Aisin 450-43LE tech tips

Diagnostic Tech Tips for Aisin 450-43LE transmissions in Isuzu trucks, Nissan UD trucks, GMC and Chevrolet W 3500 and W4500 trucks, Mitsubishi Fuso trucks, Hino trucks and Bering trucks
The recommended fluid for the Aisin 450-43LE transmission is DexronIII.
The Aisin 450-43Le transmission was first introduced in 1998 model year trucks.

aisin transmission


For some Free technical information such as pressure testing transmission, component locations, applied elements and some simple tests you can perform on the Aisin 450-43LE transmission please follow this Link to view PDF document.


Symptom 1: Transmission goes into reverse ok, when put into drive, seems like slipping or sluggish. pull shifter to low and goes in 1st and off you go! go to 2nd and shifts to 2,drive and you have 3rd, harsh shifts on all. will not have o/d and lock up, stop and stays in 3rd unless you pull manual low again. Cause: likely blown trans. fuse. (can be other power or failed Tcm problems) Mitsubishi truck and UD truck fuses are in the fuse panel. Isuzu truck and GM hide the fuse: remove glove box and look to the left, wrapped up in harness are two fuses. one is for the transmission Tcm power. you may notice that the o/d cancel light and trans lights do not work without power.


Symptom 2: Transmission goes into reverse ok, when put in drive wont move or intermittently wont move. when put into low 1 off you go and can then up shift to 2nd,3rd and o/d and lock up all works fine until you come to stop again and happens all over. Cause: low Sprague inside transmission has or is failing. this requires disassembly and or overhaul of the unit. can be mistaken as electrical problem but is not. will require total disassembly and or rebuild of transmission. Planetary part number from Isuzu: 8-97257013-0 and Isuzu part number for sprague is


Symptom 3:  Transmission will not shift out of first gear at all.  Cause: two possibilities; one is both speed sensor 1 and 2 are not working, thus Tcm doesn't know truck is moving (should be trouble codes set) or debris from internal failure or if replaced unit already may be from cooler etc has gotten into valve body and stuck shift valves and so on. (this may require an overhaul) Good way to test this is unplug trans. harness and drive truck. should start in 3rd when in drive, pull 2 and have 2, low have 1st. if this works then you don't have stuck valves etc internally.


Symptom 4: Transmission fluid is severely burnt, unit overheating very badly in short time of driving, slipping, may not move at all. Cause: possible cooler restriction, can be due to cooler lines hooked up backwards to filter adapter on Isuzu/Gm trucks, Thermostat stuck or damaged on Mitsubishi trucks, kinked lines, clogged cooler. Mitsubishi trucks also have stand alone cooler and fan, switch, which can be failed fan, switch etc and will cause severe overheating! there's test explained in diagnostic information we offer for these trucks under Mitsubishi in the 450-43LE section. Its also very common someone has installed lines at filter head, cooler etc. backwards thus deadheading the cooler flow thru unit. this makes a very distinct odor and can ruin the transmission!


Symptom 5: Engine stalls when Transmission put into gear, no lock up, pressure problems (slipping)  fluid may be very clean if caught soon enough, no codes. Cause: can be cutback valve sleeve in valve body has broken which can cause any or all of the above issues. can also ruin unit if run too long. Sonnax makes kit to repair: 94987-01K


Symptom 6:  Severe leak out of front seal area, after removing unit you may find converter hard to remove, stuck, and or comes out with pump bushing stuck to hub.

Cause: first and obvious one is an alignment issue of some sort, Isuzu/Gm trucks can have flex plate assembled incorrectly causing this, Mitsubishi and others can be missing alignment dowels in bell to engine holes, worn pilot bushing in rear of crank, broken flex plates-loosened up bolts etc. if bushing seems welded to hub can be bad ground from battery to frame, or engine to frame and what happens is when cranking starter all those Amps are going thru the transmission and arcing like an arc welder between the converter hub and pump bushing which equals transmission destruction!


Symptom 7: Transmission wont shift up unless foot taken off throttle or shifts very late, may or may  have TPS code 21. Cause: this scenario usually means the throttle cable from pedal to injector pump linkage is loose. Depending on how loose is how late may be shifting or not at all. Reason for this is TPS is inside cab on pedal assembly, you have pedal and TPS floored and engine is actually maybe only 1/2 throttle. Adjust throttle cable so there's no slack in it. This can be adjusted under cab by linkage bracket. Possible code 21 set in memory, perform TPS relearn procedure after any adjustments/repairs


Symptom 8: Vehicle speed sensor code for either speed sensor 1 or sensor 2 in memory but truck works fine with no problems. Cause: one of sensors is dead, Ckt open etc. this will not effect the operation of transmission but if the other sensor goes bad etc you will not be shifting at all! should be fixed, there's two of them, ones for backup if other fails.

Sprague or one way clutch rotation direction for Aisin 450-43LE transmission: Click Here to view PDF file of both overdrive sprague and low reverse sprague proper rotation and hold directions.


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