All of the below refers to the Laycock Type "J" Overdrive fitted to an SE5a.
Although there could be ideas on this page that would help with other Types. All pictures are expandable by clicking on image.
To do most of what is suggested below will require the Car to be raised.
I use the "Normal" Type ramps for the back and then Jack up the Front and Use Axle Stands.
Whatever method you use always make sure that the Car is well supported, fixed and cannot move or fall.
A Scimitar landing on you is guaranteed to give you a sense of humour failure.
Maybe there is something here that could help with Overdrive problems. This could be intermittent operation, not engaging or sticking on. The first thing to check should be the Gearbox Oil Level as this is the easiest ( See here how to check Gearbox Oil Level) then Voltage at the Overdrive Solenoid. Take care that although the Dash Warning Light may be coming on, due to the way the Scimitar is wired voltage may not be getting to the Solenoid. Note that Reliant did not fit an Overdrive Warning Light so if fitted it was done by a previous owner. If there is no Dash Light Illumination check the Bulb in the Dash Light and 12 Volts to and through the Dash Switch. If the Dash Light illuminates check for Voltage on the Solenoid, the best way is to connect a 12 volt bulb across the Solenoid. With a Scimitar and its fibre glass body and many earth points one of the best Tools you could have is a 12 Volt Reversing Light Bulb with about 2 to 3 metres of wire attached, and if there is a small needle attached to one wire you are able to push the needle through the insulation of the wires to check for voltage. Take care if using a Multimeter, An Analogue one (With a Needle) could be better as it will give a more accurate indication of Voltage. A Digital one (With Numbers) could give false reading due to it's high internal resistance. With the Bulb across the Solenoid Terminals ensure Ignition is on, the Gearlever is in the third or fourth position and the Dash switch is in the on position. If the bulb illuminates there is Voltage on the Solenoid, if no bulb illumination you will have to find where it's gone. First thing to check is the Earth Connection from the Chassis, This should be a short wire to the Overdrive Casing, try a wire from this point to the Battery Negative Terminal. Take care you connect to the correct battery terminal or things could get "Interesting". The Battery Negative Terminal is normally the one nearest the front of the Car. If still no Bulb Illumination you need to find where the 12 volts has gone. There are two directions to go, From the Solenoid back or from the Fuse Box Forward. Whatever way you go it will be the wrong way ! but if the Dash Light is coming on that will prove that voltage is getting to and through the Dash Switch. Due to the environment under the Car this should be the first place to try. With the Bulb still connected to the Battery Negative Terminal (Sods Law dictates that you have more than on problem so keeping the wire Connected to the Battery Negative Terminal during Tests will help) use the other Bulb Wire to trace. The Solenoid Feed comes from the Gear Override Switch (Only operates in third and forth) then from the Dash Switch and then from the Fuse. Using the "other" Bulb Wire try both sides of the Override Switch. Not easy to get at (Just below the Gearlever) if no illumination you need to go to the Dash Switch. If the Dash Warning Light is on and there is no voltage at the Solenoid or through the Gearbox Override Switch there is a break or bad connection between the Two. You could trace it using the Wiring Diagram, Click Here for Diagram. Or you could run a separate feed from the Dash Switch to the Gearlever Override Switch. If you measure the current that the solenoid requires it should be about 2 Amps. Note that the Overdrive Solenoid does not make any noise ( Or very little ) when engaging, do not expect to hear clicks etc. Don't be tempted to run the Wire direct to the Solenoid as although you would get Overdrive on all four gears you would also get Overdrive operation in reverse. You will "Kill" your overdrive if it is engaged and you select reverse and go backwards.
The Overdrive Switch is located at the base of the Gear Lever assembly. Not the easiest place to get at. To see where the Switch is and what activates it click on pictures.
If you do not know where the Solenoid is click on Picture.
The other possibility of Overdrive problems could be Low Gearbox Oil Level. To check there are two "Screw Plugs" on the right of the Gearbox. The bottom one is the drain Plug, the Top one is the one to check. Remove the Top one (Best to have a Container and Big Rag etc at the ready). A small Mirror and Torch (Flashlight) can be used to see inside. Better still buy a Litre of Gearbox Oil in the type of container that has a Long Flexible Tube/Spout and Top up Oil until overflowing from Top Gearbox Plug. The one litre containers are small enough to get between the Transmission Tunnel and Gearbox. Remember to have the Car on level ground when checking.
The following will require the draining of the Gearbox Overdrive Oil except possible the removal of the solenoid. To ensure the minimum of Oil down the Arm Syndrome I removed the Sump Cover (See Below) after as long a Drive as possible. This will allow the Oil to be Hot and Drain easier. This was left overnight to make sure as much Oil as possible drained away. Just draining from the Gearbox Drain Plug will not remove the Oil from the Overdrive Sump.
If all of the above does not cure the problem the next thing to Check is the Solenoid. Removal should be easy once you have the Correct Spanner. You will need a Spanner that fits on the Front of the Solenoid. The Spanner will need to be thinner than normal. I "attacked" one that was close with a grinder until it fitted, there is also a problem with the Length of the Spanner due to the Chassis etc. Best you look and compare with the Spanners you have. Once Solenoid removed the first check is to apply some voltage to it and see what current it draws. 2 Amps is normal, No current drawn means the Coils are faulty, Open Circuit (Not repairable). More than 2 Amps means there are shorted turns on the Solenoid Coils. Do not expect a "Click" when operating, the thing to look for is a small movement of the inner shaft. If Current is 2 Amps the fault could lie in the Shaft Sticking so a clean may help. Remove the Solenoid, disassemble and clean. To see what to look for click on picture.
Remove circlip and the small piston and spring will come out, then remove the shaft. To remove a small self tapper screw can be used by screwing into the hole and pulling out. Take care not to expand/damage things by screwing self tapper to hard.
If this does not cure the problem the filters and pistons etc will have to be removed and cleaned. Click on pictures for more details. Due to the nature and operation of the Overdrive cleanliness is very important. There is an exploded View of the Overdrive at bottom of page.
I have not gone into details how to clean. I'm sure you know how to clean. Allowing for a few problems this should not take more than a couple of hours (Including Tea/Coffee breaks).
Remove the Sump Cover. Clean Inside. Note Magnetic Strip to "Hold" metal particles.
You will now see this, remove the Sump Filter, It is a "Push" on Fit.
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Remove the Large Plug Nut on the Left, You will now see this. Clean Filter etc.
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Remove the Middle Plug Nut, You will now see this.Take care as when the Plug Nut comes out there is a Spring with a Ball. Ball can be seen in place in middle picture but it comes out with Nut etc as can be seen in left picture. Clean all etc.
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Remove Right Plug Nut. You will see this, and how to remove all parts. Clean etc.
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Exploded View of Overdrive.
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