FLD 120 Tach / Speedo / Cruise issues

Discussion in 'Freightliner Forum' started by Big Moe, Nov 9, 2013.

  1. Chip S

    Chip S Light Load Member

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    May 27, 2013
    Texas
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    The yoke shouldnt have much play at all. Off the top of my head im not sure, but it is in the .001"s If I remember right the up and down movement is .007" I wouldnt worry too much about the yoke needing replacment as I would be with the trans output shaft bearing in the back box of the trans eventually going out. Check for play, and if there is any, remove the driveline and install a new yoke nut, and tighten with a 3/4" impact. That will take your play out.
     
  2. Big Moe

    Big Moe Light Load Member

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    Dec 22, 2011
    South Bend, IN
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    Chip S- I was under the truck the other day and inspected the yoke. (not yolk, as i have been saying. :) ) I would grab the yoke and "turn" it left to right, then right to left. In a clockwise/counter clockwise motion simulating the drive line turning. It turned a little. but not much at all. I also grabbed the yoke and pulled out twoard the rear diffs. No movement. Finally, I grabbed the yoke and moved it "up and down" and got no movement or slop.

    Based on these tests, I think I am in good shape. Do you agree?
     
  3. Big Moe

    Big Moe Light Load Member

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    Dec 22, 2011
    South Bend, IN
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    Update: I installed my Scan guage and calibrated the speed reading on the scan guage based on the factory speedometer and my gps. All 3 are reading the same mph. When i have my foot on the accelerator, it maintains a smooth increase or decrease of MPH on the speedometer. When I take my foot off the accelerator, both the scan gauge and the factory speedometer jump up 7 to 10 mph.. When I put my foot back on the acclerator and give it a little fuel, it jumps back down to the "correct speed." When I apply the brake, both the scan gauge and the speedometer drop smoothly as it should.

    The RPM function on the scan gauge is giving me accurate RPM readings and does not jump around when the Tachometer jumps around.

    What I gather from this data is that my my ECM is getting good input from the RPM sensor (at the bellhousing). There is a problem on the Truck side of the ECM going to the Tachometer. Ground issue, wireing problem, or just the Tachometer is getting bad. In regards to the Speedometer, i think the problem lays at on the Engine side of the ECM (and possibly inside of the ECM, which I am not ruling out, but I feel unlikely.) The ECM is getting "garbage" in, and giving me "Garbage" out on the scan gauge and speedometer. I am thinking it may be a ground issue, wire issue, yoke sensor dirty or failing, or the "sprocket" inside the transmission that the yoke sensor gets its signal from is loose. When it has torque on it, (i.e. the accelerator pressed) it tightens up onto the output shaft and reads the true speed. When torque is removed from drive line, it slips back a tooth or two and throws the reading off.

    When I get a chance, I am going to pull the yoke sensor off. bench test it (ohm readings) and see if it is with in spec. Clean it. And feel inside the hole if there is any play on the sprocket. Hopefully that isnt the case.



    Anything I am missing here? Suggestions?


    Thanks.
     
  4. Chip S

    Chip S Light Load Member

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    May 27, 2013
    Texas
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    Sounds like your yoke is tight! The tone wheel that reads rpm for your speed sensor should also be tight, as it is tightned by the yoke. You should have two connectors for your vehicle speed sensor, being an FLD, as one circuit goes to the ecm, and one goes to the gauge. Since both circuits on the vss are screwing up, seems like you have a faulty vehicle speed sensor.

    I would bet your tach is also faulty. As your gauges share the same ground

    Good luck
     
  5. bakersacres

    bakersacres Bobtail Member

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    Oct 2, 2016
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    Hey guys.
    I know this thread was awhile ago. but i would like to ask a couple things..... i have a 1995 FLD 120 with a 3406E on it. I have replaced my speed sensor and pulled all of the wiring out from back of tranny to ECM to check for breaks or abrasions and nothing was found. I have replaced the spedo/odometer and continue to have the same problem (erratic speedo and no odometer) i am at a loss what should i do next?
    Thanks
    Mitch
     
  6. Jarrods

    Jarrods Bobtail Member

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    Sep 2, 2016
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    Make sure all your grounds are clean and tight. Make sure the wiring has continuity and then use a headlight and put power through the wire.
     
  7. x1Heavy

    x1Heavy Road Train Member

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    10,774
    Mar 5, 2016
    White County, Arkansas
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    5000 rpm?

    Pray tell where is 5000 rpm?
     
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