Fuse 2 only hot when key is in accessory, run, or start position.
Hello Richard...
Looks like fuse #2 feeds your power seat relay.
See schematic for details.
http://www.autolib.diakom.ru/CAR/Mercedes-Benz/1992/400E/SYSTEM%20WIRING%20DIAGRAMS/4292.pdf
Thanks for responding. I should have given more info. The seats work fine. My issue is with power windows. None of the windows work, and from what I can ascertain, fuse 2 opens fuses A, G, and H which powers the windows. When I test fuse 2 hot side with ignition in run position, I only get a little over 3 volts. Do you have a similar diagram showing the window circuit or do you know where the wires going to hot side of fuse 2 originate? From what I can see under fuse box, there is a pink and a pink/yellow wire going to hot side of fuse 2. The short is on this hot side of fuse 2.
Your very welcome Richard... Fixya members love helping out.
Power window Group:
Ignition>Fuse 2/3 Fuse/Relay Box>Solid State Convenience Relay>Fuse G/H>Convenience Control Unit>power window switch.
I think what your looking for is the feed to the aforementioned Fuse Relay Box (From Ignition>control computer>Fuse/Relay Box.
I'll post a couple of links...one for the complete wiring index and one for your specific request for the power windows.
GL Richard.
POWER WINDOW CIRCUIThttp://www.autolib.diakom.ru/CAR/Mercede...
SYSTEM WIRING DIAGRAMShttp://www.autolib.diakom.ru/CAR/Mercede...
I connected 12v from battery to load side of fuse 2. all associated fuses (A, G, H) have power, but I cannot get power to switches. I can work the windows by closing contacts in the comfort module and they open and close. Tried checking connections in comfort module with lighted circuit tester. everything out of module checks ok except I get very low voltage on ground wire on A2. Changed comfort module window relay with power seat relay (seats working fine). It is the same 6 pin relay but window switches still do not get power. Is it a possibility that the ground wire showing low voltage is touching a hot wire somewhere? If so, I have no clue where that could. Also would that cause low power on fuse 2 hot side?
Yes Richard,
Poor ground connections on G302 could cause the switch to be under powered. G302 (Behind lower front of console) is the main grounding point for all the switches and seeing you have confirmed voltage from A,G,H a poor ground or
a problem with the Convenience Control Unit (CCU) should be your starting point. As all your window switches are not receiving power and the relay is not at fault I suspect the CCU may be your culprit...best guess. Check for solid clean ground at G302 and if that doesn't solve the switch no power condition check for
CCU diagnostic fault codes. Is this vehicle OBDII accessible?
Found some DTC info specific to the M119 Engine.
See Here:
http://www.w124performance.com/docs/mb/d...
it does not have obd but i have a pin blink code reader that goes in the 32 pin diagnostic connection
I didn't have time to go through the W124 DTC info..any help there? Diagnostics are often focused more on emissions issues and fail to alert you to things like a failing ABS pump..go figure.
Swapping parts out is often ineffective and always expensive so I hope the W124 provides some help.
PS: A hot short to ground would blow something
or in the worst case scenario melt some wires.
Have you done a thorough inspection of the wiring harness? and checked the plug connections on the CCU?
What do you think of the possibility of a defective "Convenience Control Unit" it seems to be the last component before the P.W. switches.
Hello Richard,
I've been reading some posts on Mercedes window issues and a lot of complaints are directing troubleshooters attention to the wiring bundle that passes into the door to the P.W. switches and motor. Apparently they are prone to breaking after several hundred or thousand door open / door close cycles. As all your windows are not working I find this solution suspect but shorts do weird things when solid state devices are involved...just a thought.
Okay Richard lets start over.
Fuse 2 is powered up by the ignition switch. I'm having no luck locating a power distribution schematic so I cannot confirm 3 volts is proper voltage to fuse #2 (But it is possible the Solid State Relay only requires 3 volts to activate the switch for the 12 volts feed to Fuse G and H) Fuse #2 powers up the Solid State Relay which in turn provides a switched 12 volts from the battery to Fuse G and Fuse H.
Fuse G and H provide power to the Convenience Control Unit which then provides power to the window switches and window motors. The window switch completes the circuit by bridging to ground G302, the interior lights are also tied into this circuit.
With the ignition on have you tested for voltage at the window switch itself? If no voltage is present at the switch but present at Fuse G and H your problem must be in the (CCU) Convenience Control Unit.
"I get very low voltage on ground wire A2."
If your getting any voltage readings on A2 from the CCU (I believe you refer to this component as the comfort unit) that to me, would be another indication that the CCU is likely defective.
I will check A2 again. Also, I do not know how to read in-depth schematics but I found some at this site, www.revbase.com
Again, I am a novice at all of this so I need to ask, If you need 12v to power the ccu, would you not need 12v starting at the fuse. I do not understand how 3v could create 12v downline.
By the way, I REALLY apprecite all the help.
i really don't know much about wiring but I found this site: www.autolib.diakom.ru
If you look under 1993 400e you can find power distribution diagrams. They should be identical to 1992 400e.
I looked at the diagrams and , to me, it looks as if fuse 2 powers the power seats , as well. The power seats are working even though it shows only about 3 volts. The other strange issue is that if I remove fuse 2 and probe power side I get about 3 volts, but if I touch power side to load side with circuit tester the power drops. I do not know how much but light in circuit gets vey dim.
This is driving me nuts!
Yes Richard I appreciate your frustation, it can get a little complicated and hard to understand automotive electrical trouble shooting.
When you find fuse 2 powering more than one accessory it is because the solid state relay
has multiple contact points feeding power to different components. Some features are powered up in the Acc/run/start ignition position (like the power windows) and some are always hot (like the interior dome light)
Consider the Solid State Relay as a switch that powers up other switches or circuits. Your Solid State Relay may only require 3 volts to do its job of closing the contact that provides 12 volts from the battery to the Fuses G and H, which supplies the power window motors with a full 12 volts.
The electrical schematics do not lay out all the functions of a particular solid state relay
in a single view (On a single page in the manual) as the drawing would become very complicated by showing systems you may not be trouble shooting...as in the case of fuse 2.
Seats, windows interior dome lighting...you are concerned with trouble shooting the power window problem so a page is dedicated to that system, free of all the other components the solid state relay provides power to.
With that said...you have tested for 12 volts at fuse G and H (G/H supply power to the CCU (Convenience Control Unit) so we can confirm by the presence of power at fuses G/H that the upstream Solid State 3 volt Relay and Fuse 2 are doing their job of making available a 12 volt supply from the battery to G/H fuses... which then powers the CCU (If the CCU is operational.)
Now that you have confirmed that the CCU has 12 volts available from fuse G/H the question now is...does the CCU supply 12 volts through its circuits to the power window switches?
Turn the ingition on, test for 12 volts at any one of your power window switches. If 12 volts IS PRESENT at the switch but the window still doesn't work the problem is not the relay or the fuses or the CCU..it is the power window motor, it's connections or the switch.(probably a motor if only one window is experiencing a problem) You can test the switch by bridging from one of the power feed wires up/dwn ect..to ground...if the motor then works, the switch is defective, if the motor still doesn't work the motor has failed.
When none of the switches are receiving a 12 volt supply you can be assured that the component tasked to control the power windows
functions is defective. In this case, the last component in the circuit...named in the Power Window Circuit Diagram as the "Convenience Control Unit" which is located "Beneath Left Side Of Rear Seat"
Confirming all plugs and connectors of the CCU are firmly seated and the CCU has ground contact through its A2 lead (brown wire) it is time to replace the CCU and fix your car. The job has been completed when your new CCU is installed and all your vehicles power windows are back in operation.
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