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MG MGB Technical - Clutch Slipping When Cold - Why?

I admit that I am new to MGB's, so maybe the "problem" I am having with my car is normal. This is my first winter in which my 73 MGB has been drive able. It runs great, starts easy and the heater will burn you out of the car. The problem I have been encountering is when I first start out when the car is cold. First gear is fine, Second gear is fine, but when I shift into Third, the engine revs (you can see it on the Tach) but the car does not go anywhere (clutch pedal is all the way out). I can see my speed dropping. When I take my foot off of the gas and let the rpm drop back down to about 1500 rpm, you can feel the clutch grab hold and then the car goes on from there just fine. After it slips the first time, it never does it again until the car is cold again. It has never done this when the weather has been warm - it just started when the weather turned cold. The temperature here in Atlanta has been in the high 30's and low 40's (F) - so it is not that cold. The clutch is new - only about 1,000 miles. Why is it doing this? Is this normal? How do I fix it? Has anyone else had this problem? Thank you.
Robert Browning

Do you have an OD unit? Sounds like slipping OD unit, if it's not doing it in 1st or 2nd.
John A

Yes, I have an OD, but is not "on" when the "clutch" is slipping. The OD seems to be working fine. The transmission is also new - only about 1,000 miles. If as John suggest, the OD is slipping, why? The gears seem to be fully engaged in the transmission. How could the OD cause the Third gear, gears to slip?
Robert Browning

To answer your first question, that is not normal behavior.

When the clutch was replaced, was the slave cylinder hose replaced? That hose (and the front brake hoses, as well) can fail internally in such a way that they act as a one way valve. When you press the clutch pedal, it moves the slave cylinder and releases the clutch. When you release the pedal, however, the hose collapses, and does not allow the fluid to flow out of the slave. So, even thought the pedal is fully released, the slave is still holding the clutch in the released position.

I've never heard of this being triggered by cold, but its possible.
Paul Noble

Paul, why would it only do it in Third gear?
When I had the clutch replaced, supposedly everything dealing with it was also replaced. I know the brake hoses and master cylinder are new and the clutch master cylinder is also new.
Robert Browning

Robert,

But when it slips, could it be in o/d dispite where you have the o/d cockpit switch? ie overdrive is electricly on and engages because it's meant to come on in third. Would that indicate electrical prob?
Cockpit switch shorted to "on"? Cold, first time of day thing maybe just that and it doesn't like it. You wouldn't shift into 3rd o/d on your first shift hot or cold day. Just guessing if you can't tell.
You'll get lots of good help here.

Have you checked and changed the oil yet on your new gearbox and o/d unit? The o/d has a drain too.

Paul
Paul Hanley

Paul, The gearbox and OD oil was changed at 500 miles. As for the OD being engaged, I do not think it is. I have a light wired in so it is activated when the OD is engaged. It is not "on" when the "clutch slips". Also, if it is the OD, why is it when I slow down, you suddenly feel the power "reconnect" and the car goes on like it should and I have not touched the OD switch on the wiper stalk. Also, why would it only be doing it when cold and only the first time you go through the gears? It seems like I have lots of questions about this, I just hope all of you "intelligent" MG folks have the solution.
Robert Browning

Robert,

Sorry I don't have any other guesses. Doesn't sound like a major deal but like I said, you'll get some great help here, especially from the pro's local and across the pond.
I like the indicator light thing. Would be very useful.

Paul

Paul Hanley

Robert: two things came to mind as I read your posts,#1 although you say everything dealing with the clutch was replaced you don't mention the hose to the slave cylinder, as Paul N mentioned this hose can fail I've seen hose failure that was so bad on brake hoses that the car will not move for about an hour while the trapped fluid seaps by the "flaps" in the bad hose.#2 When the oil was changed in the trans and OD what oil was used the wrong oil in the OD might cause this problem when cold .Good luck RIC
R E L Lloyd

What kind of lubricate and what is the viscosity -- weight of it?

Could it be it's just too thick when cold, or colder than summer temps?

Perhaps a lightwer weight oil in the transmission and OD unit might help.
glg

I may be wrong, but I believe that the transmission oil is 20w50. Is this what everyone would recommend?
The hose as Ric described could be an issue. I will check in the morning and see if it was replaced. It is just a strange problem. Thank you, Robert
Robert Browning

For What it's worth:

There was a post on the General board about MGs having a heart beat. I do believe that all mechanical contraptions, MGB engines included, have their own personality quirks. Just the nature on a whole bunch of metal parts held together to perform some sort of function.

On Oils --Engine/Trans. the manual states --
For temps above 10*F;
SAE 10W/50, 10W/40, 20W/50 or 20W/40
Temps 0* to 20*F
SAE 10W/50, 10W/40 or 10W/30;
Temps Below 0*;
5W/30 or 5W/20

Just to give you an idea as you're starting out on the MGB Journey. You might consider the option of using a lighter weight oil in transmission/overdrive as an inexpensive way of troubleshooting.
glg

Robert - Everything you have told us points directly to the O/D. The fact that the slippage occurs only when you shift into third gear and not in first or second would tel me that the clutch is not slipping. A check that you can make is the next time you are out on a cold morning, try shifting directly from second to fourth and see if the slippage occurs. It sounds to me like you may have a miswire to the O/D solenoid through the lockout switch that is applying 12 volts to the solenoid regardless of where you have the O/D switch set int he car. If this is the case and if your tattle tale light is wired to the switch in the car, you could wind up with the O/D engaging without the tattle tale light coming on. A check that you can make to see if this is happening is to take the car out on the highway and see what your speed is in high gear at 3000 RPM without the O/D engaged (from the switch in the car). Without the O/D engaged, you should see a speed of approximately 54 MPH. If the O/D is engaged, the speed should be around 66 MPH.

20W 50 oil should work fine in the Transmission. That is what I use year around here in Western Washington State, where the temperatures in the winter is mid thirties to mid forties, with occasional excursions down to the mid to lower twenties and I have never had a problem with the O/D using the 20W 50 oil. If someone mistakenly put 90W oil in the transmission, then the O/D will not work correctly (I have never experienced this, so can't say if this will cause the symptoms you are having).
Good luck - Dave
David DuBois

My understanding of slipping O/D's is that they can and will slip even when not engaged, FWIW.

Dean
Dean Lake

Dean. My understanding is similar to yours. Chris Betson, of Octarine Services, has posted some information on this subject, but not dealing with this specific problem. It would be worth contacting him for guidance. Les
Les Bengtson

Clutch slip due to clutch wear starts in 4th and will eventually happen in lower gears as the wear gets worse because the clutch experiences the greatest torque in the highest gear, the engine finding it much more difficult to accelerate the car in 4th than 1st. But because the gearbox is between the clutch and the OD any slippage in the OD will be apparent in 1st gear first because that is when *it* experiences the greatest torque, i.e. you are much more likely to get wheelspin in 1st than 4th. You don't say what happens in 4th, what does *it* do? As well as clutch slip it could be misalignment of the mechanical linkage between the clutch hydraulics and the pressure plate. See if you can push the slave piston back into the cylinder when the clutch pedal is released. You should be able to, if you can't then it is probably not allowing the pressure plate to apply full pressure to the friction plate, which will allow it to slip.
Paul Hunt

Paul, your thinking is more in tune to what I have been thinking about. As for Fourth gear, it always works fine. The only time that I get the "slipping" is when the car and weather is cold and in Third gear - only when you first start out. Once I have gone through the gear pattern once, it appears to not happen again - or at least I have not noticed it. You say to see if I can push the slave piston back into the cylinder. Can you give me a little more detail on doing this procedure?

Paul Noble asked about the slave cylinder hose. I found out this morning that the hose is new. I am stumped.

To verify if it is the OD, could I disconnect all the electrical wires going to the OD and see if this makes a difference? Will the transmission still work if I disconnect the electrical - the main four gears? If I disconnect all the electrical, would this basically lock out the OD and eliminate it as the problem?

Thanks, Robert
Robert Browning

Robert - disconnecting the wire to the solenoid will disable the O/D and leave the transmission working as a normal 4 speed. Good luck - Dave
David DuBois

Hi,
I would tear out the trans, throw in a new clutch and slave cylinder, and I bet you are good to go.

FWIW Alan
Alan

Dont forget to throw the trans back in. :>)
Andrew Blackley

Find a mild hill, and put the car in forth at a lower speed, say 35-40 or so. Floor it up the hill. If the clutch is bad it will slip real bad doing this and be very noticable. If the clutch is good in 4th and does it in 3rd only, I'd bet the OD unit. Maybe the override switch is missaligned and the OD unit is getting some power in 3rd? I'd try disconecting the wire and see if that cures it. If not, I'd bet something is amiss with the internals of the OD unit.
John A

I will give it a try see what happens. I will find a big hill and test the clutch. However, as for Alan's suggestion - yeah, right! OK Alan, I tell you what, why don't you just come on down to Atlanta and you can "throw in" a new one for me. :) Thanks for the suggestions.
Robert Browning

Maybe the 3rd gear thing being only when you first go through the sequence, is a bit like the 'may car won't start after I have bought vanilla ice-creanm' story from way back. Reputedly a true story, whenever this chap bough vanilla ice-cream from his local shop his car wouldn't start when he got back in, but it was OK when he bought choc-chip. I'll spare you the long version but it boiled down to choc-chip being right by the checkout and the door, whereas vanilla was buried right at the back of the store. It was simply related to how long he left his car parked before he tried to start it again. Maybe if you spent longer in 1st and 2nd, or got into 3rd sooner, it would happen in another gear.

Funnily enough my cousin and family were visiting last week and she happened to mention that when she parks her car with the wheels one side up on the kerb and the other two on the road it won't re-start. After questioning her it again came down to how long she left before restarting, rather than what angle it was parked at!
Paul Hunt

OK Paul, I will try the "Choc-Chip" theory and see what happens. If this works, maybe you should write a paper on it!! It would make more sense then some of the technical papers I have read and had to deal with in my life. I will let you know the results. Thanks.
Robert Browning

This thread was discussed between 12/12/2003 and 13/12/2003

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