With the age of the chassis plus engine/turbo heat, the engine harness is prone to failure.  This is the thick bunch of wires that snakes out from the passenger side firewall, down along side of the front subchassis rail, up the passenger side strut tower, and up and over to the engine.  This wiring harness includes almost all the engine sensor and control leads - this also includes the main ECU ground that runs from the ECU to the top of the engine.

Troubleshooting any electrical problems due to this harness needs to be systematically checked with the Mazda factory service manual.  A brand new harness costs over US$1,000 from the Mazda dealer!

To minimize damage from heat, cover the wiring harness with insulative tape.  Be careful when working in and around the harness to prevent accidental damage.

A common trouble area for the wiring harness is right behind the water pump housing, right below the alternator.  The wiring harness is secured by a clamp/nut that secures the lower alternator bracket.  This clamp/nut sometimes can twist out of alignment and cut the harness.  When this happens, it grounds out a lot of the wires inside.

At the very "ends" of the wiring harness is the plugs for the ACV, BAC valve, and O2 sensor.  These ends are subject to the heat radiating from the turbo just below.  These plugs/wires are typically damaged from the heat.  The O2 sensor plug is just a female spade (0.250") connector (which can be esily crimped on.  The BAC valve plug is the same as a fuel injector plug (readily available and easily spliced in).  The ACV leads are a bit more trickey, but hopefully these can just be crimped together if there are long enough usable wire leads.

Most important is the ECU ground leads on top of the engine.  If these ground leads are not securely attached (or the engine grounded properly), this can lead to a lot of engine problems.  Problems can range from minor hesitation to the engine not being able to start and idle at all.  The ground leads are a pair of black wires crimped to a single closed-end connector (circle) or lug.  This lug is secured under one of the bracket bolts that's on the top of the engine.  You need to remove the (stock) top-mount intercooler and upper intake manifold to inspect this connection.

Questions?  Comments?  Send mail to:  reted@fc3spro.com