Finding a short, dash light fuse is now blowing. 1970 Deville

[chrisntam]

Well, almost everything works.   

As you may know, I pulled my dash apart to put in a factory radio and change my instrument panel bezel.  Got it all back together, hooked up the battery to look at all the new bulbs I installed, and the dash lights worked.  A day later, started the car, took it for a test drive.  Cruise control works, idiot lights work,  radio works, headlights work, a/c works, pretty much all I disconnected and re-connected works, except (now) the dash lights.

The only thing I did different is that I tapped into one of the dash light wires to get power to the light on my aftermarket temp gauge.  I used a scotchlock.

The dash lights worked until I started/drove the car.  I changed the fuse, and it blew right away.  No dash lights, everything else works.  Never had an issue with the dash lights until I got my little grubbies all up in there.   

I was sure to get all the ground wires reconnected.  One odd thing about the ground wires, is that there were two separate ground wires connected to the same piece of metal, but each wire had it's own hole, each had it's own screw.  I put them both back together using only one hole, one screw, piggy-backing the ground wires.

So what causes a fuse to blow?  Is power going to ground somewhere?  Should I undo my scotchlock tap?  I don't think its a (lack of) ground issue because I wouldn't be blowing the fuse (I think).

Thing is, it worked for a (short) while (my initial test to look at the dash lights), now it blows the fuse.

Where should I look first?

What a PITA.  I was very careful with EVERYTHING so this wouldn't happen and now, here I am. I sure hope I don't have to take it all apart again.



Thanks in advance for lending your expertise, it is appreciated.

[tripwire]

Sounds to me like you pinched a wire somewhere when you put things back together.  Take a look at the routing of the harness(es) to see if you can find a tight spot that's squeezing the harness or a single wire.

WParo in VT

[Jeff Rosansky CLC #28373]

Check the lights for the clock and radio. Gray wires. Since you were working in that area there could be an issue there.
Jeff

[chrisntam]

Thanks for the replies.  I'll work my way backwards to see what I can find.  I did struggle a bit putting the lower dash panel back on.  I'll report back, but it may be a week or two, I need the car to be ready to go for a Memorial Day Parade, so I can't take it too far apart....Seems like I'll need to buy a couple of boxes of SFE 4 amp fuses.   

[m-mman]

....Seems like I'll need to buy a couple of boxes of SFE 4 amp fuses.   

The smiley face implies comic relief but for others who might read this I feel a need to state that FUSES DO NOT WEAR OUT. 

They blow because something is drawing too much current. They are the safety device for the electrical system. Too much electricity passing through a wire causes it to get hot. This melts the metal in the fuse and STOPS the flow of electricity. <a good thing>

You have worked on your car and plan to continue to search for the underlying problem (that is the right thing to do)
For any new mechanics on this board - replacing a blown fuse fixes the SYMPTOM, it does not solve the PROBLEM.

Eliminating fuses, putting pennies into fuses boxes, mindlessly replacing fuses or otherwise not adding a safety valve to the wiring system can result in a FIRE(!) You MUST figure out WHY the fuse blew to ensure safe operation.

Electrical fires have destroyed more collector cars than has been counted. Fires brought about because somebody did not want to fix it right. To anybody doing electrical repairs, make sure you do the job right.

[chrisntam]

To clarify, I'll need the boxes of fuses to assist in correcting the problem.

Move wires around, install a new fuse.

Fuse blows.  Try something different, move more wires around, install new fuse.

Fuse blows.  Disconnect scotchlock, install new fuse.

Fuse blows.  Check wires again, install new fuse.

Fuse blows.

See, nearly a box of fuses right there.

[Glen]

There is another way.  Replace the fuse with a light bulb.  It will limit the current flow to a safe level and as long as the short is there the light will light.  When the short is removed the light will go out or at least become a lot dimmer. 
Take a blown fuse and solder leads on it to attach the bulb.  You can make the leads long enough to put the bulb anyplace that easier to see than the fuse box. 

[Scot Minesinger]

Same thing happened to me on my dash lights - it was a power wire (only powered when lights were on) pinched as suggested by other posters. 

You very likely have a wire that is between two metal pieces and when you tightened it the insulation was crushed and compromised, grounding it out.  The light bulb instead of fuse will confirm this for you.  Then you have to find the problem.  Tug on wires gently and maybe you will find the one wire between two pieces that does not give like the others.  In my case I did not have entire dash apart and it was not too difficult to find.  Consider yourself lucky it is lights on only short and not an ignition on short - if that is any comfort.  In my case it was the wire leaving the light switch on dimmer (I think it is white - but only one of those possible - consult wiring diagram to be sure) follow it and look for the pinch.

Good luck

["Cadillac Kid" Greg Surfas 15364]

Chris
Not sure about the location of the in car sensor on a 70, but plugging in the clock wire to that sensor seems to be a common error.  Suggest you disconnect what you have connected to the insides (AC) sensor and see if that helps.
Greg Surfas

[chrisntam]

There is another way.  Replace the fuse with a light bulb.  It will limit the current flow to a safe level and as long as the short is there the light will light.  When the short is removed the light will go out or at least become a lot dimmer. 
Take a blown fuse and solder leads on it to attach the bulb.  You can make the leads long enough to put the bulb anyplace that easier to see than the fuse box.

Thanks for the tip.  Just so I'm not the dim bulb in the group, solder the positive side (of the bulb lead) to where the power side is in the fuse box and solder the ground side to the other side?  Seems obvious now that I think it through.

That sir, is a brilliant idea.

chris.

[m-mman]

A basic circuit test light that is connected across the prongs in the fuse box works also

To see HOW this works, turn off EVERYTHING in your car and remove the negative cable from your battery.
Connect the test light between the battery post and the battery cable and you will see no light.

Then turn on your headlights. This will simulate a 'short' or large draw on the battery.
<You can turn on anything that draws current (heater blower? Radio?) but pulling the headlight switch is one of the easiest>

The test light will illuminate as the current flows across it. Your headlights wont come on because there is too much resistance in the test bulb to make them function but it safely shows that there is current flowing between those two points.

Turn off the headlights and the test light will go out as the 'short' (current draw) is stopped.

Connecting the test light across any two electrical points will show if current is flowing between them. This is a safe way to trace shorts. Since it is just a bulb, polarity doesn't matter.

[bcroe]

There is another way.  Replace the fuse with a light bulb.  It will limit the current flow to a safe level and as long as the short is there the light will light.  When the short is removed the light will go out or at least become a lot dimmer. 
Take a blown fuse and solder leads on it to attach the bulb.  You can make the leads long enough to put the bulb anyplace that easier to see than the fuse box.   

The above is the method used here.  Blowing fuse after fuse will get you nothing
except frustration.     Bruce Roe

[Glen]

Thanks for the tip.  Just so I'm not the dim bulb in the group, solder the positive side (of the bulb lead) to where the power side is in the fuse box and solder the ground side to the other side?  Seems obvious now that I think it through.

That sir, is a brilliant idea.

chris.

You may be thinking of LEDs, they are polarity sensitive and won’t work for this.  I use incandescent light bulbs, they are not polarity sensitive.  And as others have stated you can also disconnect a battery cable and put the bulb between the cable and battery terminal. 
Pick you light bulb by what you are looking for.  A small low wattage bulb will light with very small current flows making it very sensitive.  A higher wattage bulb will be less sensitive.
On a friend’s car, I was looking for a heavy short on a car with many electronic things running, I used a headlight bulb.  The problem was isolated to the automatic power antenna that had melted gears causing it to stall.   

As mentioned above you can also just put leads on the bulb with clips

[chrisntam]

I dug out a small bulb (prolly 2 or 3 watt), a bulb socket & wires, but I didn't have any clips.

I went to the local Lowes, wondered around a bit, couldn't find them.  I asked two older workers if they had roach clips.     They took me right to them.     I told them they would be used for electrical purposes, not what I recall them being used for 40 years ago.   

Got home, rigged it up, connected it to the fuse box, connected the battery, pulled the parking lights on and sure enough, the bulb lit up.  Fiddled my hand up under the dash for a bit, really didn't do anything, bulb remained lit.

Will work on it more this weekend.

Thanks again for the tip, it is appreciated!

[wrench]

I am chasing a similar hard short, not exact, but similar. What I am doing is connecting a multimeter with a continuity tone. That way you dont have to monitor the light.

first, i reference the wiring diagram to pinpoint plan the troubleshooting effort.

I use a couple of color coded (red and black) alligator test leads and hook one end to the probes and the other end to the output side of the fuse holder of the circuit under test and the other lead to ground. I shake the suspect wires/ components and listen for the tone to be interrupted.

I also disconnect the components one at a time to try to isolate the source of the short.

Not trying to redirect your efforts, just illustrating another method of doing a 'wiggle check'...

[bcroe]

I dug out a small bulb (prolly 2 or 3 watt),

Got home, rigged it up, connected it to the fuse box, connected the battery, pulled the parking lights on and sure enough, the bulb lit up.  Fiddled my hand up under the dash for a bit, really didn't do anything, bulb remained lit. 

Gaining on the problem.  I think you have maybe 4 small dash lights fed by the fuse. 
The similar test bulb in place of the fuse would try to light them if the short cleared. 
Since the dash lights need maybe 4 times as much power, you would have perhaps
2.5V across the dash lights and 9.5V across the test light, fairly bright.  With the short
in place the test light gets 12V, the dash lights 0V.  The point is, the test light won't
change that much between 9.5V and 12V. 

To get better report, use a much bigger test bulb, than the normal load.  The test light
will still get 12V for a short, but little voltage when in series with a small load.  I would
use at least an 1157 brake/tail bulb (26W), both sections in parallel.  For a heavy circuit,
a headlight.  An alternate approach is lighten the normal load, by removing all the bulbs. 

The problem with a tone meter is the same, the normal dash lights MAY keep the tone
going even when the short is relieved.  The meter is a very light source, and the
resistance of a cold bulb may be only 1/20 of when it is hot, making its own apparent
short.  Again removing ALL of the dash bulbs will work.  good luck, Bruce Roe

[chrisntam]

So with that 3 watt bulb, when the short is "ungrounded" the light won't go out?

That's what I thought would happen.

Am I looking for the bulb to go a bit dimmer?  Thus the 1157?  I'd prolly use an 1156....

chris.

[bcroe]

Thats it, just dim some.  You can detect it by measuring the voltage at the output
side of the fuse, but that takes away the elegance of the simple bulb.  1156
should work.  Bruce Roe

[wrench]

Bruce, of course you are correct about putting a load on the circuit under test and using a simple bulb on a wire is good practice here.

Removing the bulbs one at a time would be a valid means of 'splitting the circuit' to pin point the short.

In my case, the circuit had enough components to provide the resistance to make the tone go out when I disconnected the faulty component.

Again, just pointing out another method of finding a short.

Don't get me going about time domain reflectometry...

[bcroe]

Bruce, of course you are correct about putting a load on the circuit under test and using a simple bulb on a wire is good practice here.
Removing the bulbs one at a time would be a valid means of 'splitting the circuit' to pin point the short.
In my case, the circuit had enough components to provide the resistance to make the tone go out when I disconnected the faulty component.
Again, just pointing out another method of finding a short.
Don't get me going about time domain reflectometry... 

Certainly any method that solves the problem is good; I try to start with the most
straightforward one.  There are some "short finders" that put a heavy pulse down
the line, so a "receiver" can be moved down the line to pick up the radiation until
the fault if reached.  I don't see these as useful in a dash, where the wiring may
not be accessible.  In retrospect I probably should have done that on the short in
the dome light circuit of my 79 Eldo.  Of course access to that wiring puts the
headliner at great risk.  Turned out the actual wiring was not shown in the manual;
I traced it over to a melted socket on the side.  NOW it is wired by the manual. 

We did use a Time Domain Reflectometer in the lab to check impedance, it could
tell the distance down the line to the fault.  Could be used from several points to
zero in on the fault.  But I have not been aware that the TDR technology has been
reduced in cost enough to be available in the field.  Bruce Roe