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Voltage at Brake Lights?
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rfoubi
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PostPosted: Wed Aug 26, 2020 8:59 pm    Post subject: Voltage at Brake Lights? Reply with quote

So i have been chasing down some issues related to trailer wiring. All lights function as they should, but the trailer lights are weak/flickering.

I should have +/- 12V across the brake circuit with the pedal pressed right? I got only 6-7V on the brake circuit at the back of the van when pedal pressed. So i assumed trailer adapter wiring. Went straight to the tail light assemblies, still low voltage. Then rewired and cleaned grounds. slight improvement, but still the most i can get at the back end of the van is ~9V. I checked in the engine bay junction box too, ~9V so it doesnt seem to be an issue in the tail lights themselves?

I assume now i need to trace wires under the van or in the fuse box area? Is there a relay for the brake lights or a straight run?
Any suggestions?

Brake switches on the master cylinder are both ok i think. However one is 12.4V off, 0v on, whereas the other is 12.4V and then like 0.5V? Could that small difference affect it? I suppose i should jumper that switch and then go check voltage.
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4Gears4Tires
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PostPosted: Thu Aug 27, 2020 6:56 am    Post subject: Re: Voltage at Brake Lights? Reply with quote

Tap a wire into the ground wire right before the lights and ground it right to the alternator case. Does that change the voltage?
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rfoubi
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PostPosted: Thu Aug 27, 2020 8:22 am    Post subject: Re: Voltage at Brake Lights? Reply with quote

I redid the grounds before the tail lights and it changed the voltage slightly but no major difference.
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crazyvwvanman
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PostPosted: Thu Aug 27, 2020 8:49 am    Post subject: Re: Voltage at Brake Lights? Reply with quote

The short answer is no.
Ohm's Law is always in effect.
It matters exactly where and how you measure.
When amps flow there is always a voltage drop. The more amps that flow, the greater the voltage drop. The farther the amps flow, the greater the drop. The smaller the wire the greater the voltage drop. Voltage drop occurs on both the plus voltage side and the ground side in our vehicles.

Mark

rfoubi wrote:
.....
I should have +/- 12V across the brake circuit with the pedal pressed right? I got only 6-7V on the brake circuit at the back of the van when pedal pressed.....
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rfoubi
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PostPosted: Thu Aug 27, 2020 9:42 am    Post subject: Re: Voltage at Brake Lights? Reply with quote

crazyvwvanman wrote:
The short answer is no.
Ohm's Law is always in effect.
It matters exactly where and how you measure.
When amps flow there is always a voltage drop. The more amps that flow, the greater the voltage drop. The farther the amps flow, the greater the drop. The smaller the wire the greater the voltage drop. Voltage drop occurs on both the plus voltage side and the ground side in our vehicles.

Mark

rfoubi wrote:
.....
I should have +/- 12V across the brake circuit with the pedal pressed right? I got only 6-7V on the brake circuit at the back of the van when pedal pressed.....


Yeah, i fully expect to not have the same V as across the battery terminals, especially with a long run some relatively high amp (non-LED) bulbs, with dubious 30yr old connectors in between. <9V seems too low though? If its totally normal to have such a low voltage at the brake lights, i wont waste my time chasing down wiring issues.

The reason i care mainly is the brake controller on my trailer gets the signal from the brake lights to initiate braking and then brakes based on inertia. It will manually work with the override, but wont automatically trigger the brakes, since i think the controller on the trailer doesnt see enough volts from the brake light wire.
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rfoubi
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PostPosted: Fri Aug 28, 2020 9:30 am    Post subject: Re: Voltage at Brake Lights? Reply with quote

FWIW i redid some dodgy splices at the tail lights and got enough voltage to make it work. Still not 12.5V but better. Seems to lose voltage across the fuse box (12.5V at the actual fuse, but more like 10.5 on the wire exiting the back of the fuse box to the tail lights.) Presumably this would be very hard/complicated to rectify. If anything i would hack it by doing an add a circuit through the fuse slot and run a new wire to the tail lights. but since it works now, ill save the hassle.

Oh and in classic vanagon form, i bumped some grounds in the cluster loose, lost the tach/gauge lights, everything was grounding through the cluster instead of headlight switch when i turned lights on.

Then engine was constantly dying after restart. Realized i forgot to hook up air intake after messing with engine bay grounds and lack of airflow over MAF killed engine.

Oh and lost the ground to the interior lights now, and one of my rear hatch lights now flickers every couple seconds. 1 step forward 2 steps back!

Here's my ridiculous setup:

Image may have been reduced in size. Click image to view fullscreen.
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jimf909 Premium Member
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PostPosted: Fri Aug 28, 2020 9:50 am    Post subject: Re: Voltage at Brake Lights? Reply with quote

Ah, I didn't notice you mentioning trailer lights. Does your trailer light controller have a separate power source?

I added a taillight junction box with 12v coming from the junction box on the firewall. That shortens the wire run and gets 12 v to the trailer (I also have aux. tail lights).

Here's the powered trailer light converter...
https://www.amazon.com/gp/aw/d/B010QB7QGS?psc=1&ref=ppx_pop_mob_b_asin_image

The seventh (empty) lug is where the 12 v power connects. It's empty because only the aux lights and back-up cam are wired in at this point, the converter hasn't been installed yet.
Image may have been reduced in size. Click image to view fullscreen.


Image may have been reduced in size. Click image to view fullscreen.


Image may have been reduced in size. Click image to view fullscreen.


I'm a novice at this stuff, listen to crazyvwvanman and others...
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rfoubi
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PostPosted: Fri Aug 28, 2020 10:01 am    Post subject: Re: Voltage at Brake Lights? Reply with quote

jimf909 wrote:
Ah, I didn't notice you mentioning trailer lights. Does your trailer light controller have a separate power source?

I added a taillight junction box with 12v coming from the junction box on the firewall. That shortens the wire run and gets 12 v to the trailer (I also have aux. tail lights).

Here's the powered trailer light converter...
https://www.amazon.com/gp/aw/d/B010QB7QGS?psc=1&ref=ppx_pop_mob_b_asin_image

The seventh (empty) lug is where the 12 v power connects. It's empty because only the aux lights and back-up cam are wired in at this point, the converter hasn't been installed yet.
Image may have been reduced in size. Click image to view fullscreen.


Image may have been reduced in size. Click image to view fullscreen.


Image may have been reduced in size. Click image to view fullscreen.


I'm a novice at this stuff, listen to crazyvwvanman and others...


Yeah, the trailer lights gets power from a separate fused link straight off the alternator (trailer battery charging is also straight off alternator with 4Ga cables to a battery combiner relay and then anderson plug on the bumper.)
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Wildthings
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PostPosted: Fri Aug 28, 2020 10:28 am    Post subject: Re: Voltage at Brake Lights? Reply with quote

If you have a 2v drop across the fuse box, you have located at least part of your problem. If you have the early 85 and earlier style box it is very common for the connections at the fuses to be bad. Buy new fuses with brass contacts (and not the zinc ones) and use a bit of dielectric grease on the contacts. Spinning the fuses several times in their holders can help as well.

You can use dielectric grease on the newer style fuses as well.
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