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
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
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