There are two sides to this. It may be that the control side is faulty, or you may have a defective motor and/or mechanism. Let's look at the control side first. The electric window switches are not the best, so the contacts may be failing. You can test these by substitution, with a known good switch, but please bear in mind that BOTH must be good for the left (passenger) side window to work. If either one has a fault, the window is unlikely to work since they are wired in a sort of series connection. To understand this you need to follow the wiring in a correct circuit diagram. All centre pins must have good voltage, and the pins either side of them on both drivers door switches, must have good earthing. All other pins are cross connected between either motor, or the other switch in the case of the two passenger door window control switches. There are two window relays controlled by the ignition, which feed two 16 amp fuses. Please note however, that one relay and fuse is for BOTH driver door switches, (not one for each window motor as you might expect) whilst the other is for the passenger door switch! Since everything on the fuse board passes through edge connectors and these are known to be a place for poor connections, if you have poor voltage at any switch, check these plugs and contacts and all continuity first.
Assuming all is well with the control side, then it is likely the fault lies with the motor(s) or mechanism(s). To get at these, you will need to strip off the door trim panel, and remove the assembly. Please note, you can get everything out, without cutting the inner door panel. The first thing to do is to take out the window glass. If the window is closed and will not go down, you will have to disconnect the two bolts that hold the bottom frame to the mechanism, by feel. If you can lower the glass with the electrics, then lower it about two thirds the way, until you can see the bolts through the lower opening, and you can remove them more easily with a socket and ratchet. Otherwise you will need a flat 10mm spanner; but you should be able to undo the bolts; and it will be much easier in either case, if you have a second person to hold the glass. When the bolts are out, tilt the glass to release it from the edge runners and carefully lift it out, flexing open the door slot to get the frame through. Put this in a safe place, where it cannot be broken whilst you work on the door!
Next, there are three bolts that hold the mechanism to the door - two about half way up the door panel in middle and one at the bottom, screwed up into the base of the door. Now undo the three nuts that hold the motor to the inner panel, remove the washers, and push the motor inwards. If you look carefully, near the top of the vertical slider mechanism there is a rivetted bracket holding the top to the panel. You can release the mechanism simply by unhooking it - no need to drill out the rivet. If you lower the motor, you can remove the electrical plug that is on the back of it. You can then bring the motor out through the lower opening, moving the mechanism to allow the movement, to a point where you can work on it. There are two screws that hold the main body of the motor to the worm wheel box. Remove these and carefully ease the motor and armature away. With the motor off the mechanism, you can finally remove the complete lift/lower mechanism out the the top slot where you removed the glass. You can now check it for damages, breakages, freedom of movement, and clean and lubricate it ready for re-installation. In all my experiences, it is not the mechanism that causes the motor to be slow, it is usually the motor. However, it will help if it in the best condition as it will ease the load on the motor.
If you had a 'brrr' noise at any time but particularly under load or when reaching the top or bottom, look carefully at the wheel box. The plastic housing is crimped to the metal body, and often it is no longer held tight. So when the mechanism is under load the housing pushes away from the metal and the wheel slips out of mesh with the 'worm' cable. It is this that causes the noise. You need to secure the plastic to the metal again. Now look at the motor. If you examine it closely you will often find the brushes are totally worn out at this age. Also the magnets will have lost some of their magnetism. However, the worst cases are where the motor has had water in it and the lower bearing has seized, or is very tight. It is possible to strip, clean and re-lubricate these sometimes and get them working again; but at this age and particularly if the motor had quite a bit of corrosion in it, or the bearing was seized, it is time to admit that you need a new motor. These motors are made by Rockwell, the same as many other manufacturers use, so you may find one on another car that can be fitted, but at this time they should still be available from HB Pieces or Simon Auto. After replacing the mechanism back in the door you can fit the new motor to the wheel housing, and then re-assemble the whole mechanism to the door.
One final tip. The motor body has two holes in it to allow the water that will seep down there to drain out. The motor can be fitted either way up depending on installation in various mechanisms, hence the option. One hole will have a plastic plug in it. Make sure that the lowest hole is open and the upper one has the plug! If they are the other way around, the bottom will retain the water and the bearing will eventually seize!
If this is an early Murena with the gear lever that you press down to enable you to engage reverse, this can be a common problem. To have this spring loaded gear lever, the area where one part is slotted into the other is rather fragile as the metal is thin and not very strong. These early gear levers were replaced during 1981 for a solid gear lever. Repairing a split one when it has broken is difficult if not impossible and needlessly costly. However, you don't really need the spring loaded split gear lever anyway, and you should simply make it like the later cars which have the solid gear lever, and which will be more reliable. If you strip the broken one out, clear out the workings and weld the two main parts together to make it solid, you must also remove the detent plate which is mounted at the base, and dispense with it. This is the only reason you have to press down on the spring loaded gear lever in the first place. Once it is removed you can simply select reverse by pushing right against the spring and pulling back to engage reverse gear. There is no requirement at the gearbox end for any detent.
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