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MG MGB Technical - Non-working OD in 74 MGB GT

Hopefully someone can help lead me in the right direction. I purchased and rebuilt an LH type OD transmission for my 74 B. The wire to the Solenoid was broken off and from the looks of the inside of the OD unit it has seen little usage. I purchased and installed a new solenoid and when the transmission was complete I ensured that the pressure switches worked properly for OD and reverse and that the OD would only work in 3rd and 4th. The car is finally back on the road but the OD doesn't work. I have confirmed that I have power to the solenoid and just pulled the solenoid out and made sure that the little plunger and ball were in correctly, tried it again and still no luck.

I noticed in an internet posting that the solenoid should have a copper (?) washer but this one didn't come with one. Will this make a difference and if so, how is it installed?

Any ideas gratefully accepted.

Brian
Brian Smith

Brian,

Do you have good new "O" rings in it? Is the filter screen clean? What oil are you using in the transmission? Should be 20-50. Is there enough? If you know there is power to the solonoid, I presume you have checked for continuity through the 3rd-4th gear switch. ' Sounds like time to give John Twist a call on his daily help line: 1-616-682-0800 between 1:00 and 2:00 PM (eastern time). http://www.universitymotorsltd.com/

' Hope this helps.

Allen
Allen Bachelder

Allen

Thanks for the response. The unit has new O rings, the filter is clean and I am using 20/50 oil. Prior to installing I ensured that the switches worked properly for both the OD (in 3rd & 4th) and the reverse lights. As the OD isn't working I got under the car and with the car in 3rd, ignition on and OD switch in the on position there is power to the solenoid. After removing the cover I pulled the solenoid and plunger out along with the little ball. I tried pushing the plunger back into the body to see how easily it would move in and out but it doesn't seem to want to push up into the housing. Looking at the o ring I am wondering if it is too large and thus stopping the plunger from working correctly.

Brian
Brian Smith

Hi Brian,

' Think you should call John Twist and tell him what you've told me. He's a great guy and he knows SO much about these things.

Good luck!
Allen
Allen Bachelder

Have you made absolutely sure the solenoid and electrical circuit is OK by putting an ammeter in series (where the yellow wire from the main harness joins the gearbox harness by the fusebox is a convenient point)? You should see about 800mA on an LH type. Also remove the solenoid, push the plunger right in (or partially pull it out), then power the solenoid, and it should centralise. If that's OK it must be hydraulic or mechanical. The next step would be to check the pressure, should be 400 to 450 psi at 30mph with OD engaged, there is no pressure when OD is not engaged. That should determine whether it is lack of pressure from the pump or valve, or some mechanical problem preventing movement of the cone clutch.
Paul Hunt 2

Sorry, 400 to 420 psi.
Paul Hunt 2

Thanks Paul

I drained the transmission today so that I can check the pump and relief valve. John Twist has sent me some information and I am pretty sure that it must be a fault with the oil pump. Too bad that it is raining and blowing otherwise I might have had this fixed by now ( Oh yeh, also a few little jobs that the boss wanted done today)

Anyway, I will post the results once I have it working. This car went from an every day driver to being rebuilt once I retired and I am really looking forward to getting it on the road again. Put a couple of days insurance on it a week ago and had a great time with the only thing not working.... the overdrive.

The progress can be seen at http://www.island.net/~btsmith/

if you are interested.

Brian

Brian Smith

Success! Thanks to the above and also to John Twist who supplied a written commentary and a diagram I have the OD working in my MGB GT. I did a couple of things but feel that the pump was the problem. The first thing was to cut a hole in the center of the gasket between the "black label" cover and solenoid, the second was to take the pump out. I am not sure if this OD tranny worked for the last owner or not as the solenoid wire was broken right off and the insides of the unit looked like new, anyway the pump was initially cleaned and put back in but John Twists diagram showed an "O" ring between the spring and bottom fitting. I fitted an "O" ring and put it all back together and everything is now working great.

Thanks All.

Brian
Brian Smith

Brian,
I have an LH type overdrive that I am working on now. The did the light test similar to John Twists video and checked my third and fourth gear test. In fourth position the light stays on as in the video. However, I have to hold the gear lever to the right in order to keep the light on. My guess is that I am having a problem with my switch. Did you happen to do any work on your 3/4 switch.

Also,you last comment states that you cut a hole in the gasket. What was the purpose of this cut?
thanks,
Mike
m foster

Sounds like you have duff gaskets in the US - in the UK gaskets come with the central hole! It allows the solenoid plunger to travel just that bit further out to open the valve.

There is no "O" ring between pump spring and the nut - sounds like the old problem of the valve spring being compressed, normally sorted by stretching the spring so it stands proud of the hole in the nut.

Mike, you need to remove the switch and take off one of the two washers. This can be done most easily from inside the car after removing the tunnel cover and dropping the crossmember.
Chris at Octarine Services

"most easily"

Hmmm, 'more easily than from underneath' I suggest, rather than 'very easily'!

Unlike Allen Brian's problem almost certainly is tightness or 'adjustment' of the switch.
Paul Hunt

This thread was discussed between 29/10/2007 and 14/09/2009

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