W210 Diesel keyless entry not working?
 

My 1996 300D key is not unlocking/locking the doors. I changed both batteries in the key and the mirror was replaced last year.

Is there another door lock module in the car? Is there a separate fuse for door locks?

Thanks for the guidance.

The FOBs go bad. I and others have had to replace them. Dealer only item as it relates to anti theft and has to be coded to the car. When I had to replace mine 4-5 yrs ago it was almost $300 IFIRC.

My 1996 300 Diesel (W210) does not have the later key fobs model. It is just a key with a single round button that controls both lock and unlock feature.

It sends signal to mirror that then sends signal to unlock doors. Any thoughts?

I have a 97 300D. Replaced the fob within the past year. Cost about $350. Fob is already paired to vehicle when it arrives. License, title and registration required by dealer to order new fob. Names must match on documents. Cash/payment "up front".
These particular fobs are getting scarce and will soon be NLA. When they're gone, they're gone as I was told by the MB parts dept. When I ordered mine, the dealer did a nationwide search. At that time, IIRC, there were less than 10 available.
All in all, except for the price, pretty painless endeavor from start to finish.
BTW, you can use the metal key in the trunk lock and all doors should unlock or lock. There's also an infrared sensor by the trunk lock (same type of sensor that's in the mirror) activated by a functioning fob.

Can you open it and post some detailed photos of the top and bottom side of the circuit board?

Perhaps a solder joint has cracked. I’m not sure what can “go bad” in one of these transmitters aside from a cracked circuit trace or fatigued solder. The weakest link in all these transmitters is the button and it’s surrounding traces. They see the most fatigue stress.

I recently played with the enclosures from your transmitter. Am I not wrong that it is optical and not radio frequency? The case has this red window on it that looks like it contains an LED possibly to activate a photo detector on the car handle?

ETA - oh I just read your note about the mirror. So I take it the mirror has the photo detector for the fob. Yeah so changing the mirror may have something to do with it. If you’re lucky it’s just not properly plugged in. Maybe it’s a mirror off the wrong car. You can check your IR on a web camera on a pC or phone. It is invisible but the camera should pick up the blinks. Try checking the connections behind the mirror.

changed mirror last year
 

Was your fob working properly after you changed the mirror? Everything working ok since the mirror change and then it just quit recently?
If "yes", I'd suspect the fob. Same thing happened to me with my '97.
All working good and then, one day, the fob wouldn't do anything. New batteries, checked for the infrared signal, solder traces, dirt, etc., all good and still nothing. I was fortunate to get 2 fobs with the car and the spare worked fine. That's when I ordered the new one. Smooth sailing since.

Thanks for your thoughts.

I changed the mirror last year and it worked perfectly with the key. Just recently, the key no longer unlocks/locks car. It is the only key fob I have for the car.

The key is as described above which is a square black key with single push button for both lock/unlock feature with a small red plastic window at top of key. I have never seen a light flash inside the key. I am able to lock/unlock thru key hole in driver's door.

I am trying to diagnose to avoid spending $350 for new key and same problem happens.

Have you tried the trunk lock? Also, using the camera in your phone, point the remote at the camera, key side toward the camera (as if you're gonna take a pic of it) and push the remote button. You should see the infrared light blinking into the camera.
Not trying to be negative, but the "new fob" might not even be an option, now. Call a MB dealership tomorrow, with your VIN in hand, and see if they still have the fobs available. They can do a nationwide search.
Not sure of other model years, but on my '97, there are infrared sensors at the trunk lock and drivers door handle. Not just the review mirror. The senors on the handle and trunk will light "green" when unlocking and "red" when locking.
Does your car have these?

When you manually unlock or lock the car with your key does all the other locks work? Is your vacuum pump working? If not, maybe it timed out. You can reset it by removing the fuse for a few seconds or removing the plug on the pump and plugging it back in.

I believe that my 1996 only has sensor on mirror. I don't see lights on driver door handle or trunk area.

All the locks work together when I use the key in driver side key hold. The vacuum pump seems to work fine.

Would it still help to take fuse out for a few minutes? Which fuse # should I remove?

Red light, green light
 

Sorry if that was getting a little confusing. I meant to say, "when using the button on the remote, at the driver's door handle or trunk, you'll get a red or green light".
When using the key at those locations there is no light.

If you aim your remote directly at the driver's door handle or trunk lock (meaning within a couple inches) and press the "button" everything should unlock or lock, with the accompanying red or green light (assuming your car has the lights).
This is with the key end facing the lock.

These 2 locations are independent of the "mirror sensor". If the remote "button" does nothing and everything else seems to be in order (pump, fuse, etc), I'd suspect the fob.

BTW, do you have the "owner's manual" for the car? I believe that would have the pertinent information regarding sensor location, remote operation, etc. and how the entire lock/unlock systems should operate under normal circumstances.