Hummer H1 Service Manual page 211

5-12 Transmission/Transfer Case
Road Test
1. If engine and transmission have cooled down, drive vehi-
cle 10-12 miles to return to normal operating temperature.
NOTE: The engine coolant and transmission fluid tempera-
tures must be at normal operating levels for a proper road test.
The engine and transmission temperature sensors will inhibit
upshifts otherwise.
2. Check torque converter operation as follows:
a. Operate vehicle at 50-55 mph (80-88 km/h).
b. Lightly apply brakes and check for converter clutch
release. Engine rpm will increase slightly when
release occurs.
c. Release brakes, accelerate again, and check for
converter clutch apply. Engine rpm will fall off
slightly when apply takes place.
d. If shudder occurs during converter clutch apply, or
clutch won't release, problem is with one of the
converter control valves, solenoid, or fluid level is
low. If shudder occurs after clutch apply, problem is
with engine, mounts, or other driveline component.
e. If more than normal throttle opening is required to
accelerate and maintain cruise speed, stator roller
clutch may have failed.
3. Check shifting in both D ranges. Verify 1-2 and 2-3
upshifts and 3-4 upshift in overdrive D range. Then
manually shift into 2 position at 25 mph. Transmission
should immediately shift into second gear but not upshift
to third. Stop vehicle and shift into 1 range. Accelerate to
25-30 mph and note operation. Transmission should
remain in first gear, provide overrun braking effect at
decel, and not upshift. Stop vehicle, shift into reverse and
backup. Vehicle should move smoothly in reverse. Check
kickdown operation in both D ranges. Transmission
should downshift promptly when accelerator pedal is
moved to wide open position.
4. Drive vehicle in all gear ranges. The following indicate
transmission mechanical or electrical fault:
• loss of one or more gear ranges
• engine flare during shifts (indicates clutch slip)
• rough-harsh shifts
• noise in one or more gear ranges
• delayed or harsh engagement in D or R
• shudder or surge during shifts
• engine starts in range other than P or N
• engine stalls after shift to D
• loss of drive or reverse
_____________________________________________
Road Test Analysis
Shift problems such as harsh, or late shifts, indicate a problem
with the wiring, a sensor, low fluid level, or a PCM fault. Com-
plete loss of a gear range when the fluid is cold or hot, indi-
cates failure of a drive element.
An important diagnostic tool is the Transmission Elements In
Use chart (Figure 5-15). The chart indicates what elements are
applied, holding, or overrunning in each gear range. The chart
also indicates shift solenoid status in each gear range as well.
By comparing elements in use to a problem gear range, the
number of parts to be checked can be narrowed considerably.
In cases where loss of drive, slow engagement, or engine flare
are experienced, a line pressure test is needed. This test checks
oil pump and pressure control solenoid output and condition.
A torque converter fault will require scan tool testing to deter-
mine the problem part. A fault in one of the sensors or sole-
noids will also require scan tool testing. Incorrect shift points,
no 3-4 upshift, converter shudder or slip, and harsh engage-
ment are some of the problems caused by sensor, solenoid, wir-
ing, or a PCM fault. Potential causes for common shift
problems are described in the diagnosis guides. Corrective ac-
tion is also provided. Problem causes are listed in order of
probability (most-least).
®