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MG MGB Technical - Overdrive not working

Finally got my MGB back from a garage after having sills replaced (and loads of other stuff besides). Runs brillant except that the overdrive does not engage.

I have checked the gearbox dipstick and the oil is clean and filled to Max. Where do I start looking to fix this? If I drain the overdrive to clean filters, solenoid etc does that drain the gearbox as well?

Nigel

First, you have to ascertain if you're getting power to the od solenoid. Go under the car and you will see a square plate at the back of the od unit. It will be black or blue, depending upon the year it was made. With the car in 4th gear and the ignition turned on, you should be getting 12 volts at the wire coming going to solenoid that is mounted under this plate. If you don't have power, you'll have to trace the wiring back to the od isolation switch which prevents the od operating in 1st and 2nd gears. Try this first and let us know what the results are. RAY
rjm RAY

Nigel

If it was working prior to the body work, I would say a wire may have gotten displaced. Don't mean to be insulting, but I would start with the fuses. Then check what Ray said
Bruce Cunha

No fuses anywhere in the overdrive circuit, from the factory anyway.

The only way to check the continuity of the overdrive circuit is to confirm current is flowing. Having voltage at the solenoid isn't enough if the solenoid itself is faulty. If you have an ammeter switch to the 1 amp scale, find where the yellow wires from the main and gearbox harnesses join by the fusebox, part that conenctor, and insert the ammeter. Switch on the ignition, turn on the manual switch, and engage an overdrive gear. You should see about 800mA on the meter, if so electrically the circuit is good, so the problem must be mechanical or hydraulic inside the OD. If you don't have an ammeter insert a test-lamp instead, and hopefully that will glow, again confirming continuity although not the correct current.

If no current is flowing *then* you start looking at voltages. First place is at the connectors you have just parted, there should be 12v coming from the main harness. If not then you need to check at the manual switch if on the dash or column. If you have voltage at those connectors then you need to check at the gearbox lockout switch, and the manual switch if on the gearlever.

The lockout switch is a pain to get to, you will probably have to remove any centre console, pull the tunnel carpet back and remove the large or small access panel that is there, and maybe lower the back of the gearbox on a jack after removing the bolts securing the crossmember to the chassis rails if you have to work on the switch.

Those connection points are for a typical UK 1968 to 1976 model with 4-synch gearbox and no electrical cooling fans. Earlier and later are different, but we don't know what year yours is.
Paul Hunt 2010

i have voltage at the solenoid. I think the next thing to do is remove the solenoid and check the O rings and ball (?).

Am I OK to just take of the solenoid cover and remove it without draining the gearbox / OD oil?
Nigel

When you remove the square access plate beneath the solenoid, the plunger will drop out. On top of that is the ball, which will also drop out. Above that is the valve assembly which can be pulled out with a little effort. Inspect the 2 O rings on the valve assembly as well as the one on the plunger. If any are suspect, they should be replaced. Thoroughly clean all the parts with carb cleaner. When re-assembeling, It is a good idea to re seat the ball up into the od housing with a small drift. You don't have to drain anything at this point. RAY
rjm RAY

Does the solenoid have continuity to earth? Should measure about 15 ohms.
Paul Hunt 2010

I received a new solenoid coil in the mail yesterday. It measured just above 14 ohms. The draw was .8 amps. When I install it today, I will post my results. I miss having my overdrive. It's become an integral part of the MG driving experience. RAY
rjm RAY

My fault, since rectified, was the solenoid coil and a rough, sticking plunger. I would hazard a guess that your solenoid coil is failing. When the welding was being done on your car, it is easy to see that the od wire could have been stretched and left you with an open circuit. Get a new solenoid coil, the 2 O rings that go on the valve above the solenoid, the plunger O ring, a new ball and a new base gasket. All of these parts cost me less than $60 U.S. and it was a good investment. The od works like new and I'm a happy man again. RAY
rjm RAY

Oh dear, swapping parts without proving them faulty first! That makes me cringe, being tight ...
Paul Hunt 2010

Paul, I'm not suggesting that Nigel should just assume that the coil is faulty, only that under the circumstances, with the car having sheet metal replaced that the probability of someone coming into contact with the feed wire is high. I agree that readings of ohms and amperage draw should be recorded before hand. RAY
rjm RAY

My tongue was in my cheek ...
Paul Hunt 2010

My foot was in my mouth. RAY
rjm RAY

This thread was discussed between 04/05/2010 and 09/05/2010

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