Chapter 5 Ignition and starting system

931 DITC Board Repair

Homebrew Digital Ignition - Peter Ballum pointed me to this project possibility. Why is it in a 924 specific site? Well, I assume lots of 924 people, like myself, selected the car with an eye to an economical hobby car. The ignition systems are getting pretty old. The replacement black boxes are seventies technology yet still relatively expensive. So I am examining alternatives. If I could come up with a replacement that was a couple of decades more current, cost at least no more, maybe less, and if need be could be built in such a way to hide it from the ever more intrusive politically correct OEM only police, why not!?

So far though, if I had to replace the system then I believe the Electromotive folks would get my first $400 bucks.

Bosch Spark Plugs for the 924

$11 spark plug wire set
Being the cheap guy I am (hey, I'm supporting 4 project cars) I found/made a very inexpensive set of wires for my 931. Are they Magnacores? no, but do they work? yes. When I purchased my new distributor cap I noticed the connectors looked very similar to pre-73 General Motors stuff. I also purchased a new set of plugs so I would not have to contend with the threaded ends of the current plugs. At this point I had a cap with standard connectors and plugs with standard ends, so I headed to NAPA and shopped for pre-made wires by the inch. I think I bought 1 12" length, 2 18" lengths, and 1 24" length. I also bought a different 12" wire to use as a coil wire (bring you old wires with you for comparison purposes). If I was really concerned about maximizing performance on minimum budget I would have bought a cut-to-length set of Mallory's or MSD wires and attached my own ends. I hope this will save you guys a few bucks.

931 DITC Board Repair (81-82 931 only)

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.

DITC Installed  DITC Removed

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.