[TR] TR250 LED tail light problem

Randall tr3driver at ca.rr.com
Sat Nov 2 15:07:35 MDT 2019

There may not be a good solution, Roger.  The incandescent turn signals draw a fair amount of current, which is likely more than the stock alternator can deliver (along with tail lights and so on) at idle.  So it may be that the battery voltage is dropping from 13+ volts (alternator supplying all power to car) to  12.6 volts (battery supplying some of the power) and the LEDs you’re using are sensitive enough to show the difference in voltage.

To check, connect a good voltmeter or DMM to the battery, then watch what it does when the tail lights and flashers are both on.  If I’m right, you’ll see the battery voltage sag in time with the turn signals.  The only fix would be to convert to a more modern alternator, that can keep up with the lights at idle.  (I’m not certain, but I think there is a Lester unit that would look and fit the same as the stock Lucas but give more current across the board.  Check with the Jaguar folks.)

Another fix might be LEDs that use an active current source (so are much less sensitive to supply voltage), but I have no idea where to buy such things.  I made my own using a simple 2-transistor active current limiter.

To check for grounding issues, I suggest running a wire to the negative battery terminal or negative starter cable, so you can connect the ground lead of your DMM to that.  Then you can probe at the rear lights, to see how well they are actually grounded.  0.2 volt is probably acceptable, anything more than that represents a problem that could be fixed.

I’m not sure how the TR250 tail lights are wired.  On my TR3, all the rear lamps ground only through their mounting screws, which go into clip nuts fastened to the sheet metal.  Very insecure, especially if the sheet metal has a fresh coat of paint.  

However, each lamp has a terminal inside the housing for a ground wire.  So, I made up a ground wire that daisy-chains across all the rear lamp holders, then leads around the trunk to one of the fuel tank mounting bolts.

-- Randall

From: Roger Elliott
Sent: Friday, November 1, 2019 12:50 PM
To: triumphs at autox.team.net
Subject: [TR] TR250 LED tail light problem

Here's the problem.  When the tail lights are on and I use the flasher, 
the tail/brake lights flicker with the flasher.  They don't go on and 
off but the get brighter and dimmer.  When the third brake light is 
hooked up it does the same thing.

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