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Well I have a 2001 m5 with alignment issues. Steering is off slightly left but vehicle drives straight. I was in the tire shop while the guy was doing the alignment and saw the screen and everything. While he was adjusting the tie rods he told me to turn my steering wheel left then to the right. We then looked up at the screen to see if the alignment was still in the green zone, It was not.

Why would just turning the wheel left or right cause the alignment to all of a sudden be off even while the alignment is being performed?

The mechanic told me its my steering rack but I dont think it is because my previous alignment was more spot on than this one.
He did however find that my left side thrust arm has some play to it. Can that cause a misalignment and not hold it?
 

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Well I have a 2001 m5 with alignment issues. Steering is off slightly left but vehicle drives straight. I was in the tire shop while the guy was doing the alignment and saw the screen and everything. While he was adjusting the tie rods he told me to turn my steering wheel left then to the right. We then looked up at the screen to see if the alignment was still in the green zone, It was not.

Why would just turning the wheel left or right cause the alignment to all of a sudden be off even while the alignment is being performed?

The mechanic told me its my steering rack but I dont think it is because my previous alignment was more spot on than this one.
He did however find that my left side thrust arm has some play to it. Can that cause a misalignment and not hold it?
I would say yes, because bushing is what that holds suspension components in its place. I would replace the bushings if ball joints are in great shape.
 

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Well I have a 2001 m5 with alignment issues. Steering is off slightly left but vehicle drives straight. I was in the tire shop while the guy was doing the alignment and saw the screen and everything. While he was adjusting the tie rods he told me to turn my steering wheel left then to the right. We then looked up at the screen to see if the alignment was still in the green zone, It was not.

Why would just turning the wheel left or right cause the alignment to all of a sudden be off even while the alignment is being performed?

The mechanic told me its my steering rack but I dont think it is because my previous alignment was more spot on than this one.
He did however find that my left side thrust arm has some play to it. Can that cause a misalignment and not hold it?
It is not the steering rack; you don't have one. And you need to find a mechanic who actually knows what the components of the steering system are and what type of system is on the car. By the way the e39 M5 has recirculating ball steering, not rack and pinion, the components of which are the steering gear box, Pitman arm, idler arm, center link (aka drag link) and the inner and outer tie rods. It is likely not the thrust arms as they have no effect on toe. The issue of the toe adjustment not repeating is likely in the gearbox or the tie rods.
 

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Well regarding the thrust arm "play" not affecting toe, I agree and disagree (everyone get your morning coffee and sitdown a minute...) Usually slight play in the thrust arm won't affect toe much. However the two thrust arms provide a lower pivot for the strut such that the tie rod can pivot it around it's axis and turn the tire. Techically, the two thrust arms have ball joints that are not in the same plane as the strut and therefore the strut does not pivot around a single axis. The strut rather pivots in a small arc (at the bottom). There is, then a bit of a conical movement. This geometry has been "massaged" a bit through the generations and the E28, E34 and E39 have slightly different geometry (the biggest change going from E34 to E39). In any event if there is "slop" in the lower ball joints a rigid tie rod assembly will only do "so much" to hold the position of the tire. It stands to reason that the affected side will have to move one way or the other. I admit the slop has to be pretty extreme to see results change on the aligment machine. I think in this case the converse is more likely: that there is slop in the tie rods and or pitman arm and very likely the steering box. In any of these cases when the steering wheel is moved and re-centered the toe on one side may not return fully although the total toe between both front wheels is likely to stay about the same.
 

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It is not the steering rack; you don't have one. And you need to find a mechanic who actually knows what the components of the steering system are and what type of system is on the car. By the way the e39 M5 has recirculating ball steering, not rack and pinion, the components of which are the steering gear box, Pitman arm, idler arm, center link (aka drag link) and the inner and outer tie rods. It is likely not the thrust arms as they have no effect on toe. The issue of the toe adjustment not repeating is likely in the gearbox or the tie rods.
Well regarding the thrust arm "play" not affecting toe, I agree and disagree (everyone get your morning coffee and sitdown a minute...) Usually slight play in the thrust arm won't affect toe much. However the two thrust arms provide a lower pivot for the strut such that the tie rod can pivot it around it's axis and turn the tire. Techically, the two thrust arms have ball joints that are not in the same plane as the strut and therefore the strut does not pivot around a single axis. The strut rather pivots in a small arc (at the bottom). There is, then a bit of a conical movement. This geometry has been "massaged" a bit through the generations and the E28, E34 and E39 have slightly different geometry (the biggest change going from E34 to E39). In any event if there is "slop" in the lower ball joints a rigid tie rod assembly will only do "so much" to hold the position of the tire. It stands to reason that the affected side will have to move one way or the other. I admit the slop has to be pretty extreme to see results change on the aligment machine. I think in this case the converse is more likely: that there is slop in the tie rods and or pitman arm and very likely the steering box. In any of these cases when the steering wheel is moved and re-centered the toe on one side may not return fully although the total toe between both front wheels is likely to stay about the same.
I agree and if the shop actually checked for play prior to starting the alignment and didn't find any easily discernible play, it's even more likely to be the gearbox. How many miles on the car? If the 'box isn't too bad or for a temporary fix (ie get another year out of it) you can adjust the slop out of the steering gear box.
 

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I think that a tech that thinks it could be "the rack" may not be the right one to help you out with this..Totally agree with tie rods or centre links as a solution..
 

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while the car is on the alignment rack there is no reason to turn the wheels left or right except for doing a caster sweep, take it to a shop that really does know how to perform an alignment, you have many factors that change when you do this one the vehicle is on four plates on ball bearings that can move side to side, two if the heads were not tight on the wheels and they moved a little and three did he put the steering wheel where he had it and four did he have the brake pedal tool installed to lock the wheels, five FIND A SHOP THAT REALLY KNOWS HOW TO DO AN ALIGNMENT
 

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I agree lack of experience on the techs part. When you find some one that says they know, watch them. On the steering box there are two arrows that must get lined up. One is on the shaft one is on the case. If the tech does not go there first, then he does not know. He may then check to see if the steering wheel is straight but might not, because it will be. If he just uses his machine he will be compounding old mistakes. Now that I think give him a second, he may use the machine and then check the arrows.
The reason they must be lined up is the answer one of your questions, because of camber and caster as soon as you turn the wheels the alignment will change.
 

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It's not necessary to move the steering wheel during alignment.

The alignment changes as the steering goes through its angles of operation, but when it is straightened it returns to normal
 
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