Before the Series IV tractors, there weren't a lot of differences between an original series D17 and a Series III, the most significant being the full flow engine oil filtering system that was introduced at Serial #24001 ( i believe that # is correct...I don't have my book here with me). There were other changes as well. I'll describe differences between an original series D-17 and a Series III because my father owns one of each and I've spent a lot of time on both. I don't know off hand when all these changes occurred, but I believe that no changes were made say in the middle of Series I, II, or III production. (also, D17 tractors were never officially called Series I or II...that's only for ease of describing them)So here goes a list of changes between an original series D17 and a series III.
1. Oil bath air cleaner vs dry on Series III.
2. Much heavier wide front on Series III
3. Taller fenders on III with headlights on fenders.
4. Belt driven power steering pump on III vs. location between governor and distributor.
5. Power director and hydraulic oil is common reservoir on Series III.
6. Series III has power director/hydraulic oil filter plus power director oil pressure light.
7. Series III has self energizing brakes that uses momentum of tractor to help provide more braking power...original had simple bands like a WD.
8. Series III has different style of locks that lock the wheel to the rims on the power shift rear wheels...they're less like to get rusted up and stuck. Also on Series III, each increment is less giving one more wheel tread option even though I belive that max. amd min. wheel treads are the same.
9. Series III are a lot quieter with a large oval muffler instead of the small round straight through one.
10. there are many other changes as well, such as loaction of voltage regulator, rear work/tail lights, decals/paint color, short reach (AC45) vs long reach (AC 45XL) spark plugs, etc.The Series IV is almost exactly the same as the Series III with the exception of one MAJOR change. The Series IV has a high volume lower pressure hydraulic pump mounted in the belt pulley hole. This standardized the hydraulic system with the rest of the industry. ALso, this system is live..the pump turns even when the foot clutch is disengaged. Also, the Series IV was available with factory category II three point hitch, just like what is found on 180, 190, etc. Also, the new system allowed the use of one or two double acting hydraulic rams instead of just one single acting and the hydraulic controls went down by your right hand instead of by the steering wheel. Also, on a Series IV with snap coupler, the lift rams are external, and the back end is changed as compared to the earlier tractors to accomodate the new hydraulic system and optional three point hitch.
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