Help finding power seat diode/relay in 1991 190E 2.6

[brookspw]

Guys, this originally went out last year. SteveBFL and Gilly both attempted to help but my knowledge of electrical things left them short handed.

Here is that thread: Click here

Update:
-- Stereo works fine in all ways. It no longer comes on in an irregular patternwithout the key on (now, this might be because originally the bad diode allowed the current backflow, but now burned completely an allows nothing either way).
-- Seats do NOT work, at ANY time, with the doors closed. They will work at ALL times with the doors open.
-- Other than that, all electrical in my car works fine except my left door speaker only works about 1 day per month.

* I need to locate the diode/relay or whatever. If I have an electrical box UNDER my fuse box then I see NO way to get to it. There are NO screws, and even if there were, there would be no room to get to them and/or lift the top box out once loose.

Thanks for any help.

[brookspw]

Bump...for more exposure and hopefully, some help.

[stevebfl]

Well I'll bump you again. I looked in the past and I looked again. The diode is shown to be non existant as of 1989.

I have looked in three different places and all three are slightly different in the area that would explain the key on operation. The way I read the diagram after 1990 there is no power with the door closed. The wire from 15R that should power the relay says it doesn't exist after 1990.

Since the doors are obviously powered with the key on all the cars I have seen. I presume the power must come from 15R. But the alarm system or the locking system might be in the way. I looked but I don't see the connection.

[tecqboy]

I've just come across this posting and I find it extremely interesting from an electronic standpoint. I've read the posts all the way back. I don't have a diagram of the vehicle and I can't help you find the mystery diode. But two things I've learned from experience. Power diodes fail 99 times out of 100 by shorting and this almost always happens when they are asked to pass too much current ( like when something they are in series with shorts. ) A diode will almost always fail before a fuse blows. (Fuses take time to heat up and melt and are very slow; in electronic times.) Relays can have a partial failure. If a DP (double pole ) relay is say closed ( energized ) in position A and a short occurs, the NO contacts may be vaporized and open permanantly, but when de-energized the other circuit NC contact may still be OK. NO and NC relay circuits are wired into very complex logic circuits that make decesions like "if the door is open and the key is OFF, connect power to seat controller, but if the door is closed, the key must be ON to connect power to the seat controller. So if you have the wiring diagram you might be able to figure out the relay logic. If a relay is multi poled, all contacts must be checked. That means you need to have a way to energize them while testing; like a 12 volt battery or power supply. One more thing. If a relay is closed and the short occurs, it can permanently fuse the contacts closed. I've actually seen relays "heal" over time. The movement of the contacts will rub off burnt or oxidized material and restore operation.

OK, so this doesn't help solve your particular problem, but I hope it provides a bit of insight.

And the moral of the story is. Disconnect the battery before working on anything electrical in a car. This is especially true today when so many parts of the vehicle are actually being controlled by sensitive electrical devices.
Tod
Electronic engineer with 35 years experience keeping EVERYTHING electronic running down at the TV station.

[brookspw]

And I just THOUGHT I was confused before.

I wish I had your knowledge concerning these things. I DO have the CD with the schematic. If I can figure out how to email you a pdf of that 1 page, I'll do it so you can take a look. I don't know if I can do that, though.

Thanks.

[savagetom]

I faced this problem on my 1987 190D. I ended up cabbaging together a diode connection outside the fuse box. It worked fine but wasn't elegant. I never did find the diode and holder. However, I think I know where it is. I'm pretty sure if you remove the knee bolster under the dash on the driver's side and look up to the bottom of the fuse box you'll see the holder plugged into the underside of the fuse box.

I can't check this since my son totalled the car 2 months ago.

Good luck.

[stevebfl]

I looked at the CD version, the print version, and at Alldata. Actually Alldata had the best display of what should be exactly the same drawing.

Look to the diagram that has A, B, C, D, E power arrows heading out of the bottom. above that is F13 Aux Fuse Holder Power Seats (Code B). Those two fuses are powered by K5 Power Seat Relay.

The seat relay has term 85 always grounded and looks to terminal 86 for control. Power comes to terminal 86 when the doors are open through either S17/7 or S17/10 Door Switches.

In the drawing the diode is shown wired such that when terminal 15R becomes hot (Ignition switch) power flows through terminal 2 (xF), through the diode and connects into the wiring for term 86; thus powering up the relay when the key is working or when the door is open.

The problem is that the drawing represents a number of variants over the years and the footnote states that the diode link is only applicable to model year 1989. Footnote 3 shows the new link as of 1990. The way it is drawn only connects the seat switches which would leave no relay power with the key on.

This may be as confusing as I have ever seen a MB drawing. I don't recall are there two door jam switches on that car? If not the switches are tied into the alarm system and door look pneumatics.

one interesting note on the diagram that I specifically don't understand is the intention of the dotted line going between pins 1 and 2 on the electrical center (F1).

[savagetom]

one interesting note on the diagram that I specifically don't understand is the intention of the dotted line going between pins 1 and 2 on the electrical center (F1).

Steve, on page 2 of the Electrical Troubleshooting Guide, Model 201, Model Years 1984-1993, the 3rd illustration from the top on the left side of the page (landscape) seems to indicate that the dashed line means that the terminals are part of the same connector.

I've got to agree about the drawing being confusing. Page 138/10 has several footnotes that, I guess, are supposed to negate some of the lines shown on the drawing.