Shop Discount Tail Light Circuit Boards

did you check fuses?
check if fuse gets12v when depressing brake pedal?
thinks to consider - is there a tow harness? rear plug in taillight connector.

Yes, I have checked all fuses and all are good

got to start somewhere so-
ck the fuse in underhood 25amp stop fuse,
ck orange wire at brake switch 12v,
ck left dash fuse box .'veh stop fuse' 15a- only has 12v when stepping on brake yes no?
ck brake bulb circuit ltblu wire is there 12v stepping on brake?
do not look at fuses check them and depress pedal for in cab fuse ck

Will do. Do I need the switch reinstalled to do this to complete the circuit

I am also checking the power at the trailer plug to see if I have power there as well and will report back shortly

ck the fuse in underhood 25amp stop fuse,
Fuse visual check is OK Pulled fuse checked for voltage, got 12.4 did not matter if peddle was depressed or not always hot

ck orange wire at brake switch 12v,
Power at wire harness 12.8 VDC

ck left dash fuse box .'veh stop fuse' 15a- only has 12v when stepping on brake yes no?

Reinstalled switch, removed fuse Checked both side of fuse without touching brake peddle no reading, depressed peddle got 12.45 on one side of fuse

ck brake bulb circuit ltblu wire is there 12v stepping on brake?
Checked at rear trailer plug no reading on meter

do not look at fuses check them and depress pedal for in cab fuse ck

in cab fuse box 'veh stop' - yes only 12v when stepping on brake. both sides - might want to swap that fuse...

To be clear, are you telling me I should have 12 VDC on both sides of the fuse and not just on one side when peddle is depressed? Swapped out good fuse for another good fuse no change in situation.

Just a little tip. If you have power to the socket when you press on the brake pedal, take your multimeter and back probe the black wire (ground) on the socket with the bulb installed. Make sure your meter is connected to a good ground. Then take your meter and configure it in the amp mode. Just move your red lead to the amp port.

When the meter is in amp mode, it is basically acting as a jumper. If the light comes on, you have a ground issue.

After your done make sure you move the red lead back to the voltage/ohm port. If you don't and measure voltage with it, you'll blow the fuse inside the meter. You also should make sure the fuse is good inside the meter or this test won't work.

Also don't connect your meter to the blue wire in amp mode or that will blow the fuse also.

Got tied up on an other matter will do this in the AM and report back

Tried it no luck, as I was thinking about it the clearance, signal and back up lights all work and are on the same ground wire.
Any other suggestions?

Just a note: Ground for everything must be good. Truck bed isn't grounded without straps to do so + may be a couple. Add as needed. All powered anything must make it back to battery negative - got it?
What was with that fuse that had power to one side + not the other? New blow right away or stay good? Sounded like that was your problem or the clue?
BTW- I think trailer lighting is totally separately wired but could interfere only. Clearance lights not about this on cab........
T

Don't understand your comment on Ground? When I checked the ground I used the frame of the truck to check not the box. If all other lights in cluster in the rear share the same ground and they all work then should the brake light also work?

Yes, I understand the negative must make it back to the battery. That is why I tested everything to the frame.

When I checked the fuse connection I pulled the fuse and checked the connectivity on each side of the fuse connection not knowing which side the power is coming from. No fuse has ever blown. So, as I understand the fuse wiring, there should only be power to one side of the fuse so if the fuse blows it acts like a switch and cuts power down stream or am I missing something?

There is a plug block in the rear of the truck mounted on the frame behind the bumper. There is a wire bundle from the front of the truck that plugs into it with all the signal wires and ground. Prior to that wire bundle plugging into the block there is a splice of wires that feed directly into the trailer plug. From the wire block three wire harnesses plug in, 1 for the plate lights and then the left and right rear light assembly feeds. I thought because the trailer plug was connected to the front and not through the block I would get a reading, I got nothing but I get a reading at the bulb plug.This is driving me nuts it has to be something simple I just can't find it.

I really appreciate all the help I feel like I am all alone on this and certainly do not have the answers.You guys are a great sounding board.I will keep plugging away and report back or ask for more advise

The CHMSL bulb could be blown, but without checking there you won't know.

You stated you had 12 volts sitting at the blue wire at the brake/tail lamp socket. Did you check that by back probing the connector or removing the bulb and checking inside the socket?

You should check with the bulb installed and circuit powered up. Then back probe with your volt meter. Make sure you volt meter has a good ground. If you have to, run a jumper to the negative battery terminal to connect your meter neg lead to.

When you back probe the socket with the bulb installed and circuit powered up (brake pedal depressed), the blue wire should have close to battery voltage. The black wire should be less than 0.3 V.

If you are checking a circuit that is open (bulb removed), your not reading the circuit with a load on it and that can throw you for a loop if there is resistance in the circuit somewhere dropping the voltage.

OK here is the 411

All this was done with bake peddle depressed

I put the bulb in the socket and back probed the lite blue wire No reading on meter. Still no bulb illumination

I pulled the bulb and back probed the Lite blue wire no reading

I probed the socket and there is power when the peddle is depressed no reading when peddle is released.

Maybe your back probe isn't making contact. Are you sticking the meter probe in there? You can also use a paper clip or T pin. Maybe your probe isn't long enough to reach up in there.

I actually pierced the wire down stream of the socket

Well I gave up and took it to a neighborhood garage. They tested the brake switch and confirmed it is OK. They suggested it's the signal switch in the steering column. Can even do the job till Saturday.

Does the Signal switch sound like a possible issue and if so How do I check it without blowing my airbag?

Fernie - IMO - yes possible but if some info I just looked up is accurate the flasher for turn signals might be the issue instead? Total guess but seems so is the garage you had look at it but something you could try if the flasher looks like this and has 10 pins.


If to find the right one if not in fuse panel inside listen for the clicking and feel it when turn signals are working if they work.
Pic and might be wrong!
/
If like that (different brand no doubt) take it out an tap on it with a plastic end of a screwdriver just go easy as the sucker is $75 bucks! Yikes!
GM -and others when they do this it's so full of points and mini coils the low voltage if a failure points stick or quit. Many if a love tap is administered will behave right away! Maybe not too. But if it snaps right out of this problem either just by removing and replacing same one then if with that love tap makes it work the trouble is about 99% right there.


For those bucks I'd be at a bone yard for another from a nice dry wreck.
Reason: They are not air tight or last SOB funky chase for one like it but intermittent so near impossible to be sure of anything a new one fixed it. It's still guessing and hate that which is why I would go used because of the nasty price. Bet is turn signals cancel the brake light on one side sharing that filament by all look ups and it's AFUed at this or really could be at the multifunction switch as your place suggested.


Can't compete with a hands on check by them but this shouldn't be harmed for a gentle test. No diagrams for wire colors available to me to know by test light the issue is there so remains a guess on my part,


T

We have a winner!!!!!!!!!!! got one light to work by tapping on this flasher and found the other bulb burnt out. Replaced it and BINGO!!!!!!Both lights now work the third light is not working but I don't care because it is covered by my Canopy. I am going to replace the flasher tomorrow so I don't have to keep tapping that little bugger.

Thanks everyone for your help greatly appreciated. I am out.

Still confused on this. The stop lamps and turn signals have different circuits. The only thing the stop lamps and the flasher have in common is the brake switch is used by the flasher module as an input for the trailer brakes lights.

The brake lamp bulbs are above the turn signal bulbs inside the tail lamp lense, right? I guess without being there to take some voltage measurements at different parts of the brake lamp circuit, i'm going to be baffled. Maybe you have a bad connection in your IP fuse block and jiggling the flasher reestablishes the connection.

Again - I'd go used at the price and this isn't conclusive yet IMO. DS posted the diagram which I can't blow up enough to read via my devices but also said it might be the connections at that fuse block! Might be!
Thread is bucking for the second longest in site history. Reason again that those flashers and relays go bad IMO is again moisture if only temp changes and humidity. Points inside (you can see where you can open those when another is working take a look) are a mess of points like ignition points and mini winding to pull them to touch but the current thru isn't correct and item either wont shut off or doesn't work. Some can sit there buzzing.
HEY! You found a bad bulb over this now IMO a cause perhaps at that socket or the common connector again. This thing is now 12 years of being in the elements and the OE grease now suks. Might just as well be on an oil change schedule to take them all apart and re grease them dammit.
NOW - your third brake light isn't working! I forget how many bulbs are in the damn thing but they suk too. All could be bad - same bull the rubber seal if like last one (my own) is junk rubber, damp in there so sockets compromised - grease gone again. If you go there and like ones I've messed with put rubber on needle nose pliers to get bulbs out - easy to just smash bulbs to even look.
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Side note on why I think outside the box so much: Area shops/friends in the trade even EE (electrical engineering) friends from MIT (no joke) quit or don't have time on some elusive something like this and knew I would be the sucker who would spend countless hours when other pros gave up to find some red-neck trick pointed to the problem or sometimes solved it. These people or places sent me the work as they ran out of time never finding the source problem frequently because one bit or a test that failed by tools used that gave proper diagnostic readings on a failed device or how to use it sent off on a tangent where there was no problem. I had the time.


BTW - go back to where you poked that wire and find it. Now seal up the puncture hole or that will be a problem possibly a year from now when wire inside corrodes right there,


T

Sorry - DS and I posted at the same time but still think it's worth a check. When that many connections right at a stupid item it's probably cancelling the ground inside it via points? Still a guess as said,
T

Wow lots of info I will try all the suggestions and report back. May take a bit as I am working on something else right now.

Really don't think that a turn signal switch is going to cause no brake lamps. At least according the wiring schematic. Brake and turn are on different power side circuits, but they share the same grounds.



Don't understand why you don't have battery voltage piercing the light blue wire, but you have it in the socket. Doesn't make sense.

The CHMSL bulb being ( I assume the back of cab light?) Is it interconnected?

I checked the back Brake light lite Blue wire in the socket with bulb out. I will back probe it and report back.

you have the problem isolated- there is 12v at ltbl wire- your test light will light selecting independent ground so bad bulb or bad ground...

Agreed but all three brake light are non functional. I have tested the rear bulbs so that is not the issue. I have not checked the cyclops light in the back of the cab as I have a canopy and don't care if it works or not because you can't see it. Please don't tell me they are interconnected then I have to pull my canopy off the truck to check it.

Sorry if already checked and said but have you taken a brake light bulb out and checked it with a test light with known ground if you use jumper cables all the way to the battery that it has power when empower test light and it will light for ground.
You can use good jumper cables to go from frame to body as a temporary ground.


Things you can count on: Vehicles will use the least freaking wire possible so each bulb doesn't send a wire all the back for ground but would for most positive power.
Straps are those bare braided wire things you find and some seen some not. When lost even if one looks good clean those. Most are not machine threaded, rather lag bolted thru an eyelet.


Plug connectors: Originally should have been smeared with grease by now hard or water got into a plug, one or multi wire connection. Those are trouble spots too to cause hair loss. Other hair puller is plain knowing test light or meter of any sort is really making a connection. Typical test light with alligator clip end on wire can't just go to painted anything and iffy if rusted too.


Just trying to help with what causes things and when testing isn't working and might not know it,


T

Update.I pulled the bulb in the rear light fixture and checked the bulb base. There is 12.45 VDC on the Lt Blue when the peddle is depressed but bulb does not light up. My guess is now a ground issue. Am I on the right path?

brake switch must be installed - we are trying to see where power is lacking - going from front to rear.

Sorry Kev....you got this

Clearance lamps do not share the same circuit as the brake lamps. Running and brake share the same bulb, but different filaments.
Do you have good power and ground at the brake lamp connectors?

Light blue wire is the brake lamp feed circuit at the brake lamp socket. Black wire is the ground.

Maybe I am not clear, there is power at the wiring harness so there must be power at the fuses,correct?Yes there is a factory trailer harness.