got a car with electrical gremlins

[Real_ILYA]

ok, so I recently got this 1991 SL300 that has a few electrical issues that I will list below... I did, by the way, replace each and every single fuse in both front and rear panels... that made my windows work again, then the windows blew both 8A fuses in the rear box again for some reason, I replaced them and they've been working since... other fuses are still ok since replacement...

Now, my main issue is that the instrument cluster is only working partially, all gauges (speedometer, tach, fuel, etc) are inoperational - with the exception of engine temp and all warning lights seem to work OK too... I read around, and it doesn't seem to be the dreaded intermittent cluster issue related to some resistor in the cluster itself... however, I've read about some other people having a similar issue to mine due to a "bad" switch, or "bad" light - so my (1) question is how do I test for a "bad" switch or light with a multimeter, and are there any particular circuits I should concentrate on???

also, some power options dont work, such as the power hardtop release... when I push the switch nothing happens (it does blink red though)... I am guessing there must be a relay somwhere - so my other question (2) is where is that relay and can I test it???

the seats lost power, but I was told by previous owner that the switches tested out OK and the seats move if power is jumped to them with a wire... so, another question (3) is there a relay for the power seat adjustments?

is there a place anywhere to download a wiring diagram for my car (or similar)???

[tecqboy]

Welcome to the forum. All of your problems have been much discussed in the past on this forum and the MBC forum. You need to use the search tool to find the information you need. Use key words like cluster, VDO, Soft top controller, ignition switch. If you have the original aluminum fuses, switch to newer brass type, especially the 8 amp window motor fuses. The top controller needs a total reset. The seat switches are known to fail. The VDO has a voltage regulator which has a habit of failing. And the ignition switch is the cause of many electrical issues. You need to buy a service CD to get an electrical diagram.

[tecqboy]

Mercedes Soft Top Controller Reset Procedure. R129 1990 thru 12/93.

Locate the diagnostic connector: Under the black plastic cover over the hood-up switch on
the US passenger side of vehicle near the strut mount.

To recover codes from the soft top controller:
Connect a normally open (N.O. S.P.) switch between pin 1 and pin 10. A simple push button switch with leads or a short jumper lead will also work OK.


Short (close switch) 2 to 4 seconds to read error codes.
The lamp in red (operate) top switch ( in the console ) will flash out a number of pulses.
Count the pulses and see the chart.

Short (close switch) 6 to 8 seconds to erase error code.

There may be multiple codes stored, so read / erase until all are cleared or the problem
Is located. Some codes may require reading / resetting twice before they will erase.

The codes that will suspend operation of a soft top on the R129 are 21, 22, 26, and 30.

A code of "1" indicates no faults and no hard top in place, while a code of "20" indicates no faults and the hard top is assumed to be on the car.

2 = Limit switch, LEFT locked, soft top storage compartment cover.
3 = Limit switch, RIGHT locked, soft top storage compartment cover.
4 = Limit switch LEFT closed, soft top storage compartment.
5 = Limit switch RIGHT closed, soft top storage compartment cover.

6 = Limit switch LEFT locked, soft top fabric bow.
7 = Limit switch RIGHT locked, soft top fabric bow.
8 = Limit switch LEFT closed, soft top fabric bow.
9 = Limit switch RIGHT closed, soft top fabric bow.

10 = Limit switch LEFT front locked, soft top.
11 = Limit switch RIGHT front locked, soft top.

12 = Limit switch soft top storage compartment open.
13 = Limit switch soft top fabric bow raised.
14 = Limit switch soft top down in storage compartment.
15 = Limit switch soft top up ( secondary closing speed ).

16 = roll bar retracted switch.
17 = Left window failed to lower (limit switch open), bad fuse, or window relay on controller.
18 = Right window failed to lower (limit switch open), bad fuse, or window relay on controller

19 = Vehicle speed signal to controller missing or illogical.

20 = Hard top installed, no faults.

21 = Controller module / soft top switch circuit open.

22 = Roll bar switch activated.
23 = roll bar control unit not functioning.
24 = Roll bar crash deployed.

25 = contradicting switch inputs.

27 = insufficient voltage.
28 = Speedometer signal is missing.
29 = Wheel speed from ABS control module missing.
30 = Top operation disabled (due to #29).

[Real_ILYA]

nice! thanks for the top reset procedure - I will give that I shot and post up the results... appreciate the details

the seat switches - how could I test them myslef to see if they work or not?

the instrument cluster - I did some searching, and my problem does not appear symptomatic of the voltage regulator on the cluster - those issues are intermittent, like when you use your turn indicator... on the other hand, my problem is constant, all gauges are dead - other people have fixed that by figuring out a "bad" component elswhere (such as a hazzard switch, or a third brake light that has gone bad) - which is why I was asking how can someone test a swich or a light to see if it's "bad"... and if there are any particular circuits to concentrate on...

[tecqboy]

The power seats are tied into the same memory controller circuit as the center review mirror and the outside rearview mirrors. Are those devices working? Do you have the owners manual which explains how to set the 3 memory switches? Don't just look at fuses, measure them. Removing any module or the VDO in this vehicle without disconnecting the battery can be fatal to the component. If the car has been recently jump started, check the Over Voltage Protection relay fuse. You need to follow proper procedure and remove the VDO to inspect for burnt foil or obviously burnt components. There are a number of small companies which will repair a VDO.

[Real_ILYA]

re: power seats - yes, the center and side mirrors work fine powered by the 3-way switch in the center console... I was told the seat mechanisms move fine in all direction when 12V is applied... the switches in the doors seem to do nothing though unfortunately...

I did not just look at fuses, I replaced all of them with new ones however, I will measure them now to double-check

I dunno what kind of stoopid things the previous owner did to the car, so I wouldn't put it past him to having jumped started it... where is that relay (or its fuse?), and how can it be "checked" ? are there any other relays (main relay, or combination relay of some sort) that may be resposible?

removing the vdo will be my last resort, after I fix all the numerous little electric anoyances like missing light bulbs, etc - just so that I know that it isnt something simple...

[Real_ILYA]

by the way, I looked at my 16-pin diagnostic connector, and apparantly it is a California model, as it has a button + LED already mounted on the connector... so, I understand how a regular code-reader works, but which pins do I bridge on my connector for the switch+led to work???

[RadioTek]

Beware: There are code readers and then there are Mercedes code readers.

[tecqboy]

I think if you do a search on this forum or the "other forum" for Baum Tool CS1000 scanner, you'll turn up a pdf file with a very nice list of all the diagnostic connectors and pinouts. These cars evolved a lot over the years and the early ones like my 1991 300SL do not have OBD I or OBD II. I think that started in 1995. But even within the first 5 years, the diagnostic connectors and procedures changed on many of the systems from year to year. You really need the manual for each particular year car, as the early cars have many diagnostic connectors scattered throughout the vehicle. When looking at info on these forums, you need to keep the model year in mind, as what is said in generalities about something may not apply to your car, particularly in the 129 series. BTW, the diagnostic connector for the soft top on a 1991 is located on the passenger side. You do not need a LED for this test as you read the flashes on the top accuator switch.

[deanyel]

Originally Posted by tecqboy

These cars evolved a lot over the years and the early ones like my 1991 300SL do not have OBD I or OBD II. I think that started in 1995.

A 91 300SL is OBDI.

[Real_ILYA]

I got the CS1000 manual already, kinda hard to read, but its a start...

I know where my connector is... I can make the LED tool to read the other codes (besides the top), however, my car is California model, and it already has an LED and a Switch built-into the connector - I am unsure however which pins to jump to tell the car which system I want to diagnose...

[Real_ILYA]

made some progress today... changed the battery, still nothing different... still no gauges...

never found that voltage protection relay... any info? also are there any other main relays?

figured out my X11 diagnostic connector - the California's LED and Switch *only* apply to the DM (Diagnostic Module, OBD) system... all other systems still need to be tested with a tool (either built or bought)...

I read the codes for the tops - was giving me #20 with hardtop on, I removed it now it says #1 - so everything is good... reset them anyway... still a weird issue - the soft and hard tops lock IN just fine with the switch, lock mechanism moves and clicks in... but they dont UNLOCK, and the soft-top is not moving with power, although works fine manually...

so yeah, lots of work still ahead... but then I drove on a winding country road for a few miles with the tops off (first time), and it made it all worth it...