Parasitic drain, LED interior lights ...

So this is making me think.
1960's Mk2 Jaguar has an intermittent parasitic drain. Amp clamp the battery cable and its around 150-200ma.
Only 4 fuses for the whole car, mV drop the fuses and trace it to the circuit that includes the part that switches on/off the interior lights when a door opens. The interior lights are earth side switched by a switch in the door that grounds the circuit for all 4 lights. These torpedo type lights have for some reason been changed for LED's. Thinking that a fwd bias LED should basically have about the same "resistance" as a fuse I did a mV drop test and get about 200mV when the doors are all closed so definitely some current flow.
Putting a normal bulb in and I get no (measurable?) voltage drop across it.
Concluded that one of the door switches was not fully open when the door was closed giving a high resistance path to ground, but also that the draw would go away if the LEDs were replaced with normal bulbs.
Was I right or was I missing something ?
 

This post has been making me want to build a test circuit!
So you're thinking a resistive ground from a failing switch provided enough of a path for current to flow through the LED but not illuminate it?
Do you think current flow could be deduced from the 200mv voltage drop across the LED? (And multiplied by the number of bulbs probably)
Very curious...

wrote:

This post has been making me want to build a test circuit!
So you're thinking a resistive ground from a failing switch provided enough of a path for current to flow through the LED but not illuminate it?
Do you think current flow could be deduced from the 200mv voltage drop across the LED? (And multiplied by the number of bulbs probably)
Very curious.






 

Hmmm, quoting didn't work out on the tablet, sorry for that....

Noah, I'd look at this as a serious circuit with 2 loads, the first one being the led with its 200mV drop and the second one the door switch where most of the system voltage is dropped. The 200 mV potential difference over the led would not be enough to let current flow thru it, imo.

The LED's probably are arranged in parallel circuits, otherwise current flow - if any was present thru the LED's, which I still doubt at this point - can't sum up to the mentioned 200,mA.

I'm inclined to think there was another problem within this circuit but you know what: I've been proven wrong before

Concluded that one of the door switches was not fully open when the door was closed giving a high resistance path to ground, but also that the draw would go away if the LEDs were replaced with normal bulbs.

Makes sense that the voltage drop across the lamps would disappear with a change to incandescent. Very low cold resistance. I don't see how it would make the actual draw go away though??

Hmmm, quoting didn't work out on the tablet, sorry for that....

It's not your tablet. Just did the same thing to me with firefox on windoze............

Check that it hasn't been fitted with an aftermarket voltage sensing car alarm.
The other possibilities are the dash clock or an anti-theft tracker.