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Old 06-20-2019, 11:24 AM Thread Starter
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Question 1998 ML320 transmission questions

I'm new here, and new to working on Mercedes-Benz vehicles, but I've done quite a bit of car repair on everything from a 1930 Ford AA truck to a 1992 BMW 318iC. I recently picked up a 1998 ML320 in pretty good shape (and with a great running engine) for very little money, with the caveat that it has a transmission problem. I did my research before buying the car, and I've read up on the electrical gremlins that are known to plague the M-class 722.6 transmissions. However, I have yet to find a thread that gives me a starting point on my particular set of symptoms. My local junkyards have a number of W163 M-class donors, only one of which was tagged as having transmission troubles. They also have plenty of other Mercedes of similar era, along with non-Mercedes vehicles that used the 722.6.

Symptoms and relevant information:
*It won't move much. Specifically, in any shifter position other than park (including neutral and reverse), it tries to move forward. Very, very slowly, and with basically no torque, no matter the engine RPM. It behaves the same in any shifter position that is not park. With the engine not running, it will shift into neutral with no problem. In park, it stays put.
*The shifter moves and illuminates the selected gear normally. As noted above, it will go into neutral with no issues as long as the engine isn't running.
*There are no weird noises generated when the engine is running, no matter the RPM or shifter position.
*There are no error codes coming up on my code scanner.
*The battery is virtually new and my multimeter says that it's putting out around 14 volts.
*The previous owner said that it was running fine until it fell out of gear on a hill. It was his daughter's car, and she wasn't particularly fond of it, so he decided to let it go without further attempts at diagnosis. Her brother is a mechanic at the local BMW dealership, and gave it a going-over when they bought it.

I have the special dipstick but have not yet checked the transmission fluid level, nor have I dropped the pan or filter to take a look at quality of the transmission fluid.

As previously noted, I plugged in my OBDII scanner, and it recognizes the vehicle properly, but it appears that no error codes are being generated or are saved in the system.

If anyone has any possible diagnosis or troubleshooting suggestions, especially given the lack of error codes, it would be greatly appreciated.

I know the 722.6 was used in a large number of M-B and other vehicles for quite a few years, but I don't know anything about variations in internals, control systems, or gear ratios. Are there any particular years or models that would be a better source of parts?
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Old 06-21-2019, 05:37 AM
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It sounds to me like a clutch is stuck. There are a few things that can cause that. I'd start with the "low hanging fruit" which is the TCU and its wiring. Check the connector for oil leaking and the wiring for oil. Check the TCU for oil intrusion. After that, the possibilities are that a valve or its return spring is broken, a solenoid is bad, or clutch is fused (that would happen due to low fluid and heat buildup). For the valve and solenoid, you can drop the valve body and clean and rebuild it fairly easily. If it's a clutch, that's a transmission-out rebuild job (or a replacement). The ML320 uses a version of the 722.6 that is specific to it, so you'd need to get on from another ML320.

In case you want to dig into it yourself:

http://benzbits.com/722_6/722_6Service.pdf

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Old 06-21-2019, 10:17 AM Thread Starter
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Quote:
Originally Posted by rudeney View Post
It sounds to me like a clutch is stuck. There are a few things that can cause that. I'd start with the "low hanging fruit" which is the TCU and its wiring. Check the connector for oil leaking and the wiring for oil. Check the TCU for oil intrusion. After that, the possibilities are that a valve or its return spring is broken, a solenoid is bad, or clutch is fused (that would happen due to low fluid and heat buildup). For the valve and solenoid, you can drop the valve body and clean and rebuild it fairly easily. If it's a clutch, that's a transmission-out rebuild job (or a replacement). The ML320 uses a version of the 722.6 that is specific to it, so you'd need to get on from another ML320.

In case you want to dig into it yourself:

http://benzbits.com/722_6/722_6Service.pdf

Thanks for the suggestions! I was planning to start with that connector that seems to be the source of so many transmission issues with these trucks, and the associated issues with oil intrusion into the harness and TCU. The lack of error codes made me suspicious that something else was going on.

Did all of the W163 ML320s use the same 722.6, or are there certain years that are better or worse for transmission internals? There are 10 W163s at one of my local salvage yards: one 1998 (ML320), four 1999 (three ML320, one ML430), one 2000 (ML320), one 2001 (ML320), two 2003 (one ML320, one ML500), and one 2004 (ML350). One of them (I don't remember which) had an auto auction note written in grease paint on the windshield that it had a bad transmission. Most of them still have intact drivetrains with few to no parts removed.
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Old 06-22-2019, 12:50 PM
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The transmissions have different internals based on year and engine output.
Also ZF and MB made several updates as they discovered the weak points and many internal parts were updated over the first few years.
The 722.6xx trans was used by Porsche, Jag and Chrysler for a long run. Overall it turned out to be one of the more durable and reliable
transmissions for all the car makers that used it.
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Old 06-23-2019, 02:15 PM Thread Starter
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I was back at the junkyard yesterday scavenging parts to improve the ML320. I was able to verify that the truck with the "bad transmission" note on the windshield was the 1999 ML450.

Did the V6 and V8 transmissions use the same gear ratios? Did the V8s have stronger internals? The 2003 ML500 seems to be in good shape and mostly intact. I remember reading a thread about installing a later W163 transmission in an earlier truck, and that there were some differences in the electronic controls. I'll have to look that up again.
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Old 06-23-2019, 02:25 PM
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Quote:
Originally Posted by Sam Cogley View Post
I was back at the junkyard yesterday scavenging parts to improve the ML320. I was able to verify that the truck with the "bad transmission" note on the windshield was the 1999 ML450.

Did the V6 and V8 transmissions use the same gear ratios? Did the V8s have stronger internals? The 2003 ML500 seems to be in good shape and mostly intact. I remember reading a thread about installing a later W163 transmission in an earlier truck, and that there were some differences in the electronic controls. I'll have to look that up again.
ML450? typo error?
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Old 06-23-2019, 02:37 PM Thread Starter
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Originally Posted by khomer2 View Post
ML450? typo error?
Typo. Should have been ML430, obviously. I'll fix it.
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Old 06-23-2019, 04:11 PM Thread Starter
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Upon some more research, I see that the V8 models used different transmission gear ratios that will not play nice with my ECU. I have yet to see anything comparing the ML320 and ML350 gear ratios, but I have found at least one thread where a 722.666 transmission was successfully transplanted in place of a 722.6. Unfortunately, it's rather light on details, but it would appear that if I were to go that route, I would need the shifter, TCM, and the transmission from the newer vehicle.

https://www.benzworld.org/forums/w16...ru-needed.html
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Old 06-23-2019, 04:35 PM
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Quote:
Originally Posted by Sam Cogley View Post
Upon some more research, I see that the V8 models used different transmission gear ratios that will not play nice with my ECU. I have yet to see anything comparing the ML320 and ML350 gear ratios, but I have found at least one thread where a 722.666 transmission was successfully transplanted in place of a 722.6. Unfortunately, it's rather light on details, but it would appear that if I were to go that route, I would need the shifter, TCM, and the transmission from the newer vehicle.

https://www.benzworld.org/forums/w16...ru-needed.html
There is significant difference in shifter interlock between early and late models. Not worth the trouble. Stick with what fits your vehicle.

Until you get scanner and read out ETC codes and access live data you are mocking in the dark.

1972 280SEL 4.5
1999 ML320
2000 E320
2003 ML350
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Old 06-23-2019, 04:38 PM
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Sam,

here is a tad more info - https://www.benzworld.org/forums/w16...ear-ratio.html
perhaps you read thru this in your previous searches.
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