CON­VERT­ING A TRANS­MIS­SION FROM 2WD TO 4WD

Mak­ing a good deal on a trans­mis­sion a great 4WD deal

4WDrive - - Contents -

Ev­ery time we go to find a com­po­nent for a 4x4 in our sta­bles, we strive to se­lect parts that don’t carry monikers like “rare” or “ob­scure”. Gen­eral Mo­tors had a 15-year pro­duc­tion run start­ing in 1969, with the Turbo Hy­dro­matic 350, or TH350 au­to­matic 3-speed trans­mis­sion. They are plen­ti­ful, strong, com­pact, eas­ily adapted and a GREAT choice in an off-road ve­hi­cle for all these rea­sons, and can be found in any­thing from 2WD gro­cery-get­ter mid­size cars to 4WD trucks. Our buddy Derek scored a sweet deal on a TH350 to land firmly un­der the hood of his TJ based snow wheeler.

That was a few years ago and Derek’s luck with the 3-speed au­to­matic fi­nally ran out. As is nor­mal for Derek’s luck, the TH350 died AFTER a deep snow wheel­ing trip only a mere me­tre from his house. If only we had his luck. In or­der to keep costs in check and get him back into the fluffy stuff, Sum­mit Rac­ing was tapped for a re­built re­place­ment unit (PN SUM-700310 $1199.99 USD) and a torque con­verter that matches the weight, gear­ing and planned us­age (PN SUM-G2699 $129.99). The only fly in the oint­ment was that Sum­mit only sells their house brand TH350 trans­mis­sions in 2WD form. Luck­ily, we have one of the best trans­mis­sion re­builders not far from us to help make the changeover to 4WD.

After ex­press­ing our grat­i­tude and mut­tered some­thing about “ow­ing favours”, Ivar Hoi­land of Kelowna Trans­mis­sion & Auto Re­pair (KT&AR) fi­nally broke down. He caved and let us hang out at his “heal­ing bench” while he dis­sected and de­ter­mined the cause of death of the old trans­mis­sion and swapped the re­quired parts to con­vert the new Sum­mit TH350 to a 4WD unit. Typ­i­cally, we’re not afraid to tackle just about any au­to­mo­tive job we have at home, but au­to­matic trans­mis­sions re­quire a “feel” and very strict at­ten­tion to de­tail that you only get after decades of hands on work. We just don’t have what it takes and are happy to shell out the clams re­quired to do this kind of work. The best ad­vice if you are search­ing for a shop is do your re­search and ask ques­tions. KT&AR is one of a se­lect few where we can con­fi­dently drop parts and ve­hi­cles off and KNOW it’s done right, and the only one for trans­mis­sion work.

You would ex­pect that with a 15-year pro­duc­tion run, there would be a mul­ti­tude of vari­ants of the TH350 but there are only a few; Ob­vi­ously a 2WD ver­sion and a 4WD ver­sion are out there, but this is only a dif­fer­ence of a few parts. Bell­hous­ing bolt pat­terns con­sisted of a Chevro­let ver­sion, a BOP (or “Buick, Oldsmo­bile, Pontiac”) and in some rare cases, a dual pat­tern. And for a few years dur­ing the end of its pro­duc­tions run, there was a ver­sion with a “lock up” style torque con­verter known as the TH350C. Know what ver­sion you have as there are a few crit­i­cal parts in the TH350 and TH350C that are not in­ter­change­able.

Ex­ter­nal con­nec­tions, or lack thereof, is an­other rea­son the TH350 is a great swap can­di­date as it is al­most en­tirely con­trolled in­ter­nally. The only “re­quired” con­nec­tions are a vac­uum source to de­ter­mine en­gine load and a gear se­lec­tor. The TH350C ver­sion had an elec­tri­cal con­nec­tor to ac­tu­ate the lockup torque con­verter and gain some ex­tra mileage. A kick­down ca­ble at­tached to the en­gines car­bu­re­tor or throt­tle body and lets the trans­mis­sion au­to­mat­i­cally “gear down” when the motor is floored. The kick­down is a sim­ple hookup but it is not re­quired and of­ten omit­ted on many off road ve­hi­cles for sim­plic­ity sake and bet­ter driver con­trol.

So with our over­view out of the way, let’s hang over Ivar’s shoul­der and see what it takes to make a good deal on a TH350, a GREAT deal when we make it 4WD com­pat­i­ble.

Sum­mit Rac­ing www.sum­mi­trac­ing.com

Kelowna Trans­mis­sion & Auto Re­pair kelow­na­trans­mis­sion.com

1. Ex­ter­nally, the only dif­fer­ence be­tween the old and new is the ex­ten­sion hous­ing on the new grey unit in the back­ground.

2. Ivar de­cided that we should go in and get the short rear out­put shaft we needed to make the new trans­mis­sion work. He started by re­mov­ing the pump assem­bly. All com­po­nents ei­ther come out this end of the trans­mis­sion or the pan area in a TH350.

3. Pry­bars, screw­drivers, picks and spe­cialty tools from here on out.

4. Our first “Eureka!” mo­ment came after re­mov­ing the pan. Ivar pro­claimed “Torque con­vert’s dead” after look­ing at the fluid. Good thing we or­dered a new one!

5. Although it looks like this fil­ter gas­ket may be po­si­tioned cor­rectly, it isn’t seal­ing the hole above it and is al­low­ing air and un­fil­tered fluid into the sys­tem.

6. The valve body needs to be re­moved to re­lease a few bands and al­low the rest of the in­ter­nals to come out.

7. Prob­lem #2 made it­self known as the Di­rect Drum came out in mul­ti­ple pieces of shrap­nel. Good thing we don’t need it to con­tinue.

8. Ap­par­ently, this is a rel­a­tively rare oc­cur­rence. Well done, Derek!

9. Ivar de­cided that he would pop apart the other drums and clutch hous­ing to see what else may be lurk­ing. We were not dis­ap­pointed…

10. “You should not be able to shave clutch ma­te­rial off this eas­ily” we were told. “This is what hap­pens when wa­ter gets to the clutches”. This old trans­mis­sion was run­ning on bor­rowed time since it was in­stalled. YIKES !!!!

11.A few more com­po­nents still had to come out be­fore the rear out­put shaft would be re­leased to us.

12. After some strug­gling, the re­verse clus­ter and rear out­put shaft came out.

13. The strug­gle was due to the rear bush­ing be­ing se­verely side loaded at some point. This can hap­pen when a trans­fer­case is not per­fectly aligned to the trans­mis­sion.

14. Like a botched game of Op­er­a­tion, there was no hope for this pa­tient. Just load it all into a box and ship it back to Sum­mit Rac­ing as a core.

15. All that work just to re­trieve the short out­put shaft seen in the back­ground. We need to delve just as deep into the new TH350 to get the out­put shafts swapped.

16. If only it were as sim­ple as pop­ping off that 2WD ex­ten­sion hous­ing and swap­ping the shaft out. Nope. Other than this hous­ing, the rest of the work get­ting into the new TH350 is ex­actly the same as the old one.

17. Once we were at “ground zero”, Ivar checked all the clear­ances and com­po­nents in the new trans­mis­sion. All the re­fur­bished com­po­nents looked to be in ex­cel­lent shape.

18. An af­ter­mar­ket “case saver” was in­stalled by Sum­mit to keep wear to the trans­mis­sion hous­ing to a min­i­mum - a very nice touch!

19. After adding all the “clutches” and “steels” to the drum assem­blies, Ivar would check for clear­ances and then align all the com­po­nents so that re­assem­bly would be that much eas­ier.

20. This par­tic­u­lar TH350 from Sum­mit came with a few up­grades in­clud­ing the hard­ened sprag race shown here.

21. When in­sert­ing the loaded drums, Ivar can tell by feel and sound if every­thing dropped into place prop­erly. This is where we know we would have screwed up.

22. The steels and clutches here are name brand Raybestos. We are told they may not be “race spec” but are a ma­jor step above stock re­place­ments.

23. After all the snap rings, doohick­eys, thingam­abobs, uni­corn farts, hens’ teeth and what­ever else it takes to make our wheel­ing hopes and dreams come true are in place, Ivar takes yet an­other mea­sure­ment.

24. This en­sured that clutch pack en­gage­ment is within spec. Once the jig is set up, it gets moved to the pump unit it­self while it is still on the bench.

25. Tech­ni­cally, our unit was within spec, but for­tu­nately for us, Ivar is a per­fec­tion­ist and shimmed the bear­ing un­til it was at the height he pre­ferred.

26. The valve body went back on and all the link­ages were re-con­nected. We learned that there are three holes on a TH350 valve body that are a smaller di­am­e­ter than the oth­ers, these are to align it per­fectly. And for once, the en­gi­neers made things easy; all the valve body bolts are the same size!

27. Sum­mit in­cludes this very nice alu­minum oil pan with a drain hole with their re­built TH350’s com­plete with stain­less Allen head cap screws. An alu­minum pan with a qual­ity pa­per gas­ket should not re­quire any sealant. This is good as many fail­ures Ivar sees are from ex­ces­sive sealer clog­ging pas­sage­ways.

28. The fi­nal part of our re­assem­bly was re­in­stalling the gover­nor and cap. The washer was in­stalled here to put a lit­tle ex­tra ten­sion on the re­tain­ing clip. Then it was re­turned to the Jeep be­fore the win­ter wheel­ing sea­son came to an end!

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