OK, I'm working on my buddy's truck and we have this problem that I don't really understand, hopefully someone here does.
Simply put, the brake lights do not function on this truck. They used to and then one day they just quit.
When the switch on the dash is pulled, all the lights on the truck turn on including the tail lights, but when the brake is depressed the brake lights do not light up.
I've checked the grounds for the lights and they are fine. The problem is that no electricity is getting to them. I looked up the brake light wires in a Haynes Manual, then found the wires on chassis. I've traced them back into the engine compartment where they link into the wiring harness and I have tested them this far. They are stone dead. I have also located them below the dashboard but not tested them there.
We replaced the brake switch and this did not solve the problem.
According to the wiring diagram, the wires should at some point run to the Multi-Function Switch. Would this switch malfunctioning cause the problem I have related above? Where is this switch, and how hard is it to replace?
Additional information: This truck blew it's right head gasket last fall, so we pulled the engine apart down to exposing the cylinders, cleaned it up and put all new seals on it. After putting her back together she functioned perfectly (hell, better than before with all the gunk taken out of parts of the motor). Then all of a sudden this happened a few weeks later... is it possible that we unknowingly did something that eventually caused this problem?
Also, all the fuses are fine. According to the fuse diagram all the lights on the back of the truck run off of fuse 1 (15amp). This fuse is fine, I even went so far as to test it with my multi-meter.
I'm used to working on later model than this & I don't have the resources for these older models but let's get you started and maybe someone will correct me if I'm wrong. Does the stop lamp (CHMSL) on the roof of the cab work? Do the turn signals work? "Assuming" the '90 is like later models, the CHMSL does not run through the multifunction switch (MFS). If the CHMSL works then the stop light circuit out of the SL switch is fine. If the turn signals work then the wiring from the MFS to the tail lights is fine. You then must check the incoming SL wiring to the MFS, which I think in your case will be a Lt.Green wire. This is '92 pinout but it "should" be the same on your '90. You'l be checking circuit 511 for 12V when brakes are applied.
You must check the wiring to (LG/R) and from (LG) the brake light switch to see if it's hot. If the CHMSL is not working then no power is getting to it nor the MFS. Fuse # 1 does power the stoplight switch and hazards. You must either have a bad brake light switch or a broken wire. Make sure the brake light switch is installed properly and actually operates as designed.
On this particular vehicle there isn't a combination switch like that. The stalk on the left of the steering column is just a single piece of plastic that controls your turn signals and high/low beams.
The switch to turn on the lights is a pull switch left of the steering column, closest to the door. Wipers are controlled by a switch next to this, to the right of the light pull-switch and to the left of the steering column.
They are both just above above the fuse panel, the back of which is the general area where both the wires from the brake switch run to, or at least into that general area (I assume to the pull switch for the lights actually). Once they get up there I am unable to continue to follow them and see or feel where they actually go.
I get the feeling I'm going to have to remove the dashboard?
Here's the schematic for a 88. Can't say it's exactly the
same as a 90 but it sounds like it's very close.
Since Bubba has worked on the truck you could have
more than one problem.
Fuse F1 is hot at all times and comes from a fusible link
at the Starter Solenoid (Battery side) terminal lug.
It supplies 12 volts to a Red/White wire at the Hazard
flasher which is located behind the fuse panel.
The turn signal flasher is the one located in front of the fuse
At the Hazard flasher the Red/White wire
is also connected to a Light Green/Red wire that goes to the
Brake Switch. When the Brake switch is closed 12 volts passes
through the switch to a Light Green wire that goes
to the turn signal/hazard switch assembly.
12 volts passes through the turn signal/hazard switch assembly
to the brake light bulbs.