Dead Short

[TexomaAv8r]

My little 91 Mazda B2200 has a dead short in the tail light/instrument light circuit. <br />I jumped across the fuse block to try to have a chance to locate the short and sure enough got lots of smoke pouring out from under the dash, so I killed the circuit and tried to figure out what I had. The haynes book offers little insight into what I am looking at downthere, just a bundle of wires, one with the insulation burned off, stretched across a plastic block of some sort,..any insight or sugestions to diagfnosing/solving this would be appreciated.<br /><br />And if we can do it without the well deserved scolding for how I located the short would be nice since I have allready beaten myself up over it

 

[rattana]

Re: Dead Short

Some manuals have a color coded wiring diagram that may help. If not you have follow the burnt wire until you will find your short. You may have to cut some ties and undo some cable grips to expose the wires to trace it out. You can also disconnect your instrument cluster plug. Check the wires under the truck going to your tail lights. It will take some tracing, but you can find it. Have some fuses on hand to test the circuit as you proceed.

 

[fixin]

Re: Dead Short

Do you have a plug for trailer lights?I'd start in the back of the truck and work my way forward,Allot less wires,and more than likely it's a wire grounding on the frame.

 

[TexomaAv8r]

Re: Dead Short

I do have a plug for the trailer and am naturaly suspicious of that since I didnt install it, but the smoke came from under the dash so I am committed to at least rewiring there.....I'm just assuming thats where the original short was.<br />The manual I have has some color coded diagrams but the schematic on paper seems much simpler than what is in the truck, it seems some areas of the vehicle are represented by big boxes on the schematic leaving me to interpret..oh well, I guess its just one of those kind of chores

 

[Xcusme]

Re: Dead Short

Hmmm, there is another alternative way to check for the short. Substitute a 12VDC light bulb for the fuse. Since the circuit is shorted, the bulb will light, when you clear the short, the light will go out. This saves on fuses too.

 

[TexomaAv8r]

Re: Dead Short

"Hmmm, there is another alternative way to check for the short. Substitute a 12VDC light bulb for the fuse. Since the circuit is shorted, the bulb will light, when you clear the short, the light will go out. This saves on fuses too. "<br /><br />That is brilliant, thanks!

 

[dolluper]

Re: Dead Short

Under the dash smoke probably will lead to your light switch as when you jumped it at the back it wasn't grounded, check all your harness " GROUNDS" black wires it sounds like a bad ground or light switch bleeding across terminals of switch EDit don't forget to check door switches as they ground the inst lights

 

[Bob_VT]

Re: Dead Short

I will put my money on a faulty light switch. They have been the source of shorts in 2 of my trucks.<br /><br />Bob

 

[TexomaAv8r]

Re: Dead Short

Originally posted by Bob_VT:<br /> I will put my money on a faulty light switch. They have been the source of shorts in 2 of my trucks.<br /><br />Bob

This goes on long with my own logic that most electrical problems will be traced to the most remote, complicated. expensive or hard to locate component in the system

 

[dolluper]

Re: Dead Short

Is your dome light working?

 

[AK_Chappy]

Re: Dead Short

TexomaAv8r,<br />The reason the wire "smoked" was because of heat generated by the short. The direct short resulted in a high amperage draw on the wire. High Amps = High Heat. This is the reason for larger wires for high amp circuits.<br /><br />I would start by looking in the general area of the "burnt wire" though. Look for places that move, i.e. the steering column or brake pedals. If the wire bundle shifted and is rubbing, this will chafe the wire and rub off the insulation. If all of that is good, you need to inspect where the wire bundles go through the "bulkhead" (that's what we call it in airplanes) again looking for chafing. Next, I would inspect the wire bundles going back to the taillights. <br /><br />One more thing to consider. Did the fuse blow before you turned on the light switch or after? If it was before, that means your short is between the fuse and the switch. If it was after, it is between the switch and the lights. <br />It is easier to try and break up the wiring to sections when troubleshooting. In other words, if you can break the circuit in half by disconnecting a plug, test for shorts in one half. If not there, the short must be in the other half. You have now eliminated half of the wiring. I personally would suspect the switch or around the steering column.<br /><br />The replacement of a light for the switch is a brilliant idea, if the short is happening when the switch is off. If it is happening when the switch is on, that is a different story. <br />The reason being, if the switch is on, there is supposed to be a ground on the other side. The light could possibly dim, due to other bulbs using up some voltage, but should stay on.<br /><br />AK Chappy

 

[Dunaruna]

Re: Dead Short

The fuse/globe substitution is an old trick - use a headlight QH globe (the reason will become obvious). A short will illuminate the globe, wriggle looms and wires - if the brightness intensifies you are close to the short, a QH globe will handle AMPS and allows you to visually gauge brightness easily.<br /><br />Aldo

 

[TexomaAv8r]

Re: Dead Short

The fuse blew with the lights on, subsequent fuses blew more or less instantly after I turned on the headlight switch. The switch is built into the turn signal stalk, headlights, highbeams and park/tail lights and instrument lights. The head lights and turn signals are on a different circuit with their own fuses. The dome light works.<br />I will follow the advice of Cappy and others..just need a warmer/dryer day to work<br /><br />What is a "QH globe" Dunaruna?<br /><br />I will also begin the search for a replacement switch, my gut is pointing that way as well

 

[Dunaruna]

Re: Dead Short

quartz halogen - run of the mill headlight globe.<br /><br />Aldo

 

[TexomaAv8r]

Re: Dead Short

Originally posted by Dunaruna:<br /> quartz halogen - run of the mill headlight globe.<br /><br />Aldo

Oh yeah, gotcha.....language barrier

 

[TexomaAv8r]

Re: Dead Short

Okay I finaly had some time in day light to work this problem. I isolated the short to be in the instrument light circuit somewhere, dont know if it was in the switch or the instrument cluster itself but I removed the burnt wires from that circuit and was able to make the taillights work.<br />Then I wired 2 small 12v after market courtesy lights below the speedo to illuminate it, that worked but was very bright, lots of glare and eye strain SO I added a small potentiometer to the circuit as a dimmer and it works great.<br />Thanks for all your help

 

[AK_Chappy]

Re: Dead Short

A little ingenuity went a long way there Tex.<br /><br />Glad you got it working.<br /><br />AK Chappy