1990 300SL not shifting into overdrive 5th gear
 

I recently purchased a 1990 300SL R129 automatic 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!

I'm not 100% sure with the 300 but I think 1990-1995, SL's only came with 4 speed automatics. :confused:

I will take a pic of the shifter, but it does have OD. The dealer is still working on it.

1990 SL 300 transmission
 

The 300's did have a 5 speed auto trans, but the V8's only had the 4 speed auto.

It is the 722.5 transmission. I was able to get it working better?...by putting in a new crusie control module, not I have cruise control. I also checked the three prong plung and actually moved it away from the valve cover. Shifted perfectly this morning, crusie works perfect but after stop and go driving, it stops working again. I removed the N15/1 module and it skips 4th and goes directly to 5th smoothly which rules out a tranny probelm. I don't have the proper diagnostic equipment so off to the shop.

Deak
 

I have the same problem as you on your 300SL with it not shifting into 5th gear. Did you solve the problem. It is a 91

ME TOO!! ME TOO!!
 

Pristine 300sl & Deak60 ~

Ive got the same issue. What did you guys do to remedy inoperable 5th gear?

THANX MUCH : )

Does anyone have an update on this - I've the same issue - someone told me their is a small switch that contacts the shift lever in the console that could be an issue - I'm going to gingerly remove the beautiful wood console to see if I can see anything. This car only has 74,000 KM ( 46,000 miles ) on the highway it's revving at about 3500 RPM @ 110KPH ( 65 MPH) which seams to be high... someone mentioned the N15/1 module removing to trick it moving into fifth ( where might that reside )

I just had the transmission serviced - new fluids and gaskets etc...

It took me a while to figure out where overdrive (5th) was. I simply moved the shift lever over to the right and it switched up! Keeping it on the left side of the "Drive" slot keeps it in 3rd or 4th, depending on which trans you have.
But this may not be your problem ... just mentioned it because I had a "Doh" moment when I read this in the owner's handbook.

Did the 1990 have a 5-speed? I thought it was 4-speed.

It resides with the other electronics under a plastic cover in the right-rear engine compartment. Atop it is a black plastic knob which you twist to remove it. The module should have a white label which reads "HGS".

I will try to remove the module and see if it moves into 5th - My gear selector has a D and then if you move lever to the left it locks into 4th - driving at HWY speeds no difference in RPM in either 4 or D ( which should be 5th )

I do have a Obd1 code reader but I do don't have any dash lights on - will the code reader identify a problem with a part even if no dash light is on?

the local dealer hasn't been much help as they don't know much about these vintage units,

The solenoid they speak about will that throw a code that you can read to know that is your issue... Is the solenoid external to transmission or do you have to pull the pan?

so i pulled the N15/1 module and the car skips right into 5th when driving skips over 4th. so my question is do i replace that module? or do I pull the console out to check the operation of the switch? i can find the replacement module fairly easy but don’t want to replace if that part isn’t a common issue...

mine is a 300sl and it has the 5 speed the V8 cars had 4 speed

I would see if the HGS module is receiving the signal from the "D" switch on the shifter gate.

You remove the electrical connector to the HGS switch and use a multimeter to check continuity between sockets #17 and #21 with the selector in "D" and also in "4". In one position you should see a short circuit and in the other an open circuit.

If no signal, then I would suggest that you remove the "D" switch and clean its contacts. If you have the signal, then the control module is suspect.

thanks much .. i’ll will give that a try...

I tried removing the module and checking the continuity on the sockets when moving the shifter from D to 4th and yes the switch at the shifter does work - so the HGS module is bad - I bought a used replacement and installed still the same problem won't shift into 5th gear - so my question is do you have to perform some kind of "reset" to initialize the new module or should it just work when installed - I'm hoping I don't have a second dud unit... but maybe I just need to do a reset ? if so what is the procedure to do one... thanks much

So I now determined that the micro switch should be good at the gear selector - I have now replaced the NGS module three times - all with the same results ( I have not bought new but hard to think that everyone is bad with the exact same issue )

the car if pull the HGS module does shift down into Fifth gear - I can tell due to the RPM at highway speeds - so the transmission is shifting fine -

I plugged in the OBDi reader this morning with the HGS module installed - and I got THREE flashes when plugged into socket 13 ( ETC - Electronic automatic transmission control )

Not sure what that 3 flashes mean but I could do a reset and got the flashes to go away..

I've been lookign at the MB Tech star manual someone sent me a link to and it doesn;t talk about what codes or flashes come up ... Can anyone help...

OK so I found a manual on line - 3 flashes from the ETC socket #13 - says "transmission overload protection switch 4th/5th gear defective" well I guess that might solve what the problem might be but what is a transmission overload protection switch? where is it and how do I replace it... Any ideas on a part number / location etc... 1990 SL300 -

I think I have found the part number but the description of the issues related to the switch make me think it might be part of the problem but not the source. I see it's on the left side of the transmission do I need to drain fluid prior to changing or can it be done quickly on the fly...

https://cimg6.ibsrv.net/gimg/www.mbw...96d237dd7e.jpg
https://cimg7.ibsrv.net/gimg/www.mbw...7654087588.jpg
https://cimg8.ibsrv.net/gimg/www.mbw...2da1d8f708.jpg
OK so now I've purchased the overload protection switch - drained the transmission fluid - removed the pan - removed the OLD switch which because of the o ring on the end pulled the valve out of the transmission. I see it has a notch so it can only fit one way but I'm having a heck of a time fitting it back in - see the pictures of the assembly and the NEW overload switch - is there some trick to get this valve to seat - the new overload switch doesn't tread all the way down to the seat and it's getting really stiff to turn it in... any ideas. I've attached some pictures of the pieces.. I've looked with an inspection camera into the hole and it doesn't look like there are any obstructions - does it just fit that tight?

help would be appreciated...

So I solved the problem of getting the valve assembly back into the transmission - new switch installed - pan back on - filled with fluids - no leaks so that is all great. I started this process trying to figure out why the car won't go into 5th gear - as you can read earlier in this post - My contact switch in the console is operative - if I pull the HGS N51 module the car does go into 5th - I was getting a three blinking light code from test port #13 on the 16 pin connector with my OBDI tester - I could clear the code but it would come back after driving and the car wouldn't switch into fifth - So I found a diagnostic manual which said the three flashing lights are the transmission overload switch error - so I changed the switch - I have changed the HGS module three times all the same results... I've checked all the vacuum lines and they are all fine...

Where do the two wires on the transmission overload switch go to? I've heard the EZL unit I should be able to "test" them somehow - I really hope to stop just replacing parts until I find the issue - other than this the car is in near perfect condition so it's pretty annoying...

Any help or advice would be appreciated...

One side of the overload switch is wired to ground; the other is wired to the EZL controller.

You can test the EZL controller by disconnecting the coil, substituting a 1K-ohm resistor in its place, cranking the engine, and measuring the DC voltage across the resistor. I think you would measure 2-4 volts, but I would have to search for one of my old Benzworld posts to be sure.

So the resistor goes into the plug where the coil woudl go on the EZL - would the car run properly if the DC voltage is out of spec ?

the wiring and vacuum lines on this car are in exceptionally great condition - Any ideas as to where the overlaod switch is wired into the EZL maybe I can test the cable between the two..

If the engine is running fine, then I wouldn't bother with the resistor substitution test.

The transmission overload switch is wired to ground and female socket #3 of the EZL module's "B" connector. If you were to hold the connector with the cable pointing down, socket #3 is at the 7 o'clock position.

The "B" connector is distinguishable from the "A" in that it's "key" angles to the left when you hold it as described above, while the "A" connector angles to the right.