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OverDrive Oddities - '77 Mk2 J-Type - Reluctance to 'drop out'

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    OverDrive Oddities - '77 Mk2 J-Type - Reluctance to 'drop out'

    For the first 5-10 mins OverDrive (OD) skips in and out without problem.

    After a period of running in OD it hesitates when asked to drop out; i.e. when the switch is moved to ‘out’ ... it fails to drop out immediately.

    On continued driving, if the switch is left in the ‘out’ position, it may then decide to drop out at random times, although somewhere between 30 and 90s is typical, longer than that is rare.

    Once it drops out … it is happy to engage / disengage again until it left in overdrive again for while. At which point the cycle starts again.

    The gearbox has been examined by Faversham Classics who cleaned the filters, changed the oil (using Millers XF Premium 75W90) , fitted new gaskets and advised that nothing untoward for the mileage’ was found on the magnetic pick up. The box has done 22k miles from new.

    My next idea is to check the electrical circuit, given that this is the next easiest to do. I will monitor the current through the control circuit. This would enable me to monitor the effect of the gearlever switch, inhibitor switch and solenoid during various control request events. The intent being to help me identify whether the gremlin is electrical … or mechanical.

    Am I looking at this sensibly ? Is there anything I’m missing ? What else might be worth checking ?

    #2
    Maybe clean up the switch. Most likely its keeping it on.

    Comment


      #3
      Maybe clean up the switch. Most likely its keeping it on.

      Comment


        #4
        Yes, I would definitely look at the switch first..

        The green box ones are notorious, I was on my third until I found an NOS one which is behaving well.
        Terry Hunt, Wilmington Delaware

        www.terryhunt.co.uk

        Comment


          #5
          Thank you. Really appreciated. Suggestion on 'to do' list ... will report back when done.

          Comment


            #6
            Now I might be going completely mad or confusing the J-type overdrive with something else but I thought that the solenoid needed a continuous 12v supply in order to stay engaged.

            so for instance change from over drive 3rd overdrive 4th without switching the overdrive off, the inhibit switch briefly breaks the circuit and the overdrive will disengage as you move out of 3rd gear, resulting in a crunch as you engage 4th and the overdrive re engages

            With that in mind I would have thought an overdrive that sticks in the engaged position will be unlikely to be electrics related. provided of course the inhibit switch is still in place and operating as intended..

            I have a problem where my overdrive will not engage until the gear oil has warmed up, this is something to do with the o-rings or solenoid as far as I am concerned.

            in OP case I might be looking in the general direction of the mechanical parts of the solenoid

            Yes greenbox switches are ****
            Stags and Range Rover Classics - I must be a loony

            Comment


              #7
              Agree Richard, but I have had my switch stuck on as well, though admittedly not heat related, and not that long.. but it’s an easy first step - at least move the switch in and out a few times and try changing 3/4 as you say.
              Last edited by trunt; 18 July 2025, 11:40.
              Terry Hunt, Wilmington Delaware

              www.terryhunt.co.uk

              Comment


                #8
                Originally posted by trunt View Post
                Agree Richard, but I have had my switch stuck on as well, though admittedly not heat related, and not that long.. but it’s an easy first step - at least move the switch in and out a few times and try changing 3/4 as you say.
                We really have reached a new low of quality reproduction parts if we get switches that remain closed circuit when they should be open, just horrendous
                Stags and Range Rover Classics - I must be a loony

                Comment


                  #9
                  I think the switch is not a simple slider, it’s a spring loaded rocker and in my case I suspected the frame holding the slider down had lifted and the slider had raised up enough that it sometimes no longer engaged the rocker. ISTR pushing down on the slider as I moved it helped.
                  Terry Hunt, Wilmington Delaware

                  www.terryhunt.co.uk

                  Comment


                    #10
                    In the interests of science I opened up an old one, Its not a Rocker.. Its a Roller

                    The slider has a pin into a spring and there is a brass roller at the bottom of the spring that snaps across as you slide it. The brass roller makes the contacts.

                    thumbnail_IMG_6320.jpg

                    Terry Hunt, Wilmington Delaware

                    www.terryhunt.co.uk

                    Comment


                      #11
                      That's chaps. Additional input appreciated. I really hope its not mechanical ... as this sounds a tad more expensive to fix. So I'm pinning all my hope on it being electrical ... fingers crossed !
                      .

                      Comment


                        #12
                        Interim update ... (in other words I really haven't gone far enough yet to convince myself of the final outcome) ...
                        1. I rigged up a multimeter to measure the current in the in / out electrical circuit.
                        2. I dropped a new OD gear lever switch in ... for good measure ... as it's the cheapest and easiest thing to get to
                        3. Took it for a drive ...

                        Results ...
                        1. It didn't exhibit the unwillingness experienced before ... but I'm not yet convinced I've done enough miles with OD 'in' to replicate the original problem ... so more to come
                        2. Measured current when solenoid cold was ~ 2.31 amps ... which ... interestingly ... and not overly surprisingly (if that's even a word) ... it dropped to ~ 2A when 'warm' ... which suggests the solenoid is working away at ~ 24-30W.

                        Next up ...
                        1. Do some more miles allowing for OD to 'stay in' for 10 - 15 mins prior to switching it off ...
                        ... so again ... watch this space when I get around to it (and the weather stops being unpredictable).

                        Comment


                          #13
                          ... and ... the problem has not yet been banished. So ... now know it's not the gearlever switch ... will put the ammeter back in place and see what that tells me. Back soon (ish).

                          Comment


                            #14
                            Wiring for the overdrive regardless A or J type takes a tortuous route! Hannibal and his Elephants would have been proud.

                            Lift the gearstick surround and find the two yellowy wires, one will have an ignition controlled 12v to it. this will be the feed wire, the other will have nowt when overdrive switch is off, that is the wire that goes up to the and through the bulkhead, over the transmission tunnel, down to the boat shaped connector by the starter and then along the gearbox case to the inhibit switch...... which is just below the gearstick....

                            Would have made sense in assembly but not now.

                            So I would be inclined to isolate the wire the goes from the gearstick to the inhibit switch. run a new wire from the gearstick and down through the gaitor direct to the inhibit switch
                            Stags and Range Rover Classics - I must be a loony

                            Comment

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