I know I've explained this before, but here goes again:
The click is caused by the current being insufficient to throw the solenoid of the starter, this can happen for a couple of reasons OR A COMBINATION OF SOME OR ALL OF THEM:
1. The current at the solenoid is lower than it should be
2. The solenoid is stickier than it should be
1. The current at the solenoid is lower than it should be - causes:
- There are one or more 'dry joints' in the main circuit
- The relay contacts on the low-current side are corroded
- One or more of the wires involved have a high resistance because they get too hot
- One or more of the wires involved have been through lots of heat cycles and now their resistance is higher than it should be
- The wire(s) carrying the current from the battery to the solenoid are too long (causes higher resistance)
- The FIA switch's terminals are corroded
2. The solenoid is stickier than it should be - causes:
- There is dirt in the solenoid
- The piston is corroded
- The solenoid has expanded because it's getting too hot
You can test a number of these issues - eg. If you short the terminals on the starter (make sure the car is out of gear first), does it start the car? If 'yes' then there's nothing wrong with your starter motor; if the starter motor doesn't spin, but does if you short the terminals on the MFRU, then your relay is to blame etc...
HOWEVER, any number of these problems can occur simultaneously, and each component might work fine on its own, or if you swap any one of them out, but put a slightly dirty solenoid together with a slightly corroded piston and run a (old, slightly higher resistance) long line from the battery via the (old, slightly corroded terminals) FIA switch to the starter, and they lower the current enough that the solenoid won't throw. You also might get a situation where replacing any one of the components in a system where all the items are 'tired' will fix the symptoms because they push the current back up over the 'throwing' threshold, or decrease the stickiness of the solenoid; yet when the symptoms return, replacing the same component that you did last time won't fix it!
Many many people on this forum have posted that getting a different kind of starter motor/solenoid fixes it, or cleaning the piston, or getting a new FIA switch, or the relay mod, or rewiring etc... but if you really want to get to the bottom of it, I think you need to understand what's going on in YOUR car, and think about how you can test the components together and in isolation to determine the actual root cause of YOUR problem.
Edited by - charlie_pank on 17 Sep 2012 14:12:22