JST

GKN Overdrive Service

42 posts in this topic

Havent changed the oil in it for a while (25k miles) so went under today to do it whilst doing the G & T box oils etc.

Anyway drain plug - nothing bar a dribble came out, open the level hole and tonnes of it came out! then took the cover plate off to get the remaining old oil out.

Question is how come when it was filled to the correct level did it all come out the level hole? How is it held above the level hole? You can't even over fill it - its a simialr setup to the axle fillers/drain etc less you can take the sump off (well small plate with 6 x 11mm headed bolts)

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should only come out of the fill/level hole if the vehicle is in a slightly nose up attitude on sloping ground, can't see any other way for the oil to be higher than the level hole unless the fluid was still fairly hot & had expanded.

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its was level, strange that, it was an hour after a short run but i would have thought it would have drained down by then.

anyway also have a small problem with it. only happens in the wet but:

its slow to engage Over drive after pressing the button and then surges in and out of overdrive if the speed drops below 50mph, no lights come on.

When its dry again all works as it should engages straight away (as long as fast enough) and then auto knocks out if the speed drops too low with the red light coming on.

I had assumed the speed sensor was getting wet (water) and that needed a clean up/waterproof - but why wouldnt the sensor light come on and why would it surge in and out?

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On mine the gearbox leak got all over the connections on the solenoid and caused it to work every now and then. Cleaned up the contacts and it has worked fine ever since.

The user manual states to drain the oil you have to drop the plate. I spoke to the person who was responsible for it at GKN and he stated that people often confuse the bolt at the bottom for the drain hole. He stated that it had something to do with the fitting of it to American Campervans.

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IIRC the red light only comes on at start up [circuit check] & below 30mph controlled by the speed sensor. mine was slow to disengage but after a oil change it's been fine, might be worth removing & flushing the soleniod unit with a cleaning solvent like genklene or similar as thats the only part that makes it engage/disengage, only other thing I can think of is the ECU box has got wet or damp & possibly causing the problem.

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The user manual states to drain the oil you have to drop the plate. I spoke to the person who was responsible for it at GKN and he stated that people often confuse the bolt at the bottom for the drain hole. He stated that it had something to do with the fitting of it to American Campervans.

Steve Goodhall the man? any way read the manual when i got back in and yes you are right plate needs to come off the bottom bit isint a drain hole. Anyway plate did come off to clean the filter so its all done.

As Ralph suggests i will check the control box and clean up the solenoid contacts, just strange that if its not the speed sensor or ECU thing why it surges in and out below 50mph but doesn't when above and only does it when its wet.

I may just connect the solenoid straight up through a relay switched from the gear knob if cleaning the solenoids doenst cure the problem

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Steve Goodhall is the GKN chap I spoke to when mine was mucking about, He gave me top class info that cured the fault I had, darn shame GKN have stopped making the overdrive units I love mine :D

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As has been said Steve Goodall is the man....

My control box is still sitting on the didning room table after having been checked.

Next job is to remove the gear lever gator and change those little connectors to Lucar ones.

Then it might work correctly again.

I have a fag lighter plug into the fag lighter socket. A switch and warning light taped to the gear lever.....

JUST DO NOT FORGET TO SWITCH IT OUT OF OVERDRIVE AND SELECT REVERSE.

mike

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Treading carefully here ...

Going on the clues in this thread, the GKN is electrically controlled, and must be disengaged before selecting reverse. There is also a minimum speed limit before it will engage.

The MGB I used to have had a Laycock OD with the same limitations.

I'm wondering if these are essentially the same units.

In MGB format it was electrically controlled. The circuit contained a simple switch which only worked in 3rd and 4th, this took care of the minimum speed and reverse requirements.

The OD shared it's oil supply with the main gearbox. The specified oil was engine oil, not gear oil.

There was a magnetic plate fixed to the inside of a bottom cover. This collected a dark grey gunge, and you didn't realise how much there was until you came to wipe it off.

Gear selection was done by a cone clutch, operated hydraulically from a pump on the OD input shaft. The electrical control merely operated a solenoid, which when operated, closed a leak-off path, putting the pump output onto the operating pistons.

Does this sound at all familiar?

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Dave, the GKN unit is a updated Laycock Type 'R' similar to those fitted in MG's & Triumph 2000 mk2's IIRC.

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OK, I've just looked up the paper information I had, and turned up a letter from Laycock dated 1975 in response to an enquiry I had made about upgrading the overdrive to go with an overbored B series engine.

They responded with positive advice, and the obligatory caution, together with a service guide for an LH Type OD, so that's obviously what was installed at that time.

They pointed out that the 3.5 V8 MGB was configured to have OD on 4th only, whereas I had the 1800 4 cylinder.

Cheers

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Just cleaned up the two contacts so will see what its like on the next run, if its still jumpy then i will have a look at the control box.

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Just cleaned up the two contacts so will see what its like on the next run, if its still jumpy then i will have a look at the control box.

If it's still jumpy look at the block connector at the base of the gear lever. Silly small electronic connectors. Known to give trouble.

mike

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If it's still jumpy look at the block connector at the base of the gear lever. Silly small electronic connectors. Known to give trouble.

mike

Will do cheers Mike.

Base of gear lever being base of supplied OD gear knob?

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No James.

Gator off at transmission tunnel.

Have a grope under there. You will come up with the connecter block.

I'm changing mine next week. I was going to do it this week but I've been ill.

mike

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My OD is in bits at the moment with bearing rumble, simple to pull apart and only 2 main bearings. There is a double lipped seal on the input shaft that keeps the oils apart.

There is a one way roller bearing in it which is why something will explode in reverse and O/D. I'll post some piccies up sometime. If you do a search on Laycock and Volvo it leads to plenty of info on how earlier versions work. The Defender version is basically the same with fancy electronics.

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Nice pictures :) Does that double lip oil seal has a part number? My relative new overdrive filled itself up with EP from the transfer box. Probably due to pressure inside the transfer box but still have to make a long test run to if my pipe and vent cleaning has sorted out the problem.

Cheers

Marco

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Nice pictures :) Does that double lip oil seal has a part number? My relative new overdrive filled itself up with EP from the transfer box. Probably due to pressure inside the transfer box but still have to make a long test run to if my pipe and vent cleaning has sorted out the problem.

Cheers

Marco

I'll have a look to see if I can see a number, if not, I'll take dimensions.

Got to press bearings out and go to the local bearing supplier.

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thanks for the replies so far guys.

i have so far:

cleaned up the solenoid contacts

cleaned up the speed sensor and refitted.

checked the wiring in to the control box (none loose all dry etc)

cleaned up the connector at the bottom of the gear lever.

changed the oil

and on test drive

still the same

it wont engage until over 50mph, and still surges in and out (no light on gear knob though) as the speed fluctuates around 50-55mph, above that speed it holds it in and is fine.

even above 50mph is still hesitates a while before engaging the OD.

I am contemplating bypassing the whole system and wiring the solenoid in via a relay controlled off the gear knob switch.

So not being particularly good at wiring how would i do it using the current wires?

Gear knob switch is black and red wires for 12V and earth

i guess 12v ign to red wire that goes to the switch then black wire from switch to 85 on relay with 86 to earth

Solenoid is controlled via a yellow wire from the control box to earth.

so if i connect the yellow wire to 30 on relay and 12V ign to the other and last relay post would that work

ie switch the solenoid in and out with the gear knob switch.

I am not normally this bad with connections etc but having a mind F*rt today on it!

How i would then connect up the wng light to show when its engaged i haven't fathomed yet. earth the other (ie 3rd) gear knob wire) it would take its 12v feed from its old earth wire which is now 12v with wiring above.

i intend to bypass the control box and the speed switch.

I am assuming the solenoid is normally off - ie no power to it and when 12v is applied that engages the OD?

Any help greatly appreciated - if someone wants to stretch to a sketch that would be great.

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run it straight off a switch and 12v direct feed you mean

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Yes James. Yes.....

Remember I was in South Africa when mine went AWOL.

Getting the switch was the hardest part. Then I had the brilliant idea of using a cig lighter lead. It's still working since November.....

I must put it bak correctly.

A ROUNTUIT I think

Or as my daughter says. A GUNNER job

You know gunner do that.........

mike

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I have so far: .... changed the oil ....

On test drive, still the same, it won't engage until over 50mph, and still surges in and out, (no light on gear knob though) as the speed fluctuates around 50-55mph, above that speed it holds it in and is fine.

Even above 50mph is still hesitates a while before engaging the OD.

Any help greatly appreciated - if someone wants to stretch to a sketch that would be great.

Obviously I am concentrating on one aspect.

When I first posted in this thread I wasn't sure of the exact arrangements. Having seen the pictures posted here I now feel more confident in making definate statements.

The overdrive operates by oil pressure.

This pressure is created by a plunger pump that runs off the OD Input shaft. This is the 'same' as the gearbox output shaft, and therefore the speed of shaft rotation reflects the road speed of the vehicle, not the engine speed.

Given the slow engagement above 50 mph, and the surging when the speed is close to 50, my suggestions are all based around the fact that there is not enough oil pressure.

Causes for this could be the wrong oil, a worn pump, a badly seating solenoid valve, a leaking pressure relief valve.

I'd discount worn clutch linings, as these would allow slipping at all speeds when the OD is engaged, whereas the symptoms point towards oil pressure not building until the pump speed increases. This could actually be insufficient oil flow, where in conditions of low flow (50mph) the pump is not capable of compensating for the leakage through poorly seated valves.

To recap normal operation.

The pump runs all the time, but because the solenoid controlled valve is open, the oil is leaked off into the bottom of the OD housing.

When the solenoid is operated, the valve closes the leak off path.

The pump now builds pressure, to the extent that the return springs are overcome, and the clutch is moved into it's operated position.

The pump continues to build pressure until a spring loaded pressure relief valve opens, this maintains the pressure at a constant high level.

When the electrical operate path for the solenoid is broken, the solenoid controlled valve is released, opening the leak off port, causing the oil pressure to drop to zero.

The restore springs force the clutch from it's operated to it's normal position.

My conclusion is that initially you check the oil is of the correct viscosity (which doesn't just mean 'is it the same as that oil you drained out?').

The MGB gearbox used 20/50 engine oil, not the thicker EP oil. The provision of a double lipped oil seal between the gear box and the OD suggests the oils could have been of different grades, or it could have meant they just wanted a better chance of keeping the OD oil clean.

Next you strip and clean the pump and the valves.

I think you can do this without removing the OD, as there are access plugs in the side of the unit, adjacent to the plate with the magnetic filter. I'm not sure if there is a gauze filter, as well as the magnetic one, but if there is, a blocked filter is another possible cause of low flow and pressure.

For these last details to be clarified and confirmed you need someone with the appropriate manual, or hands on experience.

Cheers.

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Thanks David, i hope it isint that but thank you for the excellent description.

It does have a gauze filter which i cleaned when i changed the oil , i refilled it with ATF.

the issue that made me think it was electrical initially was that it only failed to engage in the wet, once dried out it would work fine. i had assumed some connections were a little damp but this doesnt appear to the be the case.

I will take it out for another test tomorrow after resetting the speed sensor and see how it goes, if its the same then i may be connecting up a direct feed to the solenoid as Mike suggests.

if its oil pressure then i guess i am bu**ered!

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