School me on automatic transmissions...

[jeffbebe 1559]

So I've just stumped up some cash to service the 5hp24 transmission in my 540i. New filter, new gasket and fluid flush and replacement with fully synthetic Castrol Dexron VI transmission oil.

I took the car for a proper spin at lunchtime up and over the Harbour Bridge and a 20kms or so up the motorway and back. I've noticed a few things that have noticeable improved:

• Smoother and quicker gear changes in Drive and Sport mode
• Better fuel consumption (not an actual measure - just going by the gauge on the instrument cluster)
• Lower engine core temperature (using on board diagnostic, not the gauge) by 3-5°

I get why oil that will flow more freely when cooler (the old oil was disgusting) has improved the gear changes noticeably and I'm guessing that also impacts on the fuel consumption (i.e. the gears are changing as they should and not over-revving before changing) but I'd really like to know why/how it might affect the engine's core temp?

Is it because the fresh fluid is not as viscous at lower temps so the engine is not having to work as hard to accelerate in each gear, etc? It would make more sense to me if it was the engine oil but I'm not totally sure of the physics with the trans.

Presumably the lower core-temp of the engine is also helping improve fuel economy.

Apologies in advance if this is a dumb question but I'd really like to know.

Edited July 18, 2013 by jeffbebe

[M5V8 337]

there's alot of variables in this too Julian.

what was the ambient temp on the two days you are comparing?

what was the fuel level or car weight on the two days?

was the road the same? & the reading taken at the same spot?

man,... I watch too much myth-busters..........

Edited July 18, 2013 by M5V8

[jeffbebe 1559]

there's alot of variables in this too Julian.

what was the ambient temp on the two days you are comparing?

what was the fuel level or car weight on the two days?

was the road the same? & the reading taken at the same spot?

man,... I watch too much myth-busters..........

I've been monitoring core temp at peak operating temperature for a while because the cluster gauge regularly crept up over the median vertical position - which my 528i never did. It's core temperature would range from was usually around 108-111° when cruising but sometimes getting as high as 113-115° at idle (apparently it is normal for the M62tu to run this hot). Today it was running at 105-106° cruising and around 108° at idle and the needle was sitting slightly to the left of vertical with little or no movement.

Fair point about fuel level but previous measures were at different fuel levels. Weight will have been relatively unchanged (apart from weight of fuel).

Definitely same road.

PS I am replacing my thermostat next week but these comparisons were directly before and after, similar conditions and with no other changes besides auto trans service.

Edited July 18, 2013 by jeffbebe

[M5V8 337]

by ambient temp i was meaning the outside temp or more importantly the temperature of the air coming in the front of the car and surrounding the engine.

If it was a hot day vs. a cooler one you could have a difference right there.

I note on a hot day I can see the temp gauge rise when I pull over and idle for a bit.

This either doesn't happen or is less obvious when it's cooler.

[jeffbebe 1559]

Sorry, see what you mean. Couldn't tell you exactly but if anything today was warmer than it has been lately yet the core temp readings were definitely a few degrees cooler.

[vanossa 0]

Hi. If you don't mind me asking.....How much did that cost? I would like to do similar maintenance on my 93' 325i, E36...(My oil should be horrendous) Although it still shifts through the gears fairly well, it has been playing on my mind...?

[slybma 38]

I'd say that some of the reason behind the temp drop is due to the temp of the trans fluid being lower now that its new. If the trans fluid is colder now than before that means the temp in your trans colder so its not heating the engine radiator up as much which may give you the slight drop.

[jeffbebe 1559]

Hi. If you don't mind me asking.....How much did that cost? I would like to do similar maintenance on my 93' 325i, E36...(My oil should be horrendous) Although it still shifts through the gears fairly well, it has been playing on my mind...?

Usually around $450 might be more if you go for OE oil rather than a compatible substitute.

I'd say that some of the reason behind the temp drop is due to the temp of the trans fluid being lower now that its new. If the trans fluid is colder now than before that means the temp in your trans colder so its not heating the engine radiator up as much which may give you the slight drop.

I wondered that... As much as 5°?

[slybma 38]

Usually around $450 might be more if you go for OE oil rather than a compatible substitute.

I wondered that... As much as 5°?

[/quote

Well if your trans temp is at 70 degrees and runs through the cooler which is normally on the return side of your engine cooling system then drops by 5 degrees it will have some flow on effect. I have seen (in a Toyota) a trans temp drop of 12degrees just by servicing the trans