Hi,

To type the code, it's sort of like tapping out the Morse Code with the button.

I was able to reset the service light on my '95 GLT with the following instructions, which I found on the internet (it might have been on Matthew's Volvo site):

To Reset Volvo 850 service light

There are two diagnostic units(it says A and B on the cover) located adjacent to washer reservoir under the hood.

1. Turn on the ignition and open the module A and insert the flylead into socket number 7
2. Press the button on the diagnostic unit 4 times, briefly but firmly.

Then the LED illuminates and stays illuminated, and the system is ready to accept manual code input. A code number for resetting the service reminder light is 1-5-1.

3. To do this, press the test button once and release it. And LED illuminates again.
4. Then press 5 times and wait for the LED to illuminate.
5. Press the button once and LED should flashes several times in succession and the reminder should have been reset.
6. Switch off the ignition and replace the flylead in its holder and close the cover.