I have solved the problem of dropping from 5th to N myself, it took long time so I'll share what I know.
Off course, first make sure that you have the correct oil level and the pump builds pressure as it should. Then It's a fairly easy fix !
The main culprit, withe other things being fully operational, is so called "stop" solenoid. It changes the movement of gear selector shaft between 1-2 , 3-4 and R-N planes.
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After you unscrew it, there is a spring loaded nipple which, when activated, protrudes into the barrel behind it to generally block the internal plane seletion barrel :
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The problem is that the internal mechanism is oiled and greased, over time gunk gets under the nipple and further into the solenoid, causing it to stick. The spring is not strong enough to push back the solenoid shaft quickly, this is why sometimes people cannot engage any gear for a couple of minutes after dropping to N -it needs time. The lower the outside temp, the more likely this can happen - greasy gunk being thicker.
option 1 - quick fix - It should be possible to reach and unscrew the solenoid - 500 offers less space than my Lancia Musa , but it should be possible after removin battery holder plate. Put some foil beneath so as not to drop screws, spring and nipple. Put the car in N first. Screws are phillips , there is no pressure in this part of robot. Tighten back with care.
Clean the solenoid by slowly spraying something like WD40, then brake cleaner, rubbing alcohol etc over the little shaft, let them drop sink inside. Do it many times in turns until the shaft moves fairly freely. The liquids will drain through plug assembly. Apply brake cleaner as final, give time it to evaporate, you can even heat it up to make sure it's dry inside. At first, you might need to use a 9v battery with two cables to pop the shaft back out.Don't worry about polarity. New solenoid is pricey and there is no need to change it if it works.
While solenoid is out, also clean the nipple socket in the barrel from any old grease to prevent it from sticking.
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Option 2 : Full, long term fix is to take the robot out and clean the whole "barrel" inside and then re-grease it with high quality, wide-temperature-graded synthetic grease . You can see this being done here:
screenshot:
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Option 2 best made by a mechanic, unless you really know what you are doing, followed by factory calibration in
MES
Of course, there might be some other underlying issues. Then it needs to be properly diagnosed and possibly overhauled.Find a mechanic whoe deals with dualogic.
Hope that Helps.
I have actually done a complete overhaul of the robot myself, it is quite a durable and bulletproof device with just a few soft-spots. It requires all components to work properly, with very little margin for error and then it will serve years on.