truck transmission diagnostics
truck transmission trouble codes

      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 Trucks)

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 clutch. click HERE for free 4L80E technical information such as solenoid identification harness identification, pressure testing and technical charts.

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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 truck.)
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 body
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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