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Discussion Starter #1
Ok now this sub tank is really doing my head in. A few weeks ago I cleaned all battery terminals and added a body to chassis earth strap and it worked for a bit but the light still came back on. I disconnect the battery yesterday and the same thing happened, it was working but today the light is back on. I managed to pump 1/3 of the sub out so it can't be over full (which I've heard is an issue) I bought a manual and am going through the fault code finding process stated of turning the ignition on, pressing the sub tank switch on and off 5 times within 3 seconds but it's doing nothing and it's not going into self diagnosis mode. Is the button supposed to switch on then switch off when pressed again? Because mine only switches on then the button pops back out and the only way to stop the sub tank pumping is when it's empty or to turn the car off. I really don't know where to go from here if the fault codes won't even work ? mine is a 2004 td42ti ST wagon of it makes any difference
 

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The sub tank switch is only a momentary on switch, it doesn’t lock on. When the switch is pushed in it sends a signal to the sub tank control unit to either start pumping fuel or stop pumping fuel.

Do both of your fuel gauges read correctly?.

Does it stop working while it is transferring fuel? Maybe you have a faulty fuel pump.
 

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Discussion Starter #4
Both guages work properly. After I disconnect the battery and reset it the system works 100% normal, fuel transfers, guages work etc. I can turn it off and on and do it again a few times. But after a few starts or the next day I drive it the fault comes back.

And no I haven't installed a turbo timer, the fault just happened randomly one day.
 

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GUII ZD30DI Wgn
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Both guages work properly. After I disconnect the battery and reset it the system works 100% normal, fuel transfers, guages work etc. I can turn it off and on and do it again a few times. But after a few starts or the next day I drive it the fault comes back.

And no I haven't installed a turbo timer, the fault just happened randomly one day.
The subtank module sits above the trans tunnel, not easy to get at, but from troubling experience I know earthing issues can wreak absolute intermittent havoc with our GU instrumentation and the effects can be many and varied, I have had this on and off for a few years after adding a particular device. The transfer pump is earthed to the chassis. You indicate that disconnecting the battery fixes the problem short term but it returns, I have a second earth strap running from chassis to body, have had for years, but a recent return of a fuel gauge problem which I recognised straight away as an earth issue caused me to earth that component directly to the battery, then remembering something Yendor said in a post some time back I ran an earth directly from the battery to the chassis, this has stabilised my gauge. Basically I have 3 earth straps now and a stable fuel gauge, none of the other intermittent things have returned so far. Worth a try and cheap as chips in the process of elimination.
 

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I had my module disconnected for ages and used a paper clip to activate the pump.

One day I pulled the PCB out of the case on the module and tested it, and it worked flawlessly for a couple of weeks. It now sits in the case without the end cover on and has worked perfectly for a couple of years. Must have been flexing a solder joint and causing issues.
 

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Discussion Starter #7
So today I thought I'd try again. I started with doing the usual battery disconnect and sure enough the sub tank worked but this time I was actually able to turn the pump off when I pressed the button again, which i've never been able to do. I tried and tried to get it into self diagnosis mode which didn't seem to be working except one time when the car was running I got the light to start flashing (indicating that it was in self diagnosis mode) but haven't been able to get it to flash again. The switch is still turning the pump on and off, but sometimes I'll press it and it won't work straight away. So I'm thinking the switch is stuffed? What do you reckon? I might just drive it for a few days and see if the warning light comes back on.
 

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when you start the car the sub tank pump automatically runs for a minute or 2, also the pump will not switch on if your sub is empty or the main is full.
 

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GUII ZD30DI Wgn
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So today I thought I'd try again. I started with doing the usual battery disconnect and sure enough the sub tank worked but this time I was actually able to turn the pump off when I pressed the button again, which i've never been able to do. I tried and tried to get it into self diagnosis mode which didn't seem to be working except one time when the car was running I got the light to start flashing (indicating that it was in self diagnosis mode) but haven't been able to get it to flash again. The switch is still turning the pump on and off, but sometimes I'll press it and it won't work straight away. So I'm thinking the switch is stuffed? What do you reckon? I might just drive it for a few days and see if the warning light comes back on.
Switch or just a dodgy contact somewhere in the link. I would do as you say, drive it for a few days and see what happens, you never know your luck in a big city.

when you start the car the sub tank pump automatically runs for a minute or 2, also the pump will not switch on if your sub is empty or the main is full.
Hi mate, the pump runs for seconds not minutes, it just a self test mechanism.
 

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Discussion Starter #10
I tried a mates switch (his sub tank works) and the dash light still come back on. So do I
A) try his control module
B) buy a new control module hoping that it will fix it
C) pull the sender out and clean the connections (guage works fine)
D) hard wire the pump to a switch and operate it manually.
Now I would rather have it working properly than wiring in a manual switch but with a Simpson desert trip in 2 weeks time is running out. Why must this be so difficult ?
 

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GUII ZD30DI Wgn
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I tried a mates switch (his sub tank works) and the dash light still come back on. So do I
A) try his control module
B) buy a new control module hoping that it will fix it
C) pull the sender out and clean the connections (guage works fine)
D) hard wire the pump to a switch and operate it manually.
Now I would rather have it working properly than wiring in a manual switch but with a Simpson desert trip in 2 weeks time is running out. Why must this be so difficult ?
D) Will give you a quick easy answer to one question if you are you still having the issue where sometimes the switch doesn't work?
 

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Discussion Starter #13
The switch always turns the pump on when the system is functioning, there's just some gremlin that's causing the system to fault and bring up the SUB TANK light on the dash
 

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GUII ZD30DI Wgn
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So today I thought I'd try again. I started with doing the usual battery disconnect and sure enough the sub tank worked but this time I was actually able to turn the pump off when I pressed the button again, which i've never been able to do. I tried and tried to get it into self diagnosis mode which didn't seem to be working except one time when the car was running I got the light to start flashing (indicating that it was in self diagnosis mode) but haven't been able to get it to flash again. The switch is still turning the pump on and off, but sometimes I'll press it and it won't work straight away. So I'm thinking the switch is stuffed? What do you reckon? I might just drive it for a few days and see if the warning light comes back on.
My suggestion related to this part of your earlier post.
 

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I just replaced my sub tank switch turned vehicle on waited a few seconds and light on dash came back on. Only happens when I fill tank up regardless whether it’s main or reserve. Once light comes on sub tank switch won’t work. I have run a third earth from engine to chassis and still come on.
 
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