M177 W205 Upgraded Radiators, Now Transmission Overheats
Thread Starter
Junior Member
Joined: Jan 2024
Posts: 34
Likes: 3
GLA45
The M177 and 9g-tronic cooling system on the C63 is very similar to the cooling system on the C43 (and C450/C400) it just has a couple of extra rads and pumps.
There's a "High temp cooling circuit" this is for the engine and the engine and engine oil only. It flows through the main radiator, the engine's coolant passages, and through the engine oil cooler/filter assembly.
The "low temp cooling circuit" cools the air to water intercoolers (turbo outlet charge coolers), and the transmission.
The transmission fluid is pumped up to an fluid interchange cooler where hot trans fluid and the "low temp circuit" coolant pass though a series of sandwiched plates and the heat from the warmer fluid is absorbed by the cooler fluid. Typically the trans is warmer (after the thermostat in the interchange cooler allows full flow).
On the M276.8 engine these two circuits are completely separate. I believe it is the same on the W205 M177.
Things to check on your low temp circuit:
1) electric pumps this is the number one failure mode.
2) leaks at the trans cooler heat exchanger
3) correct operation of trans cooler heat exchanger thermostat
4) proper trans fluid level
5) unobstructed trans cooler lines
There's a "High temp cooling circuit" this is for the engine and the engine and engine oil only. It flows through the main radiator, the engine's coolant passages, and through the engine oil cooler/filter assembly.
The "low temp cooling circuit" cools the air to water intercoolers (turbo outlet charge coolers), and the transmission.
The transmission fluid is pumped up to an fluid interchange cooler where hot trans fluid and the "low temp circuit" coolant pass though a series of sandwiched plates and the heat from the warmer fluid is absorbed by the cooler fluid. Typically the trans is warmer (after the thermostat in the interchange cooler allows full flow).
On the M276.8 engine these two circuits are completely separate. I believe it is the same on the W205 M177.
Things to check on your low temp circuit:
1) electric pumps this is the number one failure mode.
2) leaks at the trans cooler heat exchanger
3) correct operation of trans cooler heat exchanger thermostat
4) proper trans fluid level
5) unobstructed trans cooler lines
Now I need to find out how to check the electric pumps and thermostat function.
Thread Starter
Junior Member
Joined: Jan 2024
Posts: 34
Likes: 3
GLA45
UPDATE: I had the 50K transmission service performed (odometer is at 30K), along with a rear differential service, engine oil change and few other maintenance items. I did one evening of drifting through a full set of tires and the temps all stayed really controlled this time. The oil temp would rise slightly through a run, but then immediately cool back down to nominal again. The transmission temp never really moved from the 180's no matter what I was doing. Coolant gauge also stayed right in the middle of the marks (no numbers).
So it appears that the services performed made a difference, bringing everything back to the way it was before. Thanks everyone for the ideas and help along the way!
So it appears that the services performed made a difference, bringing everything back to the way it was before. Thanks everyone for the ideas and help along the way!
