The purpose of this post is to help those of you that are experiencing problems with the operation of your power seats to isolate where the problem is.
I recently had a problem with the driver's seatback on my 1985 300SD and searched the forum archives, my repair manuals and asked for help on this forum. I could never find a logical sequence to enable me to trace down my problem. So I decided to put in writing exactly how I was able to check out my seat.
You will save a lot of time and have fewer headaches if you will follow the instructions below in the order in which I have listed them:
1. Check the fuses and relays in the fuse box under the hood. I replaced any that appeared to be the least bit corroded and used steel wool and sandpaper to polish both the fuses and the holder. If this doesn't fix it go to 2.
2. Remove the door panel - Start by prying off the buttons on the power window switch. (They just snap on, but be gentle with these) Then remove the plastic broken-circle ring from the post that protrudes from the switch by gently prying the ring off. (Do Not lose this little ring) Gently pry out, from the front, the plastic trim under the door handle and remove the screw holding the metal piece under the door handle. Then pry out, from the front, the trim covering the window switch. Slide the chrome trim piece on the door handle down enough to get to the top screw in the door handle and remove it. Remove the remaining two screws from underneath the door handle and lay it aside. Remove the upholstered trim above the door panel at the rear adjacent to the window. Then ease the entry light out of the bottom of the door and unclip it. Carefully slide the door panel up to get it's clips out of the slots they fit in. (Easy to break these clips)
3. Remove the wires from the power window switch. Just gently pry them off. No need to mark them as they only go back one way. Remove the two screws holding the switch to the door panel and remove the switch. Take the switch apart and clean it. (Just gently pry it all around and lift off) Do this over a tray of some kind with a cloth underneath to catch the small ball bearings and springs that may fall out. Remove the contacts and clean them. I used steel wool and a pencil eraser together with contact cleaner.
4. Check the continuity of all the switches with an ohm meter. If some of the poles are not showing continuity, you may be able to resolder any broken connections. I will assume all contacts show continuity. Reassemble the switch and recheck the continuity. (In assembly, I put a small amount of wheel bearing grease on each spring and set the ball bearings in the grease) If you again get continuity on all the switches, you can be assured that a) you reassembled it correctly and b) it will work and c) you don't have to worry about the switch any longer. Reconnect the switch to the wires in the door, but do not put the door panel back on yet. Check the operation of the switch and if this doesn't fix it, go to 5.
5. Since we now know the switch is o.k., we need to check out the motors that perform the functions of the the seat, i.e. raise, lower, tilt, etc. and the control box that tells the motors to perform. But, first print a copy of the power seat wiring diagram and identify and mark the wires that apply to your specific problem, i.e. seatback, seat cushion forward or backward or seat tilt. (My wires, for the seatback, were orange and gray)
6. Remove the four bolts holding the seat and tilt it back. You will see the power seat control box under the right side (about 6" square) and the function motors under the left side (one for each function). Locate the function motor(s) that operates the function(s) you are fixing by identifing the wire(s) that go into the motor(s) (these wires are "hot"). You can check the operation of the function motor by inserting a probe into the wire(s) that lead into the motor(s) and then running a test hot wire from your battery to the probe. You will have a seperate wire for each function, i.e. orange tilts the seatback backward and gray tilts the seatback forward. If the function works on both wires to perform properly, you now know there are no problems with the motors, so we will now go to the control box. (My motors all worked perfectly) If this doesn't fix it go to 7.
7. Caution, this test will be a ground test, therefore disconnect the test wire from the battery. Identify the wire(s) that controls your inoperative function(s) where it enters the control box (this group of wires lead from under the floor mat to the control box) and insert a probe into one or the other of them. Take a test wire and find a ground and attach the test wire from the ground to the probe. If the function works on each wire to perform properly you now know there are no problems with the control box. (Again, the motors worked as they should on mine) If this doesn't fix it go to 8.
8. At this point, we have eliminated or fixed problems with the switch, function motors and control box - so the problem must be between the switch and the control box. Probe the same wire at the control box, then identify the same color wire coming out of the switch on the door and probe that wire. Run a test wire from one probe to the other and check the function by operating the switch. If the switch tested o.k., the motors tested o.k. and the control box tested o.k., then it has to work with the test wire going directly from the switch to the control box. We have now isolated the problem to the wire that runs from the switch to the control box. CAN'T BE ANYTHING ELSE!! I went to the bellows between the door and body to check for a broken wire. You need to take the pin out of the restraining strap to be able to access the wires more easily. I was in the process of pulling the wiring harness from out of the door with a large screwdriver when I decided to try the switch. Lo and behold, it worked!! I tried it another dozen or so times and it worked, so I went no further. I put electrical tape over my probe marks and proceed to button it up by replacing the door panel and seat.
To sum up this very long narrative, you can isolate your problem with a little testing of the various parts of the system. I am confident that if my seatback does not function properly now, it has to be in the wire from the switch to the control box. And if I can't find the broken wire, I can run another wire from the switch to the control box.
I did not offer suggestions for fixing problems, but it should be obvious that. for instance, if a motor does not work it has to be repaired or replaced. Same with switches or controls.
1985 300SD - Charcoal/grey 110,000 mi
1975 450SE - Blue/blue 101,000 mi