[TR] LEDs - a mystery

[Dave]

Tom, most DVM's with a diode check function are designed to check a single diode.  Your LED bulbs are a string of LED's in series and the DVM will not put out enough voltage to bias them all on.  Some of the cheaper DVM's will not even put out enough voltage to bias a single LED (I have one at work and I have to get the Fluke if I want to check an LED).

An old trick I use to trouble shoot shorts is to replace the fuse with a headlight, if the circuit is OK the bulb will remain off (or glow dimly) and the circuit will work.  If there is a short to ground the bulb will glow at full brightness.  If you have some spare bulb holders you can make an LED bulb tester using a standard filament bulb as a current limiting device in place of the headlight.  If the bulbs work in the fixture and not in the car check the connections.  surface corrosion may be the culprit.

I guess you already know about LED's being polarity sensitive.


Dave Massey


 

 

-----Original Message-----
From: Tom Walling <pdqtr6 at comcast.net>
To: NET Member Mailing <net at newenglandtriumphs.org>; Triumphs <triumphs at autox.team.net>
Sent: Sat, Jun 10, 2017 5:12 pm
Subject: [TR] LEDs - a mystery



Coming back to the "well of Triumph knowledge" once again...


My current (no pun intended)  project is to switch over my exterior lighting from the ancient incandescent bulbs to ultra modern Gen-X approved LEDs. Simple job eh? Just replace the bulbs, put a ballast resistor in the left and right turn signal circuits and pop in an electronic flasher to replace the bi-metal one the car came with.



Well, let me attest that nothing, absolutely nothing, is simple on my TR6! OK, I tried to test the new system today and only one of the back-up lamps work. Quick trip to my FLAPS as its finally time I get a multimeter of my own - so I bought a really good Bosch one that has a diode test position among the many tests it is capable of performing. Actually, it is really the same thing as a continuity test, but it looks cool as heck on the unit. Anyway, I touch the red lead to one of the bumps on the bottom of the bulb, and the black one to the side of the base and nada! 


I also bought 2 new white LEDs for my backup lamps as they were about 3/4 the price as Moss asks. This is really interesting: the first time I tried the test on them, there was a faint "click" from the bulb, then nothing. No light-up, no reaction from my multimeter, nothing. If I reversed the red and black probes, another faint click then nothing. Is there some polarity thing going on in the bulb? As I said, only 1 of the backup lights works. 



Is it possible that I got a bunch of duds? For what they cost, I certainly hope not! Is there some other thing I need to do? Help! We've finally got a couple of summer days here in Maine, and I don't want to waste them chasing this problem on my own.



Thanks... Tom Walling 



 

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