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| D2 - Transmission Gearboxes, diffs, drive shafts. |
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#1
7th September 2019, 02:06 PM
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I reckon MJ might know the answer to this one, but does the transmission have a thermostat controlling flow to and/or from the cooler?
If so, this may answer two questions, the first being will an additional cooler cool the transmission too much (the answer being no, if there's a thermostat). The other question is why does the transmission need to be at a specific temperature when filling - I don't buy the oil expansion theory, but making sure the thermostat is open to fill the cooler makes sense.
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Cheers Marty  ____________________ Current: 2001 Audi S8 - Brilliant Black with Black interior, C5 RS6 rims (whenever I actually put them on...), Solar Sunroof, Tinted side and rear glass, RNS-D, Grom, Bose, clunky old phone in arm rest! 2002 Audi S8 - Project Replacement head coming arrived  thanks to MJSilver with Black interior. All features as the '01, with the 'S' mode auto shifter. Dodgey rear tint (need to find a way to get rid of that). Family: 2009 Volvo XC90 V8 R Design - has a louder more obnoxious exhaust than the S8, sounds great! Love this thing - Q7 was double the price, and certainly not double the car! Sold: 1997 Audi A4 - Hamilton's Club Sport, Achat Grey (will miss the old girl) 
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#2
7th September 2019, 02:41 PM
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I used to think that there is thermostat in the box but was not sure and looking at previous posts it seems like there is no thermostat in the box.
In regards to thermal expansion different fluids will behave differently in different temperature ranges. I vaguely remember reading it somewhere that for the 5HP24 box with about 12l of ATF the volume from cold 20C to working temp of 100-130C will change by 0.7-1l. I also know that different engine oils expand differently, some oils hardly expand at all and others quite a lot. So with one oil I could check level after car has been standing overnight and compare it to running up to temp, then waiting 10 minutes for oil to drain and it would be pretty much the same level. But with different oil it could be barely minimum level cold and over max level when hot.
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Currently 8less 2011 Q7 S Line 3.0TDI, 2016 Tesla Model S 90D 8 history: 2006 A8 Sport 4.2TDI quattro SOLD, 1997 S8, reached end of life with gearbox failure Last edited by ainarssems; 7th September 2019 at 02:50 PM.
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#3
7th September 2019, 04:24 PM
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There isn't a thermostat in the 5HP - it relies on the engine temperature. The 'oil cooler' in the radiator is really properly called a heat exchanger. It uses engine heat to warm up the transmission to 90*, and then dissipates heat generated by the gearbox in to the coolant once it's up to temperature.
The oil definitely does expand with temperature. The way I fill them is to top up to level when the gearbox is cold and then run it until 40*. It will drip oil while it is warming up as the oil expands, and then as soon as it hits 40 I put the plug in. Much easier to do it that way rather than wait until its at 40 then put fluid in, since its really easy to over-shoot the temperature like that. As a side note, the 3.7 with the 5HP19 has an external thermostatic oil cooler in addition to the heat exchanger in the radiator, because the 5HP19 with a V8 on it is right at its operational limit so needs the extra cooling capacity. The 2.7TT and V8 Allroads have the same arrangement.
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