A while ago I did the first thread on why later Astra Mk4 (post 2000) were having such problems lighting up their speedos and mileage displays with blue LEDs. In case you don't know the mileage display has had a yellow diffuser installed under the glass screen, here's what it looks like in the original pic I took for the first thread
The black faced speedos also got a diffuser but in this case it was a yellow layer impregnated onto the facia, like so
You can clearly see the yellow all over the symbols, the mileage display one was removable but nothing you could do about the yellow layer----
Whilst mucking about with the Mk2 Speedo conversion I'm doing I was working with the actual facia shown in the pic, I needed to remove the centres of all the needle holes so I could illuminate the needles, on the first one I cut it out but that wasn't great and the knife slipped slightly once and scratched the front of the facia, so I decided another method would be better. Whilst in the process of doing this other method I noticed the yellow layer peeling away, this greatly intrigued me and I looked more closely at the back of the facia, I then had what can only be described as an ephiffany, 20 mins later and here's the result, this is mounted on my Mk2 speedo which has the LEDs spaced for 160mph dials not 140 so please allow for a little uneveness in the light spread. I deliberately left the MPH part of the facia covered in the yellow layer and then removed it so you can see the difference it made. Before I had tried to light this same facia up with much stronger blue LEDs and got nowhere but now it's a completely different story.
After removing the MPH layer
I've only got the speedo part converted on the MK2 and that's using low powered LEDs, might try some higher powered ones in the tacho side and see what looks best.
So good news to all those owners who thought they were stuck with orange dials