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Old 03-20-2017, 04:06 PM   #1
Mini-Skoolie
 
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Brake light switch 97 Thomas MVP

I'm guessing this is the right group to put this in. My MVP doesn't have brake lights anymore and I have been trying to trace wires to a switch but have had no luck. Found the wire that goes to the brake lights and can make them work from all the way to the harness by the distribution center but have had no luck finding a switch. Help please and thank you in advance
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Old 03-20-2017, 06:15 PM   #2
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Welcome bmartin194

I'm no electrician but that sounds like a bad fuse or breaker. On the other hand that's the first thing you'd likely check.

Other more electrical minded individuals will pitch in when they find this thread.
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Old 03-20-2017, 06:36 PM   #3
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Thank you. I have checked all the fuses and breakers I know to check that's why I'm thinking it's a switch but am having trouble tracking it down. Been an auto mechanic and offshore mechanic all my life. I have seen a switch on the brake valve but there is no power going to it and the wires do not match the closet wire in the harness that go back to the brake lights. I don't think it goes through the turn signal switch like most vehicles since the brake lights are separate.
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Old 03-20-2017, 07:58 PM   #4
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I'm no help electrically. If it goes beyond a '65 Ford PU wiring, I'm lost. I fuss with electrical problems for weeks and finally have to spend to get it fixed, but even then I learn something.

I'm still hoping some of the others will lend their electrical expertise. While you're waiting I'd suggest reading other current electrical issue threads and get to know who the electrical minded people are.

Electrical, Charging and Solar - School Bus Conversion Resources

This link looks like mainly solar information but you might find something in there. Have you tried Youtube yet?
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Old 03-20-2017, 08:08 PM   #5
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No not yet but thank you I will. If I could dig up a wiring diagram I would probably be home free. Appreciate the input
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Old 03-20-2017, 08:30 PM   #6
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There is talk about the wiring being partially designed by the chassie manufacturer and the body manufacturer wiring the body electrical system. You can sometimes get either the body wiring diagram or the chassie diagram. We use diagrams from other buses that are similar with some luck. There are experts here with years of experience. I'm not one of them.
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Old 03-20-2017, 10:46 PM   #7
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The brake light switch probably isn't what you are used to seeing. If you have air brakes it's probably air operated.
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Old 03-20-2017, 11:14 PM   #8
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Right and there is actually a pressure switch on the foot valve but wiring doesn't match up so I'm a little apprehensive on just putting power to it and seeing what happens
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Old 03-21-2017, 08:19 AM   #9
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Just how did you test them as you said tom make them work from one point to another. How did you activate them? Is it air brake or hydraulic? You have to be more specific with your request. If not, all we can do is guess what you have done or tested.
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Old 03-21-2017, 08:25 AM   #10
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Quote:
Originally Posted by BlackJohn View Post
Just how did you test them as you said tom make them work from one point to another. How did you activate them? Is it air brake or hydraulic? You have to be more specific with your request. If not, all we can do is guess what you have done or tested.
Give John whatever info he needs- he's a super smart and HELPFUL feller.
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Old 03-21-2017, 08:50 AM   #11
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You are too kind CB. Just an old hippy, retired electrician. these sorts of things are good for the brain cells which I don't have too any left.
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Old 03-21-2017, 08:53 AM   #12
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You are too kind CB. Just an old hippy, retired electrician. these sorts of things are good for the brain cells which I don't have too any left.
That's groovy, man, I dig.
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Old 03-21-2017, 11:44 AM   #13
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Brake lights

Quote:
Originally Posted by BlackJohn View Post
Just how did you test them as you said tom make them work from one point to another. How did you activate them? Is it air brake or hydraulic? You have to be more specific with your request. If not, all we can do is guess what you have done or tested.
Its air brakes. Sorry I just figured that this particular model bus with this drivetrain would be air brakes.
I have a Power Probe and I found the wire at the brake lights and found the same colored wire at the front where the weatherhead connector is and checked and applied power from the Power Probe and I had lights. Checked power at the switch with a volt meter while the power probe were still powering the brake lights and nothing. That's why I was thinking I'm wrong about that being the switch.
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Old 03-21-2017, 02:58 PM   #14
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How long have you had the bus? Familiar with the air brake system in it?
Do you drive the bus or does it sit for long periods of time without a run?
To me I have no way of knowing each bus except for the same basic colour, so I didn't know or assume what type of braking system it has.
I do have some thoughts on it but have to know how knowledgeable you are with it.
What do you mean weatherhead at the front? Is that a firewall entry point for the brakes or any/all wiring to enter the cab?
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Old 03-21-2017, 03:20 PM   #15
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Yes the weatherhead in the front where all the wires go in at. I have had the bus about a year and we've made a few trips in it. I run it a couple times a month and just before heading to Arizona I noticed the brake lights not working. I am not real familiar with this air brake system although I have replaced the diaphragms on the brake actuators on 18 wheelers before. I probably need to take a look from one end of the brake system to the other just to familiarize myself with it. I have a generic understanding of it since I have worked on some systems in the past. The switch I see on the air valve for the brakes may be the brake light switch but doesn't make sense due to what I was saying in my previous post. I did go ahead and order a book with all the schematics for the electrical systems so I'm hoping to find the wiring in there. I appreciate your trying to help me.
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Old 03-21-2017, 05:55 PM   #16
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So, you run it a couple of times a month. Would that be starting only and warmup or actually driving and using the brakes?
If you are going to keep it I would suggest understanding what you have for brakes. Generic gives you some advantage I'd think.Out on the coveralls and crawl under it. You'd be surprised by what you find about many aspects of that bus' workings. Brake layout would suggest that the brake switch is activated when you press the brake pedal.That may be either electrically or with an air valve that moves to close the brake light switch and give you brake lights at the rear, both sides.
If you can find that underneath then you may watch it operate with the aid of a helper. Make sure the air pressure has built up first before trying. Might just need some lube to free the plunger up and clean any dirt, grease off of it etc.

If that switch you were testing is it, then your test wouldn't work. One wire would or should be hot tested to ground. The other becomes hot when the air pressure makes the contact inside the switch which would light the lights.

Try that method and show in pics what you have if possible. That's about all I know of the supposed operation electrically.
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Old 03-21-2017, 06:18 PM   #17
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in my air brake bus the air system has to be fully pumped up before the brake lights work.. testing the lights before the air system is up nets no lights..

im assuming your air pressure has built fully.. as mentioned in most air brake systems, the brake light switch is activated by air pressure being put into the air valves.. on my carpenter the brake light switches (theres 2 of them) are located right on the air valve under the dash that the pedal connects to.

-Christopher
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Old 03-21-2017, 06:33 PM   #18
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There are some buses that route the brake light circuit through the spring brake so that whenever the key is in the run position and the spring brakes are applied the brake lights are on.

It has been a while since I had my head under the dash of our church's MVP EF. IIRC, the brake light switch was plumbed into the treadle valve. As it has been noted before, it won't operate if there is no air pressure in the system. It takes very little air pressure to make the switch activate the brake lights.

The switch has to have power to one side so that when the switch closes it will send juice out to the brake lights. If there is no juice on either side of the switch you most likely have a broken wire somewhere between the switch and the power supply. The power supply will be at the for all things 12-VDC supply in the outside electrical panel below the driver's window. Most of those circuits are protected by automatic resetting breakers. Many rely on relays, some of which are only operated when the key is on.

If you are able to put 12-VDC to the switch and then the lights go on if you apply the brake pedal you know your problem is upstream. In order for you to be able to get the bus on the road you may want to put a fused power source to the switch so that you have brake lights. That is until you determine where the juice to the switch got lost.

Good luck!
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Old 03-21-2017, 07:05 PM   #19
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Brake lights

I don't see any kind of manual type switch so pretty sure it operates by air. I usually start it and get full pressure then release the brakes and if I don't drive it I run it back and up about 10 feet. The air is fully charged when I am checking for lights. I'm in Arizona right now on milled asphalt and it's supposed to cool back off Thursday so come Thursday I'll give y'all and update. Appreciate all the input.
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Old 03-21-2017, 07:08 PM   #20
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I posted more info down below before I realized I wasn't replying to your thread. I am keeping it and am building it out inside as we speak. Thursday I'll post some pics since it's supposed to cool off some right now it's miserable. Thanks for the help.
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