TR2/3/3A TR3A brake light activation

[MGTF1250Dave]

Mahalo to Art and Randall for the correction. Perhaps I would be more correct to say that the older cars do not use relays in most circuits, so the switches are subject to higher amperage. For the record, I am not an electrical engineer nor do I play one on television and I did not stay at a Holiday Inn last night. Thanks again for correcting my misconception.

 

[TR3driver]

I never figured out why they don't make that copper penny contach disc out of stainless steel so they will last 50 years instead of 5 years.

That's easy, these cars were never designed to last 50 years! Remember, these were 'inexpensive' sports cars.

Besides, I would say that the evidence suggests that the original contacts were made of a good alloy, probably a phosphor-bronze developed specifically for switch contacts. After all, most people seem to say the original switch lasted for 20-30 years, while plain copper would have serious problems much more quickly than that. Today's replacements are no doubt a much cheaper alloy (if not plain copper as you say).

I wonder too if the brake light bulbs we use today don't have a higher inrush current than the originals. I know mine do, as I deliberately use 2357 bulbs; but I suspect that even the 1157 bulbs most folks use draw significantly more current when they are cold. Unfortunately, cold resistance ratings are hard to come by, so I can't prove that; but they also draw almost 30% more power hot than the original bulbs did.

Not to mention the earlier cars with only one brake light bulb.

 

[Andrew Mace]

I wonder too if the brake light bulbs we use today don't have a higher inrush current than the originals. I know mine do, as I deliberately use 2357 bulbs; but I suspect that even the 1157 bulbs most folks use draw significantly more current when they are cold.

Good point. Does a typical 1157 draw more than the original 1036?

 

[TR3driver]

Well, my handbooks show the tail/stop bulb for a TR3-4A as a Lucas 380 with a 21 watt stop filament. I don't find 1036 in my charts, but a 1034 is listed at 23 watts and 1157 is 26.8 watts. (2357 is 28.5 watts).

But that doesn't tell the whole story. Incandescent bulbs have an odd characteristic, the filaments have much lower resistance when they are cold than when they are white-hot (ie operating). That means there is a big surge of current when they are first powered on (which is why bulbs so often burn out just as you turn them on). The 2357 in particular is optimized for more light output per watt (at the expense of shorter operating life), which is achieved partially by running the filament hotter. That means it has a bigger change in resistance from cold to hot, and hence more in-rush current than it's rating would suggest.

Thinking back, it may have been only a few months from the time I 'upgraded' to the 2357 bulbs, until the factory switch gave out.

 

[Andrew Mace]

Thanks, Randall. And my bad...meant to type 1034, which is pretty much what a Lucas 380 was! :blush:

 

[6TTR3A]

<span style="font-size: 12pt">George,
This is one view of my mechanical brake switch. It's a NAPA "Echlin" stoplight switch, part # SL161.
FRank</span>

 

[6TTR3A]

George,
This is one view of the mechanical brake switch on my 3A.
It's a NAPA <span style="text-decoration: underline">Echlin</span>Part # SL161 (normally closed)

 

[6TTR3A]

This is the other view. Hope this helps.
FRank