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  #1  
Old 07-26-2008, 05:49 PM
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Talking Brake Lights went out

Both my brake lights went out at the same time and I can't figure out why. 1962 190SL. I checked all the fuses with an ohm meter and they tested good. I also checked the switch in the master cyl and when the brake is pushed, the ohm meter reads a good signal. Both bulbs are good. The brake switch has two leads attached to it. I'm assuming one is ground and one is hot. The ground wire I was able to determine by using an ohm meter and testing to the battery ground. The other one I'm assuming is the hot wire which doesn't read anything but less than .5 DC volts. Any suggestions where to go next? Is there a hidden fuse somewhere?
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  #2  
Old 07-27-2008, 01:03 AM
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Well you can't blame a computer box on that car! I can't give you specifics since I don't have that car or a schematic for it. But the wiring on that should be fairly simple (hopefully). I can give you general advice. You need to verify every connection in the circuit. So remember that the bulb sockets are a connection and check any ground connections. Also the fuse holders and the fuses themselves can get corroded. Some people recommend changing the fuses even if they are good. Also keep in mind that an ohm meter uses a tiny current compared to the bulbs so it is not necessarily a fair test of the switch.
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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra
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  #3  
Old 07-27-2008, 01:30 AM
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http://www.190slgroup.com/tech/images/190sl_schematic.pdf

First check is:
Test both sides of fuse #6 w/test lamp to ground. Check at the holder ends , not the fuse ends, as a poor connection between the holder/fuse will not show if you just go to fuse ends. Very common ..........
Second check is to try the blower. That is same fuse feed.
As you have low voltage at the sw... Look at the schematic and you will notice the problem is most likely upstream from there ..not downstream toward the lamps/connector box. [ meaning, you should have bat. V at the brake sw. feed side and at both sides of the fuse #6....]
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Last edited by Arthur Dalton; 07-27-2008 at 02:25 AM.
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  #4  
Old 07-27-2008, 02:24 PM
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Brake Light Switch

Well it looks like it's the brake light switch. I have 12.x volts DC from the fuse to the switch. I also have continunity from post to post on the brake light switch when the pedal is pushed but it doesn't appear that the 12.x volts connects inside the switch. I think it's the switch but I'm not sure why in the heck I get a good ohm reading when the brake pedal is depressed but no voltage?? I'll replace the switch. next question...when I remove the switch, a small amount of brake fluid comes out. Do I have bleed the brake lines at the wheels or just at the top of the master cyl???
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  #5  
Old 07-27-2008, 04:53 PM
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Wow, the entire schematic on ONE PAGE!! The good old days...

Quote:
Originally Posted by jfarwell View Post
I also have continunity from post to post on the brake light switch when the pedal is pushed but it doesn't appear that the 12.x volts connects inside the switch. I think it's the switch but I'm not sure why in the heck I get a good ohm reading when the brake pedal is depressed but no voltage??
Hmmmm... I seem to remember someone saying "Also keep in mind that an ohm meter uses a tiny current compared to the bulbs so it is not necessarily a fair test of the switch."

First, if you want to verify that it is in fact the switch just jumper over the switch (connect the 2 sides of the switch) and the lights should go on. If they do then it is the switch. If they don't then it is not the switch.

Now to answer your meter question. To some extent it has to do with Ohm's Law. Ohm's law says (among other things) that as you pass a current through a resistance it "develops" a voltage across the resistance. So if your switch has a couple of ohms of resistance your meter would read that as good but the lamps would see that as a bad source of current. In practice another thing that happens is the switch has some low amount of resistance that your meter reads as good. But as soon as the load tries to draw significant current through it the resistance increases.

I better way to test a suspect switch or connector is "under load". What you do is put the volt meter (set for volts) across the switch (one probe on each terminal of the switch). With the switch off you should read 12 volts or whatever the battery voltage is. With the switch on you should read 0 volts. If the switch is bad you will continue to read 12 volts or some voltage above 0. (a very small voltage like .1 is OK).
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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra
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  #6  
Old 07-27-2008, 05:04 PM
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Load drop can be caused by either the sw or the fuse connection..
So, the easy test is to simply bring a fused jumper 12v wire from battery 12V + to the lamp side of the switch..that should light the lamps.. [ you are simply bringing a NEW 12v feed to the lamps downstream from the sw.] if NO, then the connector box under the dash is suspect..if Yes, then the fuse/sw feed are suspect.
In that case bring the same jumper to the feed side of the sw and push the pedal..if yes, fuse is dropping, if NO , sw is dropping..

Sw jumper is first test I would do with your condition.

Did you try the blower???????
The reason I mentioned that is the blower would show a poor fuse connection as that would load that circuit and it is the same feed fuse for brake lamp [ as you can see in schematic]..so you are just using the blower draw as a fuse test.
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Last edited by Arthur Dalton; 07-27-2008 at 07:57 PM.
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  #7  
Old 07-28-2008, 09:05 AM
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After I change the switch, do I need to bleed all the brake lines again?
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  #8  
Old 07-28-2008, 11:50 AM
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If you are careful , you unscrew the sw off and install the new one immediately [ without touching the pedal ].. but you do not tighten it. You then have someone press the pedal to get any residule air to come out the loosened threads..then tightem the sw. You are basically using the sw as a bleeder b/c if any air did enter , it would be right at that switch port and not into the sytem.. that way you do not have to push the air all the way thru to a bleeder. Check fluid level too.........
You may hust want to change the fluid while you are at it..a good maint. procedure that most who know brake systems do every 2 years.....

Did you try the blower test??
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Last edited by Arthur Dalton; 07-28-2008 at 12:57 PM.
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  #9  
Old 07-28-2008, 02:01 PM
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Quote:
Originally Posted by Arthur Dalton View Post

Did you try the blower test??
AND did you verify the switch is bad by jumpering it and verifying the lights go on?
__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra
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  #10  
Old 07-28-2008, 02:08 PM
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What we are saying is if guys go thru the trouble of loading diagrams and answering post, it is common here on this Forum to lets us know with some feefdback as to where the problem stands....
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  #11  
Old 07-30-2008, 10:37 PM
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sorry gang..was on the road the last couple days..

I did not test the blower but will. it makes sense and completely agree it will test the fuse... the car is currently in storage and not easy to get to.

the lights do come on when touching the wires together.

take care and thanks for all the help. this was my first time using this forum...you're all a wealth of knowledge

Thanks
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  #12  
Old 07-31-2008, 01:21 PM
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>

There you go........

Owning a 121 , I would recomend you join that 190SL Group. I have a few friends over there and it is a Great site.
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