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Help! Weird electrical problem-


ledfootslim
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I have been dailying my 1948 Desoto to work for the past few weeks and she has been doing great, in 100 degree weather and everything. Today, coming home from work I noticed a clicking sound, much like a turn signal, but couldn't locate it while driving. I got home, got some chores done and went back out the the garage and it was still clicking! I finally pin-pointed it to the inside of the headlight switch. Inside of it I could see a little spark and and arm that snapped back and forth with every spark. The ammeter would jump back and forth along with the spark as well. This was all with the key off, turning the key on made no difference. The hot wire for the switch was reading a constant six volts with key on or off, so I detached that and the clicking stopped of course, then I ran a wire from the heater to the hot wire post on the headlight switch. It still clicks, but now only when the key is on. I have no lights at all, turn signal, brake, anything. I took a video of it which is very low quality of course, but you can see the spark and the arm switching back and forth. Has anyone had this problem or have a suggestion? I'd appreciate it! https://www.youtube.com/watch?v=ROHfbfYSAX0&feature=youtu.be

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The headlamp switch should feed from an unswitched source meaning hot without ignition on.  Typically it feeds from the load side of the amp gauge.  Usually there is a longer than SAE 30 amp fuse integral with the switch.  Your's seems to have a self resetting relay.  So the clicking is the relay trying to reset.  Apparently there is an inadvertent ground (short) some where between the switch and the lights.  The relay depends on a bimetallic strip.  When it over heats due to higher than spec load it pops off.  when it cools it tries to reconnect and when it finds the same higher than rated load it pops of again.  

 

I would start at the lights and back up along the wires looking for bad insulation allowing flow to the sheetmetal or another wire or corroded connections.  Start at the lamps (parking also) and check all the wires and the grounds for the headlamps ( the running lights typically ground through the assembly to the body.  Then check all the connections and insulation at the junction box where the wire split from the main feed to the lamps. You may be able to isolate the source to one side or the other by disconnecting one sides circuit while leaving the other one connected while testing. Usually on the radiator support.  then from there back through the engine compartment to where they go through the fire wall grommet, then from there to the switch.  

 

Good luck, I don't envy you working up under the dash in the 90 + temps.  (hint removing the lower seat cushion makes the job a bit easier on your back.  

Edited by greg g
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Thank you for the quick response! Yep, my ears literally filled with sweat in this position- not fun! But what I signed up for when I bought the old lady. Hopefully I can weasel away from work early tomorrow and start tracing those wires. And I guess I can go ahead and put the other one back on since it was supposed to be there  :rolleyes:  

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I think I'd leave that constant hot wire off except when testing it.   I'd also think I'd start at the switch.  With the switch off, nothing should be going through to the lights. You can eliminate circuits simply by removing wires from the switch, one at a time, then tracing out the wires from the culprit circuit or it could be in the switch itself. 

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In my '52 there is no fuse on the headlights--instead there is a bi-metal type electrical breaker...if there is a short in the headlights the breaker will cycle on and off.  I expect that you have a short within the switch itself

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I think I'd leave that constant hot wire off except when testing it.   I'd also think I'd start at the switch.  With the switch off, nothing should be going through to the lights. You can eliminate circuits simply by removing wires from the switch, one at a time, then tracing out the wires from the culprit circuit or it could be in the switch itself. 

None of the wires leading out of the switch were hot last night. I'll test it further today.

 

In my '52 there is no fuse on the headlights--instead there is a bi-metal type electrical breaker...if there is a short in the headlights the breaker will cycle on and off.  I expect that you have a short within the switch itself

How would I test if the switch is bad or shorted out? Thanks guys!

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disconnect all the out wires, reattach the power and see if the symptom still exists.  If not reattaching the load leads one at a time should isolate the problem.

Ok, I took the wire off the other post, the ground or negative, I'm guessing, and the ammeter is no longer charging and the lights work. Is this helpful?

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Ok, I took the wire off the other post, the ground or negative, I'm guessing, and the ammeter is no longer charging and the lights work. Is this helpful?

I still don't have turn signals, though. I'm about to go mess with it some more, see what I can find out.

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disconnect all the out wires, reattach the power and see if the symptom still exists.  If not reattaching the load leads one at a time should isolate the problem.

So long as the top wire is off, the headlight, tailights and everything but the brake and turn signals work. If I put it back on, nothing works. So does this mean the short is somewhere in the brake/turn signal route?

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if that is the lead that causes they symptom its likely in there some where.  Are your brake light stuck on.  Some times this happens when pressure is not fully released from the brake lines when you take your foot off.  Which type of signal switch do you have?  Seven wire types are wired in conjunction with the brake lights.  Disconnect one of the leads from the brake light switch and see what happens when you reattach the lead causing the problem to the switch.

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if that is the lead that causes they symptom its likely in there some where.  Are your brake light stuck on.  Some times this happens when pressure is not fully released from the brake lines when you take your foot off.  Which type of signal switch do you have?  Seven wire types are wired in conjunction with the brake lights.  Disconnect one of the leads from the brake light switch and see what happens when you reattach the lead causing the problem to the switch.

Ok, I tried taking a wire off the brake switch and it did nothing. With the wire off the second post on the headlight switch the headlights, brake lights and backing lights all come on. The turn signals still don't work. I am using a new switch like this one. Does anyone know where that second wire goes on the headlight switch? There are four plug ins and two posts, and I know that one post gets the hot wire on it, but I can't find what the other is for and it seems to be the one causing problems.....91062860_L.jpg

Edited by ledfootslim
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It looks like the wires on that post might go to some interior stuff I'm not even using- cigar lighter, map light, etc. If I take it off most everything works, still just no brakes or turn signals.

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Try running a dedicated line to the brake light switch.  You want this to work regardless of key off or on.  Make a fused feed wire that will connect to the battery side of the solenoid, and run down to power the brake light switch.  Then that load will no longer go through the light switch, and you will have brake lights regardless of signals or accessory lights.  Typically signals and interior lights will work only with ignition on or with key on aux.  so they wont fire up just off the light switch on.  

 

you can also make up an 8 to 12 foot wire with alligator clips on each end.  connecting it to the battery negative terminal, will allow you to independlently power your circuits to check to see if any of the items will work. An inline fuse is a good idea for a line set up like this,just in case you should encounter powering a dead short.

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I'm not that familiar with the switch layout. Looking at a P25 diagram, it looks like the circuit breaker is located on the ACC terminal of the switch. Are the two post terminals on the breaker connections and you're taking one of these off and are the plug in connections on the switch? The switch itself should have a letter at each terminal to indicate what it feeds.

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I'm not that familiar with the switch layout. Looking at a P25 diagram, it looks like the circuit breaker is located on the ACC terminal of the switch. Are the two post terminals on the breaker connections and you're taking one of these off and are the plug in connections on the switch? The switch itself should have a letter at each terminal to indicate what it feeds.

It's like this. The plug ins are marked with the letters and my manual says where they go. The lower post is where the hot wire goes and the other one has three wires leading off to somewhere.

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Try running a dedicated line to the brake light switch.  You want this to work regardless of key off or on.  Make a fused feed wire that will connect to the battery side of the solenoid, and run down to power the brake light switch.  Then that load will no longer go through the light switch, and you will have brake lights regardless of signals or accessory lights.  Typically signals and interior lights will work only with ignition on or with key on aux.  so they wont fire up just off the light switch on.  

 

you can also make up an 8 to 12 foot wire with alligator clips on each end.  connecting it to the battery negative terminal, will allow you to independlently power your circuits to check to see if any of the items will work. An inline fuse is a good idea for a line set up like this,just in case you should encounter powering a dead short

Thanks large, Greg! I I ripped out a bunch of wire and ran new a line to the brake switch and got rid of all the other wires I'm not using and I now have all my lights and everything again, and no drain on the battery!

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  • 4 weeks later...

I have a similar problem with my 47 P15.  The brake light (single cnter light on trunk) works whenever it feels like it.  Something is going on with the feed wire from the brake pedal sensor switch to the brake light.  I tested the sensor and it is working fine.   I do not want to pull out the interior inorder to run a new wire from switch to brake light afraid I will damage it.

 

Anyone have a suggestion on running a wire underneath the car (outside)?  I'm thinking of using a plastic wire cover and zip-tieing it to exterior.  I'm sure someone had a similar situation and any advice would be appreciated.

 

 

Thanks,

 

Chad

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I have a similar problem with my 47 P15.  The brake light (single cnter light on trunk) works whenever it feels like it.  Something is going on with the feed wire from the brake pedal sensor switch to the brake light.  I tested the sensor and it is working fine.   I do not want to pull out the interior inorder to run a new wire from switch to brake light afraid I will damage it.

 

Anyone have a suggestion on running a wire underneath the car (outside)?  I'm thinking of using a plastic wire cover and zip-tieing it to exterior.  I'm sure someone had a similar situation and any advice would be appreciated.

 

 

Thanks,

 

Chad

 

I have a similar problem with my 47 P15.  The brake light (single cnter light on trunk) works whenever it feels like it.  Something is going on with the feed wire from the brake pedal sensor switch to the brake light.  I tested the sensor and it is working fine.   I do not want to pull out the interior inorder to run a new wire from switch to brake light afraid I will damage it.

 

Anyone have a suggestion on running a wire underneath the car (outside)?  I'm thinking of using a plastic wire cover and zip-tieing it to exterior.  I'm sure someone had a similar situation and any advice would be appreciated.

 

 

Thanks,

 

Chad

If you disconnect the wire that feeds the brake light at the switch does your brake light still come on when ever it feels like it? If you run power from another source to this disconnected wire does the brake light come on? Have you cleaned the ground connection at the brake light fixture?

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When the brake light comes on whenever it feels like it, it functions properly until it no longer works.  Then few weeks later, it works properly.If I disconnect the wire, the brake light does not work.

If I run power from another source to the brake light wire near the pedal sensor, the brake light doesn't work.  Ground is proper since I plugged the license plate light into the brake light feed line at the trunk and light works.  There is something going on with the old original wire from the sensor to the splice in the trunk. Also, I would like the tail signal lights to also be used as brake lights.  Since one light on truck isn't enough for todays drivers.  But I think I need to add a bulb in the taillight housing since bulbs in there are already 2 way.

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when you say sensor, are you referring to the in line pressure switch?  These are in common use today on atvs and motorcycles.  As it is a hydraulic switch and requires the build up of pressure in the as you step on the brake pedal.  As brake fluid attracts moisture rust can form in the brake lines and switch causing problems.  If it works sometimes and not others it seems likely as it may be fouled either making a false connection by not releasing pressure as the brakes are released or blocking contact when the fluid is under pressure.  You can unscrew it from the line and check it to see it there is any debris is present.  If so it might respond to a good cleaning.  If not they are still available from NAPA or Harley dealers.  Most ATV and off shore bike are metric.  

 

You can wire the outer lamps or aux lamps by tapping power from the wire that feeds the stop light.  Some where near the lamp there may be a plug type connector where you can access power for another feed.  I would recommend a dedicated feed tot he switch from the battery side solenoid terminal through an inline fuse.   you can by pass the switch with the feed to the wire to the rear and determine if the wire from the switch to the light needs to be replaced.  

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Zen, I already am certain something is going on with the wire from the line pressure switch. Using a voltmeter, I am getting power out of the pressure switch when applying brakes. But nothing back at brake light. Just wondering best way to run wire since I prefer exposed because I don't want to damage the interior. Wires are original.

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