Block heater fuse
 

The block heater on my '87 300 SDL stopped working yesterday. I tried to trace the wire, but lost it under the engine. Does the block heater have a fuse, and if so where is it located. I noticed the heater stopped working the day after the car went through the car wash. Any connection? Thanks

no fuse that I know of

My guess it that the car wash might have knocked the cable off the pins on the heater.

Jack up the front end, safely, and go under the passenger side of the engine and look up to see the connection of the block heater cord to the pins on the block heater.

The cable could have come loose, the cable could be bad, or the heater element is bad.

The cable goes to the RH side of the engine (facing front), under the turbo. You have to be under the car and look above the starter. Pull the cable off and reattach it (wear gloves or have a good hand/arm cleaner available). If it still doesn't work, pull it all the way off the car and check for continuity on both wires from one end to the other. If you have continuity, the cable is OK, the element is bad. No continuity, the cable is bad, the element may or may not be OK. If the cable is bad, I can sell you a new one that I bought and installed, but took out again because the element is bad on my car. $10 plus shipping. If the element is bad, it's replaceable, but may be very hard to get out. In my case, I just installed a lower radiator hose heater for $25.

Check the power cord for continuity.

I just discovered that my power cord plug, the end you plug into your extension cord, pulled one of the wires from inside the plug the last time I forgot to unplug before backing out of my driveway!

I put a new plug on the power cord and once again have a working block heater! :) BB

The block heater on my '87 300 SDL
 

If it is like my 123 chassis 300D block heater most likely is a 400 watt unit. No fuse, not thermostat. When she is pluged in she is on. They have an electrical connection at the block heater itself (near block) which can get bad. Also according to the people who make new ones for MB Philips and Temro (phone 952 941 9700) they should not be plugged in when car is idling or as is the case driving down the street which I did once (oops!). Unplug before start up which I never did for two years. Heaters when operating right give about an 80 degree rise over ambient air temp. Contact company they will tell you how to test them.

I used to have mine dangling over front bumper. I respliced and have it in the engine compartment area
Tom

Update on block heater
 

Changed the block heater plug per 1 suggestion and success. Thanks to all who helped this become a small problem to fix. Diesel Shop forum to the rescue again!

I need a cord for the block heater on a 603 engine.
It is not like the ones usually found on 617 diesels. this is for an '87 300D. Someone informed me that there was a coupon in the owner's manual that said they would install a block heater if you asked for one on its first (5000? mile) service. He said they simply install the cord when you take it in, the heaters are already installed at the factory.
My service manual did not include any reference to a block heater, and the parts manual doesn't show one either.
I saw a couple of 603 engines at the wrecking yard, all had heaters covered by a plastic cap where the plug goes (and none had cables!). Maybe because I'm on the Left coast ? (it seldom freezes here but I plan to drive this car back East this winter. )

If anyone has a cord available please PM me.
Please note! I can't use the rectangular shaped cord used on the 123 cars, I have one like that and it won't fit the 124 block heater. The 124 uses a large threaded port for the heater cord and the pins look smaller than the ones on the 123 heaters.

If I assume that the W124 is identical to the W126, then you have no problem. Just order a block heater cord from any of the suppliers and it will fit. The protective plastic cap is removed and the cord plugs right in.

The dealer cord is slightly different, IIRC. At double the price you get a threaded cap that secures the cord to the side of the block. The typical cord that I received last year just plugs onto the pins.

I will tell you that I made the effort to put the cord on, however, never plugged the 603 in all winter. It starts at 8°F. the same way it starts at 70°F. You just need to let it run the full 40 second glow.

Out there, I really don't think you will need one.

Out where? You mean I won't need it in West Texas in 8 degree weather? I just remember the trouble I had starting 123's in 15 degree weather once.
I still want one, the kind that screws on preferably. Thanks, Brian I'll try the dealer next week