The elusive 100 amp fuse seems to have been the cause of numerous calls to the AA among TVR owners. Many are unaware of it's existence, and even those who of of it don't know where it is or what it's for.
The fuse sits between the output of the alternator and the battery, it's exact purpose is unclear. I guess it's to protect the car in the event of a major failure of the alternator.
Finding it
On my car (a '98 500) the fuse is located in a small black plastic box mounted onto the chassis. It's located under the car, near to the main chassis rail, on the off-side of the car around 18" back from the oil filter.
Here's a picture of it as I found it (yes, my sump is leaking a bit):
The new fuse
The fuse inside the holder should look like this:
Replacement
I snipped the cable tie which was keeping the lid closed and inside was what can only be described as a corroded mess. The first task was the removal of the old fuse.
NB. At this point you should probably disconnect the battery, you can probably get away with just being careful but there is ample opportunity to short out the battery against the chassis.
Removing the old fuse was just a matter of undoing the 2 8mm nuts which hold on the alternator and battery connections. The old fuse should then just pull out.
Next I cleaned up the inside of the case, and then polished (with wet-and-dry paper) the terminal connections.
Replacement was just a matter of slotting in the new fuse and re-connecting the newly polished terminals.
After a quick squirt of WD40 I put the lid back on and secured it with a new cable tie, some of the insulation looked a bit tired so I wrapped it up with insulation tape.
The final product:
In summary
It's an easy job once you have the car on stands. It's not an easy road-side repair and so I think it should really be done annually to be on the safe side.
