what is this crazy hot 68R (resistor?) under my dash? 2021 Caterham

[Austin David]

Some light googling says this might be something about LED lighting in the dash, and providing enough resistance to look like incandescent?  But if I unplug it, things seem to work as expected.

 

it gets HOT to the touch -- again, googling sort of indicates this is expected.  10k miles in and it doesn't seem to be causing any trouble, 

 

What is it?  Is it important?  It lives approximately above my steering column.

[CBuff]

I have that as well 2023 420r. Found it while trying to muffle my turn signal beeper. Reaching around back there and came across that one. Very hot for sure. 
 

sorry no insights on what it does. But can confirm I also have it. 

[Austin David]

about 25-30 mph seems to muffle that beeper just fine

[sltous]

When you unplug the resistor does the red ignition light come on in the dashboard and stay on vs. turning off after starting the engine?

[Austin David]

Yes!  The batt light on the tach gauge comes on when I disconnect the resistor while running.  I also noted it's not hot at long idle, but definitely gets hot if I leave the ignition on with engine off.

 

Other than the battery light on the tach, everything seems fine with the resistor out 

[CBuff]

Glad to know it doesn’t stay that hot when the engine is running.  I had the accessory on and ignition off when I was trying to located the turning beeper. So it was hot for sure. 
Another mystery solved. 

[sltous]

17 minutes ago, Austin David said:

Yes!  The batt light on the tach gauge comes on when I disconnect the resistor while running.  I also noted it's not hot at long idle, but definitely gets hot if I leave the ignition on with engine off.

 

Other than the battery light on the tach, everything seems fine with the resistor out 

I think this thread and link may explain it better than I can: 

https://www.caterhamlotus7.club/forums/topic/254262-alternators-leds-and-resistors/

http://www.mgb-stuff.org.uk/alternator.htm#theory

 

In essence, the ignition light on the newer gauges is LED and does not have enough resistance for the alternator charging circuit to correctly turn off the LED when the alternator kicks in without the parallel resistor.

[Austin David]

ok, for future sleuths:

- it's on purpose

- it should get hot while the little battery light in the tach is ON -- ignition on/engine off, or alternator no worky

- it should not get hot while the engine is running normally (light is not illuminated)

- it's there because the LED in the tach doesn't draw enough current on its own, so this will sink a few W (2? 5?) to keep the rest of the system happy.

 

Thanks @sltous, I wasn't able to find that set of articles.

[MV8]

68R is a good choice; better than suggestions of 40 ohm which would get much warmer and have enough current to require the extra diode. I calculated 55-59R as the lowest value in parallel. There is some current any time the key is on, running or not for the regulator. Considering the minimum current needed to operate the regulator and have a bright enough led versus excess current/heat, I might try a 100R of the same type/wattage or just add a volt meter.