Ryanp Posted April 22, 2022 Share Posted April 22, 2022 So Ive been working on this basket case MR 1/2 the winter and its time to finish the small things. Im down to 1 blink code I think. I have a 4-1 blink that indicates (transmission output) speed sensor problems. Trying to condense the situation as best I can.. Got the truck running but it wouldn't shift past 1st gear. (speedo worked) Figured out the harness from the trans controller to the output speed sensor was not connected because the wires that by the diagram were supposed to go to the sensor from the controller were stubbed out of the cab hinge connector and (factory?) heat shrink dead ended about 4" into the loom. So I went ahead and located the sensor wires, Not knowing where they actually went, (large harness going into the abyss by the oil filters.) cut them and ran my own new wires to the stubs that the diagram showed. Doing this made the truck shift properly but now the speedo doesn't work. So I figured the sensor wires must have originally ran up to the speedo in some round about way. So I spliced the wires back in so that now its the way it was, plus my new wires. Truck still shifts, but still no speedo. Speedo doesnt bother me that bad but it could be part of my 4-1 blink code? Looking for ideas and where to look next. I've got pretty good wiring diagrams but I'm not an expert. Picture of the diagram I'm looking at and some truck pictures for fun. Going to get tanks on it soon to be a sprayer tender. Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/ Share on other sites More sharing options...
Joey Mack Posted April 22, 2022 Share Posted April 22, 2022 Hmmm.. Do you have power to the plug for the speed sensor? the sensor talks to the vecu. did you disconnect that wire by accident? Did you OHM the sensor? here are some specs... Allison HD sensor ohms... 270-330 /// Allison HT sensor ohms... 2900-3400 . good luck, jojo Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-495872 Share on other sites More sharing options...
JoeH Posted April 27, 2022 Share Posted April 27, 2022 Do you mind sharing where you got that wiring diagram? I'd love to get a copy of the wiring diagrams for my MR... Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-496153 Share on other sites More sharing options...
Ryanp Posted May 1, 2022 Author Share Posted May 1, 2022 Joey Mack, Ive got 1.5 volts going to the sensor and 290 ohms across the sensor itself. What voltage should it be running at the sensor? Maybe I should check what kind of power I have on the other trans sensors, I didn't think of that. JoeH, The tube that the diagrams came in are addressed from a place called Pacesetter Enterprises. Allentown PA I looked online and cant seem to find it again. Also looking back through my emails for when it was ordered i cant find it. I will look tomorrow if there is anything on the sheets that say. Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-496483 Share on other sites More sharing options...
Joey Mack Posted May 1, 2022 Share Posted May 1, 2022 ok... I'm not going to re-read all the posts... Allison tranny?? did you see my chart that I posted? Sensor input voltage is to be 5v.. on line VECU J-2 pin 11... pin 12 is the ground line... when the driveshaft turns it creates pulses and voltage that is sent to the VECU.. Did you replace the sensor yet?? is FMI-2 present in the code?? if it is, there is another test for a 6-8 code... I will try to help, but wont write the whole book,,, jojo 1 Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-496486 Share on other sites More sharing options...
Ryanp Posted May 16, 2022 Author Share Posted May 16, 2022 joey mack, Yes it is an Allison auto. I believe the issue is low voltage on the sensor(s). I only have about 1.5 volts on the output speed sensor. Now if I unplug the bulkhead connector I get the 5v on the sensor. I have 5v at J-2 pin 11 and a good ground on 12. Is there a test for the turbine sensor also? Any leads onto good info would be very appreciated. Needing this truck up soon and Ive been too busy to dig into this more. Thanks, Ryan Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-497754 Share on other sites More sharing options...
Joey Mack Posted May 16, 2022 Share Posted May 16, 2022 sounds like a short to ground, if you get a voltage increase when you unplug the bulkhead connector.. without have a diagram, myself, I would do an ohms tests on all the pins the see if you have continuity on 2 lines that should be closed/not connected to each other.... what turbine are you asking about? these electrical issues are hard to diagnose, this way.. jojo Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-497756 Share on other sites More sharing options...
Ryanp Posted May 16, 2022 Author Share Posted May 16, 2022 joey, Should I test the pins at the bulkhead connector or at the J-2 terminal of the vcm? The diagram I posted in the first picture shows what I'm looking at. Working with the output speed sensor now but there is also a "turbine speed" sensor on the side of the transmission, right in the side of the main case. Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-497760 Share on other sites More sharing options...
Joey Mack Posted May 16, 2022 Share Posted May 16, 2022 honestly, all pins.. the 'devil inside' could be anywhere.. I dont have a go-to answer other than that.. how about isolating the wires for the sensors, and checking those pin's first. do you have back-up probes to put into the back of the plug on the correct terminals to see the voltage drop on you DVOM, when you plug it back in? I'm sorry, I am not describing this very well.. jojo Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-497762 Share on other sites More sharing options...
Joey Mack Posted May 16, 2022 Share Posted May 16, 2022 did you read the wire code on the sensor harness, to what path it takes? Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-497763 Share on other sites More sharing options...
Ryanp Posted May 17, 2022 Author Share Posted May 17, 2022 joey, Im having trouble understanding the wiring to this sensor I think. I found the schematic that shows the wire numbers and the connectors to the MPH sensor as you have been describing. I will attach it. That seems to be good I verified the bulkhead connector and now if I ohm pin 11 and 12 on J2 I get the correct 290-300 ohms off the sensor. I would say thats fine and good now right? Now the first post in this thread shows a different schematic that I thought was what I needed to be working with. The "ECM" in the diagram is the trans ECM that mounts right under the push button shifter. Then the "trans bulkhead" is the connector that goes down to the trans oil pan. Whats confusing to me is the "output speed sensor" in this diagram I would think is the "MPH sensor" from the other one? Each diagram only shows one set of wires to the sensor. Seems they usually make note of splices if there is one? Also I did find the PDF for test procedures for the VSS. That was very helpful. Thanks for all the help. Ryan Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-497864 Share on other sites More sharing options...
Joey Mack Posted May 17, 2022 Share Posted May 17, 2022 Im not sure I am following you, sorry. The mph sensor is the VSS.. The wires have the numbers printed on them, you can peel back the loom and read the code, it tells you which module it goes too, the plug in the module, and the wire terminal number.. Did you see that on the wire or just the schematic? also, are you checking the power at the sensor plug? ...... if you unplug the bulkhead connector and the speed sensor, did you ohm both wires individually, to see if there is resistance? there should be about, .02 ohms resistance on a good closed line.. it would be good to have a helper on this due to the distance of the wires. with the plugs unplugged, did you ohm the 2 wires together to see if they are shorted? if there is continuity, then you have an issue in that harness.. which would explain the voltage drop you stated. sorry I cant be more help on this, I really need to be there, I dont know how many different segments there on these wires.. jojo Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-497899 Share on other sites More sharing options...
JoeH Posted May 18, 2022 Share Posted May 18, 2022 (edited) My MR688 tripped the 4-1 code a few times. We changed the crankshaft sensor when we changed the flywheel housing last week. If your flywheel housing says "Renault" on the top then I'd check it real close for cracks. Ours was cracked from the starter motor area over the top, and halfway down the passenger side. Apparently it's common for the Renault housings to crack. Ours was also leaving pretty hefty puddles of oil that would dribble out of the flywheel housing whenever the truck parked. Been running the truck on the road a week and a half no issues now, no leaks. Edited May 18, 2022 by JoeH Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-497925 Share on other sites More sharing options...
Mackpro Posted May 18, 2022 Share Posted May 18, 2022 That top wiring diagram looks to be for a manual transmission with the 2 wires going from the road speed sensor all the way to the Mack VECU Vehicle electronic control unit . And of course the bottom diagram is on the Allison side . I think ( and I’m only 60 percent sure) that when the VECU is programmed for use with Allison trans it disregards the normal road speed inputs and looks for the info over the J1939 data link from the Allison ECM . 1 Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-497936 Share on other sites More sharing options...
Ryanp Posted May 18, 2022 Author Share Posted May 18, 2022 Yesterday I replaced the signal wire from the vecu to the VSS. Now I can get the code to go away if I wiggle the wire in the vecu connector. Its not easy to say how to move it and it only goes away for a second or 2. Can I take the vecu apart to inspect the pins inside the box? I put dielectric grease on the connector and made sure every wire on my J2 was in tight and clean. Only thing I can think is one of the pins has cracked inside the vecu? My brother is pretty good at repairing circuit boards so that's who Id have look over that. Thanks for the help guys seem to be gaining on this thing. Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-497953 Share on other sites More sharing options...
Mackpro Posted May 18, 2022 Share Posted May 18, 2022 You can pop the plastic cover off the circuit board pretty easy on the VECU. The 1999 and up RD’s had problems with the VECU mounted with the connectors facing straight up . The windshields would leak water into the VECU since it was facing up . Took several of the VECU apart to dry and possibly save them 1 Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-497969 Share on other sites More sharing options...
Ryanp Posted May 21, 2022 Author Share Posted May 21, 2022 Well Ive gone a few ways with this this last week and I believe I'm ready to wrap it up. Just looking for so recommendations or so I hope. The output speed sensor has 2 wires. (Say A and The Allison reads off a 270-330 ohm sensor. (2 wires) The VECU has 2 wires to the "MPH Sensor" (vj2-11 and vj2-12 i believe) All this agrees with the drawings that i posted earlier and the Allison tech confirmed this. The problem is there is one output speed sensor, when I tried to tie them both onto the one sensor it messed with the signal to the TCM and did work for the VECU. So basically I can do one or the other but not both! Put the sensor to the TCM and it drives and shifts, but no speedo and limp mode with 4-1 code. Sensor to the VECU and it wont shift but the truck has power and speedo works. Throws a code on the trans for speed reading. My temporary solution was to put a second sensor to keep them both reading 300 ohms. TCM is hooked to the output sensor on the trans, VECU is hooked to another sensor that is dangling there. This seems to have solved it!! There is a second hole on this output housing with a threaded hole. I have a (mack?) sensor coming that has the old thread in and jamnut style mounting vs the slide in and clamp down one that was already in the other spot. Does this seem reasonable? Seems the TCM and VECU should be able to take readings off the same sensor by communicating between eachother but for some reason they arent? But then the wiring schematic shows them both to individual sensors? I dont have a part # for the threaded one coming, hoping it reads the right signal? Also I think the second hole only has 4 square tongs vs 24 or whatever the first sensor has. Any input on this? I may be getting into the redneck make it work to get by zone, but its time to get this truck out of the shop! Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-498172 Share on other sites More sharing options...
Joey Mack Posted May 21, 2022 Share Posted May 21, 2022 seems to me you are doing some Kick A$$ Sh!t... go for it.. jojo Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-498173 Share on other sites More sharing options...
JoeH Posted May 24, 2022 Share Posted May 24, 2022 FWIW, here's a "Note" on 4-1 in the VMAC III book. Not sure what is meant by "engine load threshold" Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-498448 Share on other sites More sharing options...
Joey Mack Posted May 24, 2022 Share Posted May 24, 2022 my guess is it a torque limiting system. Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-498450 Share on other sites More sharing options...
Ryanp Posted June 8, 2022 Author Share Posted June 8, 2022 Alright guys so I got a sensor from napa that he looked up for a 2004 mack truck. I told him I needed the thread in one with a pigtail not the bolt down one thats stubbed out at a 90. Got it in and hooked up to the "truck" speedo wires and everything seems to work. The trans is seeing what it needs to and the truck is seeing what it needs to. Psimple fix i guess. I dont think its supposed to be that way but it worked out. My other 2001 mr688 does not have that hole drilled/ tapped but there is a boss on the tailhousing for it. So to be clear for anyones future reference I am running two separate speed sensors in the allison trans tailhousing. Thanks guys the truck runs out pretty nice and is acutally smoother than the 2001 for some reason. Wouldnt expect that with a 20k front and 46 camelback! 1 Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-499589 Share on other sites More sharing options...
Joey Mack Posted June 8, 2022 Share Posted June 8, 2022 Awesome... Glad you posted the fix,and with some creativity as well.. ... Great..... jojo Quote Link to comment https://www.bigmacktrucks.com/topic/69584-2004-mr688-transmission-output-speed-4-1/#findComment-499590 Share on other sites More sharing options...
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