park brake light (3 Viewers)
- Thread starter jbmareno
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Yeah, all the problems I have ever had electrically (on the 40) have been due to ground issues.
bj40green
Tssss, tssss
This is the diagram that fits my 78 BJ40.
The trick is in the brake pedal switch. It's a double switch.
The parking section works through the pressure sensors under the MC.
The low oil section works through the brake pedal switch.
So check the wiring at the pressure switches. They are normally closed and open up when you hit the pedal and build pressure.
If the oil is low or you have a leak, the switches stay close and the light wil come up.
Good luck,
Rudi
The trick is in the brake pedal switch. It's a double switch.
The parking section works through the pressure sensors under the MC.
The low oil section works through the brake pedal switch.
So check the wiring at the pressure switches. They are normally closed and open up when you hit the pedal and build pressure.
If the oil is low or you have a leak, the switches stay close and the light wil come up.
Good luck,
Rudi
Last edited:
Living in the Past
SILVER Star
Is it just me or is this diagram wrong. The wire out of the parking brake switch should go to ground not be tied into the brake pedal switch. The way it is now the parking park light will only have a path to ground when one of the two brake waring switches on the bottom of the brake master is made. The waring light circuit is fine but the ground for the parking brake light should be indepenent of the waring light ground which is thru the master cylinder low fluid switches.
bj40green
Tssss, tssss
As I said before, this is the diagram of my '78 BJ40.
Yes it is a funny set up but it works. I asked myself the question, why did Toyota merge 2 systems into 1 brake light?
Why not 2 seperate circuits with a light bulb each?
It took me an hour to figure out the double function of the pedal switch. Let me try to explain.
Situation: engine off, key on.
Pull the parking brake, light comes on. Light has ground via the 2 pressure switches which are normally closed.
Now step on the pedal, light goes out. Because the pressure switches open up and disconnect to ground. This is also a check function to see if your brake system works okay. If you step on the pedal and the light stays on it means trouble under the hood.
Start engine, light is on. Release the parking brake, light goes out. Step on the pedal. Light stays out. So far so good.
Now you have a oil leak or no oil situation. Here is the trick:
When you step on the pedal the rear brake lights goes on via the A-A' section of the switch. But you have a leak so the pedal goes deeper. Now the B-B' section takes over the parking brake switch to keep the circuit going. So the power route is now: fuse - light - B-B' section - pressure switch(es) and because of the leak or no oil the pressure switch stays closed so the light comes on.
This is how it works in my BJ. I know that there are different configurations with oil sensors in the lids, 2 light bulbs and more funny set-ups. I think that this is the most common configuartion for the late 70's series.
Rudi
bj40green
Tssss, tssss
I see a lot of pictures here on Mud, where the pressure switches are not connected. If they are not connected your brake warning light will not come up. See the 2 red wires under the MC. The red wires are my colors not the OEM colors (thanks to the PO).
Rudi
The bronze whisker type contacts inside the switch of my '78 broke off - a rare Toyota switch failure. Remove and test the switch.
I knew it worked this way but I never connected the dots to see this as a system check at start-up. Thanks for that.
According to the factory literature U.S. FJ40 models have a red w/white tracer wire
coming out of the harness to the first brake light warning switch (on the MC) and then a red wire to the second switch. Mine has a faded solid red wire coming out of the harness that's a pretty close match to what's in your pic so there was probably some variation. Minus the vacuum source for the booster and the probability that you have front drums vs. the U.S. spec front discs, I don't know if there's much difference in the wiring/switch set-up for FJ models.
bj40green
Tssss, tssss
Take your tester and select the resistance/ohm (Ω) function.
Disconnect wires to the Pressure Switches.
Connect the black lead to ground.
Connect the red lead to the bottom of the PS (Pressure Switch).
The reading should be 0 Ω. The switch is closed.
Now let somebody step on the brake, the reading should be infinity (∞) on an analog meter or OL (Open Loop) on a digital meter. The switch is open.
Test the second one. Same procedure.
Good luck
Rudi
Last edited:
Living in the Past
SILVER Star
Just got back to this thread. Thanks for clearing this up it's makes sense now. I plan on updating my 68 to disc brakes with 76-80 master cylinder booster. I'm sure I have both the double pole switch for the brake pedal and a spare hand brake with switch. Switch the green light in the dash for the 4WD to red which isn't used with a floor shifter and I can have a brake warning system too.
I just started trying to figure this out the other day. I tried to get the light to come on by replacing the bulb (2x) and tried three spare switches. I could only get the light to come on by grounding the light at the switch (just used a 6" piece of wire in experimental fashion), but couldn't figure out where the system had lost ground. When Rudi explained that the system grounded at the MC, I knew that was the problem because the PO had just snipped the little red wire that goes to the sensors (another 'why, Why WHY did you do that?! moment). I just grounded that wire to the body/frame and the parking brake warning light suddenly functioned normally. Of course, I'll need to actually connect the wire back up to the low fluid sensors in order for that warning function to work, but the parking brake light works now--and that's the mystery I was trying to solve. JD
bj40green
Tssss, tssss
I think you're talking about a "low fluid" sensor which has two wires. Maybe this diagram is helpful.
This a set up for the "ARL" market but gives an impression of different systems.
Rudi
This a set up for the "ARL" market but gives an impression of different systems.
Rudi
bj40green
Tssss, tssss
More info on the brake switch
I wrote a chapter about "how the brake switch works" and why it's so difficult to understand the FSM in another thread.
You can find it in posting #14 https://forum.ih8mud.com/40-55-series-tech/511958-electrical-problems-99-solved.html#post6917602
Rudi
I wrote a chapter about "how the brake switch works" and why it's so difficult to understand the FSM in another thread.
You can find it in posting #14 https://forum.ih8mud.com/40-55-series-tech/511958-electrical-problems-99-solved.html#post6917602
Rudi
Rudi, do you happen to know what the pressure rating for those switches is, both mine are open circuit and its quite difficult to get them in Australia and was wondering if they are common to other cars, or if I could just use normal oil pressure switch
