AW-4 TRANS­MIS­SION REVIVAL

FIX­ING A SLIP­PING SLUSH­BOX

4 Wheel & Off Road - - CONTENTS - Jay Kopy­cin­ski BY EDITOR@4WOR.COM PHO­TOG­RA­PHY JAY KOPY­CIN­SKI

Fix­ing a slip­ping slush­box.

THE AISIN AW4 IS A FOUR-SPEED au­to­matic trans­mis­sion used in some Jeeps and as the A340 in var­i­ous Toy­otas. It is elec­tron­i­cally con­trolled in the for­ward gears and has been used in nu­mer­ous vari­a­tions in 2WD and 4WD mod­els. It uses a trans­mis­sion con­trol unit (TCU) along with valve body so­le­noids to con­trol shift­ing based on in­puts from a speed sen­sor, throt­tle po­si­tion, and brake pedal ac­tu­a­tion. Elec­tronic torque-con­verter lockup is also con­trolled by the sys­tem.

This trans­mis­sion has proven it­self over the years to be highly re­li­able. It may last sev­eral hun­dred thou­sand miles with proper care. His­tory seems to in­di­cate that its big­gest en­emy, as with many fluid trans­mis­sions, is ex­ces­sive heat, which dam­ages the hy­draulic fluid, lead­ing to in­ter­nal parts dam­age. Ex­ces­sive heat can oc­cur from run­ning with too low a fluid level, from strain­ing the trans­mis­sion by run­ning too tall a tire with poor match of axle gear­ing, or from sim­ply work­ing the trans­mis­sion too hard too long and gen­er­at­ing sus­tained high fluid tem­per­a­ture.

In our case, we had a 1989 Jeep Chero­kee that had been act­ing oddly for months de­spite hav­ing a proper sup­ply of good fluid. It was a high-mileage trans­mis­sion of un­known his­tory, and its be­hav­ior was con­tin­u­ing to de­grade. A quick check of the fluid at this point re­vealed it looked and smelled as if it had over­heated.

We de­cided we would tear into this trans­mis­sion and per­form a re­build. OmixADA of­fers a full re­build kit for the AW4/ A340 that in­cludes all seals, gas­kets, fil­ter, clutch discs, and steels, plus other wear items. Af­ter re­mov­ing the trans­mis­sion from the Chero­kee, we be­gan a re­build on the bench. We did the work over sev­eral days us­ing mostly com­mon me­chanic’s tools. We found that the big­gest chal­lenge of the pro­ject was deal­ing with the many snap rings of var­i­ous sizes. We made good use of both pointed snap-ring pli­ers and flat-billed re­tain­ing ring pli­ers, plus some long screw­drivers or pry bars to ex­tract snap rings deep in­side the case.

What fol­lows is a ba­sic over­view of the re­build us­ing the Omix-ADA parts kit. Space here does not per­mit us to pro­vide a lot of de­tails. We used the very com­plete AW4 fac­tory ser­vice guide from Au­to­matic Trans­mis­sion Ser­vice Group (ATSG). It con­tains de­tailed in­for­ma­tion on the trans­mis­sion op­er­a­tion along with full dis­as­sem­bly, in­spec­tion, and as­sem­bly pro­ce­dures for it.

There are a lot of parts in­volved in the build, and suc­cess takes good at­ten­tion to de­tail. Once our pro­ject was done, we also added a sup­ple­men­tal cooler to the sys­tem as a fur­ther safe­guard against fu­ture over­heat­ing.

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