dover telescopic twin post synching problems

Hi , I am having issues with a dover twin post telescopic ( 3 floor) going out of synch. I am not too familiar wither dover stuff so bear with me if its something obvious!.. as you are looking from the landing when it gets to the ground floor the left piston stops sooner than the right causing the car to rack and make a pretty horrible thump on take off. if I lower onto buffers it re-synchs. it will sometimes then be ok for two weeks sometimes goes within the hour . I have tried adding a quart of caterpillar additive in case the packing was tight, there are no visible leaks , I have bled the air best I can from the bleed screws at the top of the jacks, tried resynching it by both lowering AND running it up onto the stop ring . The cab isn't 'binding ' in the hoistway as far as I can see and I softend up the take off in down direction ( was a bit harsh ) and the transition/stopping. this is the only dover I have on my route and as yet do not have a tool ( thanks to my salesman for taking on a job without even asking if I had the tools to work on it ! ) but I did manage to lay my hands on one yesterday and checked the autosynch function was set as in the booklet with the tool (if I recall there are 3 settings in timers and all were set as factory and one setting in CLC adj to enable the function which was set to 2 for twin post ).
I presume that there must be internal seals at the bottom of the smaller diameter piston , I look after some cemco jobs that when they go causes similar issues . Are they likely to be the cause ? I don't want to start trying to figure a way to pull pistons out if its something obvious im missing
any help or advice will be greatly appreciated.

I had a dover cimmaron with the same problem. If you salve up the Pistons with the additive it will usually fix the problem for months. Of course the sync proximity switches need to be even on both pistons.

Thanks john jay , i will try that . I had some of the synch sensors out and have replaced them but i only have a basic set of drawings and not much else when it comes to the DMC-I controller. Does anyone know of any resource online or other for write ups for this equipment?

I wonder what you would find if you put quick connects & gauge at each head. You could swap gauges and test again for accuracy. You said they are telescopic? Could the left side be binding? Is there a section damaged or bent? Did you drop plumb lines at 45 & 90 (clock 3 & 6) with the car all the way up and measure from the lines to the cylinder at several points of each section? Did you check the rails? Where is the "T" in the line between the two pistons?

Maybe I grasping at straws?

Thanks JKH , trust me I have been grasping at straws for a while on this one... yes they are 2 stage telescopic . visibly I can see no damage to either jack although I haven't dropped lines and measured. I did check that there are no tight spots in the rails and rails are lubed up . when the fault occurs and it is at ground floor level the top of the first piston on the right is say 3 inches from the cylinder head and the one on the left is maybe 5 inches from the head..there is still clearance on the bolt heads going into the tops of the jacks so I discounted the car binding in the guides. the T joint is more or less in the middle and the unit has ran fine for 10 + years ( according to the client ). I have some cemco telescopic jobs ( different tho in that they are 3 stage direct single ) made by ALGI ..they have similar problems with say the second stage travelling too fast and causing 'bumping' at the bottom and it is down to the internal seals at the bottom of stages 2 and 3...( not easy to get now that cemco are no more) so I wondered if it may be a similar problem ? pulling pistons is a last resort tho'.

There is 2 likely things that could be causing your sync issues within the jack; either a bad check valve or internal seal.The check valve is located on the bottom of the lowest larger piston as well as a internal seal on the bottom piston. The check valve may intermittently get stuck closed or open and will cause issues or if the seal is bad it will allow oil to pass and the piston to settle. It is a pain and an oily mess to get to either of them as you will have to physically remove both pistons separately from the jack casing to change them. If your going to change the check valves I would recommend changing all the seals, bearing strips, and o-rings along the way just to be safe while you have it apart on both sides.

Replace the 2 relays for the re sync function and make sure the bolts in the top of the piston don't bottom out when you do a manual re sync. Shut the car and remove the buffer springs and manual lower the car on the stands, then check the bolts going into the pistons to make sure they are not bottomed out. If they are add longer bolts so they are not bottomed out.

gs264 is on the same thought as I am. If you have time, and perhaps another hand onsite, you may want to even remove the platen bolts and do a manual resync with no springs, let the pistons completely settle and measure back to the platen hats. Might find one jack has never been hitting that check valve to resync. If the measurements are different, you should be able to use some washers to make up any small differences, think they were 1 1/4 or 1 1/8. This also should help you verify that the jacks are aligned, giving you a shot now at getting a chance to stringing the pistons.

It sounds like the sync valves in the jack to me too, i would check to see if each stage of the piston is in the same position from the cylinder head on both sides and that the first stage is in proportion to the second stage on each jack, i had much the same problem on a side/direct acting furse a little while ago, one of the stages of the jack would hit the back of the cylinder causing the lift to stop an inch high at the lowest floor...
As replacement parts were unavailable a new jack was fitted :-)

this is all great advice ...as I am a only tech on a small island in the Caribbean I have to try the simpler stuff first before going down the route of pulling jacks from casings although I am beginning to think it is the check valve. It has just come back after two weeks of ok running. Pete, that is what is happening...it looks like the bottom stage of the left jack is 'hanging up' . I am going back today and I will try a manual resynch with no springs or with the bolts removed in case they are bottoming out as suggested.
anybody had any experience pulling these jacks or have any literature showing part numbers and what I will find when I do would be very helpful. thanks again guys .

Thyssen = Dover (sort of). I have a Thyssen Package (twin post telescoping) that does the same thing. Thyssen sent me the exact instructions and I can also give you the guys name at A Thyssen to call. Give me your email address and I'll upload the instructions with the middle managers name that will walk you thru it.

thanks monte I will pm you my email address.

I've had this problem happen a couple times myself. Try removing the buffer Springs and land the car on manual lowering and let it sit there for a minute. Then put your springs back on. This will definitely make sure that both pistons fully bottom out.

+1 What ABE said.