard to reach both fillers without moving the truck.

 

[NVW]

tinman, at the top of the page click on "User CP" then on the left of the page click on "Edit Signature" and fill out your trucks' info. In the future it helps if we know what you have.

 

[Matt Bachand]

Yes Miles. I average around high 12's MPG's, but a fair amount of idling too.

My rear sender was so rusty it took me 1/2 day of PB Blaster and scraping just to break the bond free from the plastic rear tank.

The plastic tank, has built in metal tabs which were also rusted out , I used some permatex RIGHT STUFF gasket maker around the ring, and used what remaining metal tabs werent rotted off and to my surprise it didn't leak at all when I filled the rear tank.

However, changing the rear sending unit didn't effect my defective fuel gauge what so ever....................... At least it needed replacement anyhow.

So I'm assuming the rear tanks being plastic are factory then if yours has one also.

 

[tinman]

Well I just went through hell trying to get my rear sender out, and of course it is fine. So I put it back in and of course the gauge didn't work. Turned out to be a bad connection in the plug, works great now, and I may have even stopped the leak as well.
Somewhere in this thread I think someone wanted to know how to calibrate the fuel gauge? All I did was remove the cluster, pull off the plastic front, put the cluster back in the dash, remove the needle from the gauge, ground the sender wire and replace the needle at the empty mark. Worked fine for me, and saved the $400 fee to send it in and have it done.
I guess there is no worry about these tanks rusting out, just the attachment for the sender, mine is in pretty sad shape. I figure if it gets bad enough I'll just find an aftermarket sender that screws in.

 

[Matt Bachand]

Well I just went through hell trying to get my rear sender out, and of course it is fine. So I put it back in and of course the gauge didn't work. Turned out to be a bad connection in the plug, works great now, and I may have even stopped the leak as well.
Somewhere in this thread I think someone wanted to know how to calibrate the fuel gauge? All I did was remove the cluster, pull off the plastic front, put the cluster back in the dash, remove the needle from the gauge, ground the sender wire and replace the needle at the empty mark. Worked fine for me, and saved the $400 fee to send it in and have it done.
I guess there is no worry about these tanks rusting out, just the attachment for the sender, mine is in pretty sad shape. I figure if it gets bad enough I'll just find an aftermarket sender that screws in.

So a bad connection in the rear tank sending unit harness was the fault of your fuel gauge problems?

 

[tinman]

Not totally, the transfer module was fried as well. The hard part was that the gauge was stuck on empty and when I tested the module for some reason it was sending that signal to the gauge. Usually in the past with any other vehicle I have had the problem is too much resistance, not too little.
I've only had this truck for a couple of months and these problems have just started, it all worked fine when I bought it.
Just for clarification, I read in a previous post someone thought that the gauge only gets a signal from the front sender, but I know for certain that both senders are averaged and that signal goes to the dash gauge. When I first bought this truck and brought it home I only used the front tank and the gauge never went above 1/2 until I filled the rear as well.
At least this new system uses the same gauge for dual and single tanks, my old Topkick ran through both senders so it went from 0-180 ohms which made it impossible to find a propane sending unit that would work properly.

 

[Matt Bachand]

Very interesting.

Did you replace the balance module as well then?

 

[tinman]

No I was going to but decided to try and rewire the system myself. I used the old balance module as a connector for my new wiring in case my mods didn't work and I had to go back to the old way.
Everything works fine now. Only one small problem of the fuel gauge needle moving around a bit while driving due to the sloshing in the tank, but I can live with that.
I have been playing with the idea of just changing the tank lines and using the transfer pump as a lift pump for the second tank. The lift pump and transfer pump are the same part # so they are identical. I kind of like the idea of having 2 separate tanks and lift pumps for backup, if one were to die, just switch tanks and run off that one until the first gets replaced.

 

[Matt Bachand]

They sure are the same pumps. You would need some sort of checkvalve to stop the fuel from going into the other tank? I like that idea too. Apparently a switch from the older 6.2's can do this for you, and show the appropriate fuel level in each tank that is running at that time.

I'm just going to wire the rear xfer tank to a toggle and let it fill the front tank on command.

The fuel gauge needle moving around is as normal as sunlight for all the GM's I've ever owned. That and it holds strong at full for a while, then goes down to under 1/2 tank to empty alot faster than full to half.

What always baffled me is the OHM reading on the tanks changes when the key is on vs off. This must have something to do with the PCM/Balance module reporting the 2nd tank's value instead of the raw reading of the signal. It's been a while since I've been under there with an ohm meter.... Hopefully I never will again. That got old fast.