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MG MGB Technical - overdrive solenoid

Just purchased a 1977 MGB with overdrive. Overdrive is not engaging, so did some searching, on the car and through the archives. I removed the solenoid and that energizes, interesting though it energizes and pulls the rod down, towards the ground, not up. I reinstalled and am planning to put up on jacks, and power solenoid directly to see if overdrive is engaging, I'll worry about the gear switch later.

All I can think right now is that in the normal state the fluid pressure must be pushing the solenoid valve and ball "up", and when the solenoid engages the valve gets pulled down or open, fluid is redirected somewhere, and the overdrive engages. Is this right? I looked through all the books, and I see no explanantion how the hydraulics work in the overdrive. Would appreciate any input.
bob

No - other way. The solenoid goes up and shuts off the ball valve on the top to pressurise the cylinders to operate the clutch.

Check the ball for seating and the pump valve also.

To check the pump valve, drop the sump cover - 6 bolts - and undo the plug with 2 holes in - under this you will find the pump valve - clean it and check the spring holds the ball about 1/8 inch clear of the plug.
Chris Betson

Bob,
Yes, the solenoid plunger should move up when it's engaged, pushing on the little ball and shutting the valve.
Actually, if you test it on the bench, when energized the solenoid plunger centers in the coil. When it's up it will go down...and if it's down it will go up. With the solenoid in place in the car, it's normally below center so power to it will make it move up. (That probably didn't make any sense)
I emailed you a diagram of the oil flow, both engaged and disengaged. I tried to keep it small but still readable.
Reinstall everything and power it direct like you plan and see what happens. Your problem may just be a faulty switch or broken wire.
Rick
Rick Jaskowiak

The piston will centralise in the solenoid when powered. In normal use oil pressure pushes it to the outer end of the solenoid, so powering the solenoid moves it towards the o/d. That's for the later o/d, the early o/d worked the other way round according to the diagrams in the manual. In either case, when out of the o/d if the piston is pulled out past the centre powering the solenoid will pull it back in.
Paul Hunt

Thanks everybody, I'm much more educated and Rick, the drawing was a great help. Must be that when I was checking the solenoid outside the car I was checking with the piston in the up position, not knowing it was important where it was starting from.
Will let you you know
boB

Well I had time to take my 77 on a drive to test overdrive. I bypassed the gear switch and used a toggle switch insode to energize the solenoid. Low and behold the overdrive was engaged,...I did this twice. And it worked twice, and then it stopped working. I changed the oil to castrol 20/50, the old oil I believe was transmission fluid, and it was pretty goopy
from sitting idle for 6 years.

So what's the best next step? pull the solenoid, then the relieve valve, then the pump in that order? Anyone know where I can get all the rubber O-rings, and seals,etc, to do a rebuild?

I should say I think I downshifted from fourth to third when overdrive was engaged, would this have caused a problem?
r schwartz

Where did you pick up the 12v from to feed through your toggle switch? Are you absolutely sure that you still had 12v there when it stopped working? Try again with a test lamp connected to the solenoid side of the toggle switch, just to be absolutely sure it is not electrics before you start delving into the box.

Shifting between gears with the overdrive engaged will not in itself cause a problem.
Paul Hunt

Thanks Paul, good idea, just finished with your idea, the test light went on and off with the switch, was hoping that that would be the problem, but....so I suspect the solenoid is getting power.
r schwartz

Bob,
Don't know how you have that thing wired....are you still going through the inhibitor switch? If so maybe you have power to your toggle switch but not to the solenoid because the inhibitor is sticky.
I had my OD quit without warning last summer. Turned out that the pump stuck and wasn't riding on the cam. If you pull the sump cover and filter you can see it up there. Make sure it goes up and down when you turn the shaft. I was able to free mine with a tap of a screwdriver and haven't had a problem since.
As Chris Betson said earlier, the pump valve may also have to be checked. If you pull the pump, you'll notice one side of the plunger by the wheel is flattened. That flattened side should face the rear. I found the pump kind of a pain to reinstall.
Moss, Victoria British, etc carry all the gaskets, o-rings, etc.
If you want to see a visual of anything let me know. My family gave me a digital camera for Christmas and I have an OD torn down on my bench.
Rick
Rick Jaskowiak

Rick, when you say turn the shaft, are you talking about the drive shaft that goes to the differential and can you turn it easily by hand?
r schwartz

Yes, with the tranny in neutral and the rear wheels off the ground, you should be able to turn the driveshaft by hand unless your rear brakes are tight. I don't know how common it is for the pump to stick...I'm thinking it doesn't happen often, but it's worth a look if you already have the sump cover and filter off.
Rick
Rick Jaskowiak

r schwartz:

As was mentioned, you can get ALL the O-Rings, filter/gasket, magnets, "ball"s, etc etc from Moss and VB.

A brief run-down on parts and procedures for fixing some problems I had with my '77 OD is posted at http://www.britishcarforum.com/techtips.html

BY the way - I've heard it said that with the wet-clutch, and pump in the OD that sometimes several oil changes might be necessary to get things back to normal. Sometimes people use gear-oil instead of 20/50, sometimes transmission fluid, sometimes they add slick-50 or other products.
Scott K

This thread was discussed between 08/02/2002 and 16/02/2002

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