Howdy, I'm Caleb. I've got a 1995 Chevy C1500 with a recently swapped 5.7, refurbished motor, all new accessories, radiator, sensors except speed sensor and oil pressure sensor.
Howdy Caleb, and Welcome to the GMT400 forum. Based on your problem description, I opened
up the '95 FSM (Factory Service Manual) and dug into the brake light circuit. In other words, the
diagrams I'm going to share are all from the '95 FSM, and should be relevant.
The brake lights don't work when the ignition is turned to the accessory or on positions, only when in the off position. This only happened recently. I didn't think it was the brake light switch but I went ahead and replaced that. I've also got some tail lights and new boards coming in as a friend of mine suggested the boards on my tail lights went bad. I've read elsewhere that it could be an issue with the ignition switch but I'm curious if anyone could elaborate on that?
OK, after reviewing the wiring diagrams it looks to me that the Ignition Switch is not
directly involved
in the Brake lights. (aka: 'Stop lamps' in the GM service manual.) However, the symptoms you described
would make a troubleshooter think that the Ignition Switch is somehow involved. And it is, thanks to the
fact that GM has interleaved both the "Turn Signal" and "Hazard Flasher" functions
on top of the simple
ON/OFF brake light signal. This slicing and dicing is done in the Multifunction Switch, and as long as everything
works to factory specs this all interoperates seamlessly. (!)
NOTE: I am a big fan of the General's service manuals, and especially the documentation of the wiring harness...but
on occasion the person drawing the circuit tried to fit too much detail onto a single page, and it makes
a troubleshooting session harder than it needs to be. IMO, a troubleshooter would draw the multifunction switch
at least 3 times.
Mode 1: Brake input only.
Mode 2: Turn signal overlaid onto Brakes only.
Mode 3: Hazard Switch / Emergency Flashers only.
If they did this, the troubleshooter could go through the different modes, observe the switch changes between modes,
and have a good grasp of how the circuit really works.
Instead, they show everything at once. Not very helpful. To cap it off, you have to chase signals across at least 3
different pages. So in the interest of troubleshooting, I decided to draw only the BRAKE inputs and outputs on a single page:
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I've cross-checked the above against the FSM, and I'm 99% sure the above is accurate. From Reply #3 you have verified
ground
G410 on the bottom of the page. I'm assuming that because the Brake Lights
are working with Power off that the
20A 'Stop/Haz' Fuse 1 at the top of the page is good, but it won't hurt to check. (See attachments #1 & #2)
You have already replaced the Stoplamp/TCC (aka: Brake Light) switch, so when you depress the brake pedal the +12V
from Fuse 1 traverses the Brake Light switch, and via the White wire travels to pin 17 on Connector 266, where it enters the
Multifunction Switch. And when everything is off, you should see that same +12V at C266 pins 18 (Yellow) and 19 (Dark Green),
which in turn travel from the steering column to the LEFT and RIGHT brake lamps via Connectors C112 & C400.
As a troubleshooter, with the key in the off position you should be able to disconnect C266, and jumper pin 17 to 18, followed
by pin 17 to 19, and while operating the brake pedal verify that the brake lights come on. If it was me, I would then turn the key to
on, and verify that while I am still
jumpered around the multifunction switch
the brake lights still work, power on or off, as they are
supposed to.
IF they do, then by temporarily removing the multifunction switch from the brake circuit we have fixed the symptom. And to prove this,
if you turn the key OFF and reconnect everything, I would assume that your problem will return. By doing all this you can
prove to yourself
that the switch is the culprit
prior to buying another one. (!)
For what it's worth, in the 3rd and 4th attachments are pages 8A-110-2 & 8A-110-3 from the '95 FSM,
and in these 2 pages you will see the Stoplamp/TCC (Torque Converter Clutch) switch feeding into the
multifunction switch, as well as the wiring back to the Stop Lamps.
****
Don't know how much you have explored this forum, but there's a a cache of Factory Service Manuals that are
free for the downloading, so that you can take these troubleshooting hints and use them to really dig into your
problem circuit. If you follow this
LINK it will take you to where you can find the '95 FSMs that I was using above.
****
I know that we covered a lot of ground, but I wanted to show you that the brake light circuit does
not travel through
the Ignition Switch...but at the same time if there's some sort of failure involving a Key On powered circuit in the
multifunction switch that is interleaved with the brake light function then this might explain the failure symptoms
you are observing.
Hope this has provided some insight on how the circuit is documented to work.
Let us know what you find.
Cheers --
