Chad_Mack7 Posted January 28, 2022 Share Posted January 28, 2022 So I have a 1999 mack mr688s that has a concrete boom pump mounted on it. All of the park lights, brake lights, and blinkers quit working pretty much at the same time. 2 lights on either side going down the truck and everything on the back. I've been chasing wires and found nothing. I can't find or don't know of any diagram for relays or fuses. Everything on the cab works great. Clearance lights, front park lights, headlights and front/side of cab blinkers all work great, just everything from the cab back isn't working. Anyone have any ideas?? Quote Link to comment Share on other sites More sharing options...
Onyx610 Posted January 28, 2022 Share Posted January 28, 2022 Did you check under the truck/along the frame rail for a large wire/connector plug? Usually up fitters will use this style from the truck side and join it into the dump body or whatever it is on the back. A lot of times that “plug” can come loose. It takes all your functions from the cab and transfers it to the back and then splits off to everything. You might have this or you might not. Quote Link to comment Share on other sites More sharing options...
Onyx610 Posted January 28, 2022 Share Posted January 28, 2022 If you haven’t already I’d say trace some wires back towards the cab to see where they go. I bet along the way you find the connection. Might have to cut a lot of zip ties…. Quote Link to comment Share on other sites More sharing options...
Chad_Mack7 Posted January 28, 2022 Author Share Posted January 28, 2022 I traced them all up to the cab and it seems to just go into the cab with no connector. I'll have to double check, but pretty sure it all ran without connectors. Or maybe they ran wires into the cab and did the connection under the dash? Quote Link to comment Share on other sites More sharing options...
Onyx610 Posted January 28, 2022 Share Posted January 28, 2022 (edited) That’s very possible too. The fact that everything isn’t working makes me think broken connection between a truck side harness and the aux harness or whatever you want to call it. Edited January 28, 2022 by Onyx610 Quote Link to comment Share on other sites More sharing options...
Chad_Mack7 Posted January 28, 2022 Author Share Posted January 28, 2022 Good point! There wouldn't be a separate fuse or relay for just the back lights or something? I'll double check everything tomorrow and see what I find! Quote Link to comment Share on other sites More sharing options...
Onyx610 Posted January 28, 2022 Share Posted January 28, 2022 There could be! If there is it’s hidden somewhere….These things are hard because everybody has a different way of wiring things and you never can get any info on them. So it’s almost always a search and find and it’s usually a very slow process. I feel your pain. Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted January 28, 2022 Share Posted January 28, 2022 Have you looked at the left front where the large wire harness comes from the cab and bends around the the pivot point. I have made many repairs to wires there due to chaffing. MR's/LE's have a pretty fat wire harness in front. I can't remember it in detail like I used too. I havent worked on an MR in a few years. I always thought of these trucks as a wiring nightmare. I remember opening the the center console and krindging. Best of luck, Jojo Quote Link to comment Share on other sites More sharing options...
85snowdog Posted January 28, 2022 Share Posted January 28, 2022 Maybe double check the back is grounded . 1 Quote Keith Link to comment Share on other sites More sharing options...
Chad_Mack7 Posted February 9, 2022 Author Share Posted February 9, 2022 Well, I finally figured it out. Double checked the ground and re-did it to be sure it was well grounded. No change. I tore the whole dash and wiring harness apart basically, tracing wires from the switches to the harness and trying to find any issues. Then I found a junction box at the back of the truck where truck harness wires came in and then were disbursed to the appropriate lights. Well, the wires I'd been chasing (the factory wires) weren't even connected anymore. Instead, I found a newer set of wires running into the box and connected to the appropriate connections. Someone re-wired the lights at some point!! Very carefully traced the new wires and found that right between the two drive axles, the wires were hanging down and hitting the drive shaft. All the wires except the reverse wire were completely cut through... Thanks a lot previous owner! Anyways, did a legitimate splice job, covered wires well, rerouted them to a safer location, and all is functioning as it should! I appreciate the help!! 1 Quote Link to comment Share on other sites More sharing options...
JoeH Posted February 9, 2022 Share Posted February 9, 2022 Gotta love jerks! Quote Link to comment Share on other sites More sharing options...
JoeH Posted February 9, 2022 Share Posted February 9, 2022 On 1/28/2022 at 3:02 AM, Joey Mack said: Have you looked at the left front where the large wire harness comes from the cab and bends around the the pivot point. I have made many repairs to wires there due to chaffing. MR's/LE's have a pretty fat wire harness in front. I can't remember it in detail like I used too. I havent worked on an MR in a few years. I always thought of these trucks as a wiring nightmare. I remember opening the the center console and krindging. Best of luck, Jojo I know that feeling. But I've grown to like our mr688 because when you lift that center dash panel it's ALL there! Wide open, no skinny access panels in the dash like R models have. If you can't lift the center dash panel out due to the trash truck controls well... Yeah that would be awful. Especially because the factory wiring is buried below crappy wiring for the trash body. Quote Link to comment Share on other sites More sharing options...
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