Meatful

Any new info on where to source LEDs for the gear indicator lamps on the tach are appreciated. And any best practices for adding resistors (if necessary) are appreciated.


1986.5 928S PRND32 position micro-lamps, top

1986.5 928S PRND32 position micro-lamps, side

gazfish

I haven’t done it but also curious as they are the only non-led indicators I have left. Since only one is powered at a time, a single resistor could be used somewhere on the board feeding the common pins rather than 1 per led.

Meatful

Digikey sells some LEDs with built-in resistors for 12V applications; I ordered 10 of them. They are 3mm, which is bigger than the existing microlamps, so I may have to do some modification.

Eplebnista

Look here also. They have prewired SMD LEDs that are tiny but still bright. I used them to illuminate the slider ***** on my HVAC controller.

Lighthouse LEDs

Ed Scherer

There's a fellow in Australia (Bruce, whom I won't identify further without his permission, and who is also here on Rennlist, but not very active) who successfully did the LED upgrade to his tach. My involvement began on March 24 when he asked me about a buzzing noise he was hearing when he was dimming all his '86.5 928's instrumentation LEDs with an AILD-1. This was the same issue reported by Sean way back in 2013, but that we never followed up on (at that time in 2013) as you can see a few posts later in that thread. I never heard anyone else experiencing the issue since that time, until Bruce raised it again.

Bruce was great to work with and we finally tracked the noise down to a relay on the tach PCB. I'm guessing this relay is only present on automatics, but I'm not sure; there are a lot of variations of the tach. The relay has its coil driven by a transistor connected to the instrumentation lighting circuit (i.e., the BK/BL wires connected to all the dimmable instrumentation lighting). The relay doesn't react well to the AILD-1's 125 Hz modulation of the instrumentation lighting supply current.

After about a month of back-and-forth e-mails and reverse engineering and experimentation, we finally got a modification to the tach circuitry that allows its LEDs to be dimmed and prevents the relay from buzzing. The modification requires an additional four components: a transistor (maybe two if you don't want to reuse one that you need to remove and replace), a rectifier diode, a resistor, and an electrolytic capacitor.

I have lots of notes from that episode that I have yet to rewrite into sharable form. But this thread reminded me that I should do that, as I'd hate to see anybody else "be in there" and not take care of this potential problem when it's most convenient.

I still don't know why this problem hasn't been seen (heard!) by others. Like I said, my research revealed that there are many, many variations of the tach PCB (for different configurations for different countries and perhaps different options, etc.), so it's possible that the relay might be present or might not be, might be wired differently, or might be a different model that doesn't buzz when its coil is driven with non-DC.

I'll see if I can get Bruce to participate in this thread, as I was just consulting on this, while he did the hands-on work. I was just working on a protoboard, trying to replicate as best I could (with the parts I had on hand) that part of the tach circuitry related to the relay problem and the LEDs.

Ed Scherer

Regarding LED selection, I don't think I ever found out exactly what LEDs Bruce was using, but my selection criteria would be:

• appropriate color (I'm guessing red; whatever's consistent with the PRND32 lenses/filters)
• appropriate beam angle (I don't know the geometry of the tach and how far the LED sits behind the PRND32 lenses/filters); too thin and you get a central hot spot; too wide and you might waste light
• appropriate brightness (you might regret making it too bright)
• package: probably 5 mm (maybe 3 mm?), through-hole, discrete (a.k.a. "component")

I don't have one of these tachs to play with myself, so my knowledge is all based on examination of photos, thus, I can't provide an exact recommendation.

As to the current-limiting resistors, once you've selected an LED, just use an online calculator (like https://www.allaboutcircuits.com/too...or-calculator/). Go conservative: use 14 V for the supply voltage, the LED specs for voltage drop (a.k.a. forward voltage) and forward current, and then choose a resistor that's higher resistance than what's calculated. The higher the resistor value, the dimmer the LED will be, but the less heat it will generate, thus prolonging its life. Remember that these LEDs may often be operating in a very hot environment.

Meatful

Wow, that's quite a story. Do you think the reastat and/or the more basic variable pot options would produce a different result? I would assume so since they are not introducing what I assume is a pulse train of some ilk from the AILD.

Ed Scherer

It just doesn't work very well. A collection of LEDs (e.g., all the ones for the dimmable instrumentation lighting) won't generally respond (in perceived brightness) linearly and similarly to changes to voltage.

Meatful

Here is a short video (no sound) of some pics, and then a clip of shifting through the gears. Although my smartphone camera shows them washed out a bit, in person it's super crisp and not bright enough to distract. I definitely recommend this mod.

PRND32

Longer video with dash illuminated with all LEDs

The LEDs I used were the 3mm LTH3MM12VFR4400 series sourced from Digi-Key. They include a load limiting resistor and are high quality.

Thanks all for the inputs.