I have a 96 E300d for 6 years and 2 months ago ETS and ABS come on when I am driving. Speedometer and cruise control all quit working after few minutes it come back to work except cruise control. It happen to become more and more often, like every ten minutes. After surfing the web, everybody pin to the problem of the brake switch. Order one and install the week after. It seems go away for 2 days and it come back again. It happen more and more often and ETS ABS Speedo dead right after I start the car. I will not go away. 2 weeks ago when I stuck in a traffic I notice the coolant temperature keep rising. That night I open the hood and find out the auxiliary fans stone dead. I take it apart and do some testing. Find the relay 2105450232 with no power coming out to the fan. Order a used one from eBay (may be I shouldn't), same result. Fan working fine when I connect it to 12V battery. Last night I twist my friend arm and borrow his climate control unit (96 e320) with the same result. Now I come to a dead end and see if anyone can help. Need the car very bad.

leesanusa

I had exact symptoms, my first shot was at brake light switch. But it was relay 2105400272. Changed it all issues gone.

__________________
06 MB E320 CDI 123k miles
14 Sienna (wife's DD) 36k miles
11 GL350 Bluetec 136k (Sold)
96 E300; 292k+ miles (Donated)
11 Sprinter 22K+ miles (Sold)

To expand on what alipak said:

The relay 210-540-02-72 is known as the "K40" relay, this is the reference name given to it on the wiring diagrams for the car.

It is the main power distribution and voltage regulation for the electronics in the car.

It is located in the box under the hood on the passenger side (the big black box with the sticker that says "battery under rear seat").

It is easy to swap out and is known to fail and cause the issues you describe.

Once you have the cover off, you will see several electronic modules the size of paper back books (those are your engine, transmission, and ABS controllers), and a black unit in the front with about a dozen fuses. This is your K40 relay.

Go in the right rear back seat of the car, and look at the floorboard. Where your heels would be if you were sitting in the seat. There is a plastic cover, pop that off. Using a 1/2 inch or 13 mm socket, loosen the bolt and pull the cable away from the car body. That is your battery disconnect, you want the battery disconnected before swapping the K40. You will see some small sparks, that is okay, it is because your dome light and small electrical loads are going through that connection.

Now go back to the control box under the hood, and pick one of the wire connectors. Gently pull the connector out of the K40. Some of the connectors have a release tab not unlike a phone or computer network cable. Find the same connection on the new relay and plug that cable in to the corresponding place. Once you have all the wires transferred over simply lift out the old K40 and drop the new one in.

Put the cover back on the control box.

Hook up the battery in the back seat.

You will have to retrain the power windows and sunroof auto functions, the procedure for this is in the owners manual.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Thank you Alipak and Jay_bob. I located the relay but it look different (2105400372) when I compare with pictures online (2105400272). I cannot wait any longer. Ordered one online, hope I hit the jackpot before my wife make me sleep in the car. I driving her's to work and she take bus for last 2 weeks.

Part came in today. Take 5 mins. to put it in. wa la , it is K40 relay creat all these problems. Everything work . Thank you thank you

Don't trash your old one. If you are handy with a soldering iron, the common failure mode on these is cold solder joints. I would open it up and retouch the joints. If that makes it work I would hold onto the old one as a spare.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Opened the relay. I don't think I can do a better job on the existing solder but I will spend 10 mins. resoldering every single one to practice my soldering.

leesanusa, thanks for the feedback K40 is a bummer, it crops up so many issues but simple to fix.

__________________
06 MB E320 CDI 123k miles
14 Sienna (wife's DD) 36k miles
11 GL350 Bluetec 136k (Sold)
96 E300; 292k+ miles (Donated)
11 Sprinter 22K+ miles (Sold)