Copyright by Goodheart-Willcox Co., Inc.
Chapter 16 Turbocharging and Supercharging Systems 339
To ECM
Bypass
valve
Atmospheric
pressure
To plate inside
supercharger
Vacuum
reservoir
Vacuum
solenoid
Check
valve
Vacuum
source
Throttle
body
Light
load
Bypass valve To ECM
Closed
Vacuum solenoid
To
vacuum
source
Bypass valve
Spring pushes the
diaphragm up
Open
Boost pressure is
bypassed to
intake air
B
A
C
Throttle
body
Heavy
load
To ECM
Vacuum solenoid
To
vacuum
source
Vacuum overcomes
spring pressure
to pull the
diaphragm down
Closed
No boost pressure
is bypassed to
intake air
Open
All boost pressure to intake manifold
Figure 16-15. A—This is a schematic for a typical supercharger
bypass valve control system. B—When the engine is under
full load, the ECM sends a control signal to open the vacuum
solenoid. Vacuum pulls the diaphragm down, which closes
the bypass valve. All supercharger boost pressure enters the
intake manifold. C—When the engine is under light load, the
ECM closes the vacuum solenoid. Spring pressure pushes
the diaphragm up, which opens the bypass valve. Some of the
supercharger boost pressure is returned to the inlet, reducing
the amount of boost pressure that enters the intake manifold.
Supercharger System Service
Many of the problems and tests associated with
turbochargers relates to superchargers as well. Service may
be required after extended use. Usually, a supercharger
will fail because of worn bearings or damage to the impel-
ler and housing.
Some supercharger noise or whine is normal. When
a supercharged vehicle is accelerated, a loud whine or
“scream” can usually be heard. This is the sound of the
large supercharger rotors pulling air into the engine under
full boost.
The most common supercharger service operations
include replacement of the drive belt, checking super-
charger oil, and supercharger replacement. Supercharger
service can also involve tests and repairs to the intercooler,
intercooler pump, bypass valve mechanism, and other sup-
port components.
Caution: Owners sometimes install a
smaller drive pulley to increase rotor speed
and boost pressure. This can result in
supercharger or engine failure. Boost pressure that
is higher than the factory specifications can cause
severe detonation, which can result in cracked
heads, broken piston crowns or skirts, spark plugs
blown out of the head, and other major damage. Warn
customers that modifications to a supercharged
engine can void the manufacturer’s warranty.
Supercharger System Diagnosis
When diagnosing problems with the supercharger sys-
tem, inspect the unit for obvious problems first. Are there
signs of drive belt slippage? Does the belt squeal when the
engine is started? Are there any visible fluid leaks and is the
supercharger oil reservoir full? Other symptoms of prob-
lems with the supercharger system include internal noises,
low oil level in the reservoir, pressure or vacuum leakage,
or out-of-specification boost pressure.
A rattling noise from the supercharger indicates
rotor bearing failure and mechanical contact between the
spinning rotors and the inside of the supercharger housing.
A knocking noise indicates possible drive gear teeth break-
age. A loud growling noise may indicate either bearing
failure or teeth breakage.
If the supercharger drive belt is squealing from excess
slippage, there may be internal drag or the unit may be
locked up. This could indicate bearing seizure or other
internal mechanical failure.
Place a stethoscope against the ends of the supercharger
housing to isolate any bearing noises, Figure 16-17A. Failed
supercharger bearings will make a loud rumble when the
probe is placed over the bearings on the housing. If the
unit is not too hot, you can also place your hand on the