CletusLloyd Posted February 11, 2023 Share Posted February 11, 2023 Hello all, I have a 99 RD688S and the trailer brake lights are not working. All other lights are working fine. Does anyone know where I can test up at the cab for continuity with the trailer wiring plug? The sender seems to be mounted on the firewall by the valve cover is this correct? Any help is greatly appreciated. -Chris Quote Link to comment https://www.bigmacktrucks.com/topic/72505-99-rd688s-trailer-brake-lights/ Share on other sites More sharing options...
Frank Y Posted February 12, 2023 Share Posted February 12, 2023 To possibly ask the obvious question, but did you check for voltage at your trailer cord receptacle on the truck? Possibly a bad trailer cord. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/72505-99-rd688s-trailer-brake-lights/#findComment-525228 Share on other sites More sharing options...
mechohaulic Posted February 12, 2023 Share Posted February 12, 2023 defiantly isolate whether the problem is in tractor or trailer first. my day a test light was used ; now it a meter used. there is a tool which plugs in to light cord which will show what works. makes life easier when you sit in cab with light cord check each application. could be broken wire in trailer. do brake lights work on tractor?? not sure of wiring on "new "trucks. is the picture a brake light switch or low air buzzer sender etc. fuses check out ok ??? 1 Quote Link to comment https://www.bigmacktrucks.com/topic/72505-99-rd688s-trailer-brake-lights/#findComment-525229 Share on other sites More sharing options...
Joey Mack Posted February 12, 2023 Share Posted February 12, 2023 The fuse box on the firewall, has the trailer fuses in it.. and yes, check the plug at the back of the cab for brake light power.. jojo Quote Link to comment https://www.bigmacktrucks.com/topic/72505-99-rd688s-trailer-brake-lights/#findComment-525230 Share on other sites More sharing options...
Joey Mack Posted February 12, 2023 Share Posted February 12, 2023 (edited) Do you own one of these? If not, I suggest you get one.. You may not use it much, but when you do, you will be glad you have one.. $65.00 on Amazon... jojo Edited February 12, 2023 by Joey Mack poor sperring.. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/72505-99-rd688s-trailer-brake-lights/#findComment-525231 Share on other sites More sharing options...
Joey Mack Posted February 12, 2023 Share Posted February 12, 2023 Mech, That is the brake light switch. Its on the firewall in "R" models.. I think DM's too.. I forget sometimes.. jojo Quote Link to comment https://www.bigmacktrucks.com/topic/72505-99-rd688s-trailer-brake-lights/#findComment-525235 Share on other sites More sharing options...
mechohaulic Posted February 13, 2023 Share Posted February 13, 2023 16 hours ago, Joey Mack said: Do you own one of these? If not, I suggest you get one.. You may not use it much, but when you do, you will be glad you have one.. $65.00 on Amazon... jojo this is the tool of which i mentioned. wasn't too sure of the switch showed . my day most were on brake pedal valve. another positive moment old school meets new school. is that were the title intermediate school came from 1 Quote Link to comment https://www.bigmacktrucks.com/topic/72505-99-rd688s-trailer-brake-lights/#findComment-525275 Share on other sites More sharing options...
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