Switchless dual tanks/fuel pumps, how do they work?

dwragon · Apr 15, 2022

Hi all, I got my 97 3500 HD running, but due to the previous owner thinking someone stole his gas, (Actual problem, too much paint between the crank sensor and timing cover.) I disconnected the external fuel pump, mainly to prevent possible contaminated ancient gas from hitting the injectors. I decided to disconnect the pump because I could not find a tank selector switch. I just read that it is an automatic switch under the dash, which is not something I like (cant get to it, cant control it). I want to find out more about this system before I go reconnecting the rear tank, so I can drain it if I need to, and other considerations. Part of what I am wondering about is how the rear tank knows to dump fuel into the drivers side tank. Any help appreciated.

stutaeng · Apr 15, 2022

There has been some questions relating to how those tanks work. I don't recall at the moment. May be worthwhile to find a wiring diagram for it.

I think if anyone here would know, it would be @someotherguy . He ran one of those trucks and is pretty knowledgeable on the GMT 400 platform in general.

someotherguy · Apr 15, 2022

stutaeng said:
There has been some questions relating to how those tanks work. I don't recall at the moment. May be worthwhile to find a wiring diagram for it.

I think if anyone here would know, it would be @someotherguy . He ran one of those trucks and is pretty knowledgeable on the GMT 400 platform in general.

I wish I did know more about that dual tank setup. All (but one) of the HD's I've owned or worked on were single tank, primarily because the usual placement for the second one is between the rear frame rails, which must be removed when a wrecker unit is installed.

Only one truck I had which was a former wrecker came with dual tanks and the aux tank appeared to be an aftermarket piece, hung onto the passenger side frame rail with some homemade looking brackets.

It appeared the sending unit on the main tank (driver side inside frame mount) was hooked into a circuit that included a lift pump that transferred from the passenger side aux tank back to the main tank. Whether or not that was a factory setup, dunno. It would be easy enough for OP to check his truck though to see if that is the case, and simply disconnect the lift pump; there was a weatherpak style connector along one of the crossmembers near the pump, IIRC, the one just head of the rear axle.

(edit - on re-read it sounds like already found the lift pump and disconnected it)

Hope that's helpful..
Richard

dwragon · Apr 15, 2022

someotherguy said:
I wish I did know more about that dual tank setup. All (but one) of the HD's I've owned or worked on were single tank, primarily because the usual placement for the second one is between the rear frame rails, which must be removed when a wrecker unit is installed.

Only one truck I had which was a former wrecker came with dual tanks and the aux tank appeared to be an aftermarket piece, hung onto the passenger side frame rail with some homemade looking brackets.

It appeared the sending unit on the main tank (driver side inside frame mount) was hooked into a circuit that included a lift pump that transferred from the passenger side aux tank back to the main tank. Whether or not that was a factory setup, dunno. It would be easy enough for OP to check his truck though to see if that is the case, and simply disconnect the lift pump; there was a weatherpak style connector along one of the crossmembers near the pump, IIRC, the one just head of the rear axle.

(edit - on re-read it sounds like already found the lift pump and disconnected it)

Hope that's helpful..
Richard

Yes, l found it, and a casco box ahead of the tank. I will be pulling the drivers side tank, as well the bed, because the brake line has rust rupture and the utility box bed is so rusty (appearently a reutilization) some of the compartment bottoms and doors are gone. I will give a running update on what l find, as l will drop the rear tank also, to make sure it is safe to use. I have had another truck rear tank rust out due to holding dirt/moisture between the tank protector and the tank. But l would still appreciate anyone who has knowledge of the dual tank system chiming in.

dwragon · Apr 15, 2022

Got the bed off today, here are some pics.... North Dakota truck....

dwragon · Apr 15, 2022

Just an odd note, the drivers side tank is metal, the rear tank on mine is plastic, wish it was the other way around.

dwragon · Apr 15, 2022

Went back after supper and tackled the rear tank. The tank was full of sour gas, so l think the transfer pump went bad, not that the gas was stolen. And whatever engineer who designed that crap of a rear tank steel guard was either trying to justify their paycheck or was a Pinto in a past life. Had to cut the rusty piece of crap off with a torch. Also removed about 20 pounds of dirt and rocks with it. Think l will eliminate the rear tank and add another side tank, with transfer controlled by a toggle switch, if i cant get the original system to work properly. I will route the exhaust outside the frame rails.

Carlaisle · Apr 15, 2022

If it's not too great an imposition, could you take some good wide angle shots of that second tank set up and share them? I have contemplated doing something similar to the factory setup to increase range without sacrificing bed volume. Most of the OEM parts are NLA but it would still be a great point of reference.

tpass · Apr 16, 2022

I have been reading up on these dual tank HD's since i picked up my '01 3500HD, apparently there was a salt spreader on my truck at some point, the rear aux fuel pick up assembly completely rotted away and the aux tank is now full of water.

The the computer reads both the front and the rear sending units to operate the fuel gauge, if either one is inop or disconnected the fuel guage goes to empty.

If the computer reads more than a 20% difference in levels between the 2 tanks, it then pumps fuel from the rear aux tank to the front main tank using the lift pump mounted on the frame. There is no fuel pump in the rear tank just a sending unit, the pickup and vent tubes

It does not pump fuel from the front to the rear. I read when fueling up, you should fill the rear tank first.

Edit to add the cheapest rear sending unit i could find is on rockauto.. $83

dwragon · Apr 24, 2022

tpass said:
I have been reading up on these dual tank HD's since i picked up my '01 3500HD, apparently there was a salt spreader on my truck at some point, the rear aux fuel pick up assembly completely rotted away and the aux tank is now full of water.

The the computer reads both the front and the rear sending units to operate the fuel gauge, if either one is inop or disconnected the fuel guage goes to empty.

If the computer reads more than a 20% difference in levels between the 2 tanks, it then pumps fuel from the rear aux tank to the front main tank using the lift pump mounted on the frame. There is no fuel pump in the rear tank just a sending unit, the pickup and vent tubes

It does not pump fuel from the front to the rear. I read when fueling up, you should fill the rear tank first.

Edit to add the cheapest rear sending unit i could find is on rockauto.. $83

Thanks for this info, I got sidetracked over a rearend issue (I had to change out the rearend, which I posted my notes on.) One thing I have noticed on mine is that for about the first 20 seconds it reads the front tank, then drops to empty.

Switchless dual tanks/fuel pumps, how do they work?

dwragon

OBS Enthusiast

stutaeng

I'm Awesome

someotherguy

I'm Awesome

dwragon

OBS Enthusiast

dwragon

OBS Enthusiast

Attachments

dwragon

OBS Enthusiast

dwragon

OBS Enthusiast

Carlaisle

I'm Awesome

tpass

I'm Awesome

dwragon

OBS Enthusiast

Similar threads

Latest posts

Staff online

Members online

Forum statistics

Share this page