FLD 120 Tach / Speedo / Cruise issues

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

  1. Big Moe

    Big Moe Light Load Member

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    Dec 22, 2011
    South Bend, IN
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    I have a 1994 Freightliner FLD 120 with a 3406E Cat engine. I am having erratic issues with the tach, speedometer, and cruise control.

    The tach will work correctly at times, then it will jump and hold at a lower RPM. Usually around 5k rpm. It will also jump around. This issue is spontanious and doesnt happen all the time. Nor does it happen at any specific RPM range.

    The speedometer will sweep up like normal. Once it gets around 55 or so MPH it will jump down to 65 to 75 rpm. If I "burp" the throttle a little, it will sweep back down to the correct speed. If I hold the throttle about 1/4 throttle or less, it seems to hold at the correct speed. When it goes over 55, it "jumps" around erratically till get gets to around 65 or so. The Odometer does not read correctly at all. The trip mileage however does read correctly.

    The cruise control works and holds fine. Then it will "fall out of cruise" and you can feel the truck slow down. If I "burp" the throttle a bit, it will "resume" and you can feel it accelerate up to the set cruise speed. I've noticed that if you go over a big bump it will "fall out." On flat land, it doesnt seem to fall out nearly as often then if it is going up or down a small hill. It also seems to happen less if I set it lower then 55 mph.

    Anyone have this problem before? I dont want to just throw parts at it so any trouble shooting ideas would be great.


    Thanks!
     
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  3. passingthru69

    passingthru69 Road Train Member

    Check your speed sensor
     
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  4. Big Moe

    Big Moe Light Load Member

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    Dec 22, 2011
    South Bend, IN
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    Is that the sensor on the Flywheel? To check the sensor itself, is there any resistance I am looking for? I noticed that the sensor has quite a bit of oil build up on it. Perhaps a good cleaning is due?

    thanks for the reply.
     
  5. Skunk_Truck_2590

    Skunk_Truck_2590 Road Train Member

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    Feb 16, 2007
    Stonewall, LA.
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    Either the speed sensor or possible your gauge cluster just maybe finally giving up. It happens. Good thing about the older trucks is that while not all parts are available but you can find gauge clusters and such on ebay super cheap. Definitely a truck worth keeping up after since they don't make them like that anymore. Not sure about a truck buy had my moms car a couple months ago where I had to replace the wiring harness to the transmission and the speedometer cable because the gauges were doing the same thing. Just my .02 if its worth anything to check out.
     
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  6. Ezrider_48501

    Ezrider_48501 Road Train Member

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    bismarck, nd
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    i know this post isnt very helpfull but i have a bit of a erratic speedo on my fld cruse works as normal in top gear but in lesser gears the cruse will surge apply heavy throttle then let off then re-apply haevy throttle ect. my odo seems to pretty consistently read about 10% low. also the speed-o dives down 10-15mph when on the jake break unless i press the break pedal then it jumps back up to where its suppose to be.

    so any info on diagnosing erratic speedo issues will do me some good as well..lol i really don't want to replace the speedo unless i really have to cause i don't want the odo changed.
     
    Last edited: Nov 10, 2013
  7. Big Moe

    Big Moe Light Load Member

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    Dec 22, 2011
    South Bend, IN
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    I think I am going to start with the speed sensor since that is where the input is "starts" and work my way up to the ecm then the cluster. I think on some of these 3406E's they have a 2 wire and 4 wire speed sensor where two of the wires go to the ecm and the other two goes to the cluster. I'll have to check that as well.

    I dont know when I will get too doing this, but when I do, I will post any results.
     
  8. passingthru69

    passingthru69 Road Train Member

    Had an old 93 Int. with a cat I drove. Darn thing would shut off going down the road. Then you could restart it. Took it to several shops. Finally found out it was the sensor at the fly wheel went bad.. It would not see the timing mark and think the engine was not running and shut the power off. Talk about being a pain. Go down the road and it would just quit..
     
  9. Chip S

    Chip S Light Load Member

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    May 27, 2013
    Texas
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    Sounds like two different issues. For the speedo problem. Check the yoke on the back of the transmission. Make sure it isnt loose.
     
  10. earthmover

    earthmover Medium Load Member

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    Jan 27, 2009
    castalia , north carolina
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    my truck the speed o would jump up and down all the time I was going to replace the speed sensor but didn't and just kept driven by my tech...now I had to change the yoke on the back of the trans about 2 weeks ago and now my tech is going crazy .. I see that there is a sensor there as well as 1 in the bell housing....which is the 1 that I need to change or both ......sorry bout the hi jack seem we are all having about the same problem....
     
  11. Big Moe

    Big Moe Light Load Member

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    Dec 22, 2011
    South Bend, IN
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    Earthmover- From what I know. The sensor on the bellhousing would be for the Tach. The sensor on (top of) the transmission yolk would be for the the Speedo. From what I have read there is a specific "gap" that is needed between the flywheel sensor and the flyweel. The same for the yolk sensor and yolk. I have also read that the newer sensors do not require any specific gap. You may want to research this more.

    Chip S- How much play is the yolk allowed before it is considered warn? I also am agreeing with you thinking I may have two problems here. I am going to hook up a scan guage this week and see if the rpms/speedo jumps on it. If it does, I know that the ECM is getting "garbage in" and will start checking each sensor. I hope the yolk isnt shot.
     
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