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MG MGB Technical - Intermittent D Type Overdrive Problem

Hello,
I am experiencing intermittent problem with my overdrive. When starting out(engine not fully warmed upped) the overdrive will engage with no problem. After the car has warmed upped the overdrive will disengage at 60 mph and will not reengage. If I stop the car and start out slowly the overdrive will sometimes engage but will disengage at 60.
I rebuilt the gearbox and overdrive at Twist's in 2006.I installed it in May of this year. It has a NOS solenoid, vacuum switch, relay, and gearbox switch.
I have checked over the electrical connections and have tried a different vacuum switch with no luck.
Any help would be appreciated.
Thanks,Tony
Tony Shoviak

I would suggest that you check the od filter. It may be partially clogged and at higher speeds the pump is cavitating or drawing in air and letting the sliding member disengage. Ray
RAY

Tony,
If I remember correctly -
In 2006, except for some superficial cleaning, checking solenoid function and changing out the O-rings, we really didn't have enough time to totally disassemble our OD units for cleaning, much less call it a rebuild.

I was there again last February with my son and two more OD transmissions. With fewer participants (vs 2006), we had more time for the OD units and we were able to opened up and clean areas left untouched in 2006 - they were dirty. Nevertheless, although clean, I still worry about the clutch material on the sliding member. Has it "aged out" (see Previous Topics: Laycock LH type overdrive sliding member clutch material wear, John Esposito's website)?

I'm no expert. Probably like many, I can remove, disassemble, clean, reassemble, install, make the necessary connections and then hope for proper function.
When faced with a situation like yours, I can only guess - most times incorrectly.

Have you called John Twist or John Esposito? They might shorten your search for cause. What oil are you using in your transmission? Have you tried changing it? Some recommend using only 30wt N/D or ATF. Have you pulled your filter? I found that pasty material in those uncleaned areas from 2006.

If all else fails, I'll see you next seminar.
Steve Buchina

Another possibility is purely electrical. That would be the solenoid. A NOS solenoid comes with one shortcoming: its age. Even if it has never been used, its insulation can have aged to a point where it becomes unreliable. In someways a similar problem as you would have with NOS rubber.

Sometimes solenoids will develop a problem with either 1) an open in the winding which only shows up once the solenoid is heated up OR 2) a bad internal connection of the coil wiring which does essentially the same thing. Remember, as these things heat up, the metals expand, and what was marginally connected when cold and the metals shrunk, can come apart as they expand.

All the other advice is valid too, but if I were faced with your problem, I think I'd look at the solenoid first. A simple replacement procedure can help you rule out this component, if indeed you actually have a mechanical problem instead.

I know that a new/different solenoid to use may cost a few bucks, but if it resolves your problem, you then know that you should have less speculation about the condition than you would with an old one.

Another electrical problem could be the lock out switch on the trans. If its contacts are burned, or, more likely, the plunger worn to where it is marginally moving the contacts that make connection, the same thermal expansion can be enough to cause its internal contacts to separate, and you end up with the same result. Your resolution may be as simple as readjusting the depth to which the switch is installed, compensating for the wear to the plunger. Worst case is a new switch.
Bob Muenchausen

D-type solenoids have a two-stage operation - a low resistance high current pull-in winding, and a high resistance low current hold-in winding when the solenoid has moved far enough. It could be that solenoid was maladjusted, has been operating on its high-current winding all the time, and been damaged. You don't say how you checked out the electrical connections but that can mean many different things to different people. Electrical continuity of the D-type is relatively easy to confirm, with the ignition on, and in an overdrive gear, you should be able to hear the solenoid clicking in and out as you operate the manual switch. You would still need to confirmt he winding switching, which can be done by inserting an ammeter in the yellow yellow/red conenction between the main harness and the gearbox harness by the fusebox. The pull-in winding takes about 17 amps, the hold-in about 2 amps.
Paul Hunt 2

Thank you to all for all the advice. I will start checking things out now that I have a starting point.
Thanks Again, Tony
Tony Shoviak

While driving to Battle of the Brits in Sterling Height MI Sunday I played with the shifter some. I found that if I put a little foward pressure on it the overdrive would stay engaged and work without any problems. I took out the carpet and removed the transmission cover in the cockpit. I was then able to access the Gearbox/Inhibitor switch. I removed the fiberwasher that was on the switch and put everything back together. I took the car out for a spin. Everything worked well.
Thanks again for all the helpful advice.
Tony
Tony Shoviak

Tony,
Glad you found the problem. I'll file your findings away in my 'possible cause/solution box'. I'll now check this switch for proper clearance/function before my next installation. THANKS FOR THE UPDATE!
Steve Buchina

This thread was discussed between 02/09/2007 and 13/09/2007

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