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· Registered
05 ML500 SE
1,482 Posts
Discussion Starter · #1 ·

I noticed the part pictured above making a whistling sound (when on). It seems to turn on when the car first starts and stays on for 30secs or so and then cycles off. I noticed no obvious vent and yet i could feel it blowing air around the arrow in the picture. With the whistle and the air.. it made me wonder if it was leaking. But since I don't know what it is.. my worries could be baseless.


· Registered
05 ML500 SE
1,482 Posts
Discussion Starter · #3 ·
Ah.. Secondary air injection.

The secondary air injection heats up the catalytic converter more rapidly to its operating temperature and thus improves the
emission levels in the warming-up phase.
The electric air pump (M33) is actuated through the secondary air injection relay. The current consumption of the electric air pump
(M33) is as much as 35 A.
Air injection relay and air pump switchover valve (Y32) are actuated by the motor electronics control unit simultaneously for
90 seconds after engine start, if:
- Coolant temperature > 10 °C and < 60 °C
- Engine speed < 3000 rpm
- Throttle valve not fully opened.
After an actuation, the air injection remains blocked until the coolant temperature has risen to more than approx. +60 °C and then
drops again to below 40 °C.
The pump draws in air through a maintenance-free filter and pumps it to the air shutoff valve. The valve at the same time prevents
exhaust gases from flowing back to the air pump (check valve for injected air discontinued).
The air pump switchover valve is supplied with vacuum from the intake manifold through a check valve. The air pump switchover
valve, when actuated, switches the intake manifold pressure through to the air shutoff valve (126). The aspiration shutof valves open
and the air supplied by the air pump is forced through drillings in the cylinder heads to the exhaust side.
The injected air reacts with the hot exhaust gases in the outlet port. An oxidation of carbon monoxides (CO) and hydrocarbon (HC)
takes place and results in an additional increase in the exhaust temperature.
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