hhyndoy

Hi, guys.

Wanted to make a thread about my joyfull experience of troubleshooting an overheating transmission on my pre face c63s.

Background
So, Im located in Dubai and have been daily driving my car, but recently with the outside temps picking up amd me commuiting the car I ha e noticed it gaining some abnormal temps.

The temp of the engine is fine and working as expected, while the transmission is increasing a bit slower, but very steady until it hits an ambient that the highway airflow eventually will be able to keep it at.
for reference 95-98C is what I have been seeing before, but the transmission is now going to 110-120c eaisily after heavy deiving over a short period or about after 1 hr of highway driving. Outside temp 35-45C

Anyways, from reading this forum I have seen several threads where the water pumps and auxilary radiators have been pointed out as a common source for overheatings trans.

Theory
I did some research on the engine and have learned the following.
The transmission and intercoolers are cooled by the same coolant circuit with its main reservoir being the one in the (middte between the air intakes). This leads to a lower reservoir, that is not visable from the top.
thereafter we have some key elements

• Intercooler x2
• Aux waterpumps x2
• Main middle aux radiator
• Secondary driver side aux radiator.
• And last the transmission oil to coolant heat exchange. That sits on the back side of the radiator pack

now the problem with this circuit is that 1. the transmission cooler and the intercoolers are sharing the same circuit, and 2. that the transmission is getting the sloppy seconds after the coolant has been used in the intercoolers.

This system is poor by design and the mutual disadvantage of hot intake air and overheating transmission is closely related.

Troubleshooting

1. Inspecting system and looking for leaks.
After inspecting the system I could not find any leaks. Dpecifically inspected the radiators closely as it is a common problem woth punctures due to mercedes not i stalling protective meshs. Though no signs of damage besides a bit of bent fins.

2. OBD reading while driving - showed that the intake air temperatures were healthy on both sides - indicating in my head that the coolant temps were good. Coolant temp also showed a nice temp from the readings, but not sure whether it was reading engine circuit or transmission/intercooler circuit temp.

3. Healthy temp would indicate poor circulation, thus I made my decission to check the water pumps, they seemed to be working. (Felt for vibration when plugged/unplughed), but I know that these pumps could be weak even though they are working, so I replaced the one closest to the transmission cooler. On the deiver side and expected at least a little bit cooler temps, but no luck.

4. Next I went back to the coolant temp being to high theory. So I ordered and replaced the secondary aux radiator - which also did no make any difference

At this stage I had more luck cooling the transmission due to colder outside weather than my troubleshooting. Which of course is very frustrating.

Right now I have a Modal Split Cooling kit on order $$, but honestly dont have any idea what the problem is. Though I have a few bucks on the theromostat that sits on the oil line entering the transmission heat exchanger.

I gues this will be confirmed if the modal splitter dosent help, as that would completely disregard the coolant cooling circuit.

to be continued....

alan nguyen

thank you for listing the parts of the aux cooling system. Just to clarify, so the c63s has 3 total aux rads, two under the head lights and a center one( plus the main engine radiator and the horizontal oil cooler, so 5 radiators not included the ac stuff). If the middle and the DS radiators are for the Intercooler/trans cooler loop, what is the purpose of the smaller aux cooler on the passenger side? I’ve been looking on a lot of different forms and everyone has said that the two small ones are for the intercooler/trans loop. Thank you and I hope you get your problem solved because I have the same issue as well.

hhyndoy

Hey, Alan!

To my understanding the PS aux radiator is for additional cooling to the engine. This is the flow chart of the OEM intercooler/tranny circuit




UPDATE: So, after 3 long evenings of squeezing my arms between the radiator pack and the engine I was able to replace the OEM oil cooler with the modal split cooling kit. However, it did not seem to resolve my problem...
At this point I can at least say that it is not a matter of coolant circulation or temperature and must be a matter of oil circulation. My theory is that one of the following is the matter.
1. The transmission fluid pump is broken (god forbid), but I doubt it as none of the bad tranny pump symptoms have occurred.
2. The tranny fluid thermostat is broken/shut closed.

I am leaning towards 2 as the gear shifts are still smooth after putting the split cooling kit. The instructions said to fill approx. 1 quart(almost 1L) of tranny fluid, but I was only able to fill about 0.7L. Also we must assume that 1-2dl went to spill when splitting the original oil cooler.
So in short I believe the tranny has a shortage of about 5dl. I don't know if this should be noticeable, but I reckon it should. With this in mind I'm lead to believe that the thermostat is blocking the new oil cooler and the rest of the circuit to mix.

Modal Works is offering a modification to the OEM tranny thermostat which keeps it open all the time, it will lead to longer heat up times, but will also be the cheapest option to investigate next.

In retrospect I am evaluating whether modal works was the better option for me, ZAC also has a split cooling system where they completely eliminate the heat exchanger and thermostat by directly connecting to the OEM lines, however, their oil cooler/radiator is not as big as the one form modal works, so Im lead to believe that Modal Works is the more efficient option when everything is sorted out.

For reference. This is the OEM heat exchanger and the outlined red part is the thermostat.

alan nguyen

Thank you for the information! I would also assume that your thermostat might be the main cause if you aren’t getting any of the broken oil pump symptoms and getting the open thermostat seems like a good idea if that’s the most cost effective option because of where you live and I would assume that Dubai doesn’t have consistently low temperatures so taking longer to reach operating temperature doesn’t seem like a huge problem. I hope it’s just the thermostat. Good luck !

hhyndoy

Indeed, at times the transmission temp is within the whites straight from the parking - we have some wild temps here.
Its been quite an expense trip if the problem ends up being the thermostat which probably would have solved the problem from day 1. However, I have learned a fair bit thought the process regardless, so not a complete waste.

From the picture below you can see the oil thermostat inside the housing in its closed position.
The buffer spring sits in the left hole and the closing piston in the right. When it reaches the threshold temp the piston will/should pop to the left to allow ATF to pass.
The thermostat housing is fairly easy to remove from underneath the car and the thermostat can be removed though a opening on the right side.
It is only held in place by a expanding ring and should be easy to extract.

hhyndoy

So, it was in fact the oil thermostat on the transmission cooler that was broken. I must also mention that the cooler and thermostat was replaced 1 year ago, but with aftermarket replica parts. I gues it is a prime example of where OEM beats replica and how some little pieces can make all the difference.

Anyways, heres how it was foxed.
1. Unscrew oil lines from thermostat housing, no need to remove bushing or clip, just undo with a wrench.
2. Undo the three lille nuts that fix the thermostat hose to the transmission cooler.
4. open the expanding ring clip at the end of the thermostat housing and pull the parts out. (Forst picture shows the components)
5. at this stage a OEM setup would only need to change the golden piece (oil thermostat)

6. Since I have installed a spøit kit I went for full opening all the time. This is not adviced in cold countries or together with OEM cooler as it will "steal" cooling avility from the intercoolers.

7. THE THERMOSTAT DELETE STEP - cut the spring a half to a full turn more than the head of the thermostat. Keep in mind the lid will also mush about 3-4mm additional inward, so dont.
8. Now you should have a setup similar to two.
9. Clean the pieces thorughly and reassemble.
10. Final product will look like picture three where both holes will show spring and the thermostat head is blocking the path between them. Now the cooling eill be constant regardless of temp.

with this setup i got tranny temps at 72-75C while cruising om the highway.

hhyndoy

☺️

Yassbr1

Hey, I just read your thread and I am facing somewhat similar issues on my 2017 C63S and wanted to see if you had similar symptoms to try to isolate the problem. I don't have the tools or garage space to work on this myself but I have a trust worthy Mercedes shop that I can relay info to. Anyways:

when driving normal I get
engine oil temps of around 201-212F
Trans temp 176-181F

when driving hard in sport + (just after two or three 3-gear pulls)
engine oil temps 215-225F
Trans Temp 195-200F

If I push it two or three more accelerations, I can get up to 225F which is around 107 celcius and I know that is a little hot for the transmission. when driving back normal the trans temps go down fairly quick but I know I can't do one hot lap on track in this situation.
Keep in mind I live in California desert so temperatures outside of 75f to 90f so far.

alan nguyen

from what it seems like from his post, you have similar issues but I don’t think it is the thermostat just because what you are describing is the same exact problem I have with both oil and trans temps. From what I’ve read, the oil temp being around that temp is fine but the trans is def running hot. The issue for me is that all three of the front mounted aux radiators are damaged from rocks and all the fins are bent so it’s effectively not cooling the intercool/transmission loop. I would check the front of your car and see if the two side mounted rads fins are all bent up, same thing with the bottom half of the center radiator.

Yassbr1

Thanks for reply. I have checked my side mounted coolers and main one and no sign of puncture. I can see minimal sign of fin bends but I can see what you mean because the car struggles to cool only during WOTs and hard driving. As soon as I lay off it the trans temps drop in a healthy rate. Im sure I can even get to 240+ easily but do not want to push more after 210-215F trans temp. Service B including trans oil change and major fluid flushes done 3k miles miles ago. I'm leaning this is possibly one of two cooling pump not functioning properly. is there a way to rule out one another, or they run together? Why did MBenz put the engine and trans cooling under same circuit too?

hhyndoy

Hey, Yass!

I agree with Alan.
the fact that your system is cooling down by itself indicates that you have circulation on both sides(trans oil side and coolant side). So thermostat is probably working.

How is the coolant level looking in the middle reservoir? Bent fins are a possible explenation. If this is the case it could be verified by connecting a obd while driving and checking your intake air temperatures. If these are high its a good indicator that your radiators are not doing their job. To do this obd scan I bought a cheap ELM wifi obd2 from amazone - cost like 5 $ and you can download a scanner for free from app store.

hhyndoy

Right, it might be a matter of poor circulation also...

You can feel the pumps while the car is running, should be some vibration from it. You can verify by unplughing/plugging to be sure.

if you remove the front undertray you will see them right in front of the front wheels. One on either side

However, a working pump might also not be performing well even though it is running. But best case you find one of them to not be working 👍

46m3csl

You ever find out whats causing , did you ever end up swapping your auxiliary pumps out yet ? If the trans heats up on these cars, the intercooler are not working efficiently either since its one loop for the trans and intercoolers . The also gets heated soaked after a few runs easily

hhyndoy

Are you asking me or Yassbr1?

If me, then yes, and described in detail above. It was the transmission fluid thermostat(located on top of the trans cooler) that was not functioning properly.
I did also change one aux pump and one aux radiator, as well as adding a split cooling kit in the process.