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  1. #1
    Epic Contributor jinman's Avatar
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    Texas - Wise County - Sunset
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    NHTC45D, NH LB75B, Ford Jubilee

    Default Chasing a head-scratcher on my LB75B

    One of the things I find extremely convenient on my LB75B TLB is the Transmission Disconnect switches on the joystick and gear-shift lever. Pressing that button and shifting gears is so easy. When using the loader, approaching my dump trailer or the pile and pressing the button on the joystick makes holding my position while dumping just a simple operation. With the button still depressed, I can flip the shuttle to reverse and be ready to roll away. Suffice it to say, it's a feature I really like.

    Earlier this week, I noticed the gearshift came out of gear, but would not shift into the next higher gear. I suddenly realized my disconnect switch had stopped working. I tried the switch on the loader joystick and found it also was not functioning. However, when I placed the shuttle to neutral, the transmission disconnects as it is supposed to. The shuttle switching is working perfectly while the switches on the shifter and joystick are not operating.

    I have a maintenance manual and electrical diagrams. The way the transmission works is a dual solenoid valve sits on top of the transmission. The valve has forward, neutral, and reverse positions. The neutral position is just a centering of the valve with neither solenoid engaged. The forward solenoid, when energized, sends hydraulic fluid to the forward clutch in the transmission. The reverse side does the same thing to the reverse clutch. When either of these positions are engaged, pressing the disconnect button simply interrupts the electrical path to the forward or reverse side of the solenoid and lets it return to neutral the same as putting the shuttle switch to neutral. It's a pretty slick setup and the electrical circuit is pretty simple. The disconnect buttons have their own supply of 12 vdc through a dedicated fuse and either switch energizes one relay to interrupt flow to the shuttle solenoid. That's it. The switches are wired in parallel so their operation does not depend on the other one.

    After locating the proper fuse and relay on a diagram, I went to the TLB and removed the fuse/relay cover. I pulled the fuse (10 amp) and found it was blown. A blown fuse always gets my attention because I worry that just replacing the fuse without finding a short may just immediately blow the fuse again. To partially disconnect the circuit, I removed the disconnect relay from its socket, replaced the fuse, and started the tractor. I tested both disconnect switches and the new fuse didn't blow, so I turned off the tractor and put the relay back into its socket for normal operation. After starting, I tried the disconnect switches again and nothing happened. The relay did not energize and the fuse did not blow. Hmm . . . I exchanged the relay for a known good one and tried again. Same results! The fuse is not blowing, but the circuit is also not working. Next I slid under the tractor and looked for broken wires. Except for dirt and hyd oil, the wires look fine and no damage is visible. I previously had a leak from a hydraulic hose in that area, so things are a bit messy.

    My next move will be to get my voltmeter and start looking for the open circuit. I should have had my voltmeter yesterday, but didn't have it with me. I may also just jumper 12 vdc to the disconnect relay to make sure it works normally when energized. Of course, whatever is wrong is common to both switches but does not effect the operation of the shuttle lever. I can still use the tractor just fine, but can't shift on the fly. I have to stop and shift to neutral before I can shift gears. I'd guess that hydraulic fluid got into one of the connectors and caused an open circuit, but that's a real stretch. I have been working in/around water in my ponds, so my guess is water may also have splashed into the connectors and shorted a wire. That seems the most likely issue since the fuse was blown. I've pretty much reached the limit of what I can do without a voltmeter. I'll troubleshoot a bit with it and hopefully trace down the problem. It's comforting to know that the problem is electrical. The fact that the shuttle lever works normally tells me there is nothing mechanically wrong with the transmission.
    Jim


  2. #2
    Elite Member
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    Iuka Mississippi USA
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    3550 Fard Backhoe and a 1948 Farmall Cub,

    Default Re: Chasing a head-scratcher on my LB75B

    I have had this on a 455 Ford. Pull the disconnect solenoid to see if it might be sticking. Also jump one and see if its clicking.

  3. #3
    Epic Contributor jinman's Avatar
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    NHTC45D, NH LB75B, Ford Jubilee

    Default Re: Chasing a head-scratcher on my LB75B

    Taylor, the solenoid is working perfectly as shown by the shuttle switch. This is an electrical problem, I'm sure. I probably won't get back to checking it until next week since today is Father's Day and I don't feel like gettin' greasy.
    Jim


  4. #4
    Epic Contributor jinman's Avatar
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    Default Re: Chasing a head-scratcher on my LB75B

    This problem just keeps getting more strange. Yesterday, I got my digital VOM and printed out the circuit diagram so I could troubleshoot. What I found only adds to my confusion. On the attached diagram, fuse 4B is the 10 amp fuse that I previously found blown. That fuse is replaced and is holding fine now. Tracing the 12v at the fuse and down to switch S26 is normal with +12v. However, when I pulled relay K1 and measured at the terminal socket for it's coil, pressing either transmission disconnect switch produces -12v on my meter. Of course, if it is -12v, the coil diode is conducting all the current away from the coil. My guess is that when I get to the circuit board level, I'll find it is burned up and open.

    Now, I made all my measurements using the ground lug where it attaches to the circuit board. I measured the positive voltage coming in at that point and it is fine. I have two batteries bussed together in parallel, but my system is not a 24V system. Only +12 is supplied to this circuit. The only thing this can be is somehow the line to the relay coil is now ground and pressing the disconnect switch puts +12 on what should be ground. The same as swapping leads on the meter so that it reads -12 vdc. Of course, I left my ground lug attached to the input ground terminal, so I don't how reversal would be possible. I'm just not seeing what would make my voltmeter read negative voltage in a circuit only supplied with +12v.

    I experimented with the shuttle operation by pulling the relay K1. Exactly as I suspected, the transmission disconnects when I pull that relay. When I replace the relay, the shuttle lever works normally for Forward-Neutral-Reverse. I can monitor the switch closures at the relay, but instead of them sending +12, the switching is to -12v.

    I ran short of time and closed everything back up. I think the next move will be to remove the circuit board completely and start testing components. Something really weird is going on here.
    Attached Thumbnails Attached Thumbnails Chasing a head-scratcher on my LB75B-shuttlehilite.jpg  
    Jim


  5. #5
    Platinum Member DieselMonk's Avatar
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    New Holland / T4.75 Powerstar & Boomer 1030, Case 580 SM II

    Default Re: Chasing a head-scratcher on my LB75B

    It can't be grounded or shorted out, otherwise the fuse would blow again and again and again. -12V I have no idea how you got that one... as it should be +12V. Sometimes a digital VOM is not good to use... reminds me of a relay problem in my Mustang. I had voltage were there shouldn't be any. After running in circles and questioning my abilities and downgrading myself to junk status, I got a suggestion to use a test light instead. Anyway sure enough... voltage was there, no current. So maybe try that instead. In your case, you could test the relay driveline disconnect if you'd jumper 12V into the relay to energize it. The trouble lies ahead of the relay. Did you measure what you get at the disconnect switches? I am sure right now with the negative 12 V the relay isn't clicking either. Sounds like a broken wire / trace somewere.

  6. #6
    Epic Contributor jinman's Avatar
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    Default Re: Chasing a head-scratcher on my LB75B

    Quote Originally Posted by DieselMonk View Post
    It can't be grounded or shorted out, otherwise the fuse would blow again and again and again. somewere.
    Yeah, it's probably an open diode right now. I suspect the -12v across the diode to ground fried it pretty good although I don't smell anything burned. I monitored the voltage right at the relay coil. Since it is a DC relay, instead of energizing the relay, I think its coil is pulling-in harder instead of opening the contacts. I clicked both disconnect switches and watched the voltage to the coil go to -12v on the meter. It's like this is suddenly ground and ground is 12 volts. Very weird. I'll keep plugging along. This is a mystery I gotta solve.
    Jim


  7. #7
    Platinum Member DieselMonk's Avatar
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    Default Re: Chasing a head-scratcher on my LB75B

    Quote Originally Posted by jinman View Post
    Yeah, it's probably an open diode right now. I suspect the -12v across the diode to ground fried it pretty good although I don't smell anything burned. .
    I am sure the 10A was doing a good job of frying a component! I hope you figure it out, as I am kinda curious as heck to see what the problem was.

  8. #8
    Platinum Member DieselMonk's Avatar
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    Default Re: Chasing a head-scratcher on my LB75B

    Jinman, did you ever figure out what caused it?

  9. #9
    Epic Contributor jinman's Avatar
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    Default Re: Chasing a head-scratcher on my LB75B

    Quote Originally Posted by DieselMonk View Post
    Jinman, did you ever figure out what caused it?
    No, I haven't taken the time to work on the tractor yet. I'll have to pull the relay board out to do a component check. I don't want to do that unless I know I have the time to do the job right. For now, the shuttle works perfectly. I just have to stop and shift the shuttle to neutral before changing gears. Don't worry, when I take things apart, I'll document with photos and descriptions. My real problem is my legs. I have trouble with circulation and it has really flared up this summer. I can't stand on my legs for long periods of time because of pain and swelling. I can sit and operate, but can't stand in one place and work on something.
    Jim


  10. #10
    Platinum Member DieselMonk's Avatar
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    Default Re: Chasing a head-scratcher on my LB75B

    that's too bad jinman... didn't know about that. Take her easy...

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