TR4/4A Lighting Switch

[KVH]

The lighting switch for my TR4A that I have doesn’t appear to match the wire colors coming from my firewall. I remember having this problem once before. Are there any easy, hard and fast rules in matching up these wires? If I knew which of the terminals on my switch match the wiring diagram designations of 1 through 8 that might help.

 

[TR3driver]

Sounds like it might help if you got a 4A wiring diagram.

Otherwise, use an ohmmeter or continuity tester to check out all combinations of switch terminals/wires in each switch position. Write down which ones show continuity (0 ohms) in each position. I like to do this anyway, just to make sure I have the right switch. For example, there are at least two different versions of that 8 terminal monstrosity used for headlights on the TR4, and they aren't the same! Vendors also sometimes supply the wrong switch.

You should come up with a pair of terminals that show continuity in the first position (parking lights); and a third terminal that shows continuity with the first two in the second position (headlights). Should be another pair that only show continuity in the spring-loaded "flash to pass" position.

From what I've heard, there are some variations in the cars, perhaps based on the original destination country. In most cases, the "flash to pass" feature lights the filaments selected by the dimmer switch (which means it does nothing if the lights are already on). But sometimes, there is a UW wire that comes to the main lighting switch, so FTP can always work on high beams.

 

[TexasKnucklehead]

Maybe there's more than one wiring diagram for the FTP, because I'm pretty sure the Flash To Pass on the one we did was wired to the Purple wire -hot always. I never gave it a second thought until we found the lights would sometimes flash when turning the steering wheel in the garage. -Eventually the (purple) fuse blew.

Turns out (pun intended) the FTP section on our reproduction switch is slightly different from original in the way the wires attach. The rivet and terminal on one of the wires would occasionally foul on the self cancelling turn signal tab welded to the steering shaft. Found the same issue on a TR6 and TR250. We opted to disable the FTP on the TR6 but on both the TR4a and TR250 we used a tiny grinder to remove a little of the welded tab.

You have to admit that's an odd 'Lucas feature' to have the headlights flash when you rotate the steering wheel.

 

[TR3driver]

The diagrams for TR250 and TR6 do show purple; but the TR4A diagrams I've checked all show green.

And I was only looking at the TR4A diagrams, I don't actually have a TR4A. Its quite possible TR4A used purple too and the diagrams are just wrong.

Neither one actually makes a lot of sense to me. Why put a fuse in FTP but not the regular headlights? If there was a short anywhere after the switch, it would burn up the headlight wiring anyway!