Red Brake Warning Lamp; Diagnosis And Testing; Combination Valve - Jeep 1997 Grand Cherokee Service Manual

5 - 8 BRAKES
DIAGNOSIS AND TESTING (Continued)
SWITCH CIRCUIT IDENTIFICATION
• Terminals 1 and 2 are for brake sensor circuit.
• Terminals 5 and 6 are for the stop lamp circuit.
• Terminals 3 and 4 are for the speed control cir-
cuit.
Fig. 6 Stop Lamp Switch Terminal Identification
SWITCH CONTINUITY TEST
(1) Check continuity between terminal pins 5 and
6 as follows:
(a) Pull plunger all the way out to fully extended
position.
(b) Attach test leads to pins 5 and 6 and note
ohmmeter reading.
(c) If continuity exists, proceed to next test.
Replace switch if meter indicates lack of continuity
(shorted or open).
(2) Check continuity between terminal pins 1 and
2 and pins 3 and 4 as follows:
(a) Push switch
retracted position.
(b) Attach test leads to pins 1 and 2 and note
ohmmeter reading.
(c) If continuity exists, switch is OK. Replace
switch if meter indicates lack of continuity (switch
is open).

RED BRAKE WARNING LAMP

The red warning lamp illuminates when the park-
ing brakes are applied and when there is a leak in
the front or rear wheel brake hydraulic circuit. It will
also illuminate at startup as part of a bulb check.
If the light comes on, first verify that the parking
brakes are fully released. Then check pedal action
and fluid level. If a problem is confirmed, inspect the
brake hydraulic system for leaks.
MASTER CYLINDER/POWER BOOSTER
(1) Start engine and check booster vacuum hose
connections. A hissing noise indicates vacuum leak.
Correct any vacuum leak before proceeding.
TERMINAL PINS PLUNGER POSITIONS
plunger inward to
TEST
(2) Stop engine and shift transmission into Neu-
tral.
(3) Pump brake pedal until all vacuum reserve in
booster is depleted.
(4) Press and hold brake pedal under light foot
pressure. The pedal should hold firm, if the pedal
falls away master cylinder is faulty (internal leak-
age).
(5) Start engine and note pedal action it should
fall away slightly under light foot pressure then hold
firm. If no pedal action is discernible, power booster,
vacuum supply, or vacuum check valve is faulty. Pro-
ceed to the POWER BOOSTER VACUUM TEST.
(6) If the POWER BOOSTER VACUUM TEST
passes, rebuild booster vacuum reserve as follows:
Release brake pedal. Increase engine speed to 1500
rpm, close the throttle and immediately stop turn off
ignition to stop engine.
(7) Wait a minimum of 90 seconds and try brake
action again. Booster should provide two or more vac-
uum assisted pedal applications. If vacuum assist is
not provided, booster is faulty.
POWER BOOSTER VACUUM TEST
(1) Connect vacuum gauge to booster check valve
with short length of hose and T-fitting (Fig. 7).
(2) Start and run engine at curb idle speed for one
minute.
(3) Observe the vacuum supply. If vacuum supply
is not adequate, repair vacuum supply.
(4) Clamp hose shut between vacuum source and
check valve.
(5) Stop engine and observe vacuum gauge.
(6) If vacuum drops more than one inch HG (33
millibars) within 15 seconds, booster diaphragm or
check valve is faulty.
fully
POWER BOOSTER CHECK VALVE TEST
(1) Disconnect vacuum hose from check valve.
(2) Remove check valve and valve seal from
booster.
(3) Use a hand operated vacuum pump for test.
(4) Apply 15-20 inches vacuum at large end of
check valve (Fig. 8).
(5) Vacuum should hold steady. If gauge on pump
indicates vacuum loss, check valve is faulty and
should be replaced.

COMBINATION VALVE

Metering Valve
Metering valve operation can be checked visually
with the aid of a helper. Observe the metering valve
stem while a helper applies and releases the brakes.
If the valve is operating correctly, the stem will
extend slightly when the brakes are applied and
retract when the brakes are released. If the valve is
ZG