Diagnosing dual tank balance module

[racer55]

The truck had saddle tanks installed but I had to add a sending unit and also an automatic transfer pump using the stock wiring.

The only sending unit I could get was a 100 ohm unit and now when the saddle tanks are empty the balance module tries to keep moving fuel from them and the rear LP keeps running.

Someday I will have to bend the float arm so that it maxes out @ 90 ohms and then I think the problem will be solved.

 

[philmathews68]

ok if you get rid of second tank do you still need balance module and can i use an aftermarket fuel and how do i do it

 

[racer55]

ok if you get rid of second tank do you still need balance module and can i use an aftermarket fuel and how do i do it

If you remove the rear tank you would be better off without the balance module.

You will need to identify the LP circuit and sending unit circuit and connect each as required-better off with OEM sending unit or meterum rod.

The diagram looks as though you just unplug the balance module but that seems too easy?

 

[inchman]

I keep blowing this fuse, so I can't even test the system.

Anyone with dual tanks having problems with fuel gauge or the automated balance flow system this may help.
<Stuff Deleted>
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
<More Stuff Deleted>

Matt,
This was an awesome and excellent step by step to figure out the cause of the fuel gauge problem. However, I found this fuse and it keeps blowing on me once I turn the key on. Any ideas where I need to be looking to find this short?

In case it matters, I have a '98 GMC 3500HD mason dump duellie with dual tanks from the factory.

Thanks.

 

[racer55]

It's pretty clear in the above diagram-Ign-e feeds the fuel balance pump(rear pump),fuel balance module(for gauge sending units) and AC compressor relay.

 

[inchman]

It's pretty clear in the above diagram-Ign-e feeds the fuel balance pump(rear pump),fuel balance module(for gauge sending units) and AC compressor relay.

Thanks for the quick reply. Since the AC was removed from this truck years before I had it, I can only assume that this means the rear tanks fuel pump is shot. Is there anything else I should check before I drop the money on a new one?

 

[Bernie]

Simple matter to pull the two relays and see if it still blows. If it does, suspect the balance module or one of the fuel sensors. If it doesn't, plug the relays back in one at a time until it does blow and proceed from there.

 

[Bernie]

Thanks for the quick reply. Since the AC was removed from this truck years before I had it, I can only assume that this means the rear tanks fuel pump is shot. Is there anything else I should check before I drop the money on a new one?

That might be a false assumption to make. Check and see if the A/C relay is still in there. If it is, pull it and then see if the fuse still blows.

And the rear tank doesn't really have it's own pump, per se. There's the balance pump, which just pumps from the rear tank to the front tank, and it's easy to isolate that as the possible cause simply by pulling the balance pump relay.

 

[Bernie]

I also wouldn't drop money on a new pump (if pulling the relay indicates that as the possible problem) until I checked all the wiring to that pump. Wiring can get damaged awful easy sometimes, road debris, corrosion, pinched by something, and if you just need to fix a grounded wire instead of replacing the pump your wallet will thank you.

 

[racer55]

I would also just unplug the rear pump and see if the fuse still blows,maybe unplug the balance module as well-work through it in stages trying to isolate the problem.

 

[inchman]

It's pretty clear in the above diagram-Ign-e feeds the fuel balance pump(rear pump),fuel balance module(for gauge sending units) and AC compressor relay.

You guys are awesome!! That was the problem. It was the fuel balance pump. HOWEVER, down in this neck of the woods (NJ) no autoparts places call it that. I called everywhere and I finally found it called something like an 'auxiliary fuel pump." $70 bucks later and less than an hour and I am back in business. Thanks again!!!

 

[racer55]

The rear transfer pump and main lift pump are identical.

The best stock option is AC Delco from a 93 6.5TD equipped truck.

 

[napadude09]

I have a 99 6.5td 3500hd that the rear sending unit was replaced. At full it reads 90ohms from the sender with it unhooked. I replaced the balance module because the gauge constantly read empty. It wouldn't transfer fuel to the front tank. Now the gauge reads way over full. If both tanks are full shouldn't the reading be 90ohms to the gauge?

 

[racer55]

Yes it should.
A way over full gauge indicates an open circuit.

 

[THEFERMANATOR]

Posts about the 2004 5500 have been moved to there own thread.
http://www.thetruckstop.us/forum/threads/diagnosing-dual-tank-transfer-problems.45533/

 

[Bernie]

I have a 99 6.5td 3500hd that the rear sending unit was replaced. At full it reads 90ohms from the sender with it unhooked. I replaced the balance module because the gauge constantly read empty. It wouldn't transfer fuel to the front tank. Now the gauge reads way over full. If both tanks are full shouldn't the reading be 90ohms to the gauge?

My 99 had the same work done (both sending units and the module, by the dealer when the truck was new) and has always read about a 1/4 tank over full when full. I average 10 mpg and it takes about 125-35 miles to get it back down to the full mark. I also know, from experience, that when it's sitting on 1/4 tank there's about 10 gallons in it. This tells me the error isn't linear, such as a set point being set wrong, but rather a progressive error, tapered if you will, that magnifies itself. That, in turn, leads me to believe that the resistance values are wrong somewhere. Is there a difference in the resistance range for gas sending units vs diesel? Are there different modules for gas & diesel? Something in the system is obviously expecting different values than it's getting. On the other hand, I've never found the problem to be worth the effort to really try and fix.

 

[THEFERMANATOR]

My 99 had the same work done (both sending units and the module, by the dealer when the truck was new) and has always read about a 1/4 tank over full when full. I average 10 mpg and it takes about 125-35 miles to get it back down to the full mark. I also know, from experience, that when it's sitting on 1/4 tank there's about 10 gallons in it. This tells me the error isn't linear, such as a set point being set wrong, but rather a progressive error, tapered if you will, that magnifies itself. That, in turn, leads me to believe that the resistance values are wrong somewhere. Is there a difference in the resistance range for gas sending units vs diesel? Are there different modules for gas & diesel? Something in the system is obviously expecting different values than it's getting. On the other hand, I've never found the problem to be worth the effort to really try and fix.

Depends on what you have. Yes, starting in 97 gassers went to 39-250 ohm senders while most diesels stayed with 0-90 ohm senders. Cab and chassis trucks can be different though.

 

[Bernie]

Depends on what you have. Yes, starting in 97 gassers went to 39-250 ohm senders while most diesels stayed with 0-90 ohm senders. Cab and chassis trucks can be different though.

Not sure if it's even possible, but it sure looks like a gas sender, or even two, instead of a diesel, would cause exactly this problem.

 

[THEFERMANATOR]

From what I have seen in the catalogs, the rear sender is the same for gas and diesel. The only difference in the front one for C&C is the gasser has a fuel pump, and the diesel does not.

 

[Bernie]

From what I have seen in the catalogs, the rear sender is the same for gas and diesel. The only difference in the front one for C&C is the gasser has a fuel pump, and the diesel does not.

But earlier you said the gas version had different resistance values. I'm confused. Or are you saying that the different values are only on the front unit, not the rear? Hmmm, let me think about this from a different angle for a minute...

If such a mistake as putting a different resistance range sender in, in place of the correct one, were to actually happen, it would explain the "over full" issue, but how would it affect fuel transfer between the tanks? Fuel gets used from the front tank, and the balance module will transfer 2 gallons at a time from the rear tank when the front level drops far enough below the rear. Since it would be almost impossible to put in the wrong unit in the front, the rear is the only place such a mistake could happen (theoretically - do gassers have pumps in both tanks, or do they use a transfer pump like the diesels do?) If the rear were reporting it was over full then every time the front dropped enough the balance module would transfer 2 gallons from it, so by the time my gauge got down to "full" the rear tank should be much more than just 2 gallons lower than the front.

Since I just fueled up Saturday, and haven't driven much, this will be easy to check, if I remember. I'll have to put a note on the dash to remind me to fill up again when it gets down to full, and see how much each tank takes.