97 Disco auto box - different revs at same speed (above 50) day by day

This one's kinda hard to explain... and if my rev counter wasn't working, I wouldn't even know it was happening, so maybe it's nothing.

Anyway, I shall explain......

When driving below 50mph or so, everything is completely normal.

When I tip over 50mph or so the torque converter locks up (as it should do) and my revs drop to the fixed level (because the engine now revs inline with vehicle speed). At this point, flooring it does not rev the engine, it simply accelerates the vehicle and the revs go up in line with the speed, if that makes sense.

That is all normal.

However......

On some journeys the torque converter locks up and the revs drop to 1,500, for example. If I then speed up to 60mph, the revs read just under 2,000.

On other journeys the torque converter locks up (at the same speed!), but the revs drop to 1,750, for example. If I then speed up to 60mph, the revs read just over 2,000.

The torque converter is not slipping... it does slip sometimes, but when that happens, flooring it makes the revs change as if it weren't locked (but not as quickly). It's definitely locked up properly when this happens because foot off, or foot-to-floor makes no difference to the revs (unless the vehicle speed changes, of course).

So......

Alternator issue? Rev counter issue? Torque converter issue?

How is the rev counter run by the alternator? pulses or a constant voltage depending on speed? If it's voltage, then I can understand the voltage may vary day by day, hence the different readouts etc...

Any advice gratefully received!

if tc locks and stays locked then thats okay it depends how quick you get to lock up speed and pedal position etc at time gear change is achieved with mechanical govenor on bak that relies on spring pressure ,oil pressure and centifugal force,this can be effected by temp of box and oil .inertia,if you floor it while locked it will unlock and kick down hence rev difference,govenor action also depends on pedal position through kick down cable

i realise all that, but when it's locked, it's locked - if you are doing 60mph and it's locked, then regardless of everything else, should it not always do exactly the same RPM at a given speed when locked?

The wheels are turning at the same speed, and they are locked to the engine. The torque converter is locked and 4th gear has no play in it. Therefore, 60mph, with the TC locked should be the same RPM regardless of everything, no?!?

eventually once settled and locked you have direct drive and revs and speed will be the same day to day but at the time of locking depends on a collection of things been identical,the more sudden the change bigger effect on engine revs,rev counter measures pulses and yours is 13-14 years old maybe not exact as it was,these boxes dont usually suffer subtle faults and are completely mechanically controlled,there arent specific gears like a manual but different combinations of epicyclic gears and clutch packs,so it would be possible to have locked tc but slippingclutch pack not delivering full torque ,or freewheel clutch not grabbing as quick as it should

thanks for the reply...

you say "once settled and locked you have direct drive and revs and speed will be the same day to day", but for me this is not true, hence my first post.

The TC is definitely locking successfully and completely, but for a given speed when locked, the revs are different, day by day, as written in my first post...

The revs do not vary for a given speed during my journey, but for each journey, 60mph, for example, when locked equates to a slightly different RPM figure.

As for the rev counter receiving pulses rather than a constant voltage depending on speed, perhaps my alternator or dial is missing 1 or 2 in every hundred pulses sometimes, resulting in the lower or higher rpm readouts on the dial?

As for the epicyclic gears and clutch packs etc... that could easily be the cause, I suppose. For some journeys, perhaps 4th gear is slipping slightly, but for other journeys it isn't, hence the different readouts on the dial.

know what i mean?

i wouldn't worry! it sounds in fine fettle and more likely an electrical problem

fishsponge said:

thanks for the reply...

you say "once settled and locked you have direct drive and revs and speed will be the same day to day", but for me this is not true, hence my first post.

The TC is definitely locking successfully and completely, but for a given speed when locked, the revs are different, day by day, as written in my first post...

The revs do not vary for a given speed during my journey, but for each journey, 60mph, for example, when locked equates to a slightly different RPM figure.

As for the rev counter receiving pulses rather than a constant voltage depending on speed, perhaps my alternator or dial is missing 1 or 2 in every hundred pulses sometimes, resulting in the lower or higher rpm readouts on the dial?

As for the epicyclic gears and clutch packs etc... that could easily be the cause, I suppose. For some journeys, perhaps 4th gear is slipping slightly, but for other journeys it isn't, hence the different readouts on the dial.

know what i mean?

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has this just recently happend or has it been doing it for a while, if it hasn't been done yet it just mite need an oil and filter change, thats the first thing i would be doing its amazing what a difference it can make

cornish rattler said:

has this just recently happend or has it been doing it for a while, if it hasn't been done yet it just mite need an oil and filter change, thats the first thing i would be doing its amazing what a difference it can make

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don't know really... only noticed it last week, but I don't usually look at the rev counter at that level of detail to be honest.

probably been doing it for ages.

As for changing the oil & filter, 10 litres of so of ATF isn't particular cheap, so i'd rather not do it "just in case", if you know what i mean.

Also, I realise an oil and filter change can make a world of difference to an auto box, but could old ATF (which still smells sweet and has a cherry red colour) and an old filter really cause the issue I describe?

fishsponge said:

don't know really... only noticed it last week, but I don't usually look at the rev counter at that level of detail to be honest.

probably been doing it for ages.

1. As for changing the oil & filter, 10 litres of so of ATF isn't particular cheap, so i'd rather not do it "just in case", if you know what i mean.

2. Also, I realise an oil and filter change can make a world of difference to an auto box, but could old ATF (which still smells sweet and has a cherry red colour) and an old filter really cause the issue I describe?

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Hi there a couple of thoughts for you.

1. You will only need 5ltrs of atf to do an oilchange, as only 4.5ltrs roughly of old oil will come out. The rest stays in the TC.

2. As the issue is relatively subtle, ie 60mph is just below or just above 2000rpm I would be checking the rev counter wire on the back of the alternator, to ensure it has a good reliable contact.


Have you downloaded a cop of RAVE yet? I think you'll find that most of your questions would be answered in that. Do a Google search for 'Green Oval' and download the TDi 300 Rave manual (03 from memory).

thebiglad said:

1. You will only need 5ltrs of atf to do an oilchange, as only 4.5ltrs roughly of old oil will come out. The rest stays in the TC.

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That's good to know... is it a tough job, or similar to the engine oil and filter change?

thebiglad said:

2. As the issue is relatively subtle, ie 60mph is just below or just above 2000rpm I would be checking the rev counter wire on the back of the alternator, to ensure it has a good reliable contact.

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It is a good, reliable contact because throughout each journey it doesn't move around unnecessarily at all. It doesn't waver whatsoever.

It does read different from one journey to the next though, if that makes sense. i.e. on my way to work, 60mph will read just under 2,000 rpm non-stop. Then, on my way home it'll read just over 2,000 rpm non-stop. Or vice-versa.

thebiglad said:

Have you downloaded a cop of RAVE yet? I think you'll find that most of your questions would be answered in that. Do a Google search for 'Green Oval' and download the TDi 300 Rave manual (03 from memory).

Click to expand...

I do have a copy of Rave... find it very lengthy and hard to find stuff in though... if I had more time (like 5 minutes or so every so often), i could have a browse through it, but I rarely have 5 minutes going spare ever!

...and i know i spend a lot of time on here, but if I spent that time in the Rave manual instead, I'd just build up a backlog of forum updates!

"is it a tough job, or similar to the engine oil and filter change?"

Nope... not tough but it's more involved according to the manual

just bought oil and filter for the auto box on my td5. never done it before, but read loads of threads and it seems quite straight forward. seems you will need another person to help though when topping up.
will let you know how it goes unless you beat me to it.

cool... thanks for that, xat44! I'm unlikely to beat you to it, to be honest, and mine's a 300Tdi rather than a Td5, but might be similar! I think i have some reading to do