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TURN & BRAKE SIGNAL LIGHTS


autonut
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Does anyone have any articles or perhaps there is a better wiring diagram of just the brake & turn signal circuit.  I am confused. Do not have the maintenance manual.  I have checked several things but always come up with a blank. I did find a couple of things wrong.  I replaced the master cylinder switch and that did nothing. The fuse was blown on the light switch that supplies voltage to one side of the switch.  I still get no brake lights or no turn signals. Can anyone give me a clue?  Frustrated. I should be able to figure this out but it has my head scratching.

Lee

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Wow you got it!  Cannot believe I left that info out. 

Anyway I am struggling with a 1948 Olds 98!!!  Spent about 2 or 3 days already trying to get both my brake and turn signals to work.  I only have the wiring diagram which is good but it does not tell you everything!

In particular the internals and how it works of the turn signal assy is not clear and also the headlight switch.  To keep things shorter I finally did get the turn signals to work. They get their voltage from a source on the ignition switch.  The brake stop switch however get it's voltage from the Headlight switch and wondering why they do not use the same source?  Is there some reason to take it from the headlight switch assy?

Anyway to diagnose of why I am not getting any brake lights is that I found that when the brake pedal is pressed, the voltage at the wire going to the turn signal switch goes to zero.  There is apparently some bad contacts or whatever to cause this in the headlight switch??  But why be connected to the headlight switch?  My understanding is that the brake lights DO NOT NEED THE HEADLIGHTS IN ANY WAY!!  Thus confused and hate to try and  remove the switch to try and figure out how it is wired. If anyone can tell me where this fuse on the light switch gets it's power to power the master cylinder stop switch, I would certainly appreciate it. Thank you.  I might mention that I applied an outside source from the stop switch wire to the turn signal assy and the stop light lighted normally which tells me there is something wrong with the voltage source. Can anyone give me any help? Sure would be appreciated. Thanks

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I couldn't find a wiring diagram online for your car, but on MOST American cars of that period, the brake light bulbs are shared with the turn signals.

 

One 6 volt source goes through the flasher and then to the turn signal switch. The other goes through the brake light to the turn signal switch. I gather the flasher is fed from the key switch on yours.

 

The headlight switch was probably just a convenient place for them to get unswitched 6 volts.

 

To make things work, it doesn't matter that the two power sources are different. My guess is that they wanted the brake lights to work when someone was coasting with the key off, but did not want a turn signal left on while parked to be able to drain the battery, so they switched that with the key.

 

When the signal lights are off, current flows through the brake light switch, through the turn signal switch, and then out to both rear bulbs. Well, if the brake lights are on that is.

 

The important part is that the turn signal switch always has both rear bulbs connected to the brake light switch as long as the signals are off.

 

When you turn on a signal, the switch does 2 things, it 1). connects a front bulb to the flasher, and 2). DISCONNECTS one rear bulb from the brake light switch and connects it to the flasher.

 

Does that help?

 

Edited by Bloo (see edit history)
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Thanks for your help.  I did make some progress.  I got the turn signal circuit to work.  But as yet still haven't got the brake light circuit to work.  Was not getting any voltage to the brake lights with the brake pressed.  Installed a new master cyl switch.  But then I noticed that when I energized the circuit the voltage from the Headlight switch goes to zero.  Thus thought I either got a short or a power source problem.  Thus hooked up an external power supply to the brake circuit and it energized the rear brake lights fine. 

Thus my question is why is the voltage source from the light switch going to zero?  Must be a bad connection somewhere but where???  You cannot tell much from the wiring diagram.  All it shows is that the brake switch wire goes to a fuse on the light switch but no clear where from the diagram of where it gets it's power from.  Does anyone have a better connection diagram for the light switch in the maintenance manual or anywhere? In order to troubleshoot I need to know where it gets it's power from.  The light switch assembly does not look like it is that easy to remove or replace if it is bad.  HELP

Lee

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Well yes, voltage going to zero under load indicates a high resistance somewhere between the voltage source and the spot where you are measuring. I don't have a wiring diagram for that car.

 

Since you do have the wiring diagram, start at the "bat" terminal of the voltage regulator, and trace all the way to the brake light switch on the diagram. Probe at any spots along the way you can reach, and apply the brakes to see whether the voltage goes away. There must be a bad connection along the way somewhere.

 

It probably goes regulator >> ammeter post >> headlight switch, or something about like that. If there is a fuse and fuseholder along the way, it deserves extra scrutiny. Good luck.

 

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Okay I did finally find the problem. Not a easy thing to track down.  Here's the history.  No brake lights.  First troubleshoot determined that there was no voltage coming out of the master cyl brake switch.  So I purchased a new one.  Same problem.  Very perplexing and head scratching.  So I decided to use a remote power supply and see if I could get the stop lights to work from the brake switch.  They did light & drew approx 3 amps!! Thus the problem was with where the brake switch was getting its power from.  When brake light etc was all hooked up and pedal pressed the voltage at the Load side of the switch would go to zero.  It gets it's power from the headlight switch and a mounted fuse.  Not sure what the fuse value is suppose to be, as the original fuse was blown(another problem found).  Anyway a higher amp fuse was installed. Getting under the dash to work on the headlight switch assy seemed like no fun.  But anyway in order to try and localize the problem I disconnected the battery and used an ohmeter from the load side of the master cyl stop switch and measured to the fuse.  I got about 0.2ohm.  But on the other side of the fuse holder, I got approx 2.25ohm.  This still does not seem like very much resistance but when the brake light load only measured approx 0.5ohm, it is significant.  Removed fuse and burnished the fuse holder contact.  Got it to about 0.1 or.02 ohm which is a factor of 10X less.  Would it work now?  Certainly did, and I did not have to replace the Headlight switch which would have been a major job.  Wow what a relief!!!!  In a 6V system it does not take much circuit resistance to cause problems.  A 12V system is not as critical this way, since it uses about half the amperage.

Lee

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