I am in need of some expertise regarding the Porsche A 28 automatic transmission. The problem with my car is the downshift mechanism is locked up, in the kickdown position, and will not move. The year of the car is 1984 and the mileage is about 74,000 and that appears accurate from the few records I have as well as an autocheck history. The history of the car is like many, mixed. I am the 3rd owner. The second owner did a fair amount of work on the car but only drove the car about 500 miles in the last 10 +/- years. Like many cars this one has been parked for a very long time while various repair projects were completed on it. For a number of reasons I don’t think the car was ever driven after the transmission “work” was completed.

The PO, a good guy and current fellow Rennlist member, states the valve body was replaced in the transmission, when other work was done, including a new TT, as well as a lot of cleaning of everything. The torque converter “looks” new, the transmission is in new paint, etc etc….the fluid is clean, the filter is new.

The transmission will upshift through all 4 gears cleanly when warm. When cold it likes to stay in first gear, slip a bit, or hunt around for a couple of blocks, and gradually then works through the gears smoothly. Of course there is no kick down by the throttle. If left in drive and the car is stopped, the transmission will start in first and advance through the gears normally. Drive 1 and drive 2 behaves normally, as does park, neutral, and reverse.

So, I have spent a fair amount of time in the service manuals, I have the Jim Morrison extended set of CD’s, but have not really found R&R information regarding the “control pressure cable” and the assembly to which it connects.

Section 37 shows R&R of the whole transmission as well as the disassembly/assembly of the earlier version of transmission, not the A 28.

Before I drop the pan and get myself in more trouble does anybody have any recommendations for an absolute AT novice? Short of taking it to a transmission shop? If I thought I could find a shop that knew what to do I would do that, but…….we all know that story….

 

Not an expert.
Hopefully others will give advice

Do have pictures of doing work inside an 84 trans.

There is an electric kickdown (solenoid) and a pressure cable kickdown.
They work substantially independent of each other.

First two are the solenoid.
Second two pics show the mechanism and attachement point of the pressure cable.
(trans is upside down for the picture)

Last is the end of the 84 pressure cable where it attaches to the hook in the above two shots.

 

Chris,

As always thanks for your assistance. Your last few photos are on the area of my issue, I think. What all has to be removed to access the lever the cable actuates? More than just the pan?

Here is about the best representation of the assembly from throttle to kick down mechanism...

 

Just the pan. Makes it easier to attach the cable if you can manipulate the lever from below.
Cable end has a kind of squeeze lock and gets a partial turn IIRC.
Before taking it apart at the trans, why not pop the other end of the cable off of the throttle body?
You can tell real quickly if the cable itself is intact and feel the spring action.
Requires pulling the air cleaner on the 84 and getting a bit contortionist. Long needlenose with angled ends help.

 

I have already had the cable off at the throttle body end. I have also checked the cable for crimping all down the TT and the like. No crimps or binds. It seems to be bound up in the linkage internal to the transmission itself....not stiff, but locked up firm inside the tranny.....so it sounds like I will dropping the pan to see what I can see....thanks for the pointers and additional ideas....

 

Adjustment of the cable is a must for proper operation. While it is easier to attach the cable with the pan off it can be done with the pan on. The easiest way is to loosen the cable all the way, make the connection and then adjust. I find adjusting the cable a little tight makes the car more responsive. The danger is having the cable too tight and limiting the full throttle position.

You should put in an electrical by pass for the kick down. This will allow you to hold the trans in a gear longer. I integrated my with a push button under the pod and an LED on the center console.

The Mercedes has a feature where the transmission shift later when it is cold. This allows the engine to warm up faster and pollute less. Not sure if your slow cold warm up is part of this feature. I have not noticed this on my 85, but then I do not drive it in cold weather.

I am not sure the people rebuilding the auto trans are necessarily proficient. I have seen a lot of recently rebuilt transmissions acting oddly. In addition, to the cable that selects the gears, there is a vacuum modulator. You need to make sure that engine vacuum is actually getting to the modulator valve. If the modulator value is adjusted too tight the car will shift hard and can in a severe case actually snap the torque shaft.

One other thing to look at is the pan filter. If the trans got hot it will shed some of the clutch material. This material will get sucked up into the filter and block the flow of oil. When cold the flow will be slower. As the fluid heats up and thins out it will pass through the filter faster and pressures will rise in the transmission making it shift smoother.

Other than that you need to dig deeper.

 

Dan, thanks for the response. Unfortunately, the issue in my transmission is somewhere in the linkage inside the transmission case itself. The cable, its connections, and adjustments are not the issue. I have reconciled myself to dropping the pan and looking a bit further. If I can't find a reasonable solution, or a local transmission shop with MB/Porsche 928 experience, I may well do a 5 speed conversion, one such as you have done, if I can find a doner for the clutch master cylinder mount.

The transmission fluid is new, as is the filter and valve body (so the PO tells me). The car was never driven after the transmssion work was completed, and the felloww who did the transmission work (10 years ago) is long gone. So, it could be just about anything. The transmission is like new on the outside, which of course has nothing to do with anything, but does suggest a fair amount of work went into at least making it look good.

Regards the lack of expertise on these transmissions I went to the top notch shop that restores classic MB's not far from here. The owner/manager told me very explicity that Porsche cars had Porsche transmissions in them, not MB. Gee, how could we have been so wrong for so long, all of us? I counted my lucky stars that my car was not there and they had none of my money.....I did try to correct his perception re MB in Porsche, but he wasn't buying any of that hog wash.....

Further, I went on a long wet mission last night to hopefully buy an '85 auto with LSD in "like new" shape. It wasn't LSD, it wasn't like new, the diff was full of very black sludge, and there was enough dirt and grease to make me think it had never been touched.....which could be good I guess.

 

Check out the MB forums for transmission rebuilding there are more than a few threads about replacing the various ***** and springs in the valve body

 

Thanks for the suggestion Mrmerlin.....I had not thought of that...seems rather dull on my part not to have already gone there, in retrospect! But, that's why we pay you the big bucks?