#15 fuse keeps burning out

[chenxinghao]

Last year my 1987 420 SEL started to burn out the #15 fuse after I was tinkling with the power antenna unit - the motor keeps jamming. I unplugged the power antenna unit, but the #15 fuse keeps burning out. I then checked all the light sockets in the trunk and rear, as well as reading and exit lights and observe no obvious short circuits. I have been driving the vehicle without the #15 fuse since. It runs fine, except that the overhead warning light keeps flashing when the engine is turned on, the central locking system is off, so are the door and head lights.

I need advice on where to look for the problems that cause #15 fuse to burn out.

[Don350SDL]

There are about a dozen consumers on that fuse. You need a wiring diagram to really hunt it down - buy the service CD for $20 from MBUSA (see the sticky thread at the top of the "Technical Help" forum.

Also search the forum for a thread about "troubleshooting circuits that blow fuses". That'll get you started.

Don

[brandlj]

My 86 300sdl was in the same condition, doing the exact same things and it was a rear exit light in the bottom of the door shorting out. The light was cracked and it shorted itself to ground. I replaced the exit light with another and replaced my fuse and all is well 2 years later.
John

[chenxinghao]

Got wiring CD and installed and read the wiring diagram; checked every door light and other light fixtures fed by the #15 fuse, and found no short circuit evidence. The wiring diagram indicates that the #15 fuse also feeds the Central Locking Control device/unit, for which I am unable to locate it. One of the pictures points the location of the unit in front of the front passenger seat. I checked the engine compartment area on the passenger side: there are a number of small black boxes near the firewall and where the battery sites. But they all seem to be relays, and unlike to be the "Central Locking Control" unit which I guess would have a larger connect with many wires.

I need advice on whereelse I should look for the Central Locking Contrl unit and some way to identify it (code on the box, size, anything that may of help).

Thanks,

[Robert Goodwin]

If the unit is in front of the front passenger seat, you need to remove the carpeting (all of it) on that side. Under the carpeting you will find a large black panel that works as a footrest and a cover for several modules. Near the bottom of that panel is a 10 mm fastener. Undo the fastener and remove the panel.

You should now be looking at the component you have been searching for.

[Don350SDL]

Robert, I think you are describing the alarm system; the central locking pump is in the trunk, right rear corner of the spare tire well.

Also see this post (http://mbca.cartama.net/showthread.php?t=17663) for further techniques

Don

[Robert Goodwin]

Don,

I took the man at his word. He said the unit was forward of the front passenger seat.

He then described the firewall area under the hood where he couldn't find it.

The only place he could have been referring to was under the passenger footrest.

I can't imagine where the writer got his component name and location. I drive a W126 and other than the door actuators and fuse, it is all in the spare tire well in the trunk.

[chenxinghao]

Thanks for the feedbacks.

I did check the vaccume pump in the spare wheel well in the trunk. It gave no sign of a short circit. I unplug the electric wires from the pump unit and the #15 fuse contact still measures 2 ohms resistence to the ground.

The wiring diagram shows the #15 fuse branches out to a device it named as Central Locking System unit. Below is the list of things the #15 fuse feeds (according to the 2 wiring diagram:

#15 - Trunk, rear dome light, reading & exit lights, automatic antenna, central locking system, seat adjust memory.

I checked every thing else, except the central locking system unit.

Is footrest panel also known as (front passenger) side kick panel? I will take a look at it this weekend.

Many thanks,

[Robert Goodwin]

The answer to your question is yes, but I still can't figure out where you are getting your information.

I have attached a copy of the fuse box list and the wiring diagram for fuse number 15 and none of it matches what you are saying.

Fuse box attached here. Diagram on next post. Doggone size limits on attachments you know.

[Robert Goodwin]

Here is the diagram.

F 15 is third fuse to the right.

[chenxinghao]

The information is all from the software. At the secontion "Fuses $ Circuit Breakers (1986-93 Mercedes", the units fed by each fuse is listed. For the #15, here is the exact text:

#15 - 8 Amp
Trunk, Rear Dome, Reading & Exit Lights, Seat Belt Presenter (Coupe), Automatic Antenna & Central Locking System, Front Backrest Lock Valve (Coupe), Seat Adjustment Memory

In my early notes, the Coupe related units were taken out.

In my case, the central locking function is not working - I now use the key and the up-down pull notch near the windows to lock the doors (and unable to control the gas filler door). It used to be that I could use the key to open/lock one of the doors or the trunk, which would automatically open/lock other doors and the trunk, as well as the gas filler door. Except the #15 fuse, all other fuses are intake.

I have the wiring diagrams on the #15 feeding the power antenna unit, the power door lock unit, the seat memory unit. But I have not find the diagram on #15 feeding the courtesy lights and the trunk light. In the power door lock circuit diagram, the anti-theft system is fed by both the left and right front door actuators, the supply pump unit and the trunck/tailgate lid lock actuator.

Looking over your attached wiring diagram, the fuse (#13) next to the #15 at right feeds the Central Locking System. My #13 fuse is fine, but the central lock function is definitely not working. I am confused ...

Mine is a 1987 420 SEL.

I appreciate Don's posting of the methodical way of diagnosing short-circuit problems. I recall that at the beginging of diagnosing my #15 fuse problem, I took a couple hours staring at the main fuse box, tried a few simple steps and could not get the main fuse box detached from whatever is under it. If everything else don't work for me, I will need to find a way to lift the main fuse box and following Don's methodical way to trace the problem(s) down. Any detailed description on how to physically disengage the main fuse box would be very helpful.

Thanks,

[Robert Goodwin]

Exactly what software are you using?

My source is the Model 126 Service Manual Library on CD from Mercedes Benz.

[chenxinghao]

The MBCA site no longer offers the service manual for the W126 models.
The MB Maintenance Manual (USA version) I bought on eBay for $45 doesn't include electric wiring diagrams. I then waited on eBay and purchased the software (claim to be a genue MB software for my vehicle) on eBay - I think it is not the MBCA version, because on some sections the printouts are overlapped.

I am still monitoring on eBay for an MBCA version, as Don has suggested early.

For now I work on it with what I have - better than nothing. The wiring diagrams and the color code of the wires seem to match with my vehicle.

Based on your attached wiring diagram, what I don't understand is that my hazard lights and radio are working fine, so the #13 fuse must be fine too. But the Central Locking System is not. So I may have two electric problems: #15 fuse burns out and the #13 fuse wire to the Central Locking System is open. But what and where is this Central Locking System? Is it in the case with the Supply Pump unit in the spare whell well?

I measured the 12v power feed line (RED/WHITE) at the pump and the #15 fuse outgoing terminal and the wire seems to connected fine.

Thank you very much for all the inputs,

[Robert Goodwin]

Ok, lets solve this puzzle.

Using my fuse location diagram, go first to the trunk.

Remove the light bulb for the trunk light. It is near the rear windshield on the trunk lid.

Next, remove the electrical connector/s from the automatic antenna at the antenna.

Now remove the front passenger footrest under the carpeting. Unplug the module all the way to the left. It should have 2 plugs. Part number on the module is 126 820 24 26 or 126 820 04 26.

Next open all the doors and the hood. in each of the door jambs, squeeze and pull out each of the switches. There are 2 for each front door and 1 for each back door.

4 of the six are for courtesy lights and the other 2 are for the now disconnected alarm system. Inspect the switches for physical condition and broken or exposed wires.

At this point, install a new 8 amp white fuse.

If the fuse blows, I would be looking at the courtesy lights and wiring, as everything else on the circuit is disconnected except the connecting wires ( a remote possibility).

If the lights now work, push all the switches with brown wires only back in and test again. If the lights quit, you are close to the problem.

If they still work, reconnect the antenna cable/s. Try that. Turn radio on and off. Raise and lower the antenna. If it blows the fuse the antenna motor is a likely suspect.

If OK, put the 2 connectors back on the burglar alarm system. Same drill. Fuse blws, there is a problem.

If everything works well after this and only the fuse has been replaced, hang a very large piece of garlic from the rear view mirror to ward off evil spirits in the future.

Let is know how the troubleshooting part of this works out.

[chenxinghao]

OK, did everything as you said. At the step where you said to insert an 8 AMP fuse into the #15 slot, I measure the ohm value instead. It is measured 0.5 ohm!
This would lead to at least 24 amp current going through the #15 fuse - there seems no point to place an 8 amp fuse at this time.

All 6 switches at the door-jams, light housing units at the door bottoms, the switch on the trunk lid (the bulb was taken off) were pulled out and examined. No burned or sparked signs at all. The 2 black plugs to the alarm unit were unplugged as well.

I noticed that your procedure did not ask to do anything with the vaccum supply pump unit in the spare wheel well.

There is a paper template on the back of the main fuse box cover. I made a scan of it, but don't know how to insert it here.

(Don't know if the following would get the scan into this posting.)
[C:\Documents and Settings\Xinghao Chen\My Documents\My Folders\HomeUniverse\Automobile\MyMB420SEL\87MB420 SELfusebox.jpg]

The fuse box paper lists the central locking system under #15 fuse.
Is the alarm unit (that you asked to disconnect) considered as the central locking system?

[Robert Goodwin]

My error. I posted the fuse box for a 1981 W126. The correct one is attached below and has a lot of stuff on F15 including the central locking system.

Central locking and alarm are 2 different systems.

Go in the trunk. Lift the cover for the spare tire. In the right rear corner is a rubber surround for the central locking system vacuum pump. Remove the whole thing and pull the unit out of the rubber housing. When done, remove the electrical connector/s from the unit.

Now go back and check the fuse, current or resistance as you wish.

[chenxinghao]

Disconnected all 3 plugs with the vaccum supply pump unit. Not change to the #15 fuse load (still at 0.5 ohms). Attached is the scan of the fuse box paper.

What is this central locking system then?

Thanks,

[Robert Goodwin]

This is a snippet of the 11 page description of the Central Locking System.

It is made up of a vacuum pod in each door with the front door units also having a switch that turns the pump on and off, fuel door pod, trunk lock pod and the vacuum pump in the spare tire well.

I looked back at your first post and have to ask if there is a posibility that the antenna wires got pulled hard enough for the insulation to haver been damaged.

If so, it might be wothwhile to pull the radio and disconnect the connector/s from the rear of the unit and see what that does.

[chenxinghao]

The radio set is not the origial, but seems to be a pull-out replacement with a handle in the front. I removed the radio, no change to the #15 fuse load resistence mesure still at 0.5 ohm.

I checked the connector to the automatic antenna and open its back/top cover: every pin and its soldering seems fine. Then traced back with the connector wires, which are in a large plastic tube - no visible damage can been seen. I recall no strong pulling of the wires, as there is sufficent length for disconnect the plug on top of the automatic antenna and not to apply any pysical force to the wires.

[Robert Goodwin]

That leaves all the rest of the circuits for that fuse to be tested. You are going to need the manual (Service Library) to continue.

By the way, do your front seats operate properly?

How about the seat backs?

Seat belt indicator on the dome lamp?

[chenxinghao]

The power seats function perfectly with all 3 switches and for both front seats, except that the seat memory units would keep the stored positions when the key is in the ignition. After the key is pulled out, stored position would get lost.

The seat belt indicators on the doom light flashes all the time when the engine is runing.

Don's procedure is mine last resort, because it is not obviouse to me how to remove the main fuse box. Also, while I am waiting for a genue MBCA service manual for a 1987 420 SEL model for sale, I will need the wiring (or output, power distribution) diagram for the main fuse box, so I know which branch feeds what components. The one posted within this thread is of a 1981 300 SDL (I believe), which I am not sure is the same for a 1987 420 SEL.

Late last year there was a 1987 420 SEL main fuse box for sale on eBay, but someone out bitted me at the last minute! I need to understand how the output wires from the main fuse box are connected - plugs or hard soldered. I can see 2 srews holding the main fuse box down. But after taking these 2 screws out, I still cannot move the main fuse box. Don's general diagnose procedure is methodical, but it requires to have acces to the output contacts at the main fuse box.

Many many thanks to you and Don for for all the inputs,

[brandlj]

Did you just say that your seat belt warning light flashes all of the time with the car running????? Go under the dash and unplug the seat belt warning relay. It is attached to the cruise control computer box. My seat belt warning light stayed onall of the time and I had a burned out path in the seat belt relay. This burned out circuit kept my light flashing, the lights on buzzer would not work and my seat memory kept losing its mind. I opened the relay and found the burned area on the circuit board and I bypassed it with a small wire that I soldered in and I have had no problems ever since. My other symptoms were no dome light on when opening the drivers door, but it would work from the passenger front door.
John
86300sdl

[chenxinghao]

Which side (drive or passenger)? What does the seat belt relay look like? I don't know the look of the cruise control unit. Yesterday I opened the front passenger-side kick panel and did some work with the alarm unit (identified with the code provided by Bob in this thread). As I recall, there are 6 to 7 components/units behind this kick panel. I have not looked at the driver-side kick panel.

Thanks,

[brandlj]

Driver side under the dash. To the left of the steering shaft mounted up underneath. The cruise control amp is a silver box and atteched to it is a black rectangular relay. The black rectangular relay is the seatbelt and headlights on warning relay. When you have your lights on and turn your car off, and open drivers side door, does the buzzer sound to let you know you have left your lights on? I recall a whole lot of stuff goes through that little relay and it can cause a multitude of electrical problems when it is flaky.

[chenxinghao]

I couldn't find any relays behind the styeering column and much of the front-driver kick panel.

The warning buzzer seems to work fine - if I leave the headlights on without the key in the ignition lock cylinder or left the door open with the key in the ignition.

Are you sure you are describing a 1987 420 SEL and not some other models? It seems that my '87 420 SEL has its cruise controller box behind the front-passenger kick panel, a silver box without a relay but its connector at its end.

[Robert Goodwin]

Try these. Item 5 is the CC amp in CC.jpg:

[chenxinghao]

Thanks for the pics. I will take another look under and behind the steering wheel this weekend, as I am traveling.

I don't see the #15 fuse feeding the seat belt warning relay in the wiring diagram. How could it cause #15 fuse to burn out?

[Mediumkahuna]

I have the exact same problem.
My car is a W124 but I think the setup is simular.
My wiring diagram shows everything from the fuse runs thru a connector block: X30
which is behind the driver's side kick panel (which is why it looks to be in engine compartment some drawings). The circuit divides 3-4 ways at X30 so, if you could get at it, you could narrow the search a lot or least disconnect the bad leg and have more stuff working.
I'm now trying to figure out how to get the kick panel off.
Can't find it in my manual.

Check your wiring diagram and see if the circuit runs from the fuse to connector block then divides. Look up location of connector block. If the connector block is behind drivers side kick panel we're in the same boat.

[chenxinghao]

The wiring diagrams from my third-party manual CD (unable to get the MBCA version for W126) show many connectors/dividers from the #15 fuse feed. But the cheap manual dosn't code the connectors/dividers and doesn't describe where they are located.

The original MB Engine/Chassis/Units CATALOG A book (came with the vehicle) has drawings showing the wiring connectors with codes. But the book doesn't show the individual fuse feeds and I have not been able to identify the connectors with codes that are in the #15 fuse feed. Also this book doesn't say where these connectors are located - it is a collection of drawings only.

[Mediumkahuna]

...original MB Engine/Chassis/Units CATALOG A book (came with the vehicle) has drawings showing the wiring connectors with codes...
Yeah I've got that book too. There's a picture in there of the layout of some of the wires in the circuit. Its the one that goes to a bunch of interior lights and the power antenna.
It shows the wire leaving the fuse box and going down then along the left side of the car (as well as across the car etc). It would have to pass thru the "behind the kick panel" area to do that. So, its a good bet you have a terminal block behind there but I haven't figured out how to get the panel off anyway so I devised another plan:
I connected an old head light to the battery then connected what would be the head light ground to the load side of the fuse. This lights the head light and feeds about 4 amps into the circuit. I also connected a battery charger to my battery to keep the voltage constant.
This is enough current to make a slight voltage drop in the circuit wires. I checked the voltage to ground at the fuse (about 2 volts in my case). Then I checked the voltage to ground at all the places where I can get to the circuit. The lowest voltage is nearest (by wire) to the short.

Also if you can get the kick panel off you can probe around for something with the same voltage as the fuse. No other circuit will have 2.06 volts (or what ever it is in your case) on it. So you can find the terminal block that way.

[chenxinghao]

Nothing much behind the front driver kick panel - there is no panel to take off. There is a switch under the brake pedal - kick-down switch.

Did try to get to the under-dashboard behind the steering column. However, after losening the gray front section under the dashboard instrument cluster, a big blake tube, 5 or 6 inch in diameter, blocked the way - could not find way to get behind (nor take off) this large blake tube placed in horizontal for the entire section underneath the instrument cluster.

Peeked through a tiny opening and looked behind: there are many RED wires and part of some other stuff (cannot tell what). These are all seem to be behind the instrument cluster.

[chenxinghao]

Paid $50 for a photocopy of MB's Electrical Troubleshooting Manual for Model 420 SEL 560 SEL SEC (86-91); Model 300 SE SEL (88-91); Model 300 SDL (86-87) and Model 350 SD/SDL (90-91). It includes wiring distribution charts, connector codes, as well as photos of locations. Very detailed. Now I have someting to work with in resolving the #15 fuse problem.

Stay tuned and I will be back with my findings.

[Don350SDL]

Paid $50 for a photocopy of MB's Electrical Troubleshooting Manual
Pity. You could've downloaded the .PDF files of the same (and a whole lot more) from their web site http://www.startekinfo.com for under $20. Live & learn, I suppose.

Don

[chenxinghao]

The site provides repair information for model year 1994 and higher. They don't seem to have repair and diagnose manuals for my 1987 420 SEL.

I spent a couple of hours studying the electrical wiring and connector charts. There is a connector (X11) right after fuse #15 (which has a couple of branches going somewhere else). This connector is said to sit behind the driver's left kick panel. It is not obviouse to me on how to remove the driver left-side kick panel, as there are no screws or the like. Having learnt a few lessens with damaging a few fixtures before, I better ask around and understand how to take it off before applying any force to it.

[Mediumkahuna]

Nothing much behind the front driver kick panel - there is no panel to take off.

Its the panel on the left side of the drivers foot area behind (left of) the Parking Break Pedal. Its the plastic piece you rest your left foot on in my car.
It covers the left side of the drivers foot well.
I finally got mine off, found the terminal and fixed my wiring problem.
It was my trunk (well tailgate) closing assist motor.

To Remove the panel:
Remove driver's door sill cover.
Pull out that stuff that goes around the door starting at forward end of sill.
Remove screw in that area.
Remove black plastic retainers under carpet behind brake pedal
Remove drivers left kick panel by sliding it back toward rear of car.

Once I got that off I found the terminal block. Here's a picture of it:
http://homepage.mac.com/jud/posts/x30.jpg

Here's a close up of the of the terminal block with one plug (the bad circuit in my case) pulled:
http://homepage.mac.com/jud/posts/x30plug.jpg

Pull the plugs in the terminal block, one at a time, til your short goes away.
My plug had 2 wires going to the pin of the bad circuit so I unsoldered them and figured which wire went to the short.

Here's a picture of the kickpanel and the cover for the terminal block:
http://homepage.mac.com/jud/posts/driverleftkickpanel.jpg

Good luck

[chenxinghao]

Thanks for the pics - they help a lot so I know how to take the panel cover off. I will get back with my results related to the #15 fuse problem.