Question: How can I make it easier to diagnose engine troubles ?

The MAZDA RX-7 86-88 does not have a dash check engine light, but you can make one yourself very easily. This way you can find out if the Engine Control Unit (ECU) is "throwing" an error code.

To build this code checker, you will need two 12V led lamp assemblies (available at RadioShack) and 3 1/4" male spades. You need to crimp one spade on the black wire of each lamp (referred to as "black1" and "black2") and one spade on the joined red wires (referred to as "red"). There is a diagnostics connector on the driver side in the engine bay next to the healight (green with six female 1/4" slots). With the tab facing up, insert the "red" spade into the upper middle slot, the "black1" spade into the upper left slot, and the "black2" spade into the lower left slot. It is useful to install the led lamps inside the cabin so that you can keep an eye on them when driving under various conditions.

If the leds start flashing at any time during driving, the ECU has detected an error. The number and duration (short or long) of the flashes on each led should be matched with the patterns found in 86-88 ECU error codes.

Often, it is necessary to keep track of some of the ECU inputs/outputs (at idle or while driving) to properly troubleshoot a problem. The most radical way to do this is to tap wires onto each wire at the wiring harness ECU connectors. The tapping wires are then connected to female interlocking connectors for easy and comfortable monitoring (with a voltmeter) inside the cabin. I have personally used female interlocking connectors with nine (3 by 3) and twelve (4 by 3) female pins available at RadioShack.

This above diagram shows the wiring harness ECU connectors. Note that these connectors are shown as you see them plugged into the ECU, that is, from the back (not from the front).

This above chart shows what the ECU pin signals should be with key ON and with engine idling. By comparing, for a given ECU pin, the actual signal and the one given in the chart, troubleshooting becomes almost a breeze. Obviously, you check the input signals first. Once the input signals seem to be ok, then you look at the output signals. Makes sense to me !