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Thread: Idiots Guide to Tracking Shorts

  1. #1
    Blessed be the lowered RickS's Avatar
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    Angry Idiots Guide to Tracking Shorts

    Needed! I have no problem installing relays, stereos, etc but, tracking down shorts is another thing. My problem is a non-critical part of the hooptie: interior and glove box lights/clock/ door pins. What I have done: removed each positive lead from each and every point that supposedly draws current and still the fuse pops. However, this is an intermittant problem (yes, it gets better). I can connect everything up and hit the door pins, switch the interior and glove box lights on and off, and the fuse holds. I return to the car 5 minutes later and the fuse is blown. I put an OHM meter on each positive and negative lead (little do I know about the devise) and each showed the same reading. I even purchased some dubious devise called a Short Finder, where one traces the wiring diagam route of the suspects hoping for the little meter to start twitching. I heard about progressively testing the wires as they leave the fuse block, but how do you access them when they run up through the well wrapped wiring harness? And then when they do emerge, how do you measure current draw without knicking the wires to get a reading? As you can tell I am lost and need the basics.
    71 914 3.0, 82 SC, ESR 376, RG 307

    "The problem with the world is, the ignorant are cock-sure and the intelligent are full of doubt." Bertram Russell

  2. #2
    Righteous Indignation 70SATMan's Avatar
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    Hi Rick,


    Been browsing this board for years gathering info and been lookin for my ride for the last couple of months. Anyway, finally I see a post where I can contribute some of MY knowledge.

    Sounds like the circuit is slowly heating up from the current draw and expanding until the short occurs.

    Step one. Remove the leads from the clock only to isolate the problem to the lights where I suspect it is. See if you can recreate blowing the fuse.(unfortunately you are probably going to go through a few more)

    Step two. If it does recreate then remove the bulb from the glove box and repeat. You do not have to remove the leads from the socket cause without the bulb there is no current draw.

    Keep repeating until you narrow down the area of the wiring.

    Does your meter measure current? If so all you have to do is make sure your leads on the meter are in the right jacks. They usually have to move from reading resistance (Ohms) to current. If it does then you have to measure in series with the hot lead to the device not hot to ground. An example would be to disconnect the hot lead ONLY to the socket and connect one lead to the wire connection and the other lead to the socket connection. You may have to install a temporary jumper wire for the other connection if you cannot isolate the hot lead only.

    Without a schematic in front of me that's about the best I can offer without a little more diagnostic info.

    Good Luck
    Michael
    “Electricity is really just organized lightning”

    -Dusty 70S Coupe
    -S Registry #586

  3. #3
    Blessed be the lowered RickS's Avatar
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    thanks Michael for the response. I will need to buy a few more fuses but the approach sounds methodical.

    If all the current draws are disconnected and the fuse still pops (not slowly heating up this time) any suggestions?

    TIA,
    Rick and keep lurking!

    A man too busy speaking will never truly hear. The big B.
    71 914 3.0, 82 SC, ESR 376, RG 307

    "The problem with the world is, the ignorant are cock-sure and the intelligent are full of doubt." Bertram Russell

  4. #4
    Righteous Indignation 70SATMan's Avatar
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    Let's see.. All bulbs out and clock disconnected. If your meter can read voltage and current do the following:

    With no fuse in the socket and measuring Voltage with one lead on the hot side of the fuse contacts and other to chassis you should read 12 V. On the Other fuse contact (load side) you should not.

    Measuring Resistance (Ohms) on the load side of the fuse contact (the contact with no voltage) with one lead and the other to chassis what is the reading? Is your meter an analog type with a needle or a digital readout? With a short the reading with a digital meter will be low like 12 Oms or less. What is the fuse rating for this circuit? What you want is a very high reading. With a typical digital meter this is going to show a reading as MOhms (Mega Ohms).

    If you are getting a high reading keep the meter connected and open/close the door pins and glove box pin one at a time to see if you start to get a low Ohms reading.

    If you are getting a low reading right away if you can, measure the contacts at the door pins and the glove box pin to chassis. As you get closer to the short in the wiring the Ohms reading will decrease.

    LMK
    Michael
    “Electricity is really just organized lightning”

    -Dusty 70S Coupe
    -S Registry #586

  5. #5
    Michael..I sure hope you stick around this board. Count me among the electrical ignorant. I don't even have a problem now, but I loved the way you explained searching for the problem. So much so, that I printed it out, am filing it for future reference. GAWD, I HATE ELECTRICAL PROBLEMS!
    Paul D. Early S Registry #8 - Cyclops Minister of West Coast Affairs
    "Now, to put a water-cooled engine in the rear and to have the radiator in the front, that's not very intelligent." -Ferry Porsche (PANO, Oct. 1973)

  6. #6
    Righteous Indignation 70SATMan's Avatar
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    I'm NOT Worthy....I'm NOT Worthy.

    and I've gained inumerable knowledge from your posts over the past couple of years PWD. I only aspire to be able to pass on some early 911 know how a few years from now.

    Thanks for the kind words.

    Any reliable sources that you know of for original wiring schematics for the 2.2's? I've been helping Rick through the ethernet in my head!
    Michael
    “Electricity is really just organized lightning”

    -Dusty 70S Coupe
    -S Registry #586

  7. #7
    I have a set of early workshop manuals, wiring diagrams inserted for several years. I don't understand any of it. If I had a snail mail address, I could send a color photocopy to what I think is the 2.2 years...(edit) Whoops..just did another read of Ricks's signature...his 73 would be a 2.4, but the wiring diagrams I have only go up to '72..(2nd edit) here's a link to a site with wiring diagrams for various years. Maybe some help there?
    http://www.pelicanparts.com/911/911_...l_diagrams.htm
    Paul D. Early S Registry #8 - Cyclops Minister of West Coast Affairs
    "Now, to put a water-cooled engine in the rear and to have the radiator in the front, that's not very intelligent." -Ferry Porsche (PANO, Oct. 1973)

  8. #8
    admin_old
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    I have some cleaned up wiring diagrams of early 911s here.

    Sherwood

  9. #9

    Delayed short?

    I was just wondering if the older clocks (pre- Quartz) use some type of solenoid to reset the winder/spring mechanism? Maybe that could explain the time delay? If you disconnect the clock and the fuse does not blow out after "5 minutes".... I didn't know if this blown fuse happens only when driving, lights on/off, or even when parked. Just a thought. HTH.

  10. #10
    Righteous Indignation 70SATMan's Avatar
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    Thanks for the links guys. I was being purely selfish asking for the 2.2 info cause that's what I'm searching for. I figured the wiring for Rick's 2.4 couldn't be much different in regards to the lighting. Interesting theory about the clock although transistorized clocks were well in use in 72-73. I'd be surprised if it had a winding mechanism.

    I've done a fair amount of car electrical rewiring(not to mention what my profession is). All on 240-280Zs. OOPS , That's like getting caught with your pants down. Hope I don't get too grilled for the J@##@$ese reference on this board!
    Michael
    “Electricity is really just organized lightning”

    -Dusty 70S Coupe
    -S Registry #586

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