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LW4000 going long at vehicle rear intermittantly

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GoBuckeyes

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Ok hopefully I get more responses on this than my door post :p. Yesterday I noticed our 4000 go long on the back of a vehicle while applying the first presoak. The rest of the wash it measured properly. Then today I noticed it went long on several passes. Not so long that it went to the rear limit, but a few feet long like it was using the wrong T-bar eyes. After talking to a tech we figured first place to start was the Middle Limit prox. The one on there didn't look too bad or beat up but I put a new one on anyway. Watched a car and it ran fine. Another car pulled up and sure as hell it went long again. After talking to another tech he figured it may be the wiring to the Middle Limit prox. Makes sense but here is what has me perplexed...if the wiring was shoddy it may miss the middle flag input and not switch to the reverse eyes which would make it go long a few feet but I watched the input for X26 Middle Limit come on as the bridge was heading toward the back of the car, ran out to the bay and saw it go long? So it got the input to switch to the reverse eyes but acted like it was using the forward eyes? How can that be? I didn't think to look at the output card Y40 & Y41 to see which amplifier was powered up until later. What could cause this and how do I diagnose it by myself? Need a plan to narrow it down. Thanks!
 
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MDrost1

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I had this issue. Ran spare wire to the rear eye. Didn't fix it. Ran new eyes and replaced Gavazzi amp that was on the fritz. No issues now. The Gavazzi amp was new so I didn't think it could be that...but it was part of the problem.
 
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Thanks for the response. I've only owned this Laser for about a year and it's our first so I'm certainly no expert. As I understand it, and I could be missing something, the middle limit switch dictates which amplifier/set of eyes the machine is looking at seeing that both sets share a common input (X26). When the bridge moves in reverse and hits the middle flag, the rear amp/eyes are powered up. When the bridge moves forward and hits the middle flag, the front amp/eyes are powered up. So if the bridge is moving towards the rear of the car and the bridge stops when the front eyes are clear rather than the rear eyes then that means the middle flag didn't switch between the front and rear amp correct? If it was a problem with the rear amp/eyes than the bridge would travel until the rear limit was made correct? Like I said, not much experience with this setup so maybe there is some nuance I'm missing. Does the VT board get involved with the T-bar eyes at any point? I don't believe it does.
 
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MDrost1

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Do you get and fail alerts? Rear Eye Fail etc.? I don't think this is a VT issue.

I'd check the rear eyes and amp. If you have a spare amp, maybe swap it our for the rear eye. Sounds like you have a great handle on the machine. Sorry I couldn't be more help!
 
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GoBuckeyes

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MDrost1 said:
Do you get and fail alerts? Rear Eye Fail etc.? I don't think this is a VT issue.

I'd check the rear eyes and amp. If you have a spare amp, maybe swap it our for the rear eye. Sounds like you have a great handle on the machine. Sorry I couldn't be more help!
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Any help is appreciated! So far I'm not getting any alerts about anything. I think there is a test mode that will run the machine dry. Tomorrow I plan on running my truck through a bunch of times and trying to isolate something. I was thinking of running without the rear amp plugged in. My thoughts are it should run long until it hits the rear limit, if it doesn't then I have some kind of issue with it not switching to the rear eyes. I might try changing the middle limit prox cable as well although it looked in really good condition. Hopefully someone will have another possible cause. Does anyone know what else the middle limit does besides switch from front to rear eyes or is that it?
 
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washnvac

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Have you checked your tires and rollers?? Every time we have this happen, it is a low or flat tire causing the machine to shift a little, which then occasionally causes the mid limit to miss. Then it runs longs. Or a worn roller will cause it to shift a bit. But I would bet on a tire.
 
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GoBuckeyes

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I did check with a shovel handle to see if a tire was flat and they were not. Tomorrow I will actually check the psi. I did notice the tire on the limit side is older and a bit worn, I may end up changing it. I have not changed any rollers yet. How do you determine if they are beyond their useful life?
 
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soonermajic

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Mine was doing that exact same thing, & then it actually got hung up on the 3x foam a couple times. Turns out my tires were way underinflated. Simple fix...
 
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I replace rollers when the edges are worn down enough that it causes too much play in the bridge, and thus it creates issues like you are having. I am sure there is an exact wear down spec somewhere, but I do not know for sure.
 
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It sounds like you need to calibrate your shovel handle.:p I would suggest that you only check the tire pressure with a gauge, as tire pressure is very important on those machines. low tire pressure can cause issues like you have seen , as well as damage expensive prox switches. If you, or your attendants, do not want to climb on a ladder to to check with a gauge, I would suggest buying some 26 lb. valve step caps that are green when above 26, and turn red when below 26#.
 
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GoBuckeyes

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"It sounds like you need to calibrate your shovel handle."
Now thats funny! I do check the psi about once a week and the tires were in fact good.

"I'd check the rear eyes and amp. If you have a spare amp, maybe swap it our for the rear eye. Sounds like you have a great handle on the machine. Sorry I couldn't be more help!"
Well, maybe I sounded like I had a handle on the machine but after spending more time staring at inputs and outputs I see that I don't. My understanding of the middle limit is certainly not correct as I noticed the power for the front and rear eyes switches as soon as the bridge moves forward or reverse not when it passes the middle limit. I'm sure the middle limit plays a role in there but I'm not exactly sure how, maybe someone can enlighten me?
The other day I noticed the bridge go a little longer than I had seen it do so before. I ordered a spare amp and set of eyes, but in the meantime, I swapped the front and rear amps not expecting to see the problem go away, however, I haven't seen it go long since. Hopefully when I spend more time there I don't see it do it again.

Thanks for the replies!
 
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