MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - overdrive problem

71 roadster. Dashboard switch.
The overdrive on my roadster has started playing up recently. When in overdrive in 3rd gear it is constantly jumping in and out of overdrive, mostly when under load. At the moment its OK in 4th. When accelerating in third it can be in and out several times in the space of 3-4 seconds, It cannot be doing the engine a lot of good as the revs are up and down rapidly.
I have not checked anthing yet, thought I would get some opinions first. Thanks Trev
Trevor Harvey

Ist job check gearbox oil level.
jim soutar

Then check the electrics. One of the few times an ammeter is useful. LH type takes about 800mA-1A. It can be connected (in series) at the dash switch or the bullets in the engine bay. If the current drops to zero then it's electrical. Usual continuity tests. If they check out it will be the solenoid, but these are generally very reliable.

Then drain the gearbox and then the O/D and see what emerges. Mine has bits of friction material (looks like bits of brake shoe). = refurb o/d....

http://www.mgb-stuff.org.uk/gearstext.htm#diagnosis

http://www.mgb-stuff.org.uk/gearstext.htm#odr
Michael Beswick

the oil in the gearbox is for a 24k-miles/24 months changes, very often forgotten, at the same time a clean of the o/d filter

a couple of recent threads covered this subject
Nigel Atkins

the o/d should slip in and out effortlessly and (almost) instantly, some believe that dipping the clutch helps the o/d which it might if the o/d isn't working properly but the clutch does not need to be dipped or accelerator pedal lifted off fully as the o/d works on load

if required do a thorough oil drain and change, have the oil in the box as warm as possible and leave to drain for as long as possible, then clean of o/d filter, you can see if there are any bits in oil or o/d filter

if the oil hasn’t been changed for a long time then perhaps a flush with ATF (idea nicked off Chris) – fill with ATF instead of engine oil and drive around with this for a short time and mileage and then thorough drain of the AFT, have the ATF as warm as possible and leave to drain for as long as possible, then clean the o/d filter and refill with engine oil as normal
Nigel Atkins

If the oil level is correct, and the electrical tests Michael describes don't show a problem (although it does sound electrical) then you will need to connect a high-pressure gauge to the OD to measure the pressure with OD engaged. This needs at least one wheel to be free to rotate with the car safely supported.
Paul Hunt

although this is aimed at US obviously in principle it covers UK too

see 'fault diagnostics', and it's from this site too (if not originally) - http://www.mg-cars.org.uk/news/news363.html
Nigel Atkins

It kind of feels like to me that the overdrive inhibitor switch is set marginally. Try holding the gear lever over to the right when accelerating and see if this helps the problem. If it does it should be a fairly straight forward (apart from the access) job of removing one of the fibre washers in the switch mounting to cure the problem
Iain MacKintosh

Yes, inhibitor switch not quite set right, or just starting to fail, will give exactly those symptoms, even down to occuring in 3rd and not top gear - though once the situation develops further it will also occur in top.

Re the pressure gauge: it's surpising how often an overdrive is removed and stripped before doing this test, makes diagnosis much harder! Generally speaking, most pressure-related causes can be addressed externally, but if the pressure is still holding while problems occur then it's an gearbox-out job.

My gauge has sufficient hose length to attach it to the 1/4 light so the car can be driven. That allows a really useful comparison with any observed symptoms and system pressure. It also has attached to it a test light which is wired in parallel with the solenoid (fit a double female bullet at the solenoid plug). Together the two provide a very useful picture of what's happening or not happening.
Paul Walbran

OK So the oil level and filter should be easy enough to check but where would an ammeter be connected to and how would it be used, also a bit more info on the pressure test would be useful, ie what type of gauge, where to connect it etc.
Thanks for the advice. Trev
Trevor Harvey

Hello - I am new here - I have the same problem and found this site while searching for an answer - I have checked the electrics on my 1972 MGB and they seem OK but the OD drops out if I rest my hand on the gearlever!

Any ideas why that happens?

Alex
A Straub

It's as Ian describes above. In the overdrive circuit there is a switch, known as an inhibitor switch, in the gearbox which is triggered when 3rd or 4th gear is selected, but makes no contact in other gears. It's purpose is to prevent damage from the greater torque at the output of the gearbox (where it enters the overdrive) when in 1st and second but particularly in reverse where working in the opposite direction could get a bit ugly.

It's a plunger type of switch, triggered by the sideways movement of the gear lever (ie across the gate) and doesn't need much movement to operate it - so the set-up of it is quite critical. When it is out of adjustment, the results vary between allowing overdrive in first and second and no overdrive in 3rd and/or 4th. Most commonly, out of adjustment results in the symptoms you have, as the switch is almost making contact in 3rd/top and pulling the gear lever over to the right is enough to complete contact.

To check for this: engage overdrive in 3rd gear and while in gear pull the gear lever as far to the right as you can. This engages the inhibitor switch just a little more so if its setting is marginal then it may be enough to make contact and bring the everdrive in. Then move it back to the left - opposite process, will make the overdrive drop out if marginal. Repeat the process for top gear.

Any sensitivity to this action indicates the need to adjust the switch, done via washers between the switch and gearbox remote casing ... accessed on 3 synchro B's by removing the top cover of the transmission tunnel, but from under the car on 4 synchro models unfortunately.
Paul Walbran

The easiest place to connect the ammeter on your car is at the manual switch. Remove the white and connect it to the ammeter positive, and connect the ammeter negative to the vacated spade on the switch, or even to the yellow wire also removed from the switch if that is easier. Bear in mind that this circuit is unfused so make sure none of the connections can short to earth. If you connect the ammeter to the white and the yellow directly then effectively the manual switch will be on all the time, unless you unplug one of the lead from the ammeter. This should be the lead going to the -ve.

You need a gauge reading up to 420psi, (yes 420 psi) and it connects in place of the relief valve plug on the bottom, which is adjacent to the solenoid cover plate with the screen-printing on it. When removing this plug there are a number of parts above it, make sure they don't fall out and get jumbled up. This obviously needs the correct fitting, and a hose capable of withstanding that pressure.
Paul Hunt

Thanks for the suggestion Paul W, pulling and pushing the gear lever over makes no difference - pushing down does so I crawled underneath and found the switch by following the wire from the OD unit, what a pain to get at!

Swapped wires to a temp switch inside the car to test the rest of the circuit before even attempting to unscrew that thing.

It does the same thing, so I have ruled out the switch.

Tried wiring the temp switch direct from the fuse box to the OD unit, same again, the OD drops out if I lean on the gear lever downwards but not side to side.

Totally confused now, I can't see how that would affect it.
A Straub

as a general overview these videos might help -

15 MG Overdrive - http://www.youtube.com/watch?v=r-WUuz4BzTw

Overdrive gear ratios (overview of internals too) - http://www.youtube.com/watch?v=3HhjFNnsKr8

76 Fitting an Overdrive Unit to the Gearbox - http://www.youtube.com/watch?v=9Xilqrw48bw
Nigel Atkins

for servicing of o/d these might help -

104 MGB Overdrive Repair Part 1 - http://www.youtube.com/watch?v=XPSgUeHAy8E

105 MGB Overdrive Repair Part 2 - http://www.youtube.com/watch?v=LCqPu77EWEE

106 MGB Overdrive Repair Part 3 – http://www.youtube.com/watch?v=FHw_o5y0HWE
Nigel Atkins

"Tried wiring the temp switch direct from the fuse box to the OD unit, same again, the OD drops out if I lean on the gear lever downwards but not side to side."

Difficult to see how that would cause the problem, unless the resultant movement of the gearbox as a whole is moving the wire or the solenoid inside the OD unit and causing an intermittent connection.

Really you need to do the current test and confirm whether it is electrical or not. If not, even more confusing, that would need the pressure test.
Paul Hunt

I have tried what was advised re. moving the gearstick over to the left when in overdrive in 3rd and yes this was making the overdrive switch in and out. So maybe a shim needs removing from the switch. is this the one that is difficult to get access to, my car is a 71 roadster. I also checked the oil level and had to put in about half a pint to bring it up to the mark. The engine and gearbox was removed 3 winters ago for an engine rebuild. whilst this was out I replaced some O rings, cleaned the filter etc. in the overdrive unit, refilled with fresh oil. I found afterwards that it had an oil leak from around the filter housing, corrected this, and can only assume it lost more oil than I had thought. I have not checked to see yet if topping up the oil has made any difference to the operation of the overdrive.
Trevor Harvey

Yes, it is the difficult one to get at! You will need to remove the tunnel carpet (and centre arm-rest if present, and the small panel on top of the tunnel. Then lower the tail of the gearbox down on a jack after removing the four cross-member the chassis rail bolts. Hopefully that will give you enough access, you may need channel-lock pliers rather than a spanner.
Paul Hunt

re the g/box - half a pint sounds a lot but perhaps it was never fully filled, when filling obviously it's best and easier to add rather than take out oil, best to check the oil level on refills again after a good drive to see if it needs topping up

the gearbox oil using engine oil is serviced scheduled for changes every two years or 24k-miles whichever is the sooner
Nigel Atkins

Trevor - good progress. The cause can be either adjustment or the beginning of switch failure. Given the low cost of the switch compared with the effort of replacing it, I would be inclined to fit a new switch.

Paul, the full synchro tunnels don't have a removable panel on the top so it has to be all done from underneath. Fortunately those tunnels are larger than the early ones and there is enough room if the gearbox is pushed as far to the right as possible once it's down.

An important point when lowering the rear of the gearbox down onto the permanent crossmember is to first remove the speedo cable. If not, it stands a good chance of getting a kink in it thanks to being wedged between gearbox and crossmember. Kink = flickery speedo initially, then eventually a broken cable.
Paul Walbran

"the full synchro tunnels don't have a removable panel on the top"

They do, but it is a very small one, as shown in the attached. It does help a little, but not as much as the 3-sych panel.

Moss show it http://www.moss-europe.co.uk/Shop/ViewProducts.aspx?PlateIndexID=9, but their drawing is incorrect, the hole in the panel for the gear lever when fitted is towards the back not the front.

Paul Hunt

I've read (but not tried) that it is possible to tap the switch round to undo it using a long screwdriver from above. Not sure how to get it back -but perhaps re-assembly is.....
Michael Beswick

Im quite new to OD gearboxes. As I uderstand it the inhibitor exists to prevent the OD being switched in whilst in reverse gear. Given this interlock could equally be achieved via a relay driven from the reverse switch, would there actually be any risk in using overdrive in 1st or 2nd forward gears ?
C P Silk

C P , I've done just this to my '67 B that's been converted to a 4 synchro/OD unit. I wired a relay, into the reverse light circuit, that cuts power to the OD solenoid just in case I leave the OD switched on when I shut the car off. I've been running the car like this for over twenty five years with no problems. However, one must be careful using OD in first and second gears as the torque from the engine is greatly multiplied in the lower gears. If driven cautiously, the OD can withstand this extra torque without shortening its life. RAY
rjm RAY

I am also going through the process of diagnosing an LH overdrive problem. A new solenoid assembly is showing 650mA, and also shows this when connected directly to a 12v supply at the fuse box, thus eliminating any switch or connection issues.
My old solenoid draws 810mA when bench tested, as the literature says it should.

Has anyone experienced this lower current draw with a new assembly?
David Overington

"Given this interlock could equally be achieved via a relay driven from the reverse switch, would there actually be any risk in using overdrive in 1st or 2nd forward gears"

Technically it can be controlled from the reverse light switch, but you are introducing more possible failure points. The OD switch is 'fail-safe' in that it has to operate to engage OD, and the circuit wiring has to be continuous. Anything off the reverse light switch works on the opposite principle i.e. it has to *inhibit* OD which depends on there being a power supply to the switch and the relay, the switch closing, the relay operating and opening it's normally closed (presumably) contact.

There is also the risk of damaging the OD in 1st and 2nd through high torque reversals, that's why the V8 had OD *removed* from 3rd gear early in production. Having said that OD 2nd would be useful occasionally, it's quite a long stretch from 2nd to 3rd. Some Triumphs had OD 2nd but then they had a different OD unit.
Paul Hunt

650mA is quite a bit lower than normal, which is closer to 1 amp when the engine is running. It depends on how much that has affected the 'push' on the plunger when it's energised. It has to be able to resist the 400psi plus generated by the pump and the high-pressure relief valve. If it's marginal you could get OD clutch slip, which will glaze the linings, which makes them slip even more.
Paul Hunt

Thanks Paul, I've asked for comments from the overdrive specialist company who supplied the solenoid.
David Overington

The magnetic field in the solenoid depends on both the current and the bumber of turns in the coil. If the lower current version has correspondingly more turns, as would be the case if there were more turns using the same sized wire, then it would generate the same total force. If however the reduction in current is because of increased resistance with the same number of turns, it will be getting marginal on force but probably still be enough.

The force required to only just shut off the approx 1/8" hole against 400psi is about 5-6 pounds. IIRC the solenoid generates about twice that force. On straight current considerations the 650mA coil with the same number of turns would be around 9 pounds. I've measured the force generated by a number of solenoids and found that they corelate with dodgy performance around 7psi.

BTW it's quite easy to test: Connect the solenoid wire and a suitable casing earth to a battery so it is activated, then sit the unit on some scales (bathroom scales will do) and push down through the little hole in the end of the snout onto the ball bearing until the solenoid plunger moves, noting the load on the scales when it does so. Make sure you test it complete with its steel casing, including top and bottom, as leaving these bits off reduces the intensity of the magnetic field. And make sure you don't lose the little steel ball in the snout!
Paul Walbran

Paul W,
Thanks, I'd gone through the same thought processes regarding current and magnetic intensity but your test procedure is a great idea. Now to try it once I get the solenoid back out again!
David Overington

I must remember to check the switch(es) before fitting my overdrive 'box!
Dave O'Neill2

Do it using an equivalent circuit i.e. 12v and a couple of bulbs, not an ohmeter.

The switches can develop an oxydised layer that shows up as high resistance on an ohmeter, as that only applied a tiny voltage and current to the switch. 12v and a 'proper' load will burn through that and give a normal connection.

Of course, the switch could well be faulty, double-check it by measuring the voltage across the switch terminals when they are closed and carrying current, anything other than the odd tenth of a volt does indicate the switch may need replacing. I get a few tenths on mine but that's testing at the bullet connectors by the fusebox i.e. includes the connections to the switch, some wiring, and the bullets.
Paul Hunt

This thread was discussed between 18/03/2014 and 11/04/2014

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now