Yet another electrical woe, blowing fuse No. 3 - PeachParts Mercedes-Benz Forum

BodhiBenz1987 · #1 08-08-2013, 11:17 PM

I just finished my wheel-well rebuilding project yesterday and was very excited to get the car on the road. Until this evening, when I entered the customary electrical problem followup stage of what seems like pretty much every repair now. I drove it about 20-30 miles with the lights mostly on earlier in the day without issue, but in the evening, the lamp-out light came on. I found the following out: right parking lamps (front and rear), right tail lamp, left rear parking lamp, warning buzzer, instrument lighting and CCU lighting (basically all interior lighting). Sure enough Fuse 3 had blown. I replaced it, all OK for a couple of minutes but as I went to pull out of the parking lot (where I had been tinkering), it blew again. Probably about five minutes in there. Drove home half-lit, it wasn't quite dark out.
My first inclination was that there was some damage to the wires for the front side marker, since I'd pulled everything out of the front wheel well area and had to move those bundles of wires around. I took it all back apart and don't see any damage. I replaced the fuse again and it isn't blowing now, but I know something's wrong if it blew twice. Too much to be coincidence right after working in the area with all the wiring.
One other thing, possibly coincidental but making me think. I seem to get no voltage to the monovalve regardless of CCU setting (constant hot air ... I wired a light bulb to the monovalve harness and get no light). This was not an issue before this repair (it did actually happen once before, but stopped after a day, a year ago). The fuses related to that seem fine, but is there anything I could have disturbed heading to the monovalve harness? And is there any possible connection between that and the fuse 3 problem? Could a CCU problem cause both? Or maybe it's coincidental ... as mentioned, it did happen, and correct itself, before.
I'm trying to rule out any peripheral issue before I go to the trouble of pulling absolutely everything back out of the wheel well and cutting all the wire sheaths away ... if it's a damaged wire, it's somewhere up in there. Considering not much moves in that area, it seems odd to me that a wire would break even with me pushing them around slightly (no repeated bending like in door jams, you know?).
One more odd thing: I lose the rear left side marker when Fuse 3 is out. It is supposed to be on Fuse 8, not Fuse 3. Do I have something gone wrong there, or is my fuse listing wrong?

jay_bob · #2 08-09-2013, 09:54 AM

I pulled up the schematics for the 124 and it looks like the headlamp wiper/washer system is on that fuse too.

To help find the problem let's try isolating the headlamp wiper/washer system first, since it has lots of potential for trouble.
Disconnect the negative battery terminal
Undo the screws holding the fuse tray in the box and carefully lift out the fuse tray
Look under the fuse tray at the load side terminal for fuse 3, there should be 3 wires
All 3 wires are gray/red but one is thicker than the others, that one feeds the headlamp wiper washer system.
I would disconnect that wire and reconnect the other two.
Put some tape on the disconnected wire so it doesn't get into trouble and then close the fuse box back up.
Reconnect the battery.

The other 2 wires from the load side of F3 are as follows
- one goes back to the headlight switch and then on to the lamp monitor. This is the control pin for the lamp monitor, that tells the lamp monitor you turned on the lights to initiate monitoring of the bulbs.
- The other is the right side marker light circuit, both front and rear. This includes the dim filament in the front corner light, the dim filament in the brake/tail light, and the small bulb pointing to the side in the rear. This line passes through the lamp monitor on to the light circuits.

I cannot tell if the license plate lights are also on the right circuit or the left circuit from the diagram.

If you are still having trouble then we are going to have to look at breaking out the light circuits after the lamp monitor to see where the problem lies.

rocky raccoon · #3 08-09-2013, 11:28 AM

No voltage to the monovalve means the valve is OPEN, meaning hot water flow into the heater at all times.

a. Does your ac work?
An open monovalve will result in heat overcoming the ac and appearing as a failed ac system.

b. As I recall, you had replaced the monovalve recently. Can you apply 12vdc across the monovalve and hear it pull in?
You can do this using your battery or a starter box.

IRT Light weirdness
a. Since the lights are cross-fused (LF fused with RR etc.) and you had some lighting, I would put my guess on a bad ground in your lighting circuitry. That is the only thing that would affect both sides. In your fussing about with bodywork, you may have disturbed a ground strap.
Ground faults are a ***** to isolate. jay-bob may be right and it is probably worth looking into what he suggests also.

BodhiBenz1987 · #4 08-09-2013, 02:56 PM

Thanks guys. I will investigate the headlight wiper system as well as any possible bad grounds. According to my wiring diagram the side markers/tail light are fused laterally (right front side marker, right rear side marker, right taillamp) ... so the only thing that's out of place is the left rear side marker, which should be on the fuse 8 with the left front side marker (which works). Maybe there's a short/ground fault that's causing one circuit to cross with another.
The diagnostic challenge I have right now is that the fuse does not blow right away ... right now it won't blow at all. It must be a specific circumstance (maybe wires jostled a certain way, or certain combination of accessories turned on). Makes it hard to eliminate potential culprits. I.e., did a change fix the problem or is the problem just latent at the moment?

funola · #5 08-09-2013, 04:18 PM

Take the fuse out and clip an ammeter in its place to see what it's drawing.

BodhiBenz1987 · #6 08-09-2013, 04:23 PM

Quote:

Originally Posted by funola

Take the fuse out and clip an ammeter in its place to see what it's drawing.

Stupid question: can this be achieved using a multimeter and putting the probes on either end of the fuse holder?

funola · #7 08-09-2013, 04:26 PM

Quote:

Originally Posted by BodhiBenz1987

Stupid question: can this be achieved using a multimeter and putting the probes on either end of the fuse holder?

Yes if ur meter has an amps input equal to or greater than the fuse rating.

BodhiBenz1987 · #8 08-09-2013, 04:38 PM

I did just find this little hole in the ground wire for the right front side marker. I doubt this is the problem because 1) You really have to jam a probe in there to get continuity, i.e. wire is still pretty buried and 2) That wire attaches to the chassis anyway, so wouldn't it need to be touching something hot to cause a problem? Will tape up regardless.
Another stupid question about the ammeter: Would I essentially be bypassing the fuse while doing this? Meaning, do I have to worry about fire?

I believe my meter has the correct measures.

funola · #9 08-09-2013, 04:49 PM

Digital ammeters should be fused (mine is). Take the cover off and look for a fuse that's rated above the highest amps scale. If you don't find one, then don't use it. Be prepared to replace fuses if it is fused and you have intermittent shorts.

BodhiBenz1987 · #10 08-09-2013, 05:05 PM

Quote:

Originally Posted by funola

Digital ammeters should be fused (mine is). Take the cover off and look for a fuse that's rated above the highest amps scale. If you don't find one, then don't use it. Be prepared to replace fuses if it is fused and you have intermittent shorts.

Ah, OK, it has two fuses, one 200mA and one 10A ... never used the amp readings so never noticed. I guess the 10A would be OK to use?

BodhiBenz1987 · #11 08-09-2013, 07:18 PM

Well, now I can't check the amperage because, genius that I am, I forgot I had messed around with the leads on the meter and checked voltage with the positive lead in the wrong hole and blew the 10A fuse. Of course, I can't find a replacement; apparently no one carries them.
Really just frustrated now ... everything is working on the car now, but since I didn't fix anything, I know whatever was wrong is still wrong. Even if I had the ammeter, I know it's not spiking right now because it's not blowing the fuse. Debating whether to disassemble all the wiring and see if there's a visible problem (take the air cleaner back out, etc.), or just drive it around and wait for it to happen again so I can try to isolate it by disconnecting things.

funola · #12 08-09-2013, 08:05 PM

You tried to measure voltage when the leads were plugged in amps? I've done that too lol.

No fuses but need fuses? What I do when that happens is cut aluminum foil into strips and use the burned out fuse to stake the foil in the holder. Start with 1/32" width and increase if not enough. A lot cheaper if you have to blow a lot of fuses to do testing. I am not suggesting anyone do this. Just relating what I do, with the caveat that I know what I'm doing.

wolf1941 · #13 08-09-2013, 08:33 PM

Bodhibenz - Wolfgang- 300 SDL
same happen to my car couple of weeks ago -
body shop -paint job and " most " rust removed welding and so
on and wires got moved around -
picked up the car - noticed later in the evening left side running lights and brake dint work
manly #8 fuse problems -
back to the shop -fix it -
OK 2 hours later shop calls pick up the car -walking distance
from my machine shop -
all lights working -nice -what did he find - nothing -except he replaced
all bulbs - not head lights -he claims and almost makes sense -
car had mostly all old bulbs -shaky connectors -- corroded contact
asking for more amperage draw as per rated fuse -
over load -fuse blows -
true or not - my bulbs all light up - something to think about -
enjoy

BodhiBenz1987 · #14 08-09-2013, 09:16 PM

Well, I found the multimeter fuses at Sears, so I can give that another go ... I'm probably not the person to be experimenting with homemade fuses, given my general track record with electrical stuff.

Wolf, thanks ... that makes some sense though I'm not sure was the cause of my problem, since I have mostly new bulbs. I replaced the front side marker right before the second time the fuse blew. I did try cleaning all the connector contacts (just with electrical cleaner). The only thing that enters my mind as a possibility (and this would be coincidence since I didn't mess with the back of the car) is that the bulb in the left rear side marker was a mess at the base ... kind of melted. I tried swapping that with the right side before realizing I had multiple lights out. But I'd swapped it back before the second fuse blew ... and the left side shouldn't even be on that circuit. So I'm very confused.
Edit to add: related or unrelated, the monovalve works again. Odd thing is the last time I had a problem with the monovalve getting voltage was also after I'd done a project with the battery unhooked for a while. It also didn't work for a day, then worked again randomly. I'm guessing it's a CCU circuitry problem, which I suppose could also affect the No. 3 fuse, since it's illumination is through there. Maybe I'm reaching.

BodhiBenz1987 · #15 08-09-2013, 10:19 PM

I am getting ammeter readings of between 3.86 and 3.97 with the engine off and lights on. With the door open it goes up to 4.06 (because of buzzer). I couldn't try to run the wiper motors because I don't have that many hands ... plus I was not running those when the fuses blew. With the engine running the amps with the lights on go up to 4.19. CCU on or off makes no difference (I was just curious). The amps went up from 3.86 to 3.97 as the lights ran for a while but don't seem to go up from 3.97. I did try jostling the wires around, couldn't get any change.
Comparatively the No. 8 fused circuit draws a little over 1 amp, 1.17 when engine running. I think that is normal because there is much less on that circuit (No. 3 has interior lights, license plate lights too).
Do these numbers sound like I should expect?
Can anyone else verify that the left rear side marker should or should not be affected by the No. 3 fuse? When you take the No. 8 fuse out of your car, you lose that light, right? I don't, heh.