Yet another sub-tank issue

[mushroob]

The sub-tank mechanism in my car was not working since I purchased it, and I figured I should attempt to fix it before a big trip.
I have read many threads regarding this issue but most seem to present different to mine.
My Sub tank fault light is not lit on the dash, however the sub tank switch itself doesnt seem to respond to a push.
Both sub tank and main gauge are perfectly fine. I have made sure to that the main isnt full once trying.
I read that there is an automatic mechanism which allows some flow of petrol (in my case) from the sub to the main tank every time the car is turned on but it seems this isn't working with mine (sub tank gauge hasnt moved a bit).
In an attempt to diagnose I open the fuse box and found the fuse for the sub to be blown. Replaced it, made sure current was flowing, yet still the switch is unresponsive, no transfer of fuel.
I have come to the conclusion that either my switch is faulty, or the fuel sender is (due to the fact there is no automatic transfer every start up) but can't figure out a reason the fault light wouldn't light up if it were the case.
I have also read about hotwiring the pump to force a transfer (if anyone knows how to do this would be appreciated), and I might turn to that if I don't get it fixed on time.
I was wondering if anyone has had a similar issue and managed to fix it.
Any help would be much appreciated

[the evil twin]

You don't mention the pump? are you sure it is OK? that would be the most likely culprit

If so then IMHO your sub tank computer is probably stuffed or there is a hard fault and possibly the fault lamp is blown.
The transfer switch won't illuminate unless the Computer is OK

If you take the fuel cap off and look in the main tank filler neck you will see the sub tank outlet about two inches down.
Cycle the ignition from Lock to On and look for a spurt of fuel
If no flow, disconnect the battery for 5 minutes (that resets the Computer) and reconnect then cycle the ignition and check for a spurt.

[mushroob]

Thanks for the reply. I realised why there was no fault light at all. There was no sub tank module connected. It seems the previous owner had a problem and decided to just leave it out. Sourced one from a wrecker, and the fault light turns on. Disconnect the battery, clean the earth connecting points, added a body to chassis connection, come back an hour later, reconnect and start the car. Fault light comes back on after about 30 seconds. No spurt at all into the main tank (this would suggest a problem with the pump?) How can I diagnose wether it’s the pump, the switch (still unresponsive) or the sender, also I thought the sender was the same as the pump, so they’re two different things? I’m now thinking I should try the Hotwire method to check wether the pump works, anyone know how to do this? I’m aware you need to connect the heavy green/black wire to the power/ignition (not sure which cable this is) via a switch

[mushroob]

There is a whizzing sound that lasts for around 5 seconds as I turn the ignition. Not sure if that’s the pump trying to work, but definitely no fuel being transferred

[the evil twin]

The 30 seconds lag is the duration of the self-test so the light coming back on after time is normal when the Sub tank computer thinks there is a fault.

It is not likely to be the switch, it could be but I have only seen one faulty switch in like 1,000 sub tank issues.

There is a way to get the fault code from the Computer so you can tell what it thinks is wrong.
The procedure is in the manuals available for download from this forum.

I am not 100% on the petrol transfer pump.
It may be in the tank (the Diesel models are all external pumps) which makes it a tad more difficult to sort out if it is...