Auto gearbox slips at 40mpp

Slips, never below 40mph, never abouve 40pmh, just when going through 40mph. Always the same speed 40mph? What is going on?

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On 04/02/2018 12:10, johannes wrote:

first and cheapest is to change the fluid.
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On 04/02/2018 12:36, MrCheerful wrote:

Already new fluid. Only happens when going through 40 mph, that's what strange. The box must have something which is triggered at 40
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johannes submitted this idea :

Double check the correct fluid was used and the level was set up by the book. Some need the level set at certain oil temperatures.
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Low fluid level usually shows up as the vehicle dropping out of gear on a hill or corner, as the level drops below the pump input.
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On 04/02/2018 12:36, MrCheerful wrote:

Auto-transmission servicing/problems are new to me.
How often do you normally need to change the fluid- I appreciate it can vary from make to make, use, etc, but a 'ball park'.
For example, changing the oil in manual gearboxes - especially Fords- is unknown (at least the ones I had). In my CRV, I think the final drive fluid needed changing at 40k (don't hold me to that).
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On 04/02/2018 18:06, Brian Reay wrote:

Generally around 60k, some say lifetime auto fluid, but that is still worthwhile changing, along with the internal filter.
I have just this week had a Merc box apart, they are supposed to be lifetime fill, but the fluid that came out looked very murky compared to the new. A new conductor plate, 13pin connector, fluid, gaskets and filter, plus reset the transmission computer and it is back to normal, the car has only 75k on the clock.
Standard manual boxes have very little oil problems since they have a very different set up internally and run very cool compared to an auto.
The CRV has a limited slip diff, and they make all sorts of noises if the fluid is the wrong type or not changed often enough.
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On 04/02/2018 18:33, MrCheerful wrote:

Thank you.

I am aware it does 'creak' when the fluid needs changing, although I changed before that stage. The fluid isn't that expensive and it is a quick job.
Having said that, I do wonder at Honda's 'special' fluids. Not that you need it much/often but they specify their 'special' power steering fluid, the DPF 2 fluid, even their 'special' brake fluid.
The main transmission was manual on the CRV and I don't recall even needing to check the oil level.
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On 04/02/2018 18:06, Brian Reay wrote:

Auto box fluid recomended every two year. But difficult operation since there is often no drain plug.
I believe that the problem in my auto is not to do with fluid, as it was changed recently. Rather to do with the control system in the box.

Yeah, manual boxes (normally) don't need oil change
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On 04/02/2018 12:10, johannes wrote:

What car is it, and what type of auto?
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Can you describe the symptoms more fully? And in which gear? And what make/type of box?
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On 04/02/2018 12:10, johannes wrote:

The auto box on my CF Diesel did that, but it was actually just locking the torque converter up, that happened at about 40 too.
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Yes. But more like an extra gear change. And of course with the TC locked out, the road speed follows the engine directly.
My car locks the TC in second gear on a gentle start from rest - and it remains locked on the higher gears too. Gives an odd gearchange (engine revs) sensation - oddly spaced ratios.
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