Auto gearbox - after some advice

cb750chris · 25th April 2021, 08:58

Hi All,
I was asked to look at a car recently which has stopped 'dragging' when reverse is engaged and the owner described it as 'reluctant to reverse'. Another symptom was that the ABS light was coming on intermittently.

My first thoughts were that this was possibly a failure of one of the rear backplates, causing the brake shoes to move and partially lock a rear wheel - with the rust from the backplate upsetting the ABS sensor. Or a second thought being a cracked reverse piston causing the gearbox to loose pressure, so needing a bit more revs to reverse.

Plugging in the car I got a different symptom to what I expected - The ABS fault was being caused by a front sensor - turned out it had been rubbing and a core of the cable had failed.
There were no recorded faults on the autogearbox (as would be expected from a duff reverse piston).

Jacking up the rear of the car showed both rear wheels rotated freely in both directions - so not a rear wheel issue (he also had a local mechanic check the rear shoes before visiting me).

However jacking up the front of the car (one side at a time). With the car in neutral (ignition off) the wheels turned freely forwards, however it was significantly more difficult to turn the wheel in the reverse direction (a lot more so than a different auto 75 that was available at the same time - this car was also more difficult to turn the wheels backwards - so I assume that is just a feature of the gearbox).

The wheel turned freely backwards a few degrees, until the driveshaft 'play' was taken up (suggesting to me it is not a front brake issue, the pads are nto dragging), at which point things went tight, and 'lumpy' - the best way I could describe it would be as though turning an engine over (albeit with very low compression)

Does anybody have any suggestions as to what I could be looking at here? It doesn't feel like any of the following:
1) Duff solenoid- I would expect the T4 to flag the fault, and there are no issues when driving forward, and the mechanical feel to the wheel turning.
2) Low/Dirty oil - Unless the gearbox is stuffed I wouldn't expect oil level to make difference when turning by hand with the engine off.
3) Cracked reverse piston - as I understand this failure is a simple pressure leak internal to the box, causing loss of clamping pressure, this shouldn't affect me turning the wheels by hand.
4) Car Brakes - as the wheels all turn freely in both directions, until the play in the driveshaft is taken up. The brake pads do not drag/catch.

I don't want to start changing things that don't need changing - I want to give the owner the best cost fix - he isn't too local to me, so I also don't want it turn in to a multiple visit solution either.

Thanks
Chris

cb750chris · 28th April 2021, 12:31

Does anybody have any suggestions, or even confirmation of a cracked reverse piston can cause an increase in resistance to movement

stocktake · 28th April 2021, 14:10

Cannot see a cracked piston alone causing any kind of drag to be honest. could the problem be in one of the CV joints?

clf · 28th April 2021, 16:32

Quote:

Originally Posted by cb750chris

Does anybody have any suggestions, or even confirmation of a cracked reverse piston can cause an increase in resistance to movement

I do not have an answer, but is there a way of investigating whilst on the ground, or the axles being under tension. Just in case the weight of the car on the axles will show an issue, I doubt it though - it reminds me of an issue with a bike I had to work on years ago, which only presented with weight, but was fine otherwise (I know a bike is considerably different lol but it highlighted an issue that was not present under normal investigation methods).

Another potential issue perhaps could be a partial blockage in the oilways. A drain and refill may dislodge it and solve the issue.

mbonwick · 29th April 2021, 19:42

It's possible (I think!) that if the reverse piston has had a slight leak for a while (not enough to cause complete loss of gears but enough to reduce the clamping pressure) for the resulting excessive slip on the relevent clutch packs to cause overheating and damage.

How likely it is that this is the problem I don't know, but it must be something in the box and like you I can't see it being reverse piston/solenoids/oil or brakes. Definitely worth changing the oil as an easy, cheap first step though.

25th April 2021, 08:58	#1
cb750chris Trader Rover 75 Tourer Join Date: Apr 2011 Location: Portsmouth Posts: 849 Thanks: 201 Thanked 292 Times in 200 Posts	Auto gearbox - after some advice Hi All, I was asked to look at a car recently which has stopped 'dragging' when reverse is engaged and the owner described it as 'reluctant to reverse'. Another symptom was that the ABS light was coming on intermittently. My first thoughts were that this was possibly a failure of one of the rear backplates, causing the brake shoes to move and partially lock a rear wheel - with the rust from the backplate upsetting the ABS sensor. Or a second thought being a cracked reverse piston causing the gearbox to loose pressure, so needing a bit more revs to reverse. Plugging in the car I got a different symptom to what I expected - The ABS fault was being caused by a front sensor - turned out it had been rubbing and a core of the cable had failed. There were no recorded faults on the autogearbox (as would be expected from a duff reverse piston). Jacking up the rear of the car showed both rear wheels rotated freely in both directions - so not a rear wheel issue (he also had a local mechanic check the rear shoes before visiting me). However jacking up the front of the car (one side at a time). With the car in neutral (ignition off) the wheels turned freely forwards, however it was significantly more difficult to turn the wheel in the reverse direction (a lot more so than a different auto 75 that was available at the same time - this car was also more difficult to turn the wheels backwards - so I assume that is just a feature of the gearbox). The wheel turned freely backwards a few degrees, until the driveshaft 'play' was taken up (suggesting to me it is not a front brake issue, the pads are nto dragging), at which point things went tight, and 'lumpy' - the best way I could describe it would be as though turning an engine over (albeit with very low compression) Does anybody have any suggestions as to what I could be looking at here? It doesn't feel like any of the following: 1) Duff solenoid- I would expect the T4 to flag the fault, and there are no issues when driving forward, and the mechanical feel to the wheel turning. 2) Low/Dirty oil - Unless the gearbox is stuffed I wouldn't expect oil level to make difference when turning by hand with the engine off. 3) Cracked reverse piston - as I understand this failure is a simple pressure leak internal to the box, causing loss of clamping pressure, this shouldn't affect me turning the wheels by hand. 4) Car Brakes - as the wheels all turn freely in both directions, until the play in the driveshaft is taken up. The brake pads do not drag/catch. I don't want to start changing things that don't need changing - I want to give the owner the best cost fix - he isn't too local to me, so I also don't want it turn in to a multiple visit solution either. Thanks Chris __________________ Now offering T4 services in the Fareham area Replacement keys programmed / supplied / cut Diesel 135 upgrade available IPK virginisation and T4 matching available Sorry, due to a big change in home circumstances, I am unable to do any large jobs at present, but can still offer evening time diagnostics.

28th April 2021, 12:31	#2
cb750chris Trader Rover 75 Tourer Join Date: Apr 2011 Location: Portsmouth Posts: 849 Thanks: 201 Thanked 292 Times in 200 Posts	no suggestions? Does anybody have any suggestions, or even confirmation of a cracked reverse piston can cause an increase in resistance to movement __________________ Now offering T4 services in the Fareham area Replacement keys programmed / supplied / cut Diesel 135 upgrade available IPK virginisation and T4 matching available Sorry, due to a big change in home circumstances, I am unable to do any large jobs at present, but can still offer evening time diagnostics.

28th April 2021, 14:10	#3
stocktake Vis Whiz Rover 75 2.5 auto Saloon Join Date: Jun 2007 Location: LEEDS Posts: 20,587 Thanks: 2,057 Thanked 3,056 Times in 1,621 Posts	Cannot see a cracked piston alone causing any kind of drag to be honest. could the problem be in one of the CV joints? __________________ Dave... Lost a few stones and a Gall Bladder and part of a bile duct and all of my dignity in the suppository incident

29th April 2021, 19:42	#5
mbonwick Gets stuck in Connoisseur SE 1.8 Auto Saloon Join Date: Jul 2015 Location: Kendal Posts: 633 Thanks: 129 Thanked 176 Times in 147 Posts	It's possible (I think!) that if the reverse piston has had a slight leak for a while (not enough to cause complete loss of gears but enough to reduce the clamping pressure) for the resulting excessive slip on the relevent clutch packs to cause overheating and damage. How likely it is that this is the problem I don't know, but it must be something in the box and like you I can't see it being reverse piston/solenoids/oil or brakes. Definitely worth changing the oil as an easy, cheap first step though.