red battery dash light stays on with no key in ignition switch
 

I'm trying to help out a friend with his 1983 380SL. The car won't start; hasn't started in a while. We got a new battery and installed it. As soon as it was installed I noticed the red battery light on the bottom of the gauge cluster was lit with no key in the ignition switch.

My friend was persistent and cranked the engine anyway, it didn't start. I felt the coil and it was very hot. so I took it off and tried it on another car and it was working fine.

Is there common reason(s) why the battery light would stay lit with the car off ?

I am assuming there is a short in the alternator or voltage regulator.

We left the battery terminals off just in case.

Also, is there anyone in the (lower) Westchester County N.Y. area that knows the M116 engine electrics?

most likely the alternator has a diode shorted out. every other alt i've seen had that problem when the light was on/key off.

i am concerned about the no-start and the hot coil. granted , ignition sw on, power to the coil, the coil will get hot-normal, if the key is left on for 5-6 minuted or so. if the coil is hot with the key off, then a wiring short or a bad ignition switch might be to blame.

i would, unplug the alternator, key off-is the light off? yes, you need an alt.
no, the light is still on, you have another problem.

unplug the alt and try to start the car while using a booster battery or a VERY GOOD GARAGE GRADE CHARGER. then you can see why the car won't start, no spark, no fuel, etc. good luck, chuck.

porkface is right it's the backout diode. $3 part

Where can we get it from? and is there a thread explaining the procedure to replace it?

Radio Shack - some stores have started to reduce small parts like this but you can still get the full buffet at radioshack(dot)com. If you can make a friend at your local alternator shop, he will probably sell you one. I have the bosch pdf file for repair that can give you the illustrations of the unit. It's to big a file to attach. (PM me an email address). If you want to rebuild your entire alternator since you are in there (bearings and such) and even make it a little hotter. Summit Racing still sells the Bosch reman kits for about $3X.XX
as of a couple years ago anyways.
Note: the diode is not connected with your starting problem.

So the only way to replace the diode is to tear the the alternator apart?
Is there an easy way to just swap it out?

Also, if the alternator is not putting out enough juice, could that be the starting problem?

"Also, if the alternator is not putting out enough juice, could that be the starting problem?"

Not really - unless the battery is half dead from not being charged.

The alternator just recharges the battery - once the car is running. It will have no effect until then.

However, if that diode was somehow shorting out in a way that drains current from the battery constantly and seriously, then it could make starting hard.

But, your battery would die overnight if it was pulling that much juice.....