Diagnosing electrical shorts

Kestas

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The Motor City
I have a dead short in one of the circuits on my car. How do I go about properly diagnosing this, rather than replacing the fuse each time?
 

JHZR2

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New Jersey
Start following the wiring and using an ohmmeter to find the point where continuity is lost and the short is located. Id imagine you could find something near the endpoint that is not switched/relayed that you can use as a basis point to follow from? That's how Ive done it before. But I lost the circuit board in my instrument cluster due to the short, so be prepared for something $ to be busted.
 

Kestas

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The Motor City
I can't access the wiring. The problem started soon after I fiddled with the overhead interior lights in my 95 Mercedes E320 cabriolet. I couldn't take it apart. Ever since then I've been blowing fuses. The switches have always been flaky. I'd like to find which switch position to leave it in that is not a dead short, and if I have to take the bulbs out to prevent a short, I'll go that route. The fuse also services the radio and clock.
 

JHZR2

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ugggh, that is rough. Id start as a temporary workaround to install a properly sized "breaker", wired to the cabin via heavy gauge wiring, so that you can fiddle and reset without issue. Ive not tried these DC automotive breakers but have seen them. Is the light in the soft top? If so, Id think that would be the first culprit. But pulling a bulb isnt going to help a short on the hot line going to metal. You'd have to interrupt it elsewhere in the harness. You can't pull the kickpanels, radio, etc? You could always rewire a circuit via a separate fuse holder from the hot side just to the radio and clock. Is the short on a switched circuit or a constant hot circuit?
 

Kestas

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The Motor City
It's a constant hot circuit. The light cluster is at the top of the window frame, and also houses the mirror mounting. I tried getting at the wiring, but I can't detrim the window frame. I was hoping I could somehow use my multimeter to signal the dead short and also signal when it would go away. I like the idea of using a circuit breaker for diagnostics.
 

JHZR2

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I dont see how a light cluster is constant hot. it should be switched. I'd switch it on with no bulb in there, leads in the two sides. If that part of the harness is short, it should show a signal when the switch is closed. The issue is that you then have to chase back on the hot side to some other point, perhaps open the circuit there and test the wires to break down the connection points where the short may lie.
 
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Upstate NY
HF sells a tool that replaces an auto fuse and displays current draw. You could get that and start disconnecting things or turn on/off or pull bulb, etc. A DVM will also come in handy as will a wiring diagram of the vehicle. Unfortunately people here cannot give you exact step by step instructions. You will need to apply some electrical knowledge and track down the culprit. Have you made sure that what you are replacing is the correct one? Maybe the wrong bulb or fuse was in there to start and is causing the problems.
 
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ME
You could put a bulb and socket in place of the fuse. When it lights, there's a dead short. When it dims, the short is gone. Not sure what sort of fancy electronics are on the same circuit, and how they'd handle the brownout when the short goes away. Should do okay; they have to handle low batteries and such, just throwing it out there as a precaution. Dome lights are usually constant hot with half a dozen ways to ground them to get them to turn on. Usually a "spider" of wires goes off to each door pin switch, that then grounds the circuit at that part of the body. The switches at the light just ground the bulb there. I suspect maybe the bulb holder got shoved/pushed around and is touching ground behind it. Gotta take it apart. frown
 
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Phoenix
At least you know the problem likely is in the overhead lights, so hopefully you can find a service manual or someone knowledgeable with that type of car who can take it apart for you. Since it is a Mercedes, I am sure it is overengineered and way more complex than a light needs to be!
 
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Chicago Area
Originally Posted By: JHZR2
I dont see how a light cluster is constant hot. it should be switched. I'd switch it on with no bulb in there, leads in the two sides. If that part of the harness is short, it should show a signal when the switch is closed. The issue is that you then have to chase back on the hot side to some other point, perhaps open the circuit there and test the wires to break down the connection points where the short may lie.
Easy. Because the circuit may have a switched ground, not a switched hot. But if a problem occurred after you worked on something, it is where I'd start re-fiddling.
 

Kestas

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That's the big problem, mechtech2, If I could take the unit apart, I'd go over it with a fine tooth comb. My first attempt at taking the unit apart was unsuccessful and resulted in many cracks in the plastic unit. My first posting for help on a Mercedes forum didn't get me anywhere. I have the wiring diagrams, but it doesn't help if you can't get to the wires. Some tasks on this car are rather straightforward. Others, like this, are a nightmare. Dishdude is right, these cars are needlessly overengineered. This car has had a serious problem like this come up eery 3K.
 
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Chicago Area
I used to work for Mercedes. Odd engineering mixed with wonderful engineering. And many parts are just plain big and beefy to get quality. I have to state another obvious thing - a wiring diagram. After a few minutes , you will identify and isolate each circuit. You can come up with a battle plan then. But this is fixable! You will prevail!
 
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Location
Mississippi
(with the fuse out)Stick one probe of your multi meter in the fuse socket(load side). Touch the other to ground and read resistance. If you have a short it will read close to 0 ohms. Keep wiggling wires,overs,whatever until you see the short disappear,resistance go up. Almost certainly pinched a wire while trying to take the light apart.
 
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Central Texas
If I remember correctly, Alldata has a membership for $25/yr for consumers where you can access troubleshooting info specific to your model. They may also have info on how to remove any necessary trim without breaking it. Otherwise, you may want to aquire the interior trim manual from MB to save you some aggravation. Note many new vehicles now switch ground instead of hot.
 
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