For what it's worth, the following is from the 1994 PC/ED Manual, rather than the 1990 manual (which I don't have):
QB1 CHECK KAPWR TO POWERTRAIN CONTROL MODULE (PCM)
Continuous Memory DTC 15/512 indicates the PCM has experienced a power interrupt in its Keep Alive Memory (KAM) circuit.
Note: If KAPWR is interrupted to the PCM, for example when installing a breakout box, or when battery is disconnected, DTC 15/512 may be stored in Continuous Memory.
-Disconnect Powertrain Control Module (PCM). Inspect for damaged or pushed out pins, corrosion, loose wires, etc. Service as necessary.
-Install breakout box, leave PCM disconnected.
-Measure voltage between Test Pin 1 and Test Pin 40 or 60 at the breakout box.
-While observing DVOM, grasp the EEC-IV harness and wiggle, shake or bend a small section while working from the PCM to the dash panel.
Does DVOM indicate less than 10.5 volts?
SERVICE open circuit. REMOVE breakout box. RECONNECT PCM. RERUN Quick Test.
RECONNECT PCM. GO to QB2 .
QB2 INSPECT ENGINE COMPARTMENT WIRING FOR PROPER ROUTING
-Inspect EEC wiring for closeness to ignition components or wires. If EEC wiring is close, reroute as necessary.
-Clear Continuous Memory DTC (refer to Quick Test Appendix, Section 5A).
-Wait five minutes to allow DTC 15/512 to reset.
-Rerun Key On Engine Off Self-Test.
Is DTC 15 or 512 still present on retest?
REPLACE PCM. REMOVE breakout box. RERUN Quick Test.
REMOVE breakout box. RECONNECT PCM. SERVICE other DTCs as necessary. If none, testing is complete.
Also, the chart listing all the codes states to replace the PCM if code #15 is read with the Key On, Engine Off (KOEO) test and the Key On, Engine Running (KOER) test...
The PCM is located inside the passenger side kick panel: