Brake Switch

[SMFire History]

In an effort to make our rig roadworthy I need to troubleshoot the brake switch. It appears to be beneath the pedal and I'm hoping I can get a little guidance on getting to it before pulling it all apart and potentially making it inoperable.

1952 125LS with air brakes. Here's a pic of the pedal. Do I need to remove the bolts near the outer collar and then once it's off remove the inner bolts near the actuator?

Thanks,

Dom

Edited December 4, 2013 by SMFire History

[blackdog2]

valve is under the floor panel pedal is on top

[j hancock]

You can get to the brake valve under the floor after removing the nuts (inner circle) and the the nuts and bolts (outer circle). This will get the pedal and base out of the way.

The valve itself won't move too much because the copper air lines going to the valve will support it.

Before doing anything does the brake pedal move the plunger (beneath the pedal roller)? If not than more than likely the valve is seized. With patience, the plunger may come out and the whole assembly can be inspected.

If you are not familiar with air brakes, it will be best to get someone who does look at it in person to see what is really going on. Information through the forum will be good but brakes are not a subject for making ANY mistakes.

[MHfred]

Are you talking about the switch that operates the brake lights? The brakes work ok but you do not have brake lights??? That switch that operates the lights will probably be away from the treadle valve. It might be easiest to follow the brake air line from the rear of the truck to the front until you see it. It will have 2 wire terminals on it.

[SMFire History]

Thank you for the replies and my apologies for a vague post initially. The brakes work fine and I have replaced the two rear cans already. The electrical switch is what I need to troubleshoot as I am not getting power to the brake lights.

Thanks,

Dom

[609albert]

could be 2 possible items ,bad brake light switch , brake light housing not making contact with the body, (macks had a positive ground system) ,also are the bulbs o.k. and check the wiring. let us know what you find causing the trouble.

[SMFire History]

I did not have a chance to pull the pedal today but we are getting tail lights and the rear blinkers worked as well prior to that switch failing so I think we're good on the ground. The bulbs are intact and the second wire leading back into the housing is not energized when applying the brake. I'll try to get to the switch this weekend and will post up results.

I found it odd that the running lights on top of the fenders are on when the battery is switched on. Is that common or were these re-wired for that? All the other running lights come on with the light switch.

Dom

[609albert]

do the fender lights have red lenses? my first mack (a 47 ) they came on with the headlights. your wiring might have been changed if not orginial wiring.

[SMFire History]

Yes red lenses. Looks like original wiring but you never know. It's low on the list but we would like to make it right. It has helped me see that I've left the battery on a couple times.

[thomastractorsvc]

I would see if you can get to it from underneath first, those bolts holding the brake pedal assembly are not going to come easy and you have all the copper lines hooked to the bottom of it. You may open a cans of worms with breaking the housing or twisting one of the copper lines. You should be able to finagle you hand and test light from underneath, it will look something like this but will probably have small screws holding the wires on, you can check to see if you have power to it with a test light one side should be hot then when you apply the brake it would close the circuit sending power to the rear lights.

Not sure how your switch is wired up but you may have to have the key in the run position.

For safety, It goes with out saying, always chalk the wheels and have the engine shut off pulled out if it is a diesel or discount the coil if it is a gasser.

[SMFire History]

Thanks for the pic. Gives me a good idea what to look for. Much appreciated. I'll give it a go from underneath first.

[fxfymn]

A squirt of PB Blaster might go a long way to fixing the problem.

[Loadstar]

I did not have a chance to pull the pedal today but we are getting tail lights and the rear blinkers worked as well prior to that switch failing so I think we're good on the ground. The bulbs are intact and the second wire leading back into the housing is not energized when applying the brake. I'll try to get to the switch this weekend and will post up results.

I found it odd that the running lights on top of the fenders are on when the battery is switched on. Is that common or were these re-wired for that? All the other running lights come on with the light switch.

Dom

. Smfire History, The fender lights on my L model are on a separate pull out switch....good luck with you brake lights

[SMFire History]

. Smfire History, The fender lights on my L model are on a separate pull out switch....good luck with you brake lights

Interesting... I'll have a look. We have a few switches on the dash that are disconnected and a few others that I don't know what they do.

I'm pretty sure I determined what the issue with the brake lights is. There's a large threaded hole under the pedal that is empty! I don't see any loose wires but they must be stuffed up near there or maybe cut off. How does the switch get activated?

Robert- Any idea where I can get this switch??

Thanks,

Dom

[fxfymn]

Follow the wires from the lights forward to see if there is a switch located somewhere else. If there are two wires under the pedal touching the two together should cause the lights to come on, if that fails use a jumper to put voltage to the wires under the pedal to see if the lights come on.

[j hancock]

Yes, follow wires is your best bet.

The switch would be available at your local truck parts store or NAPA. It is a generic item. Nothing trick or brand specific or expensive.

I think the part number is 13240 for Haldex, Euclid and NAPA brand.

[thomastractorsvc]

Yes, follow wires is your best bet.

The switch would be available at your local truck parts store or NAPA. It is a generic item. Nothing trick or brand specific or expensive.

I think the part number is 13240 for Haldex, Euclid and NAPA brand.

Ditto

[MHfred]

That large threaded hole inder the brake valve is the vent-it should be like that.

[Vladislav]

I'm pretty shure the light switch on your truck wich is an L-model is a separately one from the brake pedal/ brake actuator.

It must be fitted on the left frame rail under the cab and put into the control brake line.

My truck (NR wich is basicaly an L) has it like on the picture. Has two nuts with wires on the top.

I saw the same swich or of a nearly the same kind on other L-models I looked for that point.

By the way I also need the one because the plastic part of my switch has the tread broken and I'm afraid to loose brake air if it might be blasted off by pressure.

[SMFire History]

Thanks for the great information guys. Good to know the threaded spot beneath the pedal is normal as it looks difficult to get to. I'll check the drivers side frame rail land see what I find.

[SMFire History]

So it looks like the brake switch is pretty much like Vladislav posted and is where is suggested to look. I've attached a pic. Here's what I've determined...

1. Touching the wires together with the battery switch on does not illuminate the brake lights.

2. Neither wire has 12v to it with the battery on.

3. The wires return to the loom at the switch and the tail lights and they are hard to trace.

I will run a 12v power wire to the brake lights to be sure they illuminate. I'm thinking I may need to dig into the original wiring harness to determine whats up.

Thanks,

Dom

Edited December 9, 2013 by SMFire History

[farmer52]

Use a multi-meter and check continuity of the wires. Do you know the brake lamp bulbs and sockets are good?

[609albert]

looks like 2 wires on the switch ,1 has to be power ,the other to the light and taps off (if you have 2 brake lights). the easiest way I have found to solve electrical problems is with a temporary jumper, run a wire to a battery ,put an alagator clip on the other end ,take it back to the switch and clamp on 1 or the other studs ,before doing all this, check if the truck is run on positive ground or negative .if it is negative (like most MACKS) connect jumper to a negative terminal.

[Vladislav]

Glad you've found it.

No success on this step although you have at least the intermediate point to test electric.

[fxfymn]

Electrical troubleshooting on something this old is like the proverbial box of chocolates "You never know what you are going to get."

Most repair shops don't bother with troubleshooting wiring issues; they just run new wires and boom! it's fixed. If it is like most fire trucks of this era you will find a number of wires that don't do anything. The best thing to do is to remove them as you find them. (I pulled a five gallon bucket worth of wire out of mine and everything still worked!)

Going away the most common problem with older wiring is poor grounds. Carry a file and wire brush to give yourself a clean ground wherever you choose to test a circuit. I also try to have one hot wire with me so I can test my test light with whatever ground I'm using.

When I troubleshoot tail light/brake light circuits I like to use a trailer wiring harness that comes with a cheap trailer light kit. It is long enough to reach from the switch/firewall to the lights, has all the wires you need, and is color coded. Just add alligator clips to each end to make it quick and easy to use.