| Author | Message | ||
     Alistair Riddell Experienced User Username: ariddell Post Number: 19 Registered: 1-2013 |
Hi All, Just pondering how easy it would be to replace the TH400 3 speed auto box in 25955 with the 4 speed 4L80-E box from later years. Much as the drive is lovely as it is, I can't help thinking it would be improved by the addition of that extra ratio and lockdown capability. Given the similarity between the two units it does not seem that the physical dimensions or actuation of the shift mechanism would pose any great difficulties, rather the electronic control aspect of the later box that would be subject to having sufficient feeds and communication available between the ECUs. Has anyone heard of such a conversion being performed? And if so did it provide sufficient benefit to be worthwhile considering further? Thanks, Al | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 2881 Registered: 4-2003 |
You are brave, Alistair. Whilst anything is possible, a change to the 4L80E could go like clockwork but could be a big job needing lots of hardware and maybe some component fabrication or modification. You may be lucky and buy all the parts as a package from a wreck. Key is of course the adaptor plate. I recall that it changed at Chassis 23211and those motors are compatible with the 4L80E transmission adaptor. This is a key issue as the starter bracket was removed from the engine casting and repositioned on the adaptor plate at that time. Unfortunately, there seems to be an incorrect drawing of the crankcase in TSD6167, but the later catalogues show that the adaptor should fit as the same THM400 adaptor was used from chassis 23211 until the 4L80E was introduced. I believe that the last of the THM400 adaptors have the same pattern as the 4L80Es although the rear main oil seal is repositioned but cannot be absolutely sure. A lot also depends on the ability of the casing to fit in the transmission tunnel, so it may be trivial or may need some surgery. Apart from the flexplate, converter and transmission, you would start with a replacement adaptor plate and engine rear main oil seal. 4L80E cars have the rear main located in the adaptor plate whilst earlier cars have a separate housing for the seal. Your starter motor will fit an early 4L80E adaptor plate, and the rear engine mounts are compatible. At the other end, if the 4L80E is longer overall (Ie have never measured one or seen the spec) you would need a specially-made propeller shaft as they switched to a two-section type with a centre support and bearing for the 4L80E. I wouldn’t attempt retrofitting a two-piece shaft. The fuel injection would need changing to suit a throttle position sensor potentiometer in place of the present throttle position switch. To do a really proper job you would also need an ambient pressure transducer and a few other bits. If you manage it, bravo, but it could end in tears. RT. | ||
| Paul Yorke Grand Master Username: paul_yorke Post Number: 1099 Registered: 6-2006 |
Plenty of pitfalls as Richard has pointed out. If you are dead keen on the later boxes, I would advise selling yours and buying a car instead. Some might call me a wimp . . . | ||
| Ernest Carty Experienced User Username: edcarty Post Number: 16 Registered: 12-2012 |
Hi Allistair I would not recommend even contemplating the fitment of the later transmission ,as like RT says you would end up paying a lot of money for something which may never work properly the difference in the later transmission is greater also Engine management is totally different and like Paul says it would be easier and possibly cheaper just to change your car.Regards Ernie | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 2882 Registered: 4-2003 |
Indeed, Paul. More consideration warns of trouble. No wimp you. On second thoughts about the adaptor, I now recall that the starter still mates to the engine casting on all THM400 cars, and that the bracket was deleted from the engine block for the 1992MY cars (4L80E). That is consistent with the diagrammes in TSD6167, and would potentially lead to a requirement to fabricate a one-off adaptor plate and to address all the issues of starter gear offset and the like. All in all I do prefer the 3L80/THM400. Simpler. Cheaper. More reliable. No CHECK GEARBOX lamp to worry you. Gearing-wise, my Turbo R (2.28:1 final drive) has almost the same ratios 1-2-3 as my Conti R (3.08:1) 2-3-4, so the Turbo only lacks a little ability to smoke the tyres so easily at the traffic lights. The TCC on the 4L80 is nice to have but in reality not much of an attraction. Above 60km/h there is no difference in performance or engine speed. 120km/h in first gear is quite alright with the 2.28:1 axle. If you want to relax the highway cruising, I would suggest fitting the UK-spec 2.28:1 final drive to replace your 2.69:1 Australian-spec one. They are not expensive to buy secondhand with a two year warranty. Cheaper, easier and completely hassle free. All you need then is a new plastic speedo drive gear for the sender in the transmission. RT. | ||
| Alistair Riddell Experienced User Username: ariddell Post Number: 20 Registered: 1-2013 |
And at that point my pondering stops, and the "Too Hard" basket becomes one layer fuller...  I will continue to enjoy 25955 the way the factory intended methinks, it ain't broke so it would take a braver man than I to try to fix it. Rgds, Al | ||
| Michael Hicks Prolific User Username: bentleyman22 Post Number: 122 Registered: 12-2011 |
Hi as for the electrics yes it can be done with a Stand alone unit that can be programmed to 3 different set up's I have done it with my V12 it had a 400 it know has a E Box all depends on getting it in if you need any help net me know Michael |
