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MG MGB Technical - OD trans slipping

All,
I posted a couple of months ago regarding what I perceived to be my OD trans slipping on overrun. I received some great advice, but haven't acted upon it yet. I'm pretty sure that my OD clutch is glazed or simply worn out, or the sunwheel circlip is broken.

I've continued to drive the vehicle with this condition, but today experienced some different symptoms, leading me to believe the problem might possibly be in the electrical portion of the overdrive actuation circuit. Here's the story:

I drove about 30 miles to a destination, let the car sit, and drove 30 miles back. On the trip out, the car did as it's been doing lately: when the OD is switched 'out', I got intermittent overun upon deceleration. Then the trouble started. With OD switched 'out', the trans would shift into and out of OD on it's own accord. Felt like I was driving an automatic. At one point, I had to reverse, and when I let the clutch out, the vehicle did not move (OD still switched out). Clearly, the trans was disengaged at this point. The problem became particularly severe in first gear, at one point, with the trans shifting continually into and out of OD. After arrival at the destination, the car sat for a couple of hours and then I drove home. On the way home, the trans and OD operated flawlessly. No overun on decel, and the OD responded normally to my inputs. It appeared everything was back to how it was before the problems started. I just parked, and haven't driven again...yet. I guess my question is, does this still seem like worn OD clutch linings/broken sun wheel circlip, or could it be a bad OD solenoid or wiring?

Many thanks in advance for any pointers, and I appreciate everyone's patience.

David Tye
'71 Roadster
David Tye

For it to be electrical and happen in all gears it would likely be a short between the solinoid wire and the hot wire to the reverse light switch at the trans as the wire directly from the solinoid ends at the lockout switch at the trans. The lockout switch and the OD switch both going bad together would be somewhat unlikely, even for Lucas. The alternative is that there is a problem in the oil valve in the OD that is opperated by the solenoid causing the oil pressure to the clutch to pump up when it shouldn't. the only wrinkle to this is that at very slow speed there is normally not enough pressure to engauge the OD clutch. You may want to give John Twist a call at University Motors in Ada Michigan. He normally answers technical questions between 1:00 and 2:00 PM Eastern time.
John H

"when the OD is switched 'out', I got intermittent overun upon deceleration. Then the trouble started. With OD switched 'out', the trans would shift into and out of OD on it's own accord."

On just one part of just one journey I had exactly that on my V8 last September. I've since changed the gearbox and axle oils (not just because of that, but because I hadn't done it before in the 13 years and 75k miles of my ownership) and on a 200 mile trip since there were no problems. Mine didn't 'feel' electrical as the engagement and disengagement was 'softer' then when doing it manually, so I suspected something causing pressure to remain in the hydraulic lines at a marginal level just enough to make the sliding member move gently back and fore. Checked and washed the filters as part of the oil change, but they were as clean as a whistle.

Before digging in to it I'd try filling with ATF and running it for a while. Some report that has cleaned things out, then going back to standard engine oil was fine. Others have said that they had to keep ATF in it, which may well work, but obviously *is* covering up mechanical problems.
Paul Hunt 2

Thanks, John and Paul. I'll likely be calling John Twist for his read on all of the above.

Paul, I did change out the trans fluid to ATF (still in there). Sadly, no change. I think I have the cleanest MGB trans this side of the Mississipi.

Dave T
David

This thread was discussed between 17/03/2008 and 18/03/2008

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