940 transmission vibration - prop shaft or differential?

[martin calva]

I'm on a long quest to sort out very serious vibration on braking in my 2.4 TIC 940 estate (M90 gearbox). About 550,000 km (~350,000 miles) on the clock.

After replacing various parts around the front hubs and suspension without effect, the garage identified significant transmission vibration with the engine running and the back wheels turning. Placing a hand on the prop shaft centre bearing or on the differential housing reveals vibration which the garage thinks is related to the problem of strong vibration on braking.

The garage owner guesses it is a prop shaft problem but cannot rule out it being a back axle problem. I noticed that there is significant play when the car is in neutral and the propshaft is turned back and forward by hand.

How much differential play is normal? Is the presence of play an indication that the differential might be the cause of the vibration (or of the problem of strong vibration when braking)?

So far, I have not tracked down a suitable replacement prop shaft. For the time being, I have asked them to try replacing the universal joints and the centre bearing, although this is a bit of a long shot.

Any suggestions or advice will be extremely welcome.

[john.wigley]

Hello Martin, I think that you are bang on the money with the prop centre bearing. In an age of predominantly front wheel drive, there are far fewer mechanics who have experience of and understand rear wheel drive issues.

In the past, I've experienced similar issues on earlier cars - all caused by a failed centre bearing.

Regards, John.

[martin calva]

A new centre bearing arrived yesterday. The garage replaced it and reported no improvement; there is still strong vibration on braking, which seems worse if the car is coasting in neutral.

I asked the mechanic (by email) to try removing the rear half of the prop shaft. He replied (by email) that there was no point in trying that, as the car cannot be driven without the rear of the prop shaft.

I went there this morning and explained: They have demonstrated to me that, with the car on the lift and with the wheels turning in free air, vibrations can be felt when a hand is placed on the centre prop shaft bearing and when hand is placed on the back axle.

I reason that, if vibrations can still be felt at the centre bearing when the rear of the prop shaft has been removed, then it is unlikely that the back axle is the cause. But if the vibrations can no longer be felt at the centre bearing with the rear of the prop shaft removed, suspicion falls on the back axle as the cause of the problems.

[john.wigley]

I'm sorry that it turned out not to be the centre bearing Martin, I was sure that the fault would lie there.

I would still be surprised if it turned out to be the rear axle though. They are generally pretty robust items if the oil is kept up to snuff and I have never known one to give trouble in this respect before.

I suppose that you could have a broken tooth or a duff bearing, but you would presumably have to strip the unit to verify that.

This may seem left-field, but could the prop be out-of-balance for some reason? I admit that it is unlikely, but I had this once on a 1950s Ford. We 'tuned' it out by a number of strategically placed Jubilee clips. Could be worth a try if you are desperate, but there is a world of difference between an 8HP Ford and a latter-day Volvo 940.

Regards, John.

[baggy798]

How are your torque rod bushings? They determine the angle of the prop in relation to the diff. Too much movement one way or the other due to wear can induce vibration.

+ YouTube Video

[classicswede]

The diff will not be the problem and the garage are right that taking off the rear half of the prop is going to show nothing.

7/900 series do suffer a lot with prop vibrations.
The torque rod bushes are a good call as they can put the diff out of position.
Soggy gearbox mount can do it
The other issue can the position of the centre support. Volvo did sometimes pack them with spacers to raise or lower to solve vibration problems.
A 940 I have been working on as the bearing and the holder bracket mounted on rubber mounts to solve a vibration issue.

I suspect in your case the torque rods are allowing the pinion angle to shift under braking causing the prop to vibrate

[martin calva]

Originally Posted by classicswede

The diff will not be the problem and the garage are right that taking off the rear half of the prop is going to show nothing.

Thank you for that. I am grateful for all your comments which I know are based on your wide experience.

There are two symptoms and I (and the garage) assume they are related:

[1] With the car on a lift and the rear wheels rotating in free air, continuous vibration at prop shaft speed is noticeable when a hand is placed on the centre bearing or on the rear axle. The garage owner said that this vibration is due either to a prop shaft problem or it is due to a problem in the rear axle.

[2] With the car running at speed, in gear or in neutral, there is horrendous juddering vibration when braking.

I am waiting for the garage to see whether the continuous vibration is still present when the rear half of the prop shaft is removed. If it disappears, that will suggest to me that the rear axle has a problem which is causing the continual vibration.

I have been given a back axle from a 2.4 diesel which had done 250,000 miles and I note that it has no discernible play when I twiddle the prop shaft coupling. Whereas the rear axle on my car has very significant play when I twiddle the prop shaft (gear in neutral).

Is such play normal and acceptable? Could it be the cause of my problems?

Originally Posted by classicswede

The torque rod bushes are a good call as they can put the diff out of position.
(snip)
I suspect in your case the torque rods are allowing the pinion angle to shift under braking causing the prop to vibrate

I wondered about that. The garage says that the torque rod bushes are in good shape.

[martin calva]

Originally Posted by baggy798

How are your torque rod bushings? They determine the angle of the prop in relation to the diff. Too much movement one way or the other due to wear can induce vibration.

Thank you for that. I had wondered about the torque rod bushings. The garage says that they are in good shape. Should I insist they change them anyway?

[martin calva]

Originally Posted by john.wigley

I'm sorry that it turned out not to be the centre bearing Martin, I was sure that the fault would lie there.

I would still be surprised if it turned out to be the rear axle though. They are generally pretty robust items if the oil is kept up to snuff and I have never known one to give trouble in this respect before.

I suppose that you could have a broken tooth or a duff bearing, but you would presumably have to strip the unit to verify that.

This may seem left-field, but could the prop be out-of-balance for some reason? I admit that it is unlikely, but I had this once on a 1950s Ford. We 'tuned' it out by a number of strategically placed Jubilee clips. Could be worth a try if you are desperate, but there is a world of difference between an 8HP Ford and a latter-day Volvo 940.

Regards, John.

John, thank you for those comments.

The garage found that, with the car on a lift and the wheels rotating in free air, there was noticeable ongoing vibration at prop shaft speed that could be felt by putting a hand on the centre bearing or on the back axle. The garage owner stated that the problem was either due to the prop shaft or to the back axle.

They replaced the rear half of the prop shaft with one I had received and that made no difference. (The front half I received was wrong for my car).

I have been given a back axle from a similar car and there is no discernible play when I twiddle the input shaft. Whereas the prob shaft on my car has very noticeable play when I twiddle the prop shaft. Could this indicate that the back axle is causing the problem?

There is very serious juddering vibration when braking yet no noticeable vibration when driving normally. Is that consistent with an out of balance prop?

[360beast]

Propshaft vibration is most prominent when setting off from a standstill or when reaching certain speeds.

Did you ascertain whether or not your car has the updated strut rods and are the bushes for the front suspension in good condition? Also when was the tracking done? Are the front wheels balanced/buckled? All of those will affect vibration when braking.

My 940 is very susceptible to braking vibration if the wheel alignment is out, maybe it is to do with the polybushes but twice I've had the front suspension apart requiring a wheel alignment and both times before having the tracking done it got a bad braking vibration.

When they've got the car on ramps have they got the rear wheels hanging down or is it jacked up on the rear axle? If the wheels are hangin down this would put the propshaft at an extreme angle inducing a vibration.