MCI8 Brake Light Help Please

orfunauto/Darrell · October 07, 2007, 04:55:17 PM

OK folks, got a problem here that I hope somebody can help me with. My brake lights as well as my brake telltale light (green only) stay on. All systems function as normal. Brakes work properly when applying either the foot brake or park brake. Audible warning shuts off when air pressure reaches 60#'s. Brakes release and park brake light goes out after pressing down the release button. The lights came on while I was on my way to Carlisle, PA this past week. I stopped at a rest stop, checked what I could at oh-dark-thirty in the morning. I couldn't find anything, so I continued on, as I felt that everything was operating as normal with the exception of the brake lights being on. No loss of pressure, no excessive air compressor cycling, just normal operation except for those d--- lights! I finished the 900 hundred mile trip with no other problems, even had a panic stop for an idiot that pulled in front of me and stopped....
I just finished disconnecting the brake light switch and of course, the lights went out. But I did notice that the wires from either terminal were hot, even when disconnected from the switch. I disconnected the brake light relay and that also put out the lights. And I also noticed the relay was hot on both sides when the brakes weren't applied. As you can probably tell, I'm a novice when it comes to MCI's electrical system. I do have both the maintenance manual and the parts manual. Any suggestions as to where to look for the trouble would be greatly appreciated.

NJT5047 · October 07, 2007, 07:07:41 PM

Have you replaced the brake light relay? The unit in the RJB? There's a circuit breaker in the FJB. Isolating the circuit breaker should douse the lights. That eliminates the problem of powered shorts from other circuits. The brake light relay, is in the RJB, lower RH corner. Remove the relay and verify that it opens when no power is applied.
If the brake switch has power on both leads (and you don't have a cruise control brake switch with multiple terminals), removing the relay should eliminate the power to the other side of the brake switch. As long as power isn't applied and the park brake is set...which will set whenever the air is low. Be sure that the bus is aired up and the park brake is off, or bleed off all the air and see if the brake switch circuit is closed. This would be easy to check at the RJB/brake light relay leads
The manual that I have shows two separate sets of contacts for the service brakes and park brake systems. I'd assume that both switches would be in the spare tire compartment. If two exist??
If you still have a closed brake circuit with the relay removed, there must be a separate park brake switch located somewhere??
Any electrical repairs made to the bus prior to the brake light failure?
Sounds interesting!

Good luck, JR

Jerry32 · October 07, 2007, 07:14:26 PM

Doesn't the fact that disconnesting the brake light sw on the service brake indicate it is faulty? I don't have his wiringdiagram but min shows that park and service feed the light curcuit in paralell. jerry

orfunauto/Darrell · October 07, 2007, 07:18:56 PM

Thanks for your reply JR.

Could you please give me the definition of your acronyms? - RJB/FJB

I should have stated in my original post that all my electrical checks were made with the air pressure up, switches on, wheels blocked, and the brakes released.

orfunauto/Darrell · October 07, 2007, 07:24:24 PM

DUH

Front junction block, rear junction block......

, sorry about that JR! Jerry, disconnecting the switch still gives me a hot wire on both sides.......?, so is there a back feed from something?

orfunauto/Darrell · October 07, 2007, 07:27:58 PM

JR, I've only checked the circuit breaker in the FJB, which was mentioned in the first post as a relay..... It's too dark out now to check the relay in the RJB. Will do that first thing in the morning.

NJT5047 · October 07, 2007, 08:16:54 PM

Yeah, pull the relay in the lower RH RJB and that should remove the power to the brake light indicator.
Jerry, my thinking is that if he has power on both sides of the terminals at the brake switch, which is what is indicated, his relay may have failed and it's welded itself closed??? Now his relay is a shunt. Maybe.
I'm definitely guessing. But if removing the relay doesn't correct the "power on both leads" thing, the wiring on #11 becomes suspect. If it does, the relay may be at fault.
His lights probably operate on one switch. Even though, I see what you are seeing with regards to the two sets of contacts. MCI's schematics are crap. They are what they are however, and it's what we gotta work with. There may be two sets of contacts, and if they are still hot at the brake light switch once the relay is out of the circuit, bingo...find the other switch. Pulling the circuit breaker in the FJB will rule out shorts causing backfeeds by the absence of power once the breaker is isolated. If the leads still have power, it's coming from somewhere that's not correct.

IE: Tailight wiring such as new toad wiring, repairs or replacement to/of the taillights, or any work in the RJB or even the FJB could cause the brake light to power up inapropriately.
Gotta go, I'll check back tomorrow. Hope Orfunauto is able to access the RJB without too much difficulty. The fault can be resolved once the relay is available.
I still wonder if the park brake and service brake share the same contacts? Still could be a bad park brake switch.
We'll figure it out!
JR

orfunauto/Darrell · October 08, 2007, 09:25:32 AM

Well I've been out there about two hours now and might have narrowed it down a bit. First thing I did was to disconnect the relay in the RJB. The rear lights went out as well as the telltale. I examined the relay and found that the points were clean and free for movement. Then I started working backwards. The brake light switch was still hot on both sides. Disconnecting all wires (two on each terminal) I found that on what should be the cold side that one of the wires were still hot. Suspecting back-feed, I traced that wire over to a small pressure switch which was hot on both sides. This switch is mounted into a junction block that actually has two of the same type pressure switches mounted, one on each end, with several air lines going into the brass block. I hooked the wires back up to the brake light switch and then disconnected the wire from the cold side of that small pressure switch, the lights went out.... I have a call in to the local MCI parts distributor, and he should be calling me back to help me identify the suspected pressure switch, and possibly help me further in my diagnostics. I'll let you know what he says as soon as he calls and tells me what he thinks. Thanks again for your help!

Sammy · October 08, 2007, 12:50:04 PM

Orfun, what is the unit number of your coach?
It should be on the metal ID plate.
There are different schematic diagrams for your coach.
For example - one shows a diode on the stop lamp relay,the other does not.
Let us know,might be able to assist some more.

DavidInWilmNC · October 08, 2007, 01:27:55 PM

Like Sammy said, if we have the unit number of your coach, we can probably help more. The manual for my 8 has several diagrams depending on which unit you have. The manual isn't terribly clear, but I can scan the particular page (or fax it) if that'll help. On my 8, the dash brake light comes on when the brakes are on, which is normal. The fact that it problem originates with the switch - the problem goes away when you disconnect the switch - makes me think the switch is the problem. Do the lights work as normal if you bypass the switch? If the switch is disconnected, does the parking brake still activate the brake lights? If so, I'd definitely suspect the switch. I'd also check the resistance of it, as it appears to be stuck on, but not making good contact, which would create heat. Electrical issues sure can be a PIA! If my brakes lights were doing this, here's what I'd do:

1) release the parking brake (lights should stay on, if the switch is bad)
2) disconnect switch - parking brake released (lights should go off, if the switch is bad)
3) bypass the switch (lights should come on if the switch is bad)
4) 'un-bypass' the switch and set the parking brake (the lights should come back on if the switch is bad)
5) with the switch disconnected, and service brakes not applied, check the switch for continuity. If there's continuity, I'd say that the switch is bad.
6) with the hot side of the switch connected, disconnect the 'load' side of the switch, and apply the service brakes. There should be voltage present between that terminal and ground, if the switch is good.

Now, I haven't memorized the wiring diagrams of my 8 and can't be sure if the same switch is used for the parking brake and service brake, but I don't think so. Also, I'm much better at troubleshooting in person (very good at it, in fact)! I'm sure some of the others will let us know if any of what I've posted is incorrect. Let me know if you need any info from my wiring diagrams.

David

orfunauto/Darrell · October 08, 2007, 02:58:14 PM

Problem solved! Coach # is 30632. It was the brake pressure switch that works with the release button. (service brake switch??) I semi-diagnosed it as the problem and took it out. Performed a bench test with a regulated air supply and found that the switch would not open no matter how much pressure. Went to the local MCI parts supplier and bought a replacement. All is well!!! Thanks everyone for all your help. It was greatly appreciated.

NJT5047 · October 08, 2007, 07:22:25 PM

Darrell, now that your bus has brake lights, you might as well come on over to Kyle's rally this coming weekend! It ain't that far!

BTW, that's the "park" brake switch if it works with the park brake button. The term "service" brakes, refers to the bus highway stoppers.
I would have thought that the switch "closed" with air pressure??? I mighta thought wrong too? Sammy....??
JR

Sammy · October 09, 2007, 12:09:56 PM

Gotta do more homework on it, lol
Regards,
Sammy

orfunauto/Darrell · October 09, 2007, 04:23:04 PM

I wish I could make it to Kyle's non-rally JR, but my better half can't get the time off, and I used up my kitchen pass last week going to Carlisle, PA!!

The switch is normally open when the brakes are not applied, and closed when applied by either the foot or park brake.

NJT5047 · October 09, 2007, 06:53:13 PM

Hi Darrell,
My guess is that no air = "NO". Air on the application side of the system closes the contacts? Othewise, whenever the air bleeds off the bus, the contacts would close. Not a problem for most buses since the battery master is left off, but the same switches are used on trucks. That would be problematic on trucks.
One other thought comes to mind...and that would be if you have application air in the system when no brakes are called for. That would also be a problem...but, if you got it fixed, that isn't an issue. Always be sure that the brakes are completely released before heading out.
Ya'll still working on a spring break rally? Post the dates whenever you get'r figured out.
Try to avoid Mother's Day! Got to go to Denton.
We'll be down in Fayetteville again next July 4th...at Laurel Lakes Campground. That isn't too far from you. It's about 25 miles east of Fayetteville.
See you one of these days! JR