DCT Fluid Change - My observations

[bigjae1976]

So I've had the car since new. I did my first DCT service at 50k miles and it came out black and quite nasty. I've since done two services (80k and 105k) and both times I noted that the fluid comes out and it still looks golden brown like cooking oil.

I did the services as routine maintenance, never had an issue other than not properly filling the DCT on the 50k service and having to do it over again.

What has been everyone else's experience who has done multiple DCT services?

[M3SQRD]

I’ve done one DCT fluid change on each of my e92 M3s (‘09 and ‘12) when I installed Slon DCT pans and do88 DCT coolers. It was at relatively low mileage (~30k and ~20k, respectively) but the used DCT fluid looked brand new in both cases. Definitely had a lot of life left in the fluid. However, I’m happy with the improved performance and lower temperatures in the DCTs with the deeper Slon pan and do88 cooler.

Edit: I forgot the Slon DCT pan was installed before the do88 cooler on the ‘09. Slon DCT pan was installed at ~20k miles so the ‘09 had two DCT fluid changes - one with 20k miles + one with 10k miles. Fluid was obviously in great condition after only 10k miles.

[Helmsman]

Originally Posted by M3SQRD

I’ve done one DCT fluid change on each of my e92 M3s (‘09 and ‘12) when I installed Slon DCT pans and do88 DCT coolers. It was at relatively low mileage (~30k and ~20k, respectively) but the used DCT fluid looked brand new in both cases. Definitely had a lot of life left in the fluid. However, I’m happy with the improved performance and lower temperatures in the DCTs with the deeper Slon pan and do88 cooler.

Curious, how do you fill the new heat exchanger with fluid? As the Tstat only open around 90C and one is suppose to hold 30-40C when filling the box. You prefill before installation?

[M3SQRD]

Originally Posted by Helmsman

Curious, how do you fill the new heat exchanger with fluid? As the Tstat only open around 90C and one is suppose to hold 30-40C when filling the box. You prefill before installation?

Correct - I prefilled the new cooler with fluid.

[CamasM3e93]

Originally Posted by Helmsman

Curious, how do you fill the new heat exchanger with fluid? As the Tstat only open around 90C and one is suppose to hold 30-40C when filling the box. You prefill before installation?

I think the thermostat is starting to crack open around 40C and full open at 90C. I say this because the fillup procedure calls for filling to the fill plug level, close, idle the car, at 35-40C open the fill hole back up (still idling), and run another 1-1.5 L into the transmission until it overflows again. I didn’t use any electronics to get to temp, I just let it idle for 2-3 minutes and it was warmed up adequately by then to get some fluid into the exchanger.

But as the poster above points out, you can prefill at the exchanger too.

[Helmsman]

Originally Posted by M3SQRD

Correct - I prefilled the new cooler with fluid.

Thanks mate, asking because trying to learn more about the box. Prefill makes sense then.

Originally Posted by CamasM3e93

I think the thermostat is starting to crack open around 40C and full open at 90C. I say this because the fillup procedure calls for filling to the fill plug level, close, idle the car, at 35-40C open the fill hole back up (still idling), and run another 1-1.5 L into the transmission until it overflows again. I didn’t use any electronics to get to temp, I just let it idle for 2-3 minutes and it was warmed up adequately by then to get some fluid into the exchanger.

But as the poster above points out, you can prefill at the exchanger too.

I don't think it does mate. The Tstat is obvioulsy mechanical and I guess accordingly "slow" but spec even state 95C prior opening (or closing really, forcing the fluide via the front cooler). During my on goint little investigation I'd say that seems true, touching the cooler indicates it only start to warm up getting up at least 70-80C in temp. Again, fluid has free way to the cooler all time - not sure how that works - so guess some slippes in that way also before the Tstat close the return way into the gear box.

[bigjae1976]

Originally Posted by Helmsman

Curious, how do you fill the new heat exchanger with fluid? As the Tstat only open around 90C and one is suppose to hold 30-40C when filling the box. You prefill before installation?

A thermostat normally doesn't block all flow, just most of the flow. So when you start up the car cold there is fluid going past the thermostat into the cooler loop.

That's also why you do a cold fill and then a warm fill since the cooler lines completely fill up and the level in sump drops.