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I’m in the process of modernizing a few bits of lighting around the cabin. I was able to slot an LED into the courtesy light above the key without issue. While at it, I tried to get an LED to work for the rear fog and rear defrost switches. I don’t use them, but they sure would look nice with a bit more illumination.
This one is a bit of a challenge, and I’m stumped. I tested the incandescent bulbs on the bench with a 12v and 9v power supply. I then tested LEDs in both of the housings and they functioned perfectly. When I turn the ***** back in the car, no illumination seems to occur when the instrument lights are on. Curious if anyone has had similar issues, or found a way to get 2721 equivalent LEDs to work here.
Hi JSF,
LEDs are a diod not a resistive load like a bulb. As discussed above they only work with power in one direction, if reversed +/- they do not illuminate.
Be aware if the Immobilizer light or the seatbelt light in the clock are replaced with LEDs it will trigger an OBD fault and will stay illuminated.
Here is a link to the wiring diagrams that may give you insight into your issue: https://993servicerepair.blogspot.co...-diagrams.html
If you resolve your issue please report back here for our education.
Thanks,
Andy
JSF - There's a diode inside the body of the FOG switch that prevents backfeeding the instrument light circuit when the FOGs are off. If you install the led in the **** reversed relative to that diode, it won't work, so just try reversing it. The front and rear fog switches and the rear defrost switch all have this internal diode. The courtesy light above the ignition does not.
I don't know which is the positive terminal on the ****, but my guess is Porsche would have made the shaft GND and the small center post +. If so, the polarity of your LED in your pic is reversed from what it should be.
Last edited by jay@EZimmoblock.com; 12-03-2023 at 03:16 PM.
Thanks everybody for the wisdom and feedback. I took ***** and flipped one of the LEDs to ensure that the correct polarity was not met, hoping that one of the sockets in the car would work in either configuration. Right now, one LED works and the other does not (only when tested on my workbench power; no power in car in either config).
The batteries on my bench seem to power the LEDs and incandescent bulbs, but only the car can power the incandescent bulbs. I’m curious if the LEDs have an issue with the contacts being present on both side and causing polarity issues when they are in the bulb socket.
Very curious if anybody has found a successful 2721/74 equivalent LED that works in these *****. It might be the case that I’d have to remove one contact rail on each side to make the LEDs consistent with the traditional bulbs.
Doesn't your pic show the LED working installed in the **** on the bench? If so, mating to the socket is not the problem.
That’s exactly the source of my confusion. It works perfectly on the bench with the ground at the very bottom pin of the **** and the positive around the barrel. Once it’s in the car, it refuses to work.
The light gets power from the Instrument light circuit (dimmer controlled) when the switch is OFF but directly from 12 v when ON. Does the light not illuminate in both positions? Dimmer switch all the way up?
That’s exactly the source of my confusion. It works perfectly on the bench with the ground at the very bottom pin of the **** and the positive around the barrel. Once it’s in the car, it refuses to work.
The SHAFT is ground. The center post is +. In your pic you have + on the shaft. That won't work in the car.
The light gets power from the Instrument light circuit (dimmer controlled) when the switch is OFF but directly from 12 v when ON. Does the light not illuminate in both positions? Dimmer switch all the way up?
Tried both variations of the bulb configuration, and with both positions of the light switch and with the dimmer fully turned on.
Originally Posted by jay@EZimmoblock.com
The SHAFT is ground. The center post is +. In your pic you have + on the shaft. That won't work in the car.
I flipped one of the LEDs to follow this, and still no light. This is so bizarre… the common denominator must be that I should try a different LED.
Thanks again, everyone! Happy to report the LEDs are the culprit (obviously). They have dual rails on each side (+/-) and that seems to interrupt the socket when it's back in the car. Did a crude test again in the car and it works this way, but of course need to work on cleaning it up (may just remove/cover one side of the LED).
12-03-2023, 04:39 AM
Fog Light + Rear Defrost Switch
