GU1 Sub Tank Switch

Hello,

I have been having issues with the dreaded GU sub tank light on dash issue. It has happened before and fixed itself after a couple of weeks ignoring it. This time it seems to not want to come right. I have tried disconnecting the battery and no luck.

I started to read the manual and have found the relevant section but, I cannot seem to get the diagnoses mode to activate.

"SELF-DIAGNOSIS PROCEDURE
After turning the ignition switch ON, turn the sub fuel tank switch ON and OFF more than 5 times. Make sure the indicator lamp directly above the sub fuel tank switch flashes every 2 seconds. If the indicator lamp does not flash, repeat the check procedures. (The self-diagnosis system will then start.)"

I cannot get it to flash and do not know what it means by "repeat the check procedures". I have done some more reading and I am considering buying a new sub tank switch, though at nearly $90 I though I should ask the experts first.

Once again, thanks in advance for your time,

Cam

Sorry, 99 GU1 Wagon TD42T manual

By "repeat the check procedure" they mean repeat After turning the ignition switch ON, turn the sub fuel tank switch ON and OFF more than 5 times.

It doesn't always work first attempt and IIRC you have to turn it on and off 5 times in less than 10 seconds or somesuch.

The only time I used it on my '99 Ute about 12 months ago I got it to work on my third attempt, read the code, fixed the fault and it has been fine ever since... but I wondered at the time how it would work if the switch was faulty.

Does the switch lamp illuminate at all when you press it normally?
Most faults the lamp will come on then go out after 5 seconds or so

One other thing, the sub tank circuit is very prone to faults about the earth... have you tried an extra earth from the tank to the chassis ?

Do both the sub tank and main fuel gauges work correctly?

The sub tank switch is just a momentary on switch with a light so it's easy to test.

The switch does not light up at any stage, I pulled the connector out from the switch and made sure everything looked right. Not having a light at all led me on the faulty switch path, I was going to try to run the multimeter over the switch today but was scared off by a coil looking thing on the circuit diagram for the switch and didn't want to accidentally fry anything. Is there a way to test all the funtion of the switch this way?

As far as the earth goes, I haven't been under to look at the tanks yet, is there an existing place to just add another earth or do I just follow the original earth? I'm assuming, form your post, that the tanks have earth leads of their own to the chassis?

Sorry Yendor, you answered a question I was typing as you posted, I will go pull the switch now, and have a look at the terminals I have to test. The diagram was a little confusing, though it looks like it may be pretty simple to somebody who knows what they are looking at

The points on the back of the switch as I look at them are :
- -
----

If that makes any sense, I have 2 contacts on the top row one on the left and one on the right, with 4 contacts across the bottom row

Don't worry about pins 5 and 6 there for the switch to light up when the dash lights are tuned on.

Pin 3 should have power with the ignition switch is on. The coil looking thing is just a drawing for a bulb.

In you earth pin 4 the light in the switch should come on. If your using a multimeter pin 4 will have around 12 volts until the bulb lights up. Then it should have around 0 volts.

Pins 1 and 2 are the switch. Pin 2 is connected to earth.

You can either check the switch contacts with an ohmmeter. With the switch disconnected you should have an open circuit between pins 1 &2 until you push the switch in. or

With switch connected and voltmeter connected to pin 1 there should be a reading of somewhere between 5 to 12 volts. When the switch is pushed in it should drop down to 0 volts.

M48 at the bottom of the picture shows the pin configuration in the plug.

The top two (terminal 1 and terminal 2) are the switch contacts, open cct normally, short circuit if switch held depressed.

The bottom 4 pins are for the globes
Terminal 3 and 4 are the sub tank light,
5 and 6 are for the dash illumination so don't matter for operation

The plug terminal numbering is
1 - - 2
3 4 6 5

edit...DOH must learn to type faster

Ok so...

Terminals 1 and 2 act as they are supposed to.

#1 is showing around 5.04v until the button is depressed, then down to zero.

Pin #3 has around 500mv.

Pin #4 is where my issues start, I do not have any voltage and grounding it does not illuminate the light on the switch.

I have just bought a new multimeter from Jaycar and it has some strange induction thing that i don't understand so my 500mv may have been a false reading of zero. The car has gone to the movies now with the missus as I am about to leave for night shift, so re-testing will have to be done tomorrow.

Could a faulty light lead to a zero reading? Could this lead to the diagnoses mode not activating correctly?

Is the "Sub Tank" fault light on the dash cluster illuminated?
... and just checking that you do know the system will not transfer unless certain conditions are met IE Main Tank less than 3/4 full etc

Anyway, put the multimeter on Ohms and see if you have continuity on the switch between pin 3 and 4?

If you do (and the "Sub Tank" fault light is on)... the globe is OK and there is a fault in the Sub Tank system which the Computer is seeing during self test.

Quote:

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Originally Posted by NotTortoise

Ok so...

Terminals 1 and 2 act as they are supposed to.

#1 is showing around 5.04v until the button is depressed, then down to zero.

Pin #3 has around 500mv.

Pin #4 is where my issues start, I do not have any voltage and grounding it does not illuminate the light on the switch.

I have just bought a new multimeter from Jaycar and it has some strange induction thing that i don't understand so my 500mv may have been a false reading of zero. The car has gone to the movies now with the missus as I am about to leave for night shift, so re-testing will have to be done tomorrow.

Could a faulty light lead to a zero reading? Could this lead to the diagnoses mode not activating correctly?

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Was the ignition turned on? Was it a White wire with a Black trace that only had 500mv?. If so, check fuse 28 in the fuse box inside the vehicle. Make sure it isn't blown or have a bad connection.

500mv is not enough to power the light. It should be 12 volts.

OK, my bad on the 500mv, I don't know what I did but that wire has 12.55v with the ignition on.

However pins 3 and 4 do not show any continuity on my meter whether the switch is activated or not.
-Is this a test that I should do with the switch plugged in? (I have been doing all of my tests on either the harness or switch when I have had them disconnected.)

-If it is a faulty light, is there a way to repair it?
-Would this maybe be enough for the self diagnostic mode to not activate?
-Is a light fault enough to trigger a sub tank fault light in the dash?
-When the dash light gets activated does it suspend the system from attempting to work?

The car is in Brisbane until next Wednesday so I might order a new switch tomorrow in the hope that that will fix it.

Sorry for all the questions, but you guys are geniuses, a whole heap more helpful than the bloke I had look at it in Isa he told us that "it is what it is, carry jerry cans"

Thanks for the help

Yes, if the "sub tank" fault light is illuminated the system will not work
Even after you fix the fault that caused the computer to set the "sub tank" light to reset that fault light you still have to either disconnect the battery, pull about 3 fuses or unplug the sub tank computer (battery is usually easiest).

I haven't ever pulled a sub tank switch part so don't know if it can be done and/or the globe replaced, Rodney (or someone else) may know.

When you get a chance can you also confirm both your fuel gauges are working OK and whether the "sub Tank" fault light is on or off

Dash light is illuminated and both gauges are fully functional

Quote:

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Originally Posted by NotTortoise

Dash light is illuminated and both gauges are fully functional

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OK, that means each time you "fix" something you will need to disconnect the battery to see if that was indeed the fault and reset the fault codes.

IIRC an open cct in the switch tell tale globe (that tells you the pump is running) will cause a "sub tank" fault but I am not 100% on that

12.55v on pin 3 is good. If you earth pin 4 and the light doesn't come on (switch plugged in) light in switch is stuffed.

I haven't pulled a switch apart, not sure if you can replace the bulb.

Someone on here told me that a workshop was replacing switches all the time. Now I can't see the contacts in the switch causing problems (between pin 1 and 2) but it could possible be a blown bulb in the switch that might be causing the issue.

When the "Sub tank" light is on in the dash the system will not work.

Have you tried disconnecting the battery and then try to access self diagnostic mode?

In self diagnostic mode is it the dash light or the button light that blinks the fault codes?

Sorry, what is IIRC?

Button light... so if it is blown obviously you cannot read the codes unless you put another bulb across the terminals.

I will try the battery disconnect fix before trying diagnostic mode tomorrow. I don't think I tried that after i tried the battery. I just saw the light still on and went bavk inside to mope around

Quote:

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Originally Posted by NotTortoise

Sorry, what is IIRC?

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If I Recall Correctly

Quote:

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Originally Posted by NotTortoise

I will try the battery disconnect fix before trying diagnostic mode tomorrow. I don't think I tried that after i tried the battery. I just saw the light still on and went bavk inside to mope around

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Good idea... if the fault is still there the lamp will illuminate again after the sub tank computer does it's self test.
If the fault is gone then, voila, all OK

Cool, so is the 15-30 minutes about the right ammount of time to have the batteries disconnected?

Quote:

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Originally Posted by NotTortoise

Cool, so is the 15-30 minutes about the right ammount of time to have the batteries disconnected?

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Yeah should be.

Ok.

So I went and bought an led to try bypassing the light in the switch, thought that I should re-do all of my checks just in case and get my head back into the game.

All tested the same as yesterday except for my last test, earthing pin 4... it worked! And after it worked. So, I turned the car off and suddenly I could get it into self diagnostic mode 1st try.

The dash light is still on and my code flashed as #13 "Sub fuel pump power supply circuit".

Fun and games, forgetting for the moment, that this is starting to look more and more like a gremlin, I am now looking through the wiring diagram to see where I should test next.

Is this the type of thing that could be potentially rectified by adding a 2nd earth from the sub tank to the chassis? If i do need to add one do I have to use a different location both on the tank and chassis from the original earth cable?

Thanks for the knowledge and assistance so far you guys are really invaluable help,

Cam

First thing, I have disconnected the battery and we will see what happens...

IT WORKS!!!

I am just going to ignore the fact that it didn't work and just be happy that it works now.

Yay!!!

I can't thank the evil twin and Yendor enough, you guys are incredibly knowledgeable and helpful.

Thank you, thank you, thank you.

Cam

There isn't an earth strap fitted to the chassis from factory. You need to do this first and then see if problem occurs. It doesn't really matter where you fit the extra earth strap as long as it connects to the chassis.

You can connect it near the fuel tank, or from the negative battery post to the chassis or as Nissan recommend connect it between the body and chassis under the front passengers floor.

After you have installed the earth strap disconnect the battery again and see how it goes.

Edit: Just noticed your post above. I still recommend that you fit the extra earth strap.

X2 with Rodney... fit that earth strap anyway

It should hopefully stop that and other earthing gremlins

PS... glad we could help, makes my day actually, not having very many other sparky wins at the moment

I'll give you a choccy frog ET

Quote:

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Originally Posted by Yendor

I'll give you a choccy frog ET

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Mmmmmm, choccy Frog... nom, nom, nom