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4L80E transmission diagnostics for Isuzu
NPR and NQR Trucks. GM W3500 and W4500
trucks. GM full size 3/4 and one ton
trucks, vans and chassis. (8600+ GVW
The Gm 4l80E
transmission is found in Chevrolet and
GMC pick ups, G vans and commercial
chassis with GVW of 8600 or greater from
1991 and up thru 2006, Isuzu NPR and
NQR trucks as well as GMC/Chevrolet
W3500 and W4500 trucks with either the
5.7 liter gas engine or the 6.0 liter
gas engines after model year 2000
trucks. this is a computer controlled 4
speed automatic with torque converter
HERE for free 4L80E
technical information such as
solenoid identification harness
pressure testing and technical
Complaint: Transmission works fine forward but will not back up
Cause: very common problems
here, the band is broken or the servo cracked, band wiped out. can also be 3rd
clutches wiped out, drum broken etc. this is always a terminal problem meaning
unit needs to come apart and likely be overhauled. (Broken drum or piston can be
results of worn boost valve in regulator, stuck regulator and TPS problem in
Complaint: Transmission stuck in second gear only.
Cause: Either stuck in limp mode due to trouble
codes, ( need scanner on this one sorry) blown fuse or other power problem, and
in some cases can be internal debris have stuck the shift valve in the valve
Complaint: Transmission cracked or broken off engine
around bell housing bolts.
Cause: If you find your 4L80E has cracked around the bell housing or broken off
completely first check for corrosion build up between the transmission bell
housing and engine housing, its very common to have enough festering corrosion
build up that the transmission "hydraulically" pushes off the engine and breaks
the housing around the bolt areas. if no corrosion is found go thru checking for
bad or frozen u-joints, drive line angle issues etc as will reoccur. Be sure to
clean up rear of engine block before replacing transmission.
Complaint: Transmission will not shift out of first gear at all.
Cause: check to see if output speed sensor is working, may
or may not have code, common problem here. can also be MLPS switch showing low
gear when in drive and such, again need scanner to see. and also can be stuck
shift valves in valve body due to debris. Way to check if internal problem is to
pull harness off unit and drive, should start in 2nd gear, not first. if still
in first then has stuck valves etc. (will have to clear codes after doing this,
disconnect battery will do if no scanner)
Complaint: All shifts are hard, no trouble codes at all.
Cause: Check charging system first, may be low voltage in truck,
not enough amps to control EPC sol, may not set any codes. can also be faulty
EPC, AFL valve problems, press. regulator valve stuck and or worn boost sleeve.
check TPS operation on engine. can be wrong, stuck and so-on.
Complaint: Transmission has no overdrive/lockup, 1st 2nd and 3rd ok.
Cause: Can be some trouble codes stored in system, need scan tool to retrieve here, besides 2nd gear only, certain codes omit 4th and lock-up only. Possible stuck valves or damaged overdrive clutches as well if no trouble codes.
Complaint: Trouble code PO894
Cause: this means the Lockup clutch is not working, you can check this with scanner: when
TCC duty cycle near 99% the TCC slip should be less than 25 rpms. one big problem here is the converter clutch piston is cracked in the torque converter. if the unit is otherwise working fine and clean on pan inspection you may likely get away with a torque converter replacement.
Complaint: Transmission slipping before or after overhaul
Cause: Slipping shifts, such as drawn out 1-2 or 2-3 hot or cold: Can be the EPC solenoid is defective, the '04 and newer EPC's are very bad, always replace on rebuild, if already replaced or know is good then with gas engines such as 6.0 you should check the mass air flow sensor, this is how
TCM gets information for transmission pressure rise. a simple test can be to disconnect the MAF sensor and drive truck. if
problem goes away then replace the MAF sensor.
Complaint: No lock-up converter clutch working and no codes.
Cause: This can simply be the brake pedal switch is sticking, misadjusted or bad, Brake pedal is input to tcm to shut off converter clutch so if tcm sees brake on will never allow lock-up to come on. you can see this in data on a scan tool.
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