Swoop- that's exactly what I'm saying- thanks very much for the input- I've never had one of those valves go bad before & wasn't sure of the symptoms- I didn't want to go on a hunting expedition on my customers dime without having a direction- I couldn't think of what else woulde cause those types of symptoms.

john j- thanks very much for the lead-my local dealer was of no help when I couldn't find the stamped #'s. I always like to verify the data plate & was suspicious when I couldn't find stamped #'s. I'll be contacting your "old" friend.

Titus- thanks for the input- hyd. control valve is new- just replaced. No oil leakage out of return line when mast is drifting.

bbforks,
I used to work for Komatsu up to 2001 (not in service area) and after Komatsu I was involved in selling used lift trucks - wholesale & retail - that were in defunct dealers inventory. The Komatsu model look correct and seems to verify that it is a pneumatic tire lift vs a cushion (press-on solid), what is puzzling me is the serial number typically the letter A is all that is at the end of the s/n - this indicates the unit was built in the United States or America. The Z is a new thing for me not certain what that means but you need to verify if taht is legit. Secondly, not being to locate a stamped in s/n raises a bit of a flag - more that once a dealer has been know to grind it off & install a different data plate to fool finance company auditors. You should be able to get an engine model number & serial number off the engine name plate & there maybe be one on the transmission. With all that information contact a Komatus dealer in your area & they should be able to verify model, serial number of truck & mast model, s/n. In your neck of the woods I would recommend you contact C & C Lift Trucks (a Komatsu dealer in New Jersey/New York area) and ask for a "old" but younger than me friend of mine Ron Fuchs (he is in sales but is very knowledgeable on the technical side too, is a Marine & German we worked together at one point in time). He will point you in the right direction. Use my name John Johnson.

so what you are saying is when you are lowering , with mast collapsed only with carriage/lift bracket in the first stage, IE: free-lift mode and you stop the lowering, the carriage/lift bracket will keep traveling down on its own for a few more inches before it stops? This is what it sounds like your saying to me.
If it is i had some trucks at a customer that started doing this and it did wind up being the safety valve in the base of the cylinder rod. The only way your going to know for sure is to remove the rod and take a look.
When i did this on those trucks the safety valve was either stuck or completely blown out and in pieces hammered all to pieces at the bottom. Had to get a magnet and fish out all the pieces, clean out the lines etc.
We found out it was due to a faulty snap ring breaking or coming out that holds the safety valve in place.

BB forks, try this remove the return hose of the control valve ,attach another hose and drop it in a bucket, put your mast up and then drop it if the oil stops more or less quickly then you have a hoist cyl problem, if it runs on a few seconds this may be an indication the problem is in the valve, this valve isn't controlled by solenoids is it, I know I experienced a similar problem on a Yale truck with a triplex mast and it turned out to be a check valve in the control valve slow to seat.

Titus

Thanks for the response jophn r-the complete model & serial #'s are as follows: FG25T-12, 512485AZ. This is off the data plate on the hood. My customer bought this machine off the internet- can't find the frame or mast #'s anywhere. I was told the frame # would be on the front fender- towards the front lip & the mast # would be at the very top on 1 side of the mast. Scraped off all paint & #'s aren't there.

Might be very helpful to have the complete model # & serial number of this Komatsu. The full model # like FG25ST-12, etc will help ID the design series & the s/n will help to ID any changes that may have been mode for that design series. The mast was more than likely built by Cascade or LiftTech. The ST series is primarily sold in the N.American market (mainly US). It is a press-on solid machine (S) indicates that. There should be stamped on the mast a model & s/n too (very helpful if the mast was ever switched or if the chassis was originally shipped w/o mast).

Hi BBforks, I think that the problem might be some problem with the check valve in the main hoist and tilt cylinder valve assy, allowing oil to pass the spool assy to the return gallery,

Titus

Thanks for your input but neither of your descriptions matches the symptoms. If you lift the carriage and release the hoist lever, the carriage will stop raising and not drift down. When you lower the carriage and release the control lever, the carriage will drift an additional 6 to 8 inches. It's always the main hoist cylinder that drifts, no matter which stage is engaged (no channels raised, or 2 or 3 channels raised). There are no staging issues. There's no oil leakage, the piston is completely dry. The free lift cylinder is the type with the valve on the bottom of the piston. I've never had one of those valves go bad, so I'm trying to find the symptoms of a bad valve before I tear into it. Thanks again for your help, I appreciate it.

Sounds to me like a faulty safety valve. I've seen in some masts there were more than one safety valve, the main cylinder or FFL cylinder has one but also one of the secondary cylinders contains one also.

usually when they stick or get damaged they will cause mast sequencing issues and even some drift.
When you raise the mast up and then lower it the inner channels stay up some (meaning the outer cylinders are not fully lowered) so when it hits the bottom the outer cylinders drift down.

This sound like your symptom?

Just an off the wall thought that came to mind while reading this

If the forks drop by themselves, the packing on the hoist cylinder is no good. If there is a return line on the top of the hoist cylinder, remove it. Elevate the hoist cylinder all the way to the top, if oil comes out in large amounts, the packing is no good.

Thanks for the input 7777, the issue at hand doesn't match your description so I may have to dig deeper. This one drifts down after you're done lowering the forks. Doesn't drift when lifting, then releasing the hoist lever, only when you're initially lowering. Just replaced the control valve for other reasons & was surprised when this symptom remained

If you're talking about the FFL cylinder, it may stick closed an lower real slowly, or force the 2 main cylinders to elevate prematurely, or the FFL cylinder can drop real fast.