124 pops window fuse as soon as key is turned - PeachParts Mercedes-Benz Forum

muleears · #1 05-06-2007, 07:39 AM

I've been trying to fix up the small items on a new to me 124. Current problem is blowing fuse that powers RF and LR windows. The fuse blows as soon as the key is turned to the acc. position. I realize I have a dead short somewhere but how do I locate it? I tested the switches on the one window that works (LF) and they all worked. A search suggested I look at the wiring in the door jamb for broken/chaffing wires, but it seems ok. Any ideas on where I should look for this short? Thanks in advance.

Ramblin · #2 05-06-2007, 10:29 AM

The only way to track down shorts is to one by one disconnect the suspected devices that can draw that kind of current.. I suppose it could be wire chaffing but it is more likely that one of the two window motors is bad. You could start by disconnecting the window switches for each door separately and seeing if that cures the problem and then once you have isolated it to the door go ahead and take the door panel off and disconnect the motor itself. If disconnecting the motor does not stop the blowing of the fuse the window switch would be the culprit however I think that is unlikely unless the switch physically feels different than the others..My 2 cents...

barry123400 · #3 05-06-2007, 11:01 AM

I take my limited electrical knowledge for granted so I have to be careful when suggesting things. You might take a small 12 volt bulb or test light and extend the wires from your fuse holder. Placing the bulb where it is visible. This could be done by soldering the wires to a bad fuse and installing it.. You probably have a few by now anyways. .

Then start disconnecting things. When the bulb extinguishes you have your shorted area indicated. Just keep moving along that line and disconnecting further along until you get to it. Of course re connecting what is behind to illuminate the bulb as you proceed. Should work out and pretty straightforward. If nothing else it will really tend to localise the problem.
Another option is to use a small meter. It will indicate you are grounded until you separate the shorted item.

Ramblin · #4 05-06-2007, 11:09 AM

Upon thinking about this some more I realize that if the window switches are disconnected and the fuse still blows it cannot be the motors as they would be disconnected from the circuit thereby eliminating the motors. The switches are merely a reversing type of switch reversing the polarity of the motors and when the switches are at rest position there is no connection to the motors. Given that logic it would be correct to look elsewhere for the problem...Barry is correct in his application of the light as tracking down shorts (vs. open) is difficult and the only way to do it is to eliminate all things in the circuit one at a time till the short does not occur.

babymog · #5 05-06-2007, 12:09 PM

It's not a motor.

The motors only get power when you press the switch.

Pull the switches: Remove the carpet in the center-console bin, remove the now exposed screw at the rear of the zebrano panel, swing the panel up and forward and remove. Unplug the two switches that blow the fuse. Turn on the key. If the fuse stays, plug in the switches one at a time to see which one is bad. If the fuse blows with the switches out, it is a wiring short (unlikely unless someone has damaged the wiring, run any new screws lately?).

Carson357 · #6 05-06-2007, 02:37 PM

i had a similar issue a few years back on a customers car, i don't know if the cigar lighter is on the window circuit in your car, but their window issue had something to do with it, that problem turned out to be a nickel from the ashtray change collection had fallen into the lighter receptical.

dieseldiehard · #7 05-06-2007, 05:51 PM

124's tend to have shorts in the wiring where it passes out of the door pillar into the door thru a flexible rubber cover, its usually the insulation that is cracked apart, the same thing that causes wiring harness failures.
If your windows weren't working or started to be flaky then its for certain that the wiring has deteriorated there and this could be worth checking before you tear into the console, etc..

jbach36 · #8 05-06-2007, 06:01 PM

I had a problem once with my radio being constantly on, even when the car was turned off. Turns out it was because of the fuse that looked good, but it wasn't. It was the older metal kind of fuse, pitted over the years, though all in once piece. A mechanic told me to get rid of all of them, replace them with the new copper ones. Resistance on one fuse can cause other unrelated problems. To replace them all is only like, $8, so do them all.

jeff 1991 300d, 105k

babymog · #9 05-06-2007, 07:00 PM

Quote:

Originally Posted by dieseldiehard

124's tend to have shorts in the wiring where it passes out of the door pillar into the door thru a flexible rubber cover, its usually the insulation that is cracked apart, the same thing that causes wiring harness failures.
If your windows weren't working or started to be flaky then its for certain that the wiring has deteriorated there and this could be worth checking before you tear into the console, etc..

The wires to the (front) window motors don't have power until you press the switch. Rear windows are different, depending on the position of the child switch in the console.

Bio300TDTdriver · #10 05-06-2007, 10:04 PM

Quote:

Originally Posted by dieseldiehard

124's tend to have shorts in the wiring where it passes out of the door pillar into the door thru a flexible rubber cover, its usually the insulation that is cracked apart, the same thing that causes wiring harness failures.
If your windows weren't working or started to be flaky then its for certain that the wiring has deteriorated there and this could be worth checking before you tear into the console, etc..

I voter for the wires in the pillar too.

Chris

dieseldiehard · #11 05-06-2007, 11:14 PM

Quote:

Originally Posted by babymog

The wires to the (front) window motors don't have power until you press the switch. Rear windows are different, depending on the position of the child switch in the console.

Rears are hot with the key switch on.

Bio300TDTdriver · #12 05-07-2007, 12:59 AM

Pull the rear door panel. Free up some slack in the wires. Pull the rubber piece between the pilar and the door. Look for a bump in the wires. Cut the casing and repair the wires. Reverse steps. Too rule out the motor, disconnect the wires from the switch. Apply 12 volts to motor.

Chris

muleears · #13 05-07-2007, 04:48 AM

If I place the child safety switch in the on position (so rear windows are inoperable) and this doesn't blow a fuse, they I have isolated the problem to the rear window, correct? I have pulled the rubber boot from the joint between the B pillar and the rear door and nothing seems damaged.

dieseldiehard · #14 05-09-2007, 03:33 PM

Quote:

Originally Posted by muleears

If I place the child safety switch in the on position (so rear windows are inoperable) and this doesn't blow a fuse, they I have isolated the problem to the rear window, correct? I have pulled the rubber boot from the joint between the B pillar and the rear door and nothing seems damaged.

Correct, now you are doing a great job of troubleshooting. Did you remove the rubber boot on both sides? The child switch disconnects both the L and R side rear window power, either side could have a short (I bet its the R side that fails most often)

Huck · #15 05-09-2007, 06:06 PM

Don't want to hijack anybody's thread but I have a window issue on my 93 300D 2.5 Turbo. Sometimes none of my windows or sunroof will work until I lower my driver's front window. Then all the windows will work. The driver's window will not come back up until I cut the car off and restart it. I assume I should start in the harness in the door jamb? Any other ideas before I tear it apart?

Huck