Clark Electric EC500-50 forklift speed problems
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  1. #1
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    Default Clark Electric EC500-50 forklift speed problems

    I have an EC500-50 forklift that has not been maintained well. I am in the process of rebuilding and repairing it. The problem i am writing about it that the throttle peddle currently does not provide smooth control of the lift. It is quite jerky and hard to feather or vary speed. Especially when trying to move slowly. The peddle has two micro switches, one engaged at full speed and the other at stop plus a rheostat/potentiometer. I do not have any schematics for this lift nor have I been able to find any making it very difficult to perform diagnostics on this drive.

    Common sense tells me this thing must have some sort of chopper drive to control the average current to the traction motor but I don't see much in the way of electronics at all. Only electro mechanical items such as large relays and fuses inside the back cover. In my experience one can normally hear the whine of a chopper drive but this lift is quiet except for the hydraulic pump and I am stumped as to where I should start.

    The serial number is E355 306 2300 if that helps

  2. #2
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    1974

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    How old?

    Being an ElecTrak owner, I can tell you, PWM[chopper] drives are for wussies

    It might be just the way it is, my tractor has 3 speeds, controlled by big 36 volt solenoids, and shunted through a 'toaster' IOW a big resistor the blows off current

  3. #3
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    By the ser.# it is a 1973 machine, not that it helps. Did you open the lower side doors which should have flat head allen screws in them and see if there isnt a GE panel in there?

  4. #4
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    I am not familiar with Clark, so this is just a wag.

    The first switch should engage with minimal throttle movement.
    Start switch? It kind of turns on the drive system.

    Test the throttle position sensor with an analog ohm meter.
    You can see what is going on with a meter needle sweep, digits jumping around are tougher.

    I assume the second switch is for high speed. This should engage near the end of throttle travel.
    You can get roughly 80% of battery voltage thru a SCR system.
    Some use a bypass contractor for high speed.

  5. #5
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    If what you have on the accelerator is a pot about the diameter of a quarter that has 2 or 3 wires to it, connect a voltmeter between the two wires, or if it's 3 wire, from the center wire to either of the others. Jack up the unit, and with it able to travel, (brake off, seat down or seat switch closed, in fwd or rev), you should see about 5 volts with the pedal off, or with the low speed switch just closed, then the voltage should drop *smoothly* to less than 0.1 volt. I'd guess that you are going to see the voltage move erratically, which means the pot is bad, very common, and the most likely cause of your problem.

    Good luck,
    Ted

  6. #6
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    Also, the SCR panel will almost always be in the center of the counterweight, or in a panel at the bottom of the machine between the wheels opposite whichever side has the hydraulic tank. The most common mfr. is General Electric, but it could also be Cableform, Sevcon, or Curtis. Good luck with any of them, parts are probably still available for all brands, but GE is by far the most common.


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