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SL320: Fuse #3 in boot blows constantly

3K views 15 replies 8 participants last post by  bobterry99 
#1 · (Edited)
Hi guys, hoping for some help with my SL320.

In the boot there is a fuse rail with 7 fuses for the roof, windows, central locking etc. and number 3 persistently blows no matter what size fuse is put in place, or even if the key is in the ignition. I have disconnected the roof controller box under the rear seat and that stopped it from blowing. I replace the first connector (towards the centre of the car, heading to the front) and nothing happened with the other two disconnected. No fuse blow. I reconnected the large oddly shaped connector and the same again, nothing happened. Immediately when connecting the last connector from the boot to the module, it blew instantly.

Are there any common shorting issues or other problems that could cause this?

I have removed and examined the roof control box and it looks perfect. No signs of burning, bad smells, corrosion. Thing looks like it was just taken out of a packet.

Apologies in advance if I'm missing something obvious, it's probably a brain.

~~~EDIT~~~

I should also mention that I have no history on the car, it belonged to my late father and he said the roof didn't work. The button just flashes, roll bar does not work and the windows don't automatically lower on opening the door.
 
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#3 · (Edited)
Maybe somethings grounding out on the plug lead,wires, or bad module, those are not cheap, may be an outfit that can test it and fix it but forgot who, someone will chime in.
forgot, there's all the limit switches for top operation, pretty sure the module talks to them, not sure if a shorted switch could cause issues.
 
#5 ·
...number 3 persistently blows no matter what size fuse is put in place...
On a '93 this is one of the two fuses for the top controller. The controller switches power to the windows and the roof hydraulic solenoids through the #3 fuse. The switching elements internal to the controller are relays for the windows and probably solid state devices for the solenoids.

Since your fuse blows only with the top controller connected this indicates the controller is faulty. My guess is one or more of the solid state devices has failed and is shorting to ground. If so, this could be an easy fix for someone with soldering skills and the ability to use an electronic multimeter.
 
#6 ·
Gents
New to this forum. Hope you can help.
Am drawing 67mA on fuse 3 (trunk). On disconnecting Connector X on the roof top controller, the current drain disappears. I recall from my tests that disconnecting connector Y and Z made no difference. (need to re-check this).

I have checked resistance to ground of the solenoids from connector X (pins 3,4,5,6). They all read 12 ohms suggesting their resistance is fine. These are all located on Valve block Y56 (right hand side of the car).

Roof top light flashes but error codes are 28 and s9 suggesting no Speedo signal and no axle speed sensor. Front abs sensor resistance reads fine, have yet to tackle the rear differential sensor.

My conclusion is:
Drain may be caused by faulty relay inside the roof top controller albeit it functions OK for opening and closing the roof.
I believe the flashing roof switch may not be connected to the current drain (but am open to this being related)
I have not checked pin 7 which goes to test connector X11/12. Nor have I checked pin 8 associated with roll bar. They also may be source of the drain.

Any top tips?
 
#7 ·
Just a quick update on my 300SL 24v 1991.

Current drain only happens when the Y connector and Z connector are connected. When Y connector is removed, leaving only X connector on the RST controller, the problem goes away.

Interesting, when the drivers door is open, I now read 72mA constant current (previous 67mA - but it is colder now).
When drivers door is closed, current fluctuates from 89-92mA.

My instinct would say that with the door open, and noting the roof controller takes a signal in from door and trunk actuator circuit, I would have thought it would draw more current with door open. Instead it draws more (and fluctuates with door shut)). May be an earth problem. However this does not answer why it draws current when ignition is off.

So in summary, something associated with the Y connector is causing the current drain. Any ideas?
 
#8 ·
When Y connector is removed, leaving only X connector on the RST controller, the problem goes away.
Is the Z connector removed? Also, I would wait for a little more than an hour after ignition is switched off and then take current measurements.
 
#9 ·
BobTerry99
Thanks for the reply. Sorry for confusion in last post.

Z connector is connected to the box. This carries the main power line from fuse #3 in the trunk hence has to be connected in order to see the problem.

X connector is disconnected.

Problem of current drain goes away when Y connector is removed leaving only Z connector still connected to the box.

Ignition has been off for a long time (ie days). I do not believe there are any other systems waiting to shut down.

Theory 1
My suspicion is that a faulty component tied to the Y connector is causing the Roof controller to draw current. Recap from earlier post, I am reading Code 28 (No speedo signal) and Code 29 (Axle Vehicle speed sensor signal). The VSS (Vehicle Speed Sensor) input to the Roof Top Controller on Pins 9 and 35. I will perform a check later, but believe my front ABS sensors to be OK based on coil resistance. Have not been able to get to the rear sensor yet (assuming there is one). I may be able to disconnect sensors and see if this stops the current draw. This is one possible theory.

Theory 2
When I check for voltages on the Y connector to see what systems are live, I read 12.5V on pin 13 (to ground) of the Y connector (not the Roof top controller). This signal is termed "Door and Trunk Lid Actuator Signal Input". However this reads 12.5v when the door is both open and shut. I also read 1.5 Volts on Pin 14 which is not actually used! As an input to the Roof top controller, I assume it takes a signal in from the doors and trunk (indicating they are open) however opening and closing the door makes no difference to this Voltage. Soft top controller test procedure suggests on pin Y13 "lock via central locking system <10 Ohms". Resistance to ground here is open circuit. This is another line of enquiry.

One plan is to try build a break out box with an old Bosch Connector so I can connect the whole system and do a line by line disconnect to isolate the culprit. However, before I do this, I would welcome any answers any members may have.
 
#11 ·
Inputs Y13 and Y14 are used by the roof ECU to implement the feature of closing the windows via a keyswitch or the RF remote.

When I check for voltages on the Y connector to see what systems are live, I read 12.5V on pin 13 (to ground) of the Y connector (not the Roof top controller)...However this reads 12.5v when the door is both open and shut.
Y13 reads the locked status of the left-front door. Apparently if the door is locked the voltage on Y13 is zero; unlocked it is battery voltage.

I also read 1.5 Volts on Pin 14 which is not actually used!
If you look at a schematic for the locking system you will find a signal labeled "SN1", and I believe it is wired to Y14, though it is not shown on any Mercedes schematic of mine. Years ago I found that if a keyswitch were twisted and held or if the button on the RF remote were pressed and held Y14 would switch to ground for as long as the key/button was actuated.

It's just a guess, but I think your problem lies with the roof ECU. Installing your ECU in another vehicle would prove the guess. Alternatively, you could power-up the ECU outside the vehicle and monitor the supply current as you replicate the state of each of the Y-connector inputs one at a time. You may discover that grounding one of the inputs that would be wired to a limit switch causes the current to spike.
 
#12 ·
BobTerry99,
Quick update. Did a full resistance (measured to ground) and voltage check on connector Y, with Y connector detached from the control ECU.
Probing the Y connector, all the limit switches were reading either open circuit or zero ohms. Roof top switch and Roll bar switches all zero ohms. These seemed to be in the correct state.

The following caught my eye (all measured on connector Y):

Pin 13 (Door and trunk lid actuator signal input) 11.9 Volts with doors unlocked. Sitting inside the car and manually locking the doors, this drops to zero volts just as you had suggested. Did a further check to confirm that even when the doors are locked, and all the connectors are back in place (X,Y,Z), locked doors did not reduce the current drain. Still reads 72mA.

Pin 14 (not used) reads 1.5V and 9kOhms
Pin 15 (not used) reads 0.1V and 2kOhms
Pin 32 (malfunction warning signal output) 0V and 0.9KOhm
Pin 35 (Vehicle speed Sensor Input) 0V and 7.5KOhms

I then did a voltage check on the pins on the Roof Top Controller ECU Y socket (on the box itself).
Pin 15 read 10 Volts.

I then read the current flowing across Fuse #3 in the trunk with only pin 15 connected between ECU and connector Y. Current draw reads 47mA.

So the mystery deepens...

The ECU does not use Pin 15 but it produces 10Volts
The cable harness reads 9kOhm

Current draw read at the fuse reads 47mA indicating the bulk of the drain current is from a pin that is "not" used.

Is it time to open up the ECU and have a look?
Do I have the correct circuit diagram (mine is PE 77.39-U-2000-99YA drawing for part no 1298200097)?
 
#13 ·
I am certain you have the correct schematic, though the one I have is identified as 77.39-U-2000YA.

I'm interested in seeing what voltages I have on pin/socket #15 in my cars. That will have to wait at least a week for the temperatures to warm, however.

If the 47mA current you measured on #15 shows a problem, then I think it would be worthwhile to open the controller. I presume you would discover that #15 connects to a pin on an integrated circuit with just a few components in its signal path that you would check.
 
#14 ·
Something to possibly check is the wiring routed around/near the roll bar. Mine had frayed slightly and a single strand of the wire bent across just before entering a connector and shorted to ground... It was a devil of a problem to troubleshoot but a really simple fix.
 
#15 ·
Gents
Quick update.
Took the controller apart last weekend and did a check for any shorts/blown components.
All circuits look fine.

What is interesting is that whilst pins 14 and 15 on Y socket are stated as not used, they are all connected back to a surface mount IC "M82C43".

IC M82C43 is passing some current through a surface mount diode causing pins 14 and 15 to read 10 Volts. (note previous error where I only though pin 14 was reading 1.5 volts).

I am not sure whether pins 14 and 15 are supposed to be like this and it is a short on the wiring loom (or equivalent) is causing a current draw.

Can anyone do a quick voltage check (to ground) both on these pins on the Y socket on the controller? Note you only need to have connector Z connected.
 
#16 ·
What is interesting is that whilst pins 14 and 15 on Y socket are stated as not used...
In the electrical schematics Mercedes' stating pins are "not used" is ambiguous, since it means they are not used in the system detailed by the schematic; however, they may be used in another system. For example, on the roof control schematic pins Y5, Y6, Y7, and Y8 are described as not used, but they are certainly used -- they are the inputs from the window controls switches.

Can anyone do a quick voltage check (to ground) both on these pins on the Y socket on the controller?
With a battery supplying the controller with 12.7 volts I measured 11.5 volts on both Y14 and Y15.
 
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