Regular forum readers may recall the shifting issue I’ve been fighting. I’m now working on a theory that my transmission is not cooling properly. I have not been able to locate a shop that is willing to read my tranny temp without driving 200 miles. I haven’t ruled that out, but please bear with my questions as I’m deciding how to proceed. In the meantime, I’m trying to learn how it works so I can figure out what’s happening. I’ve never received a gearbox temp warning, but maybe I shouldn’t have assumed that that meant the temp was fine.

How is the transmission fluid pumped to and from the radiator? I assume there is a pump inside the transmission as near as I can tell from diagrams of the 722.6 that I’ve found. I’m wondering if the fluid is not getting circulated correctly, and if not, why.

1. Are there check valves or actuators that allow the fluid to circulate, or does it just circulate whenever the engine is running? I do not see any kind of valve or actuator in the diagrams, but knowing the answer will help isolate the problem.

2. Is it possible that the rubber section of one of the transmission cooling lines has collapsed internally, therefore restricting the flow to the cooler?

3. Maybe there is debris in the line or cooler that is restricting flow? Not sure how this would have happened, but I did replace the radiator when I restored the car. Maybe something got in there. I doubt it, but....

Please give me your thoughts on this.

I found the following thread. Not my transmission model, but my tranny acts exactly like he describes. It made me think about the cooling process and what might be wrong. Here’s a picture of the reply since I don’t know how to link to it:




I also found this in the workshop manual for my transmission:

 

I'm fairly sure I provided this before, but in case I didn't, ..this guy does a few videos on it, to include a breakdown one... https://www.youtube.com/c/GaryFerrar...h?query=zf+5hp

 

The ATF will circulate through the cooler all the time (under pressure), driven by the same transmission pump that circulates ATF around the gearbox itself. From cold, the "cooler" actually acts as a "heater" helping bring faster the ATF temp to the optimal level. Thereafter, it acts as a cooler.

I don't think any collapse of the rubber sections is a possibility as they operate on rather high pressure, Anyhow, the only way to check is to remove them and blow through.

I cannot remember all the discussions in your threads on the issue, but one important point for proper shifting is the correct ATF level. The MB transmission is very sensitive to the fluid level and, if your level is somewhat above the maximum, it may not affect the shifting when cold/warm but, when the temperature rises a bit higher (still within the allowed range), this will cause the fluid level to go higher and maybe to the point when it starts affecting the shifting. When the ATF level exceeds the upper limit, it will start foaming due to impact by the rotating parts, i.e. it will be partially turned into "milk-shake". This will quite seriously affect the transmission operation and may even cause damage.

So, if you have not already made sure the ATF level is correct, I suggest you look into it.

 

As said above, the fluid is pumped whenever the engine is running. The base design is pretty simple and unlikely to fail. The tranny cooler is also simple - basically a circle route heat exchanger with limited areas where things can go wrong. A leak is the most likely failure mode, I think, so I doubt that is a potential problem spot.

The diagram you shared refers to the controlled slip / lock up engagement of the Torque Converter, not to shifting in the "main gears". On the ZF 5 speed box, the torque converter initiates an overdrive lockup at predefined speeds in the top 2 gears and is a known weak spot as the frictions can cause "abrupt" shift changes as the lock up clutches operate. That wouldn't explain what I understand is a 1-2 shift issue in your case.

I think a more likely source of the trouble you are seeing is with the Valve body and it being unable to command the shift when hot - it could be fluid level related or an issue with a pressure regulator, actuator , filter screen etc. Without knowing your repair history have you had it rebuilt / replaced?

If you had a ZF box, another common source of the type of delayed shift you describe is caused by an O ring on the drum involved in the 1-2 shift failing. I expect the MB box uses a similar design, but to check requires opening up the transmission, so you don't want to go there....

 

Thank you for the input. I’m not going to rehash the details of everything I’ve tried over the last couple years to fix this problem, but if you dig into my posts you can see I’ve tried everything. Everything, that is, except for whatever the problem is. The transmission has been rebuilt, we’ve visited the fluid levels and fluid types, I have done pinpoint resistance tests on the wires coming from the transmission, TCM and ECM. I have replaced the conductor plate and solenoids even though they were replaced with the transmission rebuild. I have replaced the ECM and TCM.

I’m now in the process of revisiting everything. The shop that rebuilt my tranny supposedly checked the tranny temp, since the rebuild didn’t fix it, but I did not physically see them do this, so I am going to learn how to do it, and check it myself.

The two things that are always true are these: it has to be a hot day (85* or more), and i have to drive for 1.5 to 2 hours, followed by start/stop traffic. At that point, the tranny acts up, which tells me that it has something to do with temperature - either the temp of the transmission, or the temp of SOMETHING that tells the computer not to shift the transmission.

 

The reasons why I mentioned fluid level are 1. The MB transmission is very sensitive to the fluid level (the Jag manual states that any excess must be removed) and 2. Because setting the level correctly can be very tricky - it is not easy to push the dipstick (and feel that it is) all the way in and has hit the bottom of the sump and, if it has not contacted the bottom when the level reading is taken, the transmission will be overfilled.

It is also, however tricky and time consuming, probably the easiest step to do first and eliminate (or it may solve the problem if found overfilled). Another way to experiment with the fluid level is to take out some 200 mL (no more) and see if this makes any difference.

 

Interesting. I do believe that I have checked, rechecked, filled, drained, and rechecked fluid levels more than a dozen times due to similar suggestions (probably from you) in the forum over the years. In fact, early on during this problem, I had the exact problem you mentioned with the dipstick not going all the way in, so it was overfilled slightly. All of that being said, I like your idea of draining a bit out and trying it again. I have checked the fluid level when it "acts up", and I have never felt that it was foamy, or seemed odd in any way. That said, I like the idea of lowering the level slightly and testing it again. Maybe mine just likes it a little below "full". I will go down that path again. I have received the Autel MX808 reader that I ordered, and I am planning to see if I can get a tranny temp reading.

I have gone beyond hope and optimism here, and I have settled into studying the problem until I solve it. If I hadn't done so much work/money to get the rest of the car in pristine condition, I would have sent it to the crusher by now! But now, it's a quest.

 

I got my Autel diagnostic unit and found that it will read transmission fluid temp as suggested in another thread. I test drove today and I have more information. Please weigh in with what it means.

I only needed to drive about 45-50 minutes because it was 95* outside today. The tranny temp climbed steadily the whole time. I took an exit when it reached 230* to see if it was acting up yet. It wasn’t. I got back on the interstate. When it got to 240* I exited and found that the shifting problem was active. The tranny fluid temp never really did stop climbing until I got home. The highest it got was 249*. I don’t know how high it would have climbed if I’d kept driving.

I checked the fluid level when I got home and it seemed fine, maybe if anything just a touch low, but certainly not over full. The engine temp stayed around 206* the whole time, except it did climb up between stoplights, because the transmission refused to shift until 3000+ rpm. So basically I was driving the car at 3000 rpm between stop lights. But I am sure the engine temp climbing is an effect, not a cause, of the shifting issue.

Soooooo......what do you all think? I think the transmission cooling system has a failure someplace. Right now I think that my theory of a plugged or crushed coolant line is correct....? I’ve been wrong too many times to think I have an answer here, but 240-250 is too hot for tranny fluid, so idk.

Thank you all. Your input is very much appreciated!

 

Have you had the trans radiator checked for blockage?

 

Temperatures above 220F are certainly too high. I do not have info on max. temperature specific to MB722.6 but, as a general rule, the ATF should normally be at ~175F and, under more demanding conditions, to some 200F and 220F as a max.

When the temperature goes to the higher range, the Jag TCM will switch to a different map which probably has somewhat higher shift points and I think this is what is happening in your case. 240F and more is certainly too high and you have to find out why. It may be due to a problem in the cooling circuit or it could be due to some transmission clutch slippage generating excessive heat.

 

No, but that’s what I’m going to do next. During the restoration, I installed a brand new OEM radiator, so it never crossed my mind that the tranny cooler could be blocked. I am going to inspect and likely replace the cooling lines and banjo bolts, and check the radiator for blockage.

I suppose it is possible that the first time this happened, something made its way into the banjo bolts, or the lines, or the radiator somehow and is restricting flow to the radiator. Whether blistering hot fluid can cause the shifting issue remains to be determined, but 250 degrees is not normal, so that has to be addressed regardless of anything else. Hopefully I’ll find a blockage or restriction of some kind.

 

I have thought about slippage, and will visit that question after I find out that the cooling system is working perfectly fine. Haha. (I’ve learned not to get my hopes up!). I have a ton to learn about the Autel diagnostic tool, and what some of the readings are telling me, but I THINK I saw input shaft speed, and output shaft speed as I was driving and watching the temperature with the unit on the passenger seat. I’m wondering if there is math or ratios, depending on the gear you’re in, that would indicate whether something was slipping. I have observed zero evidence of any kind of slipping, ever. Shifting has always been very nice and consistent.

But I’m getting ahead of myself. I will check for blockage first and see what happens.

Thanks again for all the suggestions.

 

Did you have this same problem prior to replacing the radiator?

 

No - I began the restoration process shortly after I got the car, and the radiator was one of the first things I did. But I drove the car for a long time (1-2 years??) after the restoration before the shifting thing happened.

You may recall that the high speed fan fuse was blown. I do not know when that happened because I didn’t know it until I came across it as part of this long diagnostic process. But I THINK it must have happened on a trip to AZ a few years ago, because that was the first time the transmission acted up. I’ve been racking my brain to figure out what connection the fuse event might have to why the transmission continues to act up, long after the fuse issue was fixed. Maybe a chunk of the cooling line hose came loose and wedged in the port on the radiator, or in one of the banjo bolts.

It’s all speculation, but that’s all I have at this point! That’s also why I really appreciate your input (and everyone’s). It gets me thinking about what else to check.

 

A couple things to think about...

I’d recommend putting your car on the lift, put the transmission in neutral, turn the engine off and make sure the emergency brake lever is all the way down (or disengaged). By hand, spin one of the rear wheels. Does it spin relatively easy? The same goes for the other rear wheel. Check the rotation of the front wheels also. How do all 4 wheels feel comparing them from side to side and front to back?

^^^ What I’m wanting you to check for is rolling resistance. It would be beneficial to do this same test on a ‘first thing in the morning’ vs ‘after a 2 hour drive’ when the transmission is acting up so you can compare cold vs hot rolling resistance with the wheels off the ground.

I've seen brakes get hot and drag on the rotor which would cause this resistance and in turn would make your torque converter work harder while your in overdrive and cause transmission fluid to get hot. It would also help having a good infrared thermometer for taking a temperature reading at each rotor after your transmission acts up.

If the above test passes cold vs hot, then the brakes and wheel bearings aren’t an issue.

The only other test I would do if I were you is bypass the OEM transmission oil cooler and add the largest B&M (or other “high efficiency”) externally mounted trans oil cooler you can fit in front of the other coolers. I want to emphasize that I’m “not“ trying to recommend a bandaid. Even though you paid a lot of money for an OEM radiator and most likely the internally mounted TOC is not the issue, whose to say that the OEM TOC isn’t defective even after driving it for 1-2 years after you replaced it? After all, you can’t see inside the radiator to look at the TOC. One thing that stuck out is that the fluid temp of the trans is significantly higher than the engine coolant. I would think that your radiator should do a better job of cooling the trans oil temp than that.

 

I’ll have the car on the hoist off and on as I work on the cooling idea, and I’ll check the resistance. I rotate the tires every 5k miles, sometimes after a drive and sometimes first thing in the morning, and I always check the brakes and roll the wheels. All new bearings, rotors, pads, and fluid flush during the restoration, so I’m pretty sure it’s ok. But since everything is on the table, I will check again.

I’m wondering how to check for blockage in the cooling system. I plan to take the lines off at the transmission and blow thru one at a time with my mouth to see how easy it is to blow. But I won’t know if what I’m sensing is good or bad, unless I can’t blow thru at all. That’s what I HOPE happens because I’ll know where the problem is. Then ill take the lines completely off and blow again. But I also want to somehow check the cooler by itself after the lines are off.

Just thinking.

 

Here's a link for the 722.6 manual if you don't already have it.... https://www.manualslib.com/manual/13...tsg-722-6.html

This is a basic overview from Merc-land... https://www.benzworld.org/threads/me...n-faq.1463460/
...the reason I posted this is to make sure you verify which fluid you used and it does show a temperature graph.

...and an overview of a standard check procedure, this is a bear of a test for the 722.6 simply from the refill point...

There are two major causes of an automatic transmission failure.

1. Lack of fluid flow (usually caused by a restricted cooler, which is normally a part of the radiator)
2. Heat caused by (excessive load or cooling system failure)

Checklist

• Cooler Flow testing must be performed at the transmission return line AFTER the cooler/radiator
• Test MUST be performed at the transmission end (to ensure that cooler lines are also clear)
• Remove one end of cooler hose & place in a measuring container
• Start engine & idle for approx 15 seconds
• Switch off engine & record the amount of fluid removed
• The recorded fluid quantity becomes the flow test
• Refit all hoses
• Start engine & top up fluid to the correct level and check for leaks

Note: If you are unsure which is the cooler return line, remove one line & have an assistant briefly start engine to check for fluid flow. Fluid MUST come from the line returning to the transmission AFTER the radiator/cooler.

 

something that really doesn't add up for me is that a transmission cooler isn't "essential" to keep a tranny at operating temp. It will help keep it in range, but not avoid continuing rises, regardless of outside temp.

However, a quick test to see if it is blocked or not would be to disconnect the banjos and see if it pumps fluid with the engine at idle. As long as you maintain fluid levels it will run OK. On my Saab. that is the procedure for doing refills and is pretty simple.

If you are seeing temps rise, and find fluid is pumping, then the next most likely is that a clutch isn't releasing properly. A quick look for common 722.6 issues lists 2 candidates that match your symptoms (which I know you have already found). One is Torque Converter clutches not releasing and the other is seal failures. ie
"Without detailing every potential fault this transmission has been known to get, there are three particularly common faults that occur. The first, and least severe, is again to do with the torque converter. Wear and tear in the 722.6’s valve body can cause the converter to come in and out of lockup when it shouldn’t, resulting in a judder or rumbling sensation. There are repairs that can be carried out on the valve body to fix this issue, but if it is left to progress too far a replacement torque converter will be needed."

Is there a way you can lock out the overdrive function on that transmission, to see if that changes symptoms?

 

One misconception mentioned in this thread is that the dipstick has to bottom out in the trans pan. It does not. There is a small shoulder on the dipstick, on the plastic piece with the measurement graduations, that stops against the conductor plate hole. You can see it in the attached diagram from the 722.6 manual. Also, the diagram shows the measurements for the dipstick graduations, so you can compare those to your dipstick. I did all the measurements last fall when I rebuilt my valve body and put in a new conductor plate, but I don't remember the exact calculations that I came up with. I think that the measurement from the conductor plate stop to the top of the dipstick tube was 27.5 inches, and from the plastic end of the dipstick to the pan was something like a quarter-inch.

 

Damn Mark, its been awhile, ..good to see you again.