Strange wiring issue after gauge install

[Gizmo]

Hi all-

I finally decided to install a boost gauge in my '96 850R, and somehow messed the wiring up.
Gauge is mechanical. I pulled the power for the backlight off of the rear-fog switch--the pink wire and the black wire. Used the cheapo wire splices.
Gauge lights and dims fine, but now my dome lights don't work, the trunk light doesn't work, and when I open the glove compartment, the "door open" chime starts to sound.
What on earth did I do??

[yanga001]

It might be a grounding issue as that switch might toggle between a resistive bank for the rear fog light. I am not too familiar with how it works for a p80 and would review the schematics, however giving a light resistive load may mess with the power distribution in the vehicle. I would recommend soldering with a connector to one of the bulbs that backlight the instrument cluster or tapping into the radio backlight.

If you disconnect the gauge power, does it fix your issues. If it does not then a wire may be shorting out or a relay or regulator may be damaged.

[Gizmo]

Might be worth running that gauge to a separate ground, at the very least?

I did a bit more digging:

apparently I bllew the 10A fuse at position 15.
Replaced it, and my interior lights came back on, and the door chime stopped when I opened the glove.
HOWEVER, now it chimes when the key is in position II (with doors closed) and when I start the engine (with doors closed). It chimes 8 times, then stops.

If its at all relevant, the gauge I installed has LED backlighting... I know LEDs can have odd amperage demands that screw with circuits not expecting to see them...

[Gizmo]

Also Yanga, to clarify -- I tapped the wires that illuminate the rear fog *switch*, not that power the rear fog itself.

I do think now that I may have somehow shorted the ground for the door-ajar alarm, though

[yanga001]

I thought it was a backlight but confused myself thinking an 850 did not have a backlight without looking it up.

Anyways yea it is probably a grounding or short across wires.

LED's can pull high amps when they do not have proper load resistors so that is a good theory given the backlight may go through the rheostat for light dimming. Worst case scenario try to throw a load resistor in series with the gauge and see if it fixes it. I would still verify that everything works with the gauge disconnected to ensure no damage was done.

I would ground the gauge anywhere to the frame of the car with bear metal as the ground goes to the frame of the vehicle. Its a bit unconventional in the cabin area but should still work in a pinch. Otherwise try to ground to the instrument cluster or perhaps the dome lights ground.

[Gizmo]

I was going to throw a resistor in there anyway to bring the light output down a bit to match the stock instrument brightness -- Figure'd I'd start a little under 1k ohm and see. Here's hoping that solves this issue as well.

At this point the only issue I'm having is the chime sounds exactly 8 times whenever the key is in the ACC position. What I can't figure out is -- why only 8 times? that implies whatever the issue is is momentary but consistent -- It's very confusing.

(all this because the VDO gauges with incandescent backlights are entirely unavailable due to supply chain issues )

I also wonder if in mounting to the A-pillar (I did carefully drill a hole to attach the pod with a small screw) I damaged the grounding wire to the vanity lights, which IIRC is in-circuit with the door-open switch -- but the vanity lights are working perfectly, and I can't think of any other wires under the A-pillar.

For now I've just pulled the #17 fuse that on the chime circuit to keep it from happening at all. But I think I'm going to have to take apart the A-pillar and rewire the gauge if the load resistor doesn't do it.