Deciphering the identifier for a diode

Comments on your schematic and circuit:

1. A 7805 is not designed to handle the full range of car power, including short high voltage bursts. This has been mentioned to you before. See previous answers for details.

2. You have D1 the right way around, but I would make it a Schottky. Those react faster and will be fine for these relatively low voltages.

3. Fix the schematic to be less of a ratsnest. Traces worming their way all over the place makes it hard to read. See https://electronics.stackexchange.com/a/28255/4512 for details.

4. Fix the schematic to add part values! You seem to have mentioned some of them in the text, but I'm not going to remember everything you said when looking over the schematic. Do it right, especially since you are asking others to look at it.

Without part values, it is difficult to critique anything but the overall topology. That seems OK, within the detail provided by the schematic.

Added after schematic cleaned up:

That's a lot better. It looks basically OK.

I didn't look up the maximum input voltage of the LM2576. Check that it can handle car power glitches.

The 12 V sense lines driving the PIC inputs look fine.

Check that the BC547 can support the relay coil current with a 10 kΩ base resistor driven to 5V. That should provide about 430 µA base current. If the minimum guaranteed gain of the transistor is 50, for example, then this can support up to 21 mA collector current. That's cutting it close if the relay needs 20 mA, and of course is insufficient if it needs more.

First, replacing this diode will require replacing both D1 and D2 to try to keep the diode characteristics matched. Trying to match an easily available Schottky diode requires the following points: 100V reverse blocking, 100mA continuous forward current, and junction capacitance at bias (1.2pF nominal at 10V reverse). Searching Digikey gives the BAT41 (product page). It matches the blocking voltage, forward current, and capacitance (figure 5) characteristics of the original diode. A cheaper diode with more robust ratings and a lower price is the BAV20,113 (product page).

Given the age of the original diode, reverse leakage and forward voltage drop characteristics will probably meet or exceed the original diode. Since your instrument will likely require calibration after rework, you should be able to accommodate more subtle changes in the circuit performance. If you're using it for mostly qualitative or ratiometric purposes, then your concerns about calibration are less and matching each branch is more important (and, with a complete change in diode, at least the same model of diode is required).