Jeep 1997 Grand Cherokee Service Manual page 336

8C - 4 CHARGING SYSTEM
DIAGNOSIS AND TESTING (Continued)
and the Electronic Voltage Regulator (EVR) circuitry.
The second part of this test (Test 2) will determine
only generator amperage and will not include anal-
ysis of EVR circuitry. EVR circuitry is located within
the Powertrain Control Module (PCM). To test volt-
age regulator circuitry, refer to the appropriate Pow-
ertrain Diagnostic Procedures service manual.
PREPARATION
(1) Determine if any Diagnostic Trouble Codes
(DTC's) exist. To determine a DTC, refer to On-Board
Diagnostics in this group. For repair, refer to the
appropriate Powertrain Diagnostic Procedures man-
ual.
(2) Before starting test, make sure battery is in
good condition and is fully-charged. See Group 8A,
Battery for more information.
(3) Check condition of battery cables at battery.
Clean if necessary.
(4) Perform the previous Charging System Resis-
tance Tests (voltage drop tests). This will ensure
clean and tight generator/battery electrical connec-
tions.
(5) Be sure the generator drive belt is properly
tensioned. Refer to Group 7, Cooling System for
information.
(6) A volt/amp tester equipped with both a battery
load control (carbon pile rheostat) and an inductive-
type pickup clamp (ammeter probe) will be used for
this test. Refer to operating instructions supplied
with tester. When using a tester equipped with an
inductive-type clamp, removal of wiring at the gener-
ator will not be necessary.
(7) Start the engine and allow it to reach operating
temperature.
(8) Shut engine off.
(9) Turn off all electrical accessories and all vehicle
lighting.
(10) Connect the volt/amp tester leads to the bat-
tery. Be sure the carbon pile rheostat control is in the
OPEN or OFF position before connecting leads. See
Load Test in Group 8A, Battery for more information.
Also refer to the operating instructions supplied with
test equipment.
(11) Connect the inductive clamp (ammeter probe).
Refer to the operating instructions supplied with test
equipment.
(12) If volt/amp tester is not equipped with an
engine tachometer, connect a separate tachometer to
the engine.
TEST 1
(1) Perform the previous test Preparation.
(2) Fully engage the parking brake.
(3) Start engine.
(4) Bring engine speed to 2500 rpm.
(5) With engine speed held at 2500 rpm, slowly
adjust the rheostat control (load) on the tester to
obtain the highest amperage reading. Do not allow
voltage to drop below 12 volts. Record the reading.
This load test must be performed within 15 sec-
onds to prevent damage to test equipment. On
certain brands of test equipment, this load will be
applied automatically. Refer to the operating manual
supplied with test equipment.
(6) The ammeter reading must meet the Minimum
Test Amps specifications as displayed in the Genera-
tor Ratings chart. This can be found in the Specifica-
tions section at the end of this group. A label stating
a part reference number is attached to the generator
case. On some engines this label may be located on
the bottom of the case. Compare this reference num-
ber to the Generator Ratings chart.
(7) Rotate the load control to the OFF position.
(8) Continue holding engine speed at 2500. If EVR
circuitry is OK, amperage should drop below 15–20
amps. With all electrical accessories and vehicle
lighting off, this could take several minutes of engine
operation. If amperage did not drop, refer to the
appropriate Powertrain Diagnostic Procedures man-
ual for testing.
(9) Remove volt/amp tester.
If minimum amperage could not be met, proceed to
Test 2. This test will determine if the generator is
faulty, or if EVR circuitry is defective.
TEST 2
(1) Perform the previous test preparation.
(2) Fully engage the parking brake.
(3) Connect one end of a jumper wire to a good
ground. Connect the other end of jumper wire to the
generator field driver (-) terminal. The 2 field termi-
nals (+ and -) are located on the back of the genera-
tor (Fig. 1). To locate and identify the (-) terminal
and circuit, refer to Group 8W, Wiring Diagrams.
Another way to identify the (-) terminal is to start
the engine and measure voltage at both field termi-
nals. The (+) terminal will show battery voltage
(12.5–14.5 volts). The (-) terminal will show 3–5 volts
less than battery voltage.
CAUTION: Do not connect the jumper ground wire
to the generator field source (+) field terminal. Dam-
age to electrical system components may result.
Connecting the jumper wire will remove the volt-
age regulator circuitry from the test. It will also gen-
erate a Diagnostic Trouble Code (DTC).
(4) Start engine. Immediately after starting,
reduce engine speed to idle. This will prevent any
electrical accessory damage from high voltage.
(5) Adjust carbon pile rheostat (load) and engine
speed in slow increments until a speed of 1250 rpm,
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