[T3] LEDs for directionals

Jim Adney jadney at vwtype3.org
Thu May 19 08:24:27 PDT 2022

On 19 May 2022 at 6:48, Max Welton wrote:

> So I got my turn signals working. I had to replace the 9-pin relay.
> And then, because I was curious ...
> https://youtu.be/lO9mmtTSz-M

Tungsten bulbs have the interesting "feature" that they draw an initial inrush 
current that's about 10x their steady state running current. This inrush 
current is hard on those bulbs and explains why Tungsted bulbs almost 
always burn out with a flast at turnon. VW and their relay suppliers used this 
inrush feature to sense a burned out bulb. The TS relay expects two 10x 
inrush currents at turnon and if it doesn't get that, the flash rate doubles, as 
a signal that something's wrong.  

LEDs don't have any turnon inrush and they draw much less current, so they 
will tend to make the relay flash faster all the time, if that relay has the failed 
bulb sense feature.

I don't know when VW started doing this, but I'm pretty sure '68 got it. Your 
car is a '67, which uses a completely different TS relay ($$$!) and that TS 
relay may not have the burned out bulb feature. (I don't know.) If you have a 
'67 owner's manual, it might tell you.

If you had one Tungsten bulb in front and an LED in back, it's possible that a 
single Tungsten bulb might be enough to fool the TS relay, if the relay was a 
bit off spec or the front bulb was larger than stock.

For anyone out there with a bad TS relay, I can sometimes fix them if it is 
any of the styles that's in a rectangular plastic box. I can't do anything for the 
earlier style that's in a large cylindrical metal can. The metal can flasher 
relays do not have the burned out bulb sense feature. LEDs probably won't 
work with them either, because they don't draw enough current to activate 
the flasher mechanism.

Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA

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