1980 300TD transmission failure - PeachParts Mercedes-Benz Forum

jlsmall27 · #1 08-25-2014, 04:55 PM

TL;DR: 300TD starts in 2nd and slips straight to 4th. 3rd can be found manually; 1st has disappeared. Car is drivable – but for how long?

***

I drive a 1980 non-turbo 300TD with the 722.118 automatic transmission. The transmission has had some issues in the past – occasionally struggling to find second; once running very low on fluid. I have been driving this car for about a year, and have never experienced any transmission problems that prevented me from operating the vehicle as normal.

Last night, after some fairly vigorous backroads-driving – the Benz was running beautifully and everything seemed perfect – I pulled off on the side of the highway to get directions. I left the car running and popped it in neutral. Getting back up to speed to rejoin the highway, the car sounded very different – a sort of low rumbling burbling sound; the engine itself sounded kind-of clunky and rough and low. The car was not accelerating well and it seemed to shift straight from 1st to 4th, with some weirdly sliding hiccoughs.

We pulled into a parking lot and poked around a little bit – thought something might be caught on the driveshaft or axles. Nothing. Try to leave the lot – it sounds like there's something scraping the bottom of the car and it is extremely slow to accelerate. Eventually, we manage to limp the car back to a buddy's garage and do some investigating. This is what we have discovered:

***

The car appears to be starting in 2nd. If put in Drive, it will start in 2nd and slip its way up to 4th at very low speeds. It acts like it's searching for 3rd, but it never finds it.

If started in Load (or whatever 'L' stands for), it will start in 2nd. Shifting to Slope (or whatever) it will shift straight into 3rd. Drive takes it straight to 4th. So, the car can be driven fairly normally like this – it gets you to work, at least.

The transmission fluid is at a good level and a good color. The filter and fluid have been changed within the last two years by the previous owner.

The line going to the vacuum modulator is holding vacuum.

The kickdown switch has nothing to do with this issue.

I do not have a Bowden cable that controls the shift timing.

This transmission is not original to the car. The original transmission failed and was replaced with a MB-approved remanufactured unit relatively early in the vehicle's life.

The front pump in this transmission failed sometime within the last decade. It was replaced, and "no debree was found w/i trans."

***

My questions, then, are:

What the hell is wrong with my car? Is the transmission toast, or could this be a (relatively) easy and inexpensive fix?

What takes the place of the Bowden cable on the 722.118 transmission?

Research has directed me to interrogate the B2 piston and the governor – what do they do; would their failure produce these symptoms; how easily can they be fixed/replaced/adjusted?

Until I can get it fixed (please god let this be the case), is it safe (for me and the car) to commute to work? About a 15-minute drive, mostly at highway speed.

***

I would really, really appreciate any insight. There may be some things I've forgotten to mention; I will be checking this regularly and will answer whatever I can.

I love this car. I don't want to part her out and buy another Hyundai Accent.

***
Here is a link to this same thread on BenzWorld.

Zacharias · #2 08-27-2014, 12:01 AM

Quote:

Originally Posted by jlsmall27

What takes the place of the Bowden cable on the 722.118 transmission?

I also have a 1980 with a .118. I wish I could contribute more, but I have never heard of your particular set of symptoms. The usual complaint on these units is failure to shift UP out of first or second.

The edition of the .118 that you and I have uses its internal governor to determine shift points. The vacuum modulator **may** play a greater role on these than other transmissions, but in theory it controls shift quality.

Very early .118s (78-79) had control rods connected the throttle linkage, and mechanical modulators (no vacuum). But for 1980 it seems to have been vacuum only.

leathermang · #3 08-27-2014, 01:07 AM

What kind of servicing have you done to it ?
When it got really low did you change out all the fluid ?
Does your car have a transmission filter ?
If yes, did you change it at any time ?

whunter · #4 08-27-2014, 02:32 AM

This may help.

722.118 Transmission Adjustments

722.118 Automatic transmission rebuild (Monster DIY)

.