narunasalam

Was unable to find any other thread dealing with this specific problem:
The Key FOB in my 2004 W211 4 Matic Wagon doesn't open the door. I can close the door when getting out. And reopen it almost immediately but if I try to reopen after a few minutes, it won't work and I have to manually open it with the actual key. The key FOB red light blinks whenever I try to open so I know the Key FOB itself is fine. I've scanned the car using a Foxwell NT530 scanner and the consistent result is that there is a Can BUS Low short circuit. Still doesn't explain why the Key FOB closes but doesn't open after a few minutes.

I've checked the relevant 7.5 A fuse in the rear SAM (#8 Left Antenna) and that seems fine.

I've also tried to measure the CAN BUS voltages at the OBD2 port and that is inconclusive ... I get a Can HI reading of about 2.6 V and a Can Low about 2.35 V - I would have expected that to be reversed since the Can Lo operates at a higher base voltage. Also don't know if the Can B bus has a terminating resister between Hi and Lo. Some say Can B is not terminated and some say it is terminated.

Any help will be very much appreciated. This car has KeylessGO as well but stopped working a while ago and I was happy with just the remote key but that too is now having problems.

CaliBenzDriver

I have hands on the KeyGo, I may be able to help guide your steps.

This maybe a little time consuming but ultimately the temperamental keyless will work better than new:

1-- Pop the KeylessGo module out from the trunk rear right side.

2-- Take good pictures to show us circuit board sides....

3-- Be ready to resolder all the loose connectors pins.

After that...
scan/clear
then test: Any luck?


> Troubleshooting:
CAN Low shorted unplug CANbus-B members from front distribution bars until CAN-LOW no longer test shorted.

The legendary 120R module termination has morfed into a 60R at distribution bar...
What you're looking for is some kind of physical damages. If you can't spot any obvious causes you're going to need bigger guns: a SCOPE!
🤞​​​​​​

narunasalam

Hi CaliBenzDriver - Thank You. Looks like we might be neighbors of sorts as I live the SF Bay Area as well. Will take a look and contact you. But after further digging I found this link that explains the connection between the Key FOB and the Keyless Go module.

https://mbworld.org/forums/mercedes-...ml#post7647270

According to this, the path from the Keyless GO unit in the door handle to the Keyless GO module in the trunk is via the Key FOB. Since I am having the basic issue of the remote KeyFOB not unlocking after a few minutes - but locking/unlocking works immediately after getting out of the car - not sure if the problem is in the Keyless GO module in the trunk. Seems like some pathway in between gets blocked or falls asleep.

CaliBenzDriver

to all that's been written about reliably goofy KeylessGo, I can add that I now recognize EIS as a bad solderless gateway module. I'd fix this as a priority before further Keyless considerations.

narunasalam

Thank you again. I understand the issues about solderless connections in a number of W211 electronic miodules. But in my case the Remote Key FOB does work to lock and also works to unlock if I do it soon enough, like under 5 minutes. So something seems to shutoff/sleep after a few minutes and doesn't wake up when the unlock button on the key is pressed. I'd like to solve this puzzle first.

bbirdwell

When unplugging the can bus connector from each distribution bar be sure to do it one at a time! Each position feeds a particular portion of the electrical harness. If you pull off several and then put them back in incorrect order the connections will no longer match the electrical schematics. I learned this one the hard way on my w-210 when troubleshooting a short in the can bus high circuit.

narunasalam

Hi BBirdwell - had already read your previous posts on Can BUS troubleshooting (thank you). Have bought 3 new connector blocks ($4 each) and practiced how to take them apart and also have them handy if I happen to break a tab or two :-). The difficulty I have in going down that path is that when I measure the Can BUS voltages at the OBD2 port as you suggested, I get 2.6 V or so for the Can HI and 2.35 or so for the Can LO. These measurements match the normal values in this very good
StarTuned magazine article https://automotivetechinfo.com/2017/06/can-bus-review/. Yet my scanner says there is a "current" CAN B Low short circuit which leaves me puzzled. Also puzzling is why the Can LO would be lower despite having a higher base voltage.