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I recently purchased a 1900 300SL R129 with 60K actual miles. The car looks like it just came off the showroom floor. When I was test driving it everything worked fine until I got it home and after a couple of days I noticed that it was shifting into overdrive. It will when I first drive the car but won't once it warms up.

I took it to the local MB dealer (Fields) and they replaced the solenoid but that didn't do the trick. I complained to the service manager that I was out $560 and the tech explained to me he knew nothing about this vintage.

Two weeks later it is still at the dealership and they still have no clue. The cruise is working. Shifting from 4th to 5th doesn't work. They reviewed all the suspected problems and I was wondering if anyone else had this problem. BTW, they aren't charging me anymore money!

pristine300sl is online now Report Post
 

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Welcome to the forum, please could you fill in your details as to your location / country etc.
I presume that you mean 1990 for year not 1900 as this would make it somewhat rare and unusual :D
I presume it is automatic. Is it a 5 speed or 4 ?
 

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There is a 3 prong round connector on top of the intake manifold right alongside the valve cover. Check it for looseness. They tend to become quite brittle over time.
 

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Have them check the following:

1- the shifter mechanism, the micro switch in it, that gets turned on and off when you shift etween 4-5.

to troubleshoot, unplug the harness end from the floor shifter, and short it with a peice of wire. now drive the merc at about 40-50mph, undo that shorting jumper, and see if the engine speed changes (the tranny shifts). if it does replace the microswitch or the floor shifter box.

once the problem is confirmed to be in the shifter box, take it out of the car to troubleshoot it, take it apart, check the microswich to make sure its turning on and off when shifting 5-4. then check and make sure the cast aluminum shift gate that actuates the mictoswitch is not broken.

I propose starting at the floor shifter, because IT DOES NOT throw a code when it's the cause.

2- Now, if the shifter checks, the problem is in one of 3 other components:
-Throttle actuator (replace/rebuild it)
-Tranny itself in the valve body (electronic shifting valve - replace it)
- HGC control unit (check - replace)

the good news is any of these 3 components WILL throw a code when its the culprit, and there is no need to replace parts blindly with them.
 

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Have them check the following:

1- the shifter mechanism, the micro switch in it, that gets turned on and off when you shift etween 4-5.

to troubleshoot, unplug the harness end from the floor shifter, and short it with a peice of wire. now drive the merc at about 40-50mph, undo that shorting jumper, and see if the engine speed changes (the tranny shifts). if it does replace the microswitch or the floor shifter box.

once the problem is confirmed to be in the shifter box, take it out of the car to troubleshoot it, take it apart, check the microswich to make sure its turning on and off when shifting 5-4. then check and make sure the cast aluminum shift gate that actuates the mictoswitch is not broken.

I propose starting at the floor shifter, because IT DOES NOT throw a code when it's the cause.

2- Now, if the shifter checks, the problem is in one of 3 other components:
-Throttle actuator (replace/rebuild it)
-Tranny itself in the valve body (electronic shifting valve - replace it)
- HGC control unit (check - replace)

the good news is any of these 3 components WILL throw a code when its the culprit, and there is no need to replace parts blindly with them.
You seem quite knowledgeable on R129s, and I have a question for you but can't figure out how to make a new thread on this, so I will ask you in this manner... I have a 95 SL320 and when I open the driver's door the interior floor lights, seat controls etc... do not come on, but they all do (both doors, floor panel lights, and seat controls) when the passenger door is open. I have already swapped the door switches and it isn't that. I have removed the switch and just jumped it (touching the wires together), nothing but on the passenger door this gives same results as with the switch, and when the headlight warning buzzer is on with the driver door opened, pressing the door jamb switch stops that buzzer. I feel like it is one of the wires that go into the switch (seems like I remember them being doubled at the connector...). If you can make sense of that description and help with mapping a new constant power wire from where to where I would greatly appreciate it... Please don't be offended by the email title, I wanted to make just initials and it forced a 'first name' so I was upset with the profile restrictions. I currently have a 79 350SEL (avail. only in Europe), 85 300D, and this recent addition. I also am not getting my overdrive gear so I have to figure out how to take apart the console to get to that shifter microswitch. Thanks in advance.
Steve
 

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I have a similar issue - 1990 300SL - I've determined the micro switch in the shift gate is working - I was told to check the terminals where the HGS module plugs in with ignition on then switch from 4th to 5th and it will close and or open the circuit which it does. So I'm thinking the micro switch is fine, it's open and closing a circiut at the HGS module

If I remove the HGS module and drive the car it shifts normally on acceleration and will shift into 5th gear after momentarily shifting through 4th gear. I can tell I'm in 5th due to RPM at speed. So does that mean the valve body is working properly in the transmission? if not is it an easy item to replace and does anyone have a part number I should be looking for?

I have the very simple OBDi code reader and plugging into the 16 socket connector and reading socket 13 for the ETC I only get a single flash. Which I can clear off if I use the reset function. I cna't find any reference to what a single flash means...

I have "replaced" the HGS module three times - each one having the same result - I have not bought them new but I find it hard to fathom that they would all be bad. I recently had my transmission serviced - new fluids and pan gaskets, also had the rear differential serviced.

You mention that the other three items throw codes if they are bad..
-Throttle actuator (replace/rebuild it)
-Tranny itself in the valve body (electronic shifting valve - replace it)
- HGC control unit (check - replace)



My car has 75,000 KM so at a loss as to how parts would be worn out or fail at this low mileage - I'm afraid to take to dealer to try and diagnose as they know little to nothing about these older models.

What is the throttle actuator and how do replace/rebuild it? Thinking with only 75K it's not a worn out part ...

Any help would be appreciated.
 

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Check the vaccum tube from manifold to EZL. The rubber connector on mine was leaking and I had no 5th gear.

In fact, check all vaccum tubes.

RayH
 

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Check the vaccum tube from manifold to EZL. The rubber connector on mine was leaking and I had no 5th gear.

In fact, check all vaccum tubes.

RayH
thanks - what does EZL stand for? Any pictures of which vacuum line? I've checked most of them and they have all been good...
 

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My car will shift into fifth if I remove the HGS module ( N51 ) ... I will check the other line too... thanks
 
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