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MG MGB Technical - OD sometimes slow to engage after oil change

Hi,

I've had my '66 GT (Mk1, 3 synchro, presumably D-type overdrive) for 10 years now and the overdrive has always worked perfectly. After flicking the switch it has always engaged quickly (after no more than 1 second or so) and disengaged immediately.

A few months ago I had the clutch (normal gearbox clutch, nothing inside the OD) replaced by a well known and respected specialist; whilst they were at it, I asked them to change the rear axle oil, gearbox oil and to clean the OD filter as I've never done any of them in the whole 10 years I've had the car. I don't know what grade of oil they used, I just trusted them to use the right stuff. Since then, the OD has behaved differently and I don't know if it's a problem or not!

When engaging the switch, the OD sometimes engages quickly (in 1 second or so) just like it always has done. However, sometimes it will take 4 or 5 seconds to engage. It doesn't slip, it just takes 4 or 5 seconds before anything happens as if I hadn't touched the switch - it then engages smoothly and quickly as normal. In all other respects it's fine - disengages immediately, doesn't slip, etc. It's not temperature related, as I experienced both quick and slow engagement on Saturday all the way through a 120 mile round trip. Previously the only time the OD ever took more than a second or so to engage was when cold, and then it was just perceptably slower (perhaps 2 seconds, no more).

I think the vacuum switch works fine - if I de-clutch with OD engaged and flick the switch to "off" nothing happens until I blip the throttle, when you can hear and feel the OD disengage with a "click" from the transmission tunnel. Likewise, if the switch is flicked to "off" on the overrun the OD doesn't actually disengage until I touch the throttle.

Any ideas? Is it even anything to be concerned about? I'm only comparing with what it's been like before - I guess we all notice when something behaves differently, but I don't really know how it *should* behave!

Many thanks,
Tim
T Jenner

Take the dip-stick out and check the oil on it. They may have put gear oil in, it should be engine oil in the 4-cylinder. Having said that it gear oil shouldn't really be a problem as it is standard in the V8.

Other than that I'd be concerned that something wasn't done right when the filters were cleaned, there are a lot of small parts in there, have a look at http://www.mgb-stuff.org.uk/wn_gearsframe.htm and click on 'Oil change'.

Your vacuum switch is working as it should, it only affects disengagement, delaying it until you are on the throttle.
Paul Hunt

Thanks, Paul. Is the OD oil and gearbox oil shared?

Cheers,
Tim
T Jenner

Tim, yes the OD and gearbox share the same oil. It could be slow also due to a buildup of varnish, gunk, etc. You may want to try running for a while using automatic transmission fluid in the gearbox. It tends to clean things up.
John H

As John says the oil is shared. It's also true to say that ATF can sometimes 'fix' problems, and some say that that have to run permanantly with ATF, but I think that just shows they have a marginal OD with incipient problems that the ATF is just masking. But if your problems only started after the recent oil change and filter clean I'd be less inclined to think that ATF will help, assuming you have the correct oil in there now. I'd expect there to be a note on the invoice of what oil they used as it should be billed for.
Paul Hunt

Thanks guys. Just checked the gearbox oil level and it's right up at max. Looks like normal engine oil to me (ATF is pink isn't it??). Unfortunately the invoice just says "gearbox and rear axle oil" - the list of things which needed doing was quite long so they were running out of room to write!

I'll try to take it for a run this weekend and see how it goes - it doesn't get used much and last weekend was the first long run for a couple of months. If it's still bad I'll give the garage a call and see what they say.

One important question - I won't have time to do anything for several weeks as I'm getting married next Saturday and the car has duties to perform! Is this anything I should be overly concerned about? Should I avoid using the OD in case it packs up or anything? I can't imagine the wife-to-be being very impressed if we end up stranded somewhere on our wedding day...?

Cheers,
Tim
T Jenner

I'm hoping that the "gearbox and rear axle oil" listed on your invoice doesn't mean they used the same oil for both in which case one would have the wrong lube. The use of ATF in gearboxes has become quite common. Some of the manufactures both in the US and the far east use it. GM has used it in the transfer cases on 4X4 trucks since the eighties. As they keep removing additives from the engine oil it may well be a better choice.
John H

Gearbox and axle oil could also refer to two different oils, but neither are correct for the 4-cylinder gearbox! There is a very different smell to gear and diff oils compared to engine oils, should be obvious when comparing the gearbox and engine dipsticks. Even if you have gear oil in it isn't going to harm things in the short term even if it is that which is causing the slower engagement.
Paul Hunt

The way I read the invoice is that they've just written down one price as the total for the two types of oil - I did have a sniff of the gearbox dipstick and I'm pretty sure it's normal engine oil. I'll take it out tomorrow and maybe give the garage a call next week.

Cheers,
Tim
T Jenner

I took the car out for a brief run at the weekend and, predictably, the OD behaved perfectly! Fingers crossed it behaves at the weekend. Thanks for the help thus far - if it plays up again I'll be back!

Cheers,
Tim
T Jenner

This thread was discussed between 19/02/2009 and 23/02/2009

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