Is that a Toyota factory ECU? There is nothing in there that clicks..
Clicking when the key goes from both ACC to ON, and from ON to START positions are perfectly normal since relays are used to handle and split larger amperage loads to the various circuits on your truck.
That clicking noise has been there the whole time, but because the buzzer is on (when going from ACC to ON) or the starter is engaging (when going from ON to START), you just never heard them.
If it's a loud and obvious click that can be heard within 50 feet from your truck, then it's the starter solenoid not engaging the starter drive gear. This could be due to debris and/or rust forming in your starter, or worn out contacts. Usually banging the starter with a decently sized rod or a rock will loosen it up so that it will work again, but it should be repaired or replaced soon to prevent this from happening again.
If it's a quiet click that can only be heard within about 5 feet from the truck, then that clicking is normal, it is just the relays working, and the starter is not getting any voltage at all, or it is completely junk. In this case, I would just start backwards and troubleshoot from the starter itself.
--Have someone hold the key in the START position and check for voltage at the Black-White "Starter Signal" wire that runs to the back of the starter (not the side terminal post BLACK 8 gauge? wire, but rather the smaller 12 gauge? wire).
--If that does not have +12, then check the ground for the Starter Relay located in the fuse box in your engine bay.
--If the ground is good, then check the lead +12 volts be unplugging your key cylinder from the interior crowl harness, and check either the BLACK (earlier EFI models) wire or the BLACK-WHITE (later EFI models including all T100s, 4Runners, Tacomas, and Tundras) for +12 volts. Be sure to check the male side and not the female side once unplugged or else you will never see any volts regardless..
--If there is no power there, then you've got some serious power problems and you would have started this thread out by saying "Help, my truck is 100% dead and my clock doesn't even work" So I'm sure you've got power there...
I might not be able to respond because I am super busy lately, so send me a PM before you
BigMike
