688mack Posted April 13, 2023 Share Posted April 13, 2023 99 rd688s replaced speed sensor an speedo. Speedo would sweep 0-max while sitting at red light 3-4 times but would read fine while moving. Now it doesn’t work at all will still sweep 0-max when sitting still but nothing while moving. I have noticed that needle will move when u adjust the dash lights also push the clutch in it will sweep turn headlights on it sweeps. I have found a bare wire on top of trans for speed sensor but that did nothing im in the dash now looking for bare wires now not seeing any yet. There are sings of a mouse being in there though. Does anyone happen to know the numbers an letters for wire in the speed circuit any help would be great. Also lightning bolt is on an i belive it was speed control Quote Link to comment https://www.bigmacktrucks.com/topic/72725-speedo-not-working/ Share on other sites More sharing options...
Joey Mack Posted April 13, 2023 Share Posted April 13, 2023 Dash wiring grounds.... Check the grounds.. it sounds like a back feed through the dash lights.. Quote Link to comment https://www.bigmacktrucks.com/topic/72725-speedo-not-working/#findComment-529111 Share on other sites More sharing options...
Joey Mack Posted April 13, 2023 Share Posted April 13, 2023 Peel the wire loom on the speed sensor back and read the numbers.. you should see something like,,,. VJ 2 , 14, .05. That is the wire code. V= vecu plug, J2 would be the plug number, 14= pin number.. then go to the dash wires and look for that code. Then you have found the wires related to the speed sensor. Also go to the VECU, to check wire condition.. (The wire code) i gave you is a sample, not the actual numbers.. I still think it's a bad ground.. keep in touch.. Jojo 1 Quote Link to comment https://www.bigmacktrucks.com/topic/72725-speedo-not-working/#findComment-529112 Share on other sites More sharing options...
mechohaulic Posted April 13, 2023 Share Posted April 13, 2023 was never that difficult in a B model . speedo dont work broken cable. so challenging for an ole man to believe technology is a positive . mechanically speaking of course. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/72725-speedo-not-working/#findComment-529138 Share on other sites More sharing options...
Joey Mack Posted April 13, 2023 Share Posted April 13, 2023 Mech.. I do take my hat off to Mack for this.. '98ish to 2006, all the wires are white, black is a ground,,, yellow and green twisted pairs are Comm. line for the ECU's (1939 lines)... So when you read a wire, lets say under the hood.. You will see something like EJ-1 42 .. it means that the wire circuit is through the Engine computer. It would the J-1 plug,,, pin 42,,, there are a couple other numbers that will tell wire size and ''intersection'' for lack of a better word.. I should probably go to the book to refresh my memory.. but the point is, for basic wire tracing, Mack did a good job.. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/72725-speedo-not-working/#findComment-529140 Share on other sites More sharing options...
mechohaulic Posted April 14, 2023 Share Posted April 14, 2023 in most all aspects Mack did a great job building a complete truck; from small to large. might be the buy outs/take overs changed things . not for positive, Quote Link to comment https://www.bigmacktrucks.com/topic/72725-speedo-not-working/#findComment-529148 Share on other sites More sharing options...
688mack Posted May 1, 2023 Author Share Posted May 1, 2023 So wiring is checking out ok my problem must either be the speedo head or in the vecu. Quote Link to comment https://www.bigmacktrucks.com/topic/72725-speedo-not-working/#findComment-530619 Share on other sites More sharing options...
Joey Mack Posted May 1, 2023 Share Posted May 1, 2023 do you have the speed sensor that is threaded and has a silver lock nut? It has to be set a specific depth... turned all the way in, then '1' full turn out, lock it down.. I still think you have a grounding issue with what you described.. Quote Link to comment https://www.bigmacktrucks.com/topic/72725-speedo-not-working/#findComment-530625 Share on other sites More sharing options...
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