Installing an aftermarket headunit, and needed a source for illumination as the stock harness doesn't include this. I found a thread here saying to tap into the ashtray light since it was the closest. I did this, but it's only measuring 8v when lights are turned on. Rest of car is 12-14v as usual. Anyone else see this?

I would have thought that wouldn't have been an issue for my headunit however it is not working as a source so it must have a higher voltage threshold for a trigger. Someone said to use the glovebox light but that is switched to the door and not to the headlight switch.

 

Wow - not 5 seconds after posting this I figured out the issue. For anyone else that runs into this, the voltage is controlled by the gauge cluster instrumentation dimming **** near the gauge cluster. It's not PWM, it's a rheostat so it simply reduces / increases voltage. I forget how old this car is, lol.

 

If one wanted to fix this, simply use a 12v relay that can trigger down to lower voltages and then tap into the constant or switched line going to the headunit as a source of steady 12v.

 

Instrument lights including ashtray definitely are PWM - had a scope on them last week.

 

What is PWM?

 

PWM - Pulse Width Modulation. For lighting, dimming is achieved by a 12V square wave with a varying on/off time.

 

Thank you!

 

Usually reserved for LED style lighting but not incandescent. Interesting you saw a PWM signal... I wonder why my voltmeter has varied voltage when I adjust the rheostat...

 

Because your voltmeter is taking an average of the voltage. A bit like the bulb!

 

A quick way to tell it's PWM without a scope: set your meter for AC volts and watch the reading change as you vary the dimmer pot.

 

Hi Narkotic,
The existing head wiring harness has a wire for this:
https://993servicerepair.blogspot.co...ire-color.html
Andy

 

For an early '90s design, wow.
If the led radio facia lights are incompatible with the pulse width modulation dimmer consider a direct connection to the headlight switch to bypass the dimmer. Or just use the existing switched wire to the radio head that will illuminate the radio any time the ignition is switched on.

Andy

 

thanks! yeah a relay would be easier than tapping into the headlight switch methinks but not a huge deal. Yeah normally i'd just hot-wire the illum wire to switched 12v however this headunit has a huge LCD screen and i want it to be bright during day / dark at night.