931's can sometimes be struck with odd running problems. The added electronics (as compared to NA cars or carbed cars) can really complicate diagnoses. The most important thing to remember when working on these cars is that vacuum/boost leaks must be resolved before much progress can be made on diagnosis of advanced electrical problems. Furthermore, problems can also arise (with the ignition) due to bad vehicle wiring - such as to the coil/tach, or grounds. In some cases, the DITC (Digital Ignition Timing Computer) may be damaged due to age or other factors. The following steps can be of help in possibly fixing the DITC.
Note: This section only applies to the 81-82 931 with Digital timing control; 1980 931's had a different system not using the same digital controls.
Due to its location below the heater core, which can be subject to failure and leaks, the DITC can become dirty. This can only be determined through removal, disassembly, and inspection of the DITC. The unit shown removed above is clear of residue, though dusty. Often there will be a trace of leakage on the cover, though. Remove the DITC by disconnecting the boost sensing line (12mm hollow bolt), flywheel crank sensor and main harness connector. Remove the board from its cover by carefully bending back the tabs of the cover and pulling the board out gently by its end (black plastic part with connections). The board should only be cleaned using a proper circuit board cleaner, such as from Radio Shack. Use of harsher chemicals could damage the board. Clean both sides until residue is gone.
Further repairs may be necessary. Intermittent faults might be caused by poor solder joints. Using a soldering iron, this problem can be fixed by re-flowing every solder joint (from the bottom side of the board). However, this must be done carefully to not bridge connections that were not already bridged. Seek help from a friend handy in electronics if necessary. All that is necessary to reflow a solder joint is simply touching the soldering iron to the joint until the solder re-liquefies.
The final step is possible damage to the board. Visually inspect every trace and joint, look for physical breaks. If there are physical breaks, they may be bridged either using solder or a small wire, carefully soldered in. Look for damaged or burnt components; they may need to be replaced.
