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Brake lights intermittently working on a 2007 Mack MR688


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So I've got a Mack MR tractor that I'm turning into a dump truck.  Working some of the bugs out of it and today I noticed the brake lights are intermittently working. This truck has the brake lights and turn signals separate. When I step on the brake, they flicker.  Sometimes they work, sometimes they don't.  I can hear the relay clicking.  I've got a generic fuse/relay diagram and there's a couple of relays that mention stop.  Looks like many of them are the same and I switched them around with other relays and still no luck.  I put a test light on the terminal/post marked stop and the test light acts just like the brake light.  I'm pretty mechanically inclined and trying to figure out what it is.  I'm thinking it may be the actual switch that's activated by air pressure when you hit the brake pedal.  Have no idea where it's at though.  Any other ideas?

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gosh,,  i have forgotten a lot about MR's..  is there a bank of air switches above the pedals? I'm sorry.. i think there are also air switches in the center console..  the intermittent issue to me is a loose connection rather than a pressure switch..  jojo

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I didn't look above the pedals.  I don't think there's anything there.  The relays/fuses are all in that huge center console that's all accessible by removing a plates from the passengers side.

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because I suck at MR's...  i would be opening that panel, exposing the air switches, fill the air tanks , and mash the brake pedal, than loosen 1 air switch at a time till I found the ones that have pedal pressure on them..  if its a leaker,, put the shop compressor on it if running the engine is too noisy...  just saying...  sometime I have to be a boyscout if i dont know the truck by 'heart'.....   jojo

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18 minutes ago, Joey Mack said:

because I suck at MR's...  i would be opening that panel, exposing the air switches, fill the air tanks , and mash the brake pedal, than loosen 1 air switch at a time till I found the ones that have pedal pressure on them..  if its a leaker,, put the shop compressor on it if running the engine is too noisy...  just saying...  sometime I have to be a boyscout if i dont know the truck by 'heart'.....   jojo

So these air switches have a wire coming off of them which activates the relay for the brake lights?  How many air switches are there and what else are they used for?  Looks like I need to go find where they are.

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they have several functions. some signal the lights some the cruise control, some the air gauges, some for the low air buzzer,  this is what you need to study and learn about... I suggest Ebay for a book.  or find a svc manual download.. sorry bud..  i hope an MR Guru shows up to help you... jojo

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Yesterday I took a look in the center console where the fuses and relays are all at.  There was a few air switches in there and the wires were all labeled and none of them were for the stop switch.  Called the dealer and asked the parts guy where the stop switch was at.  He told me they were all down by the actually pedal.   I tilted up the cab and they were all on a valve underneath the pedal.  He mentioned there were 4 of them and they were all the same switches, just for different functions.  I was hoping to find a loose wire or something rubbed through down there.  All the switches look good with no corrosion or anything on them.  The wires all look good also.  I didn't check each switch yet with a meter to see if they were working correctly.  I'm thinking if it's not actually one of the switches that's bad, I may be better off just running new wires to one of the switches to a new relay in the dash and bypass the entire stop system.  If it's a rubbed through wire, I don't think there's any easy way to find it.  Plus I may do more damage to the existing wiring of the vehicle.  Thoughts?

 

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fill the truck with air,  if it leaks down quickly, use shop air so the truck isnt running all the time....  loosen a pressure switch gently to see if air bleeds off...  if it does, mark it and tighten it back up...  do this until you find one that doesnt leak off.....  then, put a stick or a shortened load bar like i have on the brake pedal and up to what ever will hold it down,,  and see if that pressure switch that didnt leak off , leaks off now. with the pedal pushed.. ..  that would be the brake light switch..  mark it well, and continue with youre search...  you can take the 2 wires off the pressure switch, and cross them and the brake lights should be lit....  jojo

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