concrete animal Posted March 10, 2008 Share Posted March 10, 2008 Hello all, have a problem with brake lights. they only come on if you step harder on brakes than you usualy will.. recient switch replacement done my driver not employed any more, truck is 1985 mr 688 switch is on pedal valve. what is likely cause. switch screws in so no rod to adjust.. does air pressure engage switch? THANKS Quote Link to comment Share on other sites More sharing options...
ThaddeusW Posted March 10, 2008 Share Posted March 10, 2008 Hello all, have a problem with brake lights. they only come on if you step harder on brakes than you usualy will.. recient switch replacement done my driver not employed any more, truck is 1985 mr 688 switch is on pedal valve. what is likely cause. switch screws in so no rod to adjust.. does air pressure engage switch? THANKSOn all trucks equiped with air brakes the brake lights come on with a switch that is turned on by the air pressure. Are you sure its mounted to your brake pedal? Quote -Thad What America needs is less bull and more Bulldog! Link to comment Share on other sites More sharing options...
concrete animal Posted March 10, 2008 Author Share Posted March 10, 2008 On all trucks equiped with air brakes the brake lights come on with a switch that is turned on by the air pressure. Are you sure its mounted to your brake pedal? mounted on PEDAL VALVE. If all are brake- air pressure activated- the switch is requiring to much pressure to activate? Quote Link to comment Share on other sites More sharing options...
ThaddeusW Posted March 10, 2008 Share Posted March 10, 2008 mounted on PEDAL VALVE. If all are brake- air pressure activated- the switch is requiring to much pressure to activate?Hmm ok so the switch should have two terminals on one side for the wires and the other side is threaded into one of the foot valve ports. I would buy another switch and just replace it. Just unhook the two wires and unscrew the switch and screw the new one in and hook the wires back up. If you have any more questions don't hesitate to ask. Quote -Thad What America needs is less bull and more Bulldog! Link to comment Share on other sites More sharing options...
theakerstwo Posted March 10, 2008 Share Posted March 10, 2008 mounted on PEDAL VALVE. If all are brake- air pressure activated- the switch is requiring to much pressure to activate? he may be looking at the jake brake switch. Quote glenn akers Link to comment Share on other sites More sharing options...
ThaddeusW Posted March 10, 2008 Share Posted March 10, 2008 he may be looking at the jake brake switch.This is what I am trying to figure out. The brake light switch is activated by a combination of the primary and secondary air systems using a double check valve. It may be possible it was located or re-located to the brake pedal. There might possible have been or is a foot valve with an integrated stop light port.concrete animal, Do you have a manual transmission? If so make sure you are only looking at the brake pedal and not the clutch or go pedal. If its an Allison does it have the integrated retarder? I ask because there is a foot pedal from Bendix that activates the transmission retarder depending on how hard you step on the pedal. Quote -Thad What America needs is less bull and more Bulldog! Link to comment Share on other sites More sharing options...
concrete animal Posted March 10, 2008 Author Share Posted March 10, 2008 Hmm ok so the switch should have two terminals on one side for the wires and the other side is threaded into one of the foot valve ports. I would buy another switch and just replace it. Just unhook the two wires and unscrew the switch and screw the new one in and hook the wires back up. If you have any more questions don't hesitate to ask.YES thats the switch. Truck has no JAKE. Its a new switch now.. Question is --was is it activated by air? or does something physicly (solid) open the switch. I didn't switch part employee did, if not air triggered did he lose a part? could mack have sold mE a switch (if air activated) of the wrong pressure requirment to open? If as was posted previously- air-pressure opens it and it only opens under heavy braking. its to heavy a switch.. Hate to over analise this dealer is hour away, maybe NAPA WILL HAVE REPLACEMENT. Quote Link to comment Share on other sites More sharing options...
Maxville Truck Posted March 11, 2008 Share Posted March 11, 2008 Yes the switch is "air pressure" activated, I have several of these MR's Most of mine have 2 or more of the air switches located, on the foot valve (under the cab, have to jack cab over to get at them). Any chance that there is a leak on the switch itself? also check the wiring on the "switches" mine were wired parallel. I'm guessing that you may have an issue with your brake valve not sending enough "air signal" out to the switch, Making the assumtion that your supposed to have 2 air brake switches wired in paralell and one is not functioning, (holy crap now i'm rambling) listen for a leak by your foot when you hit the brakes. generally the switches will activate with very little pressure. Quote Link to comment Share on other sites More sharing options...
Mackpro Posted March 11, 2008 Share Posted March 11, 2008 Go to a mack dealer and have them look up the correct part#. Many of the switches look the same but open or close at different pressures. We have seen this on CH style trucks where the tech or parts guys guess and the wrong switch gets put on causes weird problems. Gotta a model # (MR688S) and last 6 of VIN and I can look up the part# Quote Link to comment Share on other sites More sharing options...
ThaddeusW Posted March 11, 2008 Share Posted March 11, 2008 Maxville gets the prize! Two air switches in parallel makes sense as instead of plumbing one into the air lines just thread two right into the pedals primary and secondary ports and wire them in parallel so if one circuit goes out the other will still activate the lights. They are sensitive and have a crack pressure of 5psi. If the air pressure was less than 5 psi then you would definatly notice a slight problem with your brakes! You can fix them, see below:Here is a service data sheet from bendix: http://www.bendixvrc.com/itemDisplay.asp?documentID=4936My advice would be to just replace the switch with one of similar type and call it a day. Quote -Thad What America needs is less bull and more Bulldog! Link to comment Share on other sites More sharing options...
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