Dual Fuel Tank issue

[RunMonkeyRun]

Hey there guys! I have a 99 chevy c3500 hd 6.5 with dual tanks. I ahve done searches here but did not seem to get far. The transfer pump from the rear tank is not functioning and there is no power going to it aswell, so the primary tank is the only thing working, problem is my fuel gage was not telling me the truth about how much fuel I had in the tanks so found out why I was sputtering all the way to home.

I have changed the sending unit in the primary tank ( mechanics advice) and has done nothing. Fuel gage will read just above quarter of a tank for about 10-15 seconds with the ignition on then drops to empty. Where does the transfer pump get it power from? can someone please let me know how the dual tanks fuel system works?

Thanks
Rob:confused:

[diesel65]

Under the drivers seat inside the frame rail is a fuel tranfer module, It monitors the front and rear tank level sensors to give you a total fuel level reading on the fuel guage. When the front tank goes below a determined value it will switch the rear pump on and transfer fuel from the rear tank to the front.
It is a fairly simple system, but I don't have a wiring diaghram handy at this time to guide you.

[RunMonkeyRun]

Well it seems that it definetly is not switching the rear pump on, I guess the module would be after or before the fuel pump which is also under the driver seat too correct? how would I test if the module is working or not? which it probably isn't.

[RunMonkeyRun]

Alright found the module and a solenoid which sits on top of the module. How do I test to make sure if it is the module or sending unit in the 2nd tank?

[diesel65]

The solenoid you call is actually the fuel pump balance relay, it turns the rear pump on as needed to transfer fuel.

The connector on the module is labeled A thru F, whether A starts on top or the bottom you need to figure that out your self by matching the color wires I am going to give you.
Do all test with module disconnected.

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 a DVOM 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 wires:

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.

Hopefully you understand what I wrote.

[RunMonkeyRun]

Thank you diesel65 that is just what I was looking for, I will be getting started with it tomorrow.

Sorry if I sound like a dum ars but what is a DVOM? as I have only used test liight to fix most of my crap. I assume DIGITAL VOLT OHMS thing, Once again thanks

Rob

P.S. I do understand what you were saying and will let you know my results.

[RunMonkeyRun]

Diesel65 thank you, I know more now than I did before yesterday. I followed your instruction and here is what I came up with!

TA = 12v

TB = edit aux. tank no change in multimeter. change setting here aswell and got still no reading.

TC = 4ohms (same as TB) edit changed multimeter to setting 200 and read 120 on meter

TD = Jumped realy got power to the pump. (wasn't getting power before so felt the pump and hard to tell if was working.)

TE = test light light

TF = did as what you stated it would do.

When I connect and re-connect the plug to the module I get power to the balance pump and my fuel gages is reading right for about 10 - 15 seconds then drop to empty and now power to balance pump. Same thing happens when I turn the ignition on it will show for about 10-15 seconds then drop to empty and no power to the balance pump.

I have changed the sending unit in the primary tank, so I am thinking it 2 things, balance module or sending unit in aux. tank? how to know for sure?

Rob

[diesel65]

I am having trouble understanding your TB and TC readings,
My advice would be since you replaced primary fuel sender, hopefully you still have the old one, Take the ohm meter and on the 200 ohm scale connect it to the old sender (across the two wires) and play with it. Raise the float to the top, then lower it while watching the ohm meter to get a feel what you are looking for. Again full should be around 90 ohms and empty should be 0 ohms (like the test wires wires were shorted to each other).

When you replaced the primary sender do you remember how much fuel you have in the tank? If you were to estimate that you had a half tank left then you should get a reading on the ohm meter of about 45 ohms (just an example).

I would look and see if the old sender would plug into the rear tank sender connector and try moving the float to the full position and see if the pump will come on or as a last resort is to pull the rear tank and inspect the rear sender and test it as I said to the primary sender.

[Burning Oil]

Hi, I have a request. I have never seen a factory dual tank set up. Do you have any pictures of it. Maybe a link to more info. I am in the process as of last weekend and tomorrow of fitting a Suburban tank in mine. Is yours a cab/chassis type or fleetside bed? Any other info you could share would be great. If you want to send pictures PM me and I will send my Email.
Thanks.

[RunMonkeyRun]

Burning Oil, I don't have pictures but I can take them tomorrow if you like, it is a cab and chassis and I have 1 at the driver side and aux in rear. I will bring my camera in and take them for you if you want no problem.

Diesel65, I think I am having a wiring issue, reason is I was getting reading for the primary tank and now I am not, but when I disconnect the plug from the primary sending unit and the harness I got a reading of 94ohms which is full and I just filled the tank before I tested it. Now I plug the primary sending unit to the harness and check at the connecter for the balance module I am getting no reading. So I guess I will have to pull the wires and see if there is corrosion somewhere on the lines. Any thoughts am I on the right track or just wasting my time? and know I am not getting any reading know from the aux tank. I was geting reading on friday and today nothing.

[RunMonkeyRun]

Okay, where does the tanks get the grounds from? as I was testing I could not get readings using the chassi for a negative, however I was getting a ground just under the module useing the frame but no readings today, I put a jumper wire from the battery negative so I could use that as a ground and I got my readings,

Terminal B - Light blue wire = 88 ohms
Terminal C - Dark blue wire = 94 ohms ( just filled this tanks)

If I use the negative from Terminal E - ground for module then check terminals b & c I get my readings also, but I can't get reading using the chassis.

Where do the tanks get there grounds from? the chassi?

[diesel65]

I don't have a vehicle specific wiring diaghram but my training manual is showing that the two tanks are grounded together with Terminal E for the module, and is connect to the same wire, BLK with White tracer, also part of the PCM ground circuit. These wires run to a grounding point called G104 which is the stud on the passenger side rear of the engine (by the dipstick tube for auto trans).
The engine should also be grounded to the chassis, maybe you didn't get a good connection on the chassis?

Again I am confused by your readings from the way you explain it.

First I would visually check all your grounds, there are multiple ground wires at that G104 grounding point, also there is a grounding strap on the back side of the head that runs to the chassis.

With the module and the two tanks disconnected, See if you have continuity between each others ground terminal (tank connector to tank con, pri tank to module, sec tank to module) They should all show continuity, if ok then go from ground on block to Module terminal E.
You should be able get continuity from terminal E to chassis.



I just noticed from you signature that the engine is not original, good possibilty you have a ground problem.

[RunMonkeyRun]

I will check the grounds again but I don't recall seeing the one from the back of the engine to the f

[RunMonkeyRun]

Hey Diesel65, sorry it took me so long but thanks very much for all your help on this issue. wanted to close this by saying that I did need to change the rear fuel pump. IT was the problem.

P.S.
Could not have figured it out without diesel65's help.
TTTTTHHHHHHAAAAAAANNNNNNNKKKKKK YYYYYOOOOOOUUUU.

[diesel65]

Glad to hear that you resolved your problem, I was wondering what happened.