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Technical Short-cuts to troubleshooting shorts?

Discussion in 'Traditional Hot Rods' started by Albert Drake, Jun 9, 2021.

  1. Albert Drake
    Joined: Jun 15, 2020
    Posts: 12

    Albert Drake

    I'm having a couple problems with my dad's 29 roadster with a Mercury V8 engine.

    1) There's a short causing the ignition Resistor / ignition ballast (not sure what it's called) to get burning hot.
    2) The fuel pump was working, but doesn't now. Probably due to the short
    3) Engine doesn't fire, although at one point we were getting spark to the cylinders.

    I'm focusing on issue #1

    • This car hasn't been running for 20 years.
    • Due to neglect, the engine had to be rebuilt.
    • It was stored outside for 2 years, then inside for 18.

    The car uses
    • 12 volt system
    • Lincoln zephyr coil & distributor, adapted for 12 volt, 8 cylinders
    • Most of the gauges are mechanical: water & oil pressure, fuel, tach
    Other electrical info
    • The horn works!
    • Rear turn signals work
    • Headlights were working, but don't now

    The fact that some electrical systems used to work (a month ago)
    and don't now, indicates that probably I burned out something due to the short.
    Maybe there's a melted wire or bare metal grounding out?
    We replaced the ignition switch, from a 4-pole to 3-pole and then to 4-pole again.
    That may have also caused a problem.

    Some friends suggested starting from scratch - buy a new harness & rewire everything.
    I'd rather find the short and fix it.
    Any suggestions for a "short-cut" to tracking down the short? ;-)


    \ IMG_20210608_162313_9.jpg
  2. Albert Drake
    Joined: Jun 15, 2020
    Posts: 12

    Albert Drake

  3. brjnelson
    Joined: Oct 13, 2002
    Posts: 550


    Pull every fuse, make a 12v bulb with leads on it, jumper the fuse holder with the bulb. If there is a short in that circuit the bulb will light.
    Paul, Albert Drake, footbrake and 2 others like this.
  4. Maybe you are looking at it wrong; ballasts get hot. If only one is getting hot maybe half your dual coil system isn't working.
    Rest of the problems; pull the fuses and replace one at a time to see what works; when one blows; jump it with a light bulb as mentioned above and follow the circuitry until you find the short.
    Albert Drake, footbrake and Frankie47 like this.

  5. squirrel
    Joined: Sep 23, 2004
    Posts: 50,942


    The ballast resistor is supposed to get hot, that's it's job.

    How much voltage gets to the "hot" side of the coil (the terminal that is not connected to the distributor) when you're cranking the engine? using a test volt meter to measure it, relative to ground.

    Do you see voltage at both sides of the FUEL fuse, when the pump is turned on (ie. when the ignition switch is on)?

    If you get power there, do you also get power at the pump itself? If the pump is more than 20 years old, it could have failed in several different ways.

    Shorts usually make sparks and smoke, if things are bad, but if things aren't so bad, they just blow fuses.

    Fouled spark plugs, lack of fuel, low compression, dirty points, bad condensers, and other things will make engines not start.
  6. jaracer
    Joined: Oct 4, 2008
    Posts: 1,478


    Just read your latest book. I have noticed you don't say anything about electrical troubleshooting.

    Like Squirrel said, the ballast resistor is supposed to get hot. Your problem isn't a short, it's probably an open circuit somewhere. If you aren't too terribly far from Scappoose, I'd be willing to give you a hand. I taught electrical troubleshooting on vehicles for many years. Retired from Freightliner, now Daimler Trucks North America. I'll actually be in Portland this afternoon at Sloan's Tavern from about 4:00PM to around 7:00 PM.
  7. Marty Strode
    Joined: Apr 28, 2011
    Posts: 6,735

    Marty Strode

    Albert, electrical is only one of my weaknesses as a car builder. Today, I had to have a friend diagnose an electrical problem with my riding lawn mower. Hope to see you soon, old friend !
  8. Elcohaulic
    Joined: Dec 27, 2017
    Posts: 2,481


    Start at the fuse that keeps burning out. Start disconnecting things one at a time until the fuse holds. You will know what to disconnect, there the things that don't work because the power is off...
    Most times when I hunt short circuits its two or more things...

    I have three circuit breakers that fit in the fuse holder. Its easier to push a button in then change fuses.. I use the 10 amp breaker most..

    I make a living troubleshooting and repairing knob n tube wiring in mansions that are under historical laws... I love knob n tube, best wiring they ever came out with. A know n tube circuit can safely 500 volts without any leaks, don't try this with Romax. B.X. is absolutely the most dangerous wiring they ever came out with..

    Woops, I'm wrong, aluminum wiring other then service drops is the most dangerous..
    Last edited: Jun 9, 2021
    fauj, olscrounger and Albert Drake like this.
  9. Truck64
    Joined: Oct 18, 2015
    Posts: 5,316

    from Ioway

    Ha! That's great. I thought Knob n tube is still acceptable under the electrical codes? Not for new construction. But existing wiring? I love shit like that, just to watch people go into vapor lock.
    Frankie47 likes this.
  10. 31Apickup
    Joined: Nov 8, 2005
    Posts: 2,914


    Could just be the photo, but the wires coming out of the tunnel under the dash look blackened and not good shape. Possible one had rubbed they and shorted.
  11. Flathead Dave
    Joined: Mar 21, 2014
    Posts: 3,637

    Flathead Dave
    from So. Cal.

    Hope this helps...

    Last edited: Jun 10, 2021
  12. Albert Drake
    Joined: Jun 15, 2020
    Posts: 12

    Albert Drake

    Thanks to everyone for the suggestions, especially brnelson, rich b & squirrel.
    I found an old ford wiring diagram and used that to learn the scope
    Then pulled all the fuses and figured out which parts to ignore.
    Got down to the distributor and the ignition.
    Just like Rich & squirrel say, the ballast resistor is supposed to get hot, but one of them was broken, so the other was getting extra hot.
    Also, found out that spark was only going to one side of the engine.
    So, I ordered two ballast resistors from Mac's & will try them out next week when they arrive.

    Lessons learned:
    - Get a diagram
    - Keep track of what you're doing
    - Reduce the scope (by pulling fuses)
    - Test & keep track of what you're testing
    VANDENPLAS and fauj like this.
  13. Albert Drake
    Joined: Jun 15, 2020
    Posts: 12

    Albert Drake

    oh yes, they're very dirty. I checked for scrapes & bare wire, but luckily they're good
  14. Budget36
    Joined: Nov 29, 2014
    Posts: 8,423


    You have two ballast resistors?
  15. Johnny Gee
    Joined: Dec 3, 2009
    Posts: 9,828

    Johnny Gee
    from Downey, Ca

    For any one. Interesting how ballast's are wired(yellow, tanish colored jumper). Looking and not being able to separate wiring to see how it's routed to work, is there such a thing as doubling down?
  16. olscrounger
    Joined: Feb 23, 2008
    Posts: 4,516


    ^^ may be in series as car is 12V and orig 6v system put maybe 3 volts to coil? My 40 is 6V and has one only
  17. Budget36
    Joined: Nov 29, 2014
    Posts: 8,423


    That would drop too much voltage for a 12v coil to operate properly, and not drop enough for a 6v coil to last, IMO.
    Someone may have over thought it when converting to 12v?
    Johnny Gee likes this.
  18. Johnny Gee
    Joined: Dec 3, 2009
    Posts: 9,828

    Johnny Gee
    from Downey, Ca

    What I should have added as well. "Doubling down?" or two iteams requiring voltage drop since there is a black wire with the jumper (right, appears to be tagged A).
  19. Albert Drake
    Joined: Jun 15, 2020
    Posts: 12

    Albert Drake

    I have a lincoln zephyr distributor. it's a 6v, 12 cylinder dist converted to 12v, 8 cylinders.
    So, one ballast resistor for each side of the coil.

    In my original post you can see they're mounted under the dash, on the back of the firewall.
    Two wires go out the firewall to the coil(s).

    I saw elsewhere that someone just mounted the resistors on the coil:
    RICH B, Johnny Gee and Budget36 like this.
  20. Albert Drake
    Joined: Jun 15, 2020
    Posts: 12

    Albert Drake

    Here's the original post:
  21. I know you have some stuff figured out, but I did notice 3 fuses were blown in the picture you posted above.....I'm late posting :).

    I would make your own wiring schematic, trace all the wires, find out where they go and write it down for future reference. Also, there is a short finder you can buy and use just the beeping circuit breaker in place of the fuse so you can hear when it shorts out.
    Albert Drake likes this.
  22. Budget36
    Joined: Nov 29, 2014
    Posts: 8,423


    Geesh, I had your thread confused with another that had a SBC conversion!
  23. tiredford
    Joined: Apr 6, 2009
    Posts: 558

    from Mo.

    When I was young and dumb, we would find the fuse that blows, wrap it in tin foil, pop it in then watch for smoke. lol
    Truckdoctor Andy likes this.
  24. Guy Patterson
    Joined: Nov 27, 2020
    Posts: 369

    Guy Patterson

    knob n tube and aluminum wire just look the other way
    Budget36 likes this.
  25. Don’t laugh, a few years ago we had an absolute pigpen of a road tractor come in with electrical problems. The owner operator was a real “prince”(incorrect spelling) and was being obnoxious. The cab was so filthy that my guys refused to work on it until it was cleaned up. No matter what they did, they could not locate the short to ground. Finally I got called in to help. My electrical guy and I sat in this pigsty talking about what to do. Finally with a warning of don’t try this at home, I suggested we “smoke it out”. A 10 gauge wire and two spade terminals are good for maybe 50? amps when replacing a 20 amp fuse. We followed the smoke and found the short. The problem was we were laughing so hard that we had a hard time recovering enough to fix the short. The owner never noticed the smell of magic smoke in the cab.
    VANDENPLAS, Blues4U and Truck64 like this.
  26. Budget36
    Joined: Nov 29, 2014
    Posts: 8,423


    I recall a job I got many years ago, question I was asked was about a fuse blowing, and what I’d do. The guy asking the question asked “put a bigger fuse in or? “ and left it open ended.
    I did get the job, and no, I didn’t answer “ put a bigger fuse in it”.

    That said, when I used to go out with my dad on service calls, he’d pull out a stack of pennies that the farmers would use in place of fuses.

    I learned to pull wire at a young age, lol.
    Truckdoctor Andy and olscrounger like this.
  27. Marty Strode
    Joined: Apr 28, 2011
    Posts: 6,735

    Marty Strode

    Albert, I do need a copy of your latest book for my collection, talk to you soon !
    Albert Drake likes this.
  28. Illustrious Hector
    Joined: Jun 15, 2020
    Posts: 158

    Illustrious Hector
    from Alberta

    A number of years ago, our shop built a '37 Ford sedan that had sat since the early 60s. Sometime in its life, a blown fuse was fixed by inserting a piece of 1/4 " copper tubing in its place. Curious as to how much voltage it would take to "blow".
    VANDENPLAS likes this.

  29. lol !!! I just did the same thing this week!!!!
    Had a unit in the shop that would completely explode the 10 amp blade fuse, few other techs attempted to fix it to no avail. ( even tried going up to a 25 amp fuse and that would pop as well.

    Issue was this is a “control fuse” so this one fuse powers up just about every controller on the unit.

    so I checked all my wiring for shorts and continuity, inspected all my relays and contractors to see if they where welded.
    Everything checked out.:confused: Hmmmmmm.

    ok time to let the smoke out stuck a bit of wiring in the fuse socket and operated the unit , POOF!!!!! That lovely smelling magic smoke escaped the control handle seems a micro switch that was hidden within a cover was shorting only under load when voltage was applied.

    lucky I did not do any other damage, a 2 buck micro switch and some time and I was back in business.

    Kinda a “Do as I say not as I do “ moment but I was at a complete loss as to how to find this short
    TrailerTrashToo likes this.
  30. 1971BB427
    Joined: Mar 6, 2010
    Posts: 7,267

    from Oregon

    Ballast resistors do get hot, but depends on how hot it gets. If you turn the key on and leave it with the points closed or arcing, it will eventually smoke the resistor. But if the point are stuck closed it will do the same thing. So first thing is to check the points to be sure they aren't fried. That may be the answer to multiple issues the car is having.

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