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MG MGB Technical - Overdrive failure

I have owned my chrome bumper 1972 BGT from new, a much loved and cared for car in really good condition. Up until two months ago I engaged the overdrive by easing my foot off the accelerator and flicking the overdrive on. To disengage, I fully depressed the clutch and flicked it off. I covered 101,000 miles doing this with no problem and the overdrive worked perfectly. I recently read not to disengage this way, but simply flick the overdrive off without depressing the clutch. About 300 miles later (now!), the overdrive has failed.
It could be that the unit was about to fail and the timing is coincidental. I am interested to hear other opinions, including a supplier of a replacement unit.
JP Mitchell

I was four miles from home after a 1400 mile holiday a couple of weeks ago when my O/D failed to kick in. I discovered the following day that the switch wasn't doing what it oughta. A bit of fiddling and it started working again.

Mine was just a random event. I ask myself what I would have done if it had failed four miles AFTER I had left home!
Geoff Ev

if previous to this your o/d engaged almost instantly, as it should (when you didn’t dip the clutch) then it’s probably an electrical fault (more on this later) and just a coincidence that it just after you’ve started using the o/d switch correctly

if you read your copy of the Driver’s Handbook I’m sure it won’t say about dipping the clutch, you’re the first person I’ve heard of only dipping the clutch one way, there should be load on the o/d for engagement and disengagement - the clutch pedal does not need to be dipped or accelerator pedal lifted off fully, there should be load

oil changes in the service schedule are every two years or 24k-miles, whichever is sooner, and at this time the o/d filter would be cleaned, the engine oil in the gearbox breaks down quicker than if gear oils where used so the engine oil in the gearbox needs changing more frequently

see this article, plus if you need them I can list the John Twist video about o/d - http://www.mg-cars.org.uk/news/news363.html
Nigel Atkins

I was driving my brother's B recently and he was advising me to lift when engaging O/D and dip the clutch when disengaging.

I ignored him ;o)
Dave O'Neill2

interesting to have two lots of wrong info and mix them up, perhaps the advice is or was doing the rounds

I often open the driver's door to let the clutch out
Nigel Atkins

Almost sounds like a Monty Python skit with all of the crazy motions people use to operate a simple o/d.
Steven Rechter

I'd suggest that you test the electrics first! The most usual problem is wear on the inhibitor switch and/or low oil. I assume you have checked the oil. Personally I do not think your "change of operation" will be the cause.

Mine failed last year-the friction material was worn and breaking down. FWIW there are many opinions as to how to operate the o/d. I believe the original handbook said to keep the clutch fully engaged for both in and out. To engage I keep the clutch engaged and make sure the engine is "pulling". However disengaging without the clutch makes it a jerk-as usually there is no load as I am slowing down. I dislike this, so disengage the clutch slightly as I disengage the o/d. "Works for me".

An ammeter in series (either connected at the dash switch or the bullets on the engine bay bulkhead) should show about 850mA or 1A when it engages. Holding the gear lever towards you can be indicative of wear in the inhibitor switch. If there is no current flow, checking the voltage at each point along the circuit is next.

As it's a new o/d is an engine out job, I'd change the various O rings-see WSM or Haynes. Solenoids can fail (101K and 42years?)but are generally pretty tough. As are o/d units.

Overdrive Repair Services- 0114 248 2632 www.overdrive-repairs.co.uk
Michael Beswick

Coincidence. Probably the only thing best avoided is switching OD out on a trailing throttle at high rpms in 3rd, which can give quite a jolt. 3-synch ODs had a vacuum switch and relay to prevent that happening, but on the 4-synch it's up to the driver to learn. Other than that it can be switched in and out with no special movements of the throttle or clutch.

In what way has it failed? Broken i.e. expensive noises/no drive, or simply not engaging when you operate the manual switch.

In the latter case, and if the oil level is correct, as has been said make sure the electrics are working, which is easiest done by inserting an ammeter in the circuit, and can be done while stationary and the engine not running. Insert the ammeter at one of the connections to the manual switch, turn on the ignition, select an overdrive gear, and operate the switch. You should see 800mA to 1 amp displayed on the meter, which indicates electrical continuity is good.

If no current, or much lower than it should be, then you need track the voltage through the circuit. As this will entail the ignition being on for a while remove the white wire from the coil +ve to avoid cooking the coil.

With the ignition and manual switches on (ammeter removed and wiring replaced on the switch) check you have 12v on both sides of the switch.

If there then test both sides of the 2-way bullet connector in the engine compartment, by the master cylinders, where the yellow from the main harness joins the yellow/red in the gearbox harness for 12v.

If there then you need to test both sides of the gearbox switch, which will mean removal of the arm-rest cubby and tunnel carpet, and the small panel on top of the tunnel.

If there then the only other place you can test is the bullet connector in the wire that comes from the solenoid and goes to the gearbox switch.

If on both side of that then it looks like the solenoid winding could be open-circuit.

Needless to say of any of the test-points above show no or a greatly reduced voltage, the problem lies back towards the supply.

If you had the correct current in the first test then the problem is mechanical or hydraulic inside the OD.
Paul Hunt

Gentlemen,..many thanks for your replies to my OD problem. My garage of many years assures me they have followed through on the electrics and it appears the unit has failed. It will not engage /disengage. When engaged and I try to disengage, there is a 'coasting sensation' that the car has the OD engaged and really does not want to go anywhere fast. Almost a 'slipping clutch' sensation. Quite frustrating and costly if it boils down to taking the engine out, which is the likely outcome.
Thanks again for your comments. John
JP Mitchell

JP- You know your garage. However for most garages o/d units are a bit of a mystery, as is using an ammeter. Most rely on voltage power probes. The o/d clutch has friction material that in my case came out in bits when I drained the oil. As replacing the o/d unit will necessitate draining the oil anyway, you might ask for it to be done first to see what comes out in the oil!

If you put Overdrive in to the archives, there is a thread from Sept 2010 "Help OD drop out". Altho temperature related it covers the "coasting sensation" you describe. However as I assume the garage would be doing the tests, the cost may be more that pulling the o/d and replacing it.
Michael Beswick

If you are getting a sensation of a slipping clutch, coasting or free wheeling in overrun against the engine then it is very likely the friction material on the double sided cone clutch in the OD unit is disintegrating or has glazed. I had this problem and also had no drive in reverse, certainly a weird sensation. In my case the problem was the friction material was glazed but otherwise fine. A light sand to remove the glaze solved the problem. Also I strongly recommend that you do not use modern engine or gearbox oil or friction modifiers in your OD gearbox. Penrite make a gearbox oil "Gearbox 30" which is designed for old gearboxes with Overdrive units.
Cheers Richo
R A Prior

This thread was discussed between 30/04/2014 and 02/05/2014

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