The two front power windows aren't working on my 1983 300SD. The two back ones work so I know it isn't a fuse. How to do I go about diagnosing the problem?

i belive the front and rear are on seperate fuses.

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have no worries.....President Obama swears "If you like your gun, you can keep it

Did they work at one time, or, are you a new owner, and don't know?

I think there are relays in the electrical box under the hood. You might try swapping some of those around.

The reason is that I had a non functioning passenger seat on my -new to me--1982 300SD. I replaces swicthes and rebuilt switches. I could get the seat motors to work if I applied 12v diesctly to the wires, but not thru the switch. My problem turned out to be a previous "repair" where the "mechanic" mis-wired the switch connection. The switches test OK out of the car, but failed to work in the circut.
The morale is " Never assume anything!"
You could also have a broken wire in the door.
Try applying 12 volts to the motor connections inside the door. Then see is you have voltage at the same connectors. It will soon be obvious where the fault lies. Just be patient, and never assume anything.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags

You have no issues with the fuses.

You have no issues with the relays. Don't bother to go playing around inside the fuse box.

The problem with the windows can either be the switch (most probable) or the motor in the door.

The easiest way to troubleshoot it is to find a known good switch and try it out.

Otherwise, you'll need to pull the door panel and check for voltage on the supply lines to the motor. There are two lines. They switch from positive to ground depending on the direction of the motor. If you have voltage on one of these lines when the switch is depressed in one direction, and voltage on the second line when the switch is depressed in the opposite direction, then the motor is the culprit. If you don't, then the switch is the culprit.

If you apply voltage to the switch contacts without removing battery positive post, you may fry the voltage regulator.
DO NOT LEAVE THE KEY ON FOR MORE THAN A BRIEF PERIOD OF TIME WHILE TESTING WINDOWS IF BATTERY IS CONNECTED OR YOU WILL FRY THE VOLTAGE REGULATOR. Ya, I know ... I had a spare.
If you know the windows did work at one time and no one has modified them since that time (I have seen Ebay cars with wood supporting windows!) and jumping the switch does not work, remove the door panel and inspect the regulator.
Warning: If the motor works and the regulator slider is faulty, you can on some models force the slider rivet or pin into the aluminum rails, damaging the slider rails by operating the switch / regulator motor and you will have to remove the rails to file smooth the damage or you will be replacing sliders often.
If you have to dissemble the front doors to remove / replace the regulator, study shop drawings and instructions first. You have to remove the arm rest, door lock lever plastic 'shroud' & metal 'housing', plastic lock pull knob, chrome lock plate, vertical door trim, inner panel (remembering the light on the bottom of the door), drop the window (heavy ag. silo tape helps hold window and will not leave residue like duct tape, you will have to 'hold' the window in several positions) remove the rubber door/window seal, mirror, two small screws holding the vertical window rail, remove the rail with a twisting motion (some prefer to remove the outer trim first) ... get the point? You can do this but you can also reassemble the door in the wrong order and start over. DO not remove the seat switch cover or you will have small parts everywhere!!!!!
While in there, lubricate the window with MB lube or Lubriplate (get a glob from an engine rebuilder) and remove any wheel bearing grease (!@&#%@% PO fool!), clean and lubriplate the switches, verify vacuum lock gizmo with a mini vac, verify door water weep holes, look for any parts inside the door to give you a hint on pervious work (?) done on the door, replace the lock vacuum rubber connectors, adjust the regulator stop, graphite lube the lock ... Keep a mechanic's magnet handy and a clean ice cream pail for the small screws and after removing the inner door panel from both doors, work on one door at a time so you have a reference to go by.
I know this sounds like a very large job, but if you do everything on the door you will not have to get in there again for a LONG time. I have replaced sliders only to have the vacuum line connector fail, then ... get the point?
If you are not comfortable doing this, practice on a bone yard car but please replace all hardware!
Don't you just love how I said 'remove the mirror' not mentioning all the bolts, screws, seals, wire connection ... maybe I should have said 'disassemble the door and repair any noticeable damage'.
When done, turn toward Germany and shout "why didn't you make millions of 240Ds with the 617 turbo engine, manual windows, manual trany, manual locks ... a MB folkswagon".

Where did you come to that conclusion?

I've left the key on for an hour when testing the climate control or the headlights or a multitude of things that need the key...........the voltage regulator doesn't care one bit.

Now, if you shorted the system in some fashion while the key was on, then all bets are off.

so are you trying to tell me im not supposed to roll my windows down without the engine on? i dont think so.

also you dont need to remove anything other than the door panel and a few screws to get the motor/ reg. out.

__________________
have no worries.....President Obama swears "If you like your gun, you can keep it

Often times its neither the switch nor the motor. Its the window motor cable that needs replacing. Thats the one part that flex's every time you open and close the door. After 20+ years of opening and closing the door the wires inside will crack, stretch, and come apart not allowing the window motor to work.

The local indy explained thats why he always tells his customers that come with a non-op window to replace the cable instead of dishing out $$$ on window switches and window motors when it often times is the cable.

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Current:
05 E320 CDI
07 GL320 CDI
08 Sprinter
05 Dodge Cummins
01 Dodge Cummins

Previous
2004 E55 AMG
2002 C32 AMG (#2)
1995 E300
1978 300D
1987 300D
2002 C32 AMG(blown motor :[
1981 300SD
1983 300SD
1987 300SDL
2002 Jetta TDI
1996 S420
1995 S500
1993 190E 2.6
1992 190E 2.3
1985 190E 2.3 5-Speed

Thank you very much everybody for your advice. I'll start troubleshooting tomorrow.