SOCIETY OFAUTOMOTIVE ENGINEERS, INC. 400Commonwealth Drive, Warrendale, Pa.15096 AnAutomatic Transmission In Line Haul Vehicles After Two Years ofFleet Evaluation E.R.Cottingham Detroit Diesel Allison Division ofGeneral Motors Corporation Seattle, Washington August 11-14, 1975 August 11-14, 1975750730 750730 Downloaded from SAE International by North Carolina State Univ, Tuesday, September 18, 2018Downloaded from SAE International by North Carolina State Univ, Tuesday, September 18, 2018750730 AnAutomatic Transmission In Line Haul Vehicles After Two Years ofFleet Evaluation E.R.Cottingham Detroit Diesel Allison Division ofGeneral Motors Corporation SINCE 1956 AUTOMATIC TRANSMISSIONS have been gaining inpopularity inmedium and larger truck usage. Asaresult, in1970 Detroit Diesel Allison began theproduction release ofa family ofthree automotive truck transmissions[<_->] theAT-MT-HT family (shown inFig. 1)-forthe entire commercial truck market. The ATtransmis sion hasbecome very popular with both gasoline anddiesel engines upto200horsepower inpickup anddelivery trucks, small commercial buses, and school buses upto36,000 pounds gross vehicle weight. TheMTseries transmission isused exten sively inreardump and transit mixer applications, garbage packers andbuses. Itisalsoused with both gasoline anddiesel engines ofupto250horsepower andwith governed speed ranging from 2400 to4000 rpm. TheHTseries wasfirstintroduced inOctober1972 foruseinavariety ofapplications ranging from buses tolinehaul vehicles andforusewith, primarily, gasturbine and diesel engines ofupto 400nethorsepower andgoverned speeds from 1900 to3000 rpm. Asshown inFig. 2,abasic HT700center gear section isadaptable toseveral different input and output modules. Currently, fourdifferent transmis sion models, asidentified inTable I,arebeing provided tosatisfy industry needs. Experience oftheHT740-CT gasturbine version isvery good, butithasbeen limited todate because gasturbine engines arenotasyetfully production available forbusandtruck usage. TheHT740-D is popular inlighter weight vehicles such asfiretrucks and intercity buses and heavy duty truck applications powered with high torque riseengines. ABSTRACT Automatic transmissions inline haul vehicles hadbeen practically non-existent until theAllison HT750-CRD model was specifically designed for thatvocation. Prior tothestart ofproduction ofthis newautomatic, acomprehensive market survey was conducted todetermine linehaul carrier interest in theautomatic asasolution tomanual transmission related problems. Asaresult ofthesurvey, anex tensive Fleet Evaluation Program wasinitiated to firmly establish product acceptance, productdurability, and cost comparisons with manual equipped units. Theprogram wasdesigned tocover abroad range ofvehicle duty cycles andvehicle and engine makes. The findings ofmore than two years offleet testing arediscussed, along with theinitial design ofthetransmission and itscontrols andtransmis sion refinements and installation improvements derived from theprogram. Copyright ©Society ofAutomotive Engineers, Inc.1975 Allrights reserved. Downloaded from SAE International by North Carolina State Univ, Tuesday, September 18, 20182 Some ofthetransmissions inthese applications have successfully completed over 900,000 miles and arestillrunning well. TheHT750-DRD deep ratio, diesel version uti lizesthebasic HT700four-speed gearsetincom bination with alow-speed planetary toprovide the increased ratio coverage required insuch applications asrear dumps, heavy equipment transporters and logging trucks. The useofthistransmission isexpanding rapidly into many different types ofservice. The HT 750-CRD close ratio, diesel version differs from theothers oftheHTfamily inthatthe topfour ranges have acloser ratio split ofap proximately 24percent. Thus, itprovides thehigher horsepower output andefficiency required forthe linehaul vehicles -itsintended application. This unit istheprime subject ofthispaper. Also, asshown inFi