Trials and Tribulations
The J type overdrive was standardised on all Triumphs from 1974 so that this article covers Spitfire from FH60000, all Dolomites, later 2000/2500, late TR6 and Stag. These overdrives have also been retro fitted to many earlier cars, and can be distinguished by the solenoid on the LHS (all D type and A type have the solenoid on the RHS). I will not attempt to cover all the minor variations in specification as there are also units fitted which originally came from Volvos, Ford Transits and Sherpa vans, just to deal with common problems and what to do about them. In the Triumph range the units fitted to single rail gearboxes (Dolomite and Spitfire with reverse by 3rd gear) have a different rear housing fitted so that the gearchange can be bolted on the top, apart from this all other units can be interchanged (but pay attention to different rear flanges and speedo drive ratios) if the operating pressure is set correctly. (The more torque an engine produces the higher the operating pressure required). Oil leaks - before attempting to trace a leak make sure that the gearbox/overdrive unit has a breather somewhere that is clear. Blocked or missing breathers cause oil to be forced out.
If an overdrive fails to operate the first thing to check is the electrical circuit, most cars do not have a relay fitted (it isn't necessary with the low power consumption of the solenoid) but as a lot of solenoids make very little noise when engaging a circuit test is often required. (Note that Sprint/2000/2500/Stag have a separate switch on the gearchange for overdrive 3rd and overdrive 4th, all other cars have one cut-off switch). The units themselves are fairly reliable (the filtration system is excellent) and it is the solenoid which causes most of the problems. If there is power to the solenoid and the overdrive either won't engage, engages cold and not hot, or won't disengage then the solenoid is very likely the culprit. To remove and replace this a 1 inch spanner is required but it must be no more than 3 mm thick.
The original solenoids have the outer metal case held in position with four small roll pins, if the case is very loose (which can cause incorrect operation) then it is possible to tighten it up with care and it may work again. The only other DIY option is to remove a tiny internal circlip and withdraw the operating piston, there are then two external and two internal O rings to replace if you can get hold of them - if you don't fancy doing all this (and there is about a 30% chance of it working properly again after fiddling with) then replacement is the only option. New solenoids are around £75, but the outer case is swaged on and does not come loose. All J type solenoids are interchangeable. Later Dolomite and Spitfire overdrives (after 1978) have a badly made centre to the one way bearing and from around 30000 miles can give the impression of clutch slip, quick getaways from the lights can lead to an embarrassing lack of forward motion. This problem and in reality all other faults mean the unit has to be removed from the car. Although it just about possible to remove the sump and gain access to the fine filter (RHS), pump ball bearing (centre) and relief valve (LHS) it is very unlikely that anything can be gained by doing so. (Very occasionally the relief valve may stick which means the unit can't work). The whole assembly is not too bad to work on and as long as all bits are put back in the right order and the right way round (without too many left over) then it should work again. The one way bearing is awkward to refit, a piston ring compressor can be used if all else fails.
If you really have the inclination and a 0 - 1000 PSI pressure gauge and a fitment to screw into the test point (the plug just under the front of the solenoid) then the operating pressure can be tested, around 350 PSI for a 1500 engine going up to say 550 PSI for a Stag. Altering the pressure requires fiddling with the relief valve (two types fitted) and is beyond the scope of this article. Noisy bearings can be replaced by competent amateurs, other failures, rare but often fatal are probably best left to professionals.
Note for information there are two different ratios of J type fitted, with 2.5 cars having a 28% ratio and all others 25%, (the first two figures of the plate on the RHS of the front case is the ratio). In addition some later Volvo units are 27%.
Use good oil in the gearbox/overdrive, it is worth the extra (particularly synthetics) as it is working very hard and changing it every 50000 miles is well worth the effort.