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Thread: Bj7 Overdrive slipping when hot

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    Bj7 Overdrive slipping when hot

    My 1963 Healey BJ7 overdrive slips and appears to come out of overdrive only after driving for approx half to three quarters of an hour. It dose not go into overdrive or dose not appear to change once the gearbox is hot. When the car is cold it changes up and down as it should no problems at all.
    I have changed the oil and checked out the electrical connections and they all seem fine.
    I am very new Healey owner and not up with overdrives. Lots information on overdrive but nothing that I can find about failing when hot.
    Any advice would be appreciated.

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    Re: Bj7 Overdrive slipping when hot

    It may be that the oil pressure being put out by the transmission oil pump is on the feather edge of operation and as the viscosity drops a bit when heated up it drops too low. Also, if you have worn o rings or piston rings in the OD unit the same scenario might cause your symptoms. It is possible to make a tool to check the operational oil pressure while driving but that just answers the question of whether or not you have sufficient pressure, not what is causing low pressure. There is quite a bit of info on OD operation on this forum if you search. By the way, welcome and I'm sure others will write you their thoughts. It doesn't sound like you have a rigging problem or electrical problem based on what you've said. There is also a pressure relief valve that can malfunction in various ways that is a spring loaded ball bearing with a machined seat. If the seat is pitted or the ball bearing is sufficiently worn it can cause low pressure. The slipping would possibly be due to a worn OD clutch.
    Jon Robbins
    1956BN-2 (do it all yourself, you'll be glad you did)

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    Re: Bj7 Overdrive slipping when hot

    Thanks Rosco,
    Your reply makes a lot of sense, will do some more research along those lines.

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    Yoda Randy Forbes's Avatar
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    Re: Bj7 Overdrive slipping when hot

    While most of the problems encountered with the overdrive are electric__and easier to isolate and identify__the mechanical ailments require more in depth diagnosis (and descriptions) to zero in on.

    Have you individually eliminated__by bypassing around the terminals for testing purposes only__each of the switches? Believe it or not, at least some of your symptoms can be blamed on the gear-lever selector switch; if it's on the very edge of closing the contacts when cold, it may not suffice when fully heated up.

    When your ovd. won't engage, do you still hear the solenoid "clicking" into position? Connecting a pilot or test-light into the same connector (at the last junction to the solenoid) will be a visual indicator if 12VDC is being maintained on the circuit.

    If all the electrical circuits prove to be functioning reliably and correctly, there's only a few more steps that can be tried externally...

    Make sure the pull-in and holding solenoid coils are adjusted correctly__although failure here can usually be identified with melted white & white/purple wires!
    You can fit a test-gauge to monitor the oil pump output. Use CAUTION, as the pressures are fairing high if it's working properly (500 PSI perhaps?).

    Next likely cause, in addition to those already mentioned, is the degradation of the external/internal friction lining material on the clutch drum. If this was worn thin or breaking up, you'll find bits of it when you drain/clean the overdrive's sump and filter-screen. If yours has the magnets still, check them for metal fragments (there will ALWAYS be fine-fine metal-fuzz particles).

    You can try some fresh 30-weight non-detergent oil (what is recommended, though we've all tried others...). Trying 40-wt, or 20/50 (what mine lived on for decades) may make a difference. I'd personally NOT recommend any synthetics or modern "super oils" (in short, anything that claims to reduce friction, as that clutch drum NEEDS FRICTION to work).

    If none of that gets you anywhere, it'll probably have to come out for the further inspection of internals.
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    Re: Bj7 Overdrive slipping when hot

    What grade/type of oil are you using? The OD will disengage at random if the relay points are worn/burnt.

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    Re: Bj7 Overdrive slipping when hot

    Tassiedave, I had the same symptoms on my car. The cause was a failing relay.
    Hugh
    1965 BJ8
    1958 AN5


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    Re: Bj7 Overdrive slipping when hot

    Thanks for all replies. I am working my way thru the suggestions. Gearbox specialist will start will pressure test and if this fails he will check the accumulater and go from there.
    Last edited by Tassiedave; 11-21-2017 at 04:41 PM.

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    Re: Bj7 Overdrive slipping when hot

    G'day Dave & welcome to the forum. I had similar symptoms on my 100 a while ago. The problem was resolved by the replacement of the overdrive accumulator.
    Forty nine years in one 100, still going strong!

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    Re: Bj7 Overdrive slipping when hot

    If everything else--pressure, etc.--a worn cone clutch would cause the OD to slip.

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    Re: Bj7 Overdrive slipping when hot

    I'd like to relate my overdrive experience to you as it is almost identical to your problem. My BJ8 overdrive would work well for the first 10 to 20 minutes of driving then would become slower and slower to engage and finally not engage at all. I bought a power steering pressure gauge kit from Mac Tools and installed it and found the pressure to be too low, in the neighborhood of 200-250 psi and it would drop to near zero when the overdrive was engaged. By the way I was bench testing it using a heavy duty half-inch electric drill attached to the transmission input shaft with a short extension, a short piece of rubber heater hose and some hose clamps. Quite Mickey Mouse but it served the purpose. There is an excellent youtube video of someone showing an overdrive rebuild and he has a lovely bench testing set-up. I recommend watching it. When I disassembled the overdrive I found that the accumulator piston as well as its rings and the bore it sits in to be very badly scored. The rest of the bits seemed to be in decent condition and I may also mention here that the overdrive and transmission had supposedly been rebuilt by a local shop while the previous owner had the car. His widow gave me all the receipts for work he had had done! Obviously they never took the accumulator out to have a look. I fitted a new accumulator piston and rings, accumulator sleeve, accumulator spring, oil pump(there was slight play in the roller) and new gaskets and seals and also new check valves(2). When I bench tested it again the pressure was now well above 400psi and did not drop when overdrive was engaged. I drove it this summer and it worked perfectly for about 4 months, always engaging very quickly. Then it started to get slower and slower to engage and doing the same thing it had been doing before. So now that winter is around the corner I am getting prepared to pull it apart again and see what went wrong. First I will connect my oil pressure gauge again and see if once again it is too low and then continue on from there. Bummer! Hopefully next spring I will be able to post the results of my latest expedition.

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    Re: Bj7 Overdrive slipping when hot

    I just disassembled the OD from my BJ8. It has almost 200K miles on it and doesn't appear to have been touched before and was working fine (but I had it on the bench and figured it was time). The accumulator piston and bore were lightly scored; one of the thrust washers was toast, however. If yours is scored again in such a short period of time there has to be some contamination getting into the oil (or else the replacement parts were crap).

    Here's the video I think you are referring to: https://www.youtube.com/watch?v=HLVoM4On49k

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