the solenoid is on the left side of the valve cover when looking straight on (top of engine)
When driving the vehicle with the rpm
high enough for VTEC operation, the engine control module (ECM)
operates the variable valve timing solenoid to open the solenoid and
allow oil pressure to be applied to the pistons in the intake rocker
arms. This action causes the outer rocker arms to be pinned to the
middle rocker arm and now the three rocker arms operate as a unit,
using the high-lift camshaft lobe for improved high-end power. When the
engine rpm drops below VTEC operation, the VTEC solenoid is not
energized and hydraulic oil pressure is turned off. This allows the
return spring to return the pistons to their rest position and lets the
rocker arms operate independently again. The VTEC transition between
low and high rpm is smooth and normally not felt by the driver.
When the ECM energizes the VTEC
solenoid valve to turn on the VTEC system, the ECM expects the VTEC oil
pressure switch to open and see the voltage on the circuit. If there is
a delay with oil pressure opening the switch, the ECM will set a
trouble code and may limit fuel delivery, causing a driveability
problem.
Reasons for no VTEC
operation include low engine oil level, VTEC solenoid not opening or
the screen behind it is restricted, an engine that has low oil
pressure, or an oil pressure leak to the rocker arm assembly. In some
cases, when engine work is performed, a missing or damaged O-ring can
cause incorrect oil pressure to the VTEC system.
When diagnosing a trouble code P1259,
always look at freeze-frame data. This data will tell you if the code
is set at high or low engine rpm. If the code is set below 4,000 to
6,000 rpm (depending on the engine), check the oil pressure switch
circuit for a faulty switch/connection or poor ground. When the code
sets during VTEC operation, check for an oil pressure or mechanical
problem with the VTEC system.
