MTD 990 hydro issues

Keith R

Keith R

Tractorologist
Member
Happy new year to all.

Around the first of December I started having issues with my Hydro drive. Going up hill and under load it worked great, nice steady pull; on the level or no load it would surge. Over the holidays I changed fluid and filter, same issue. Starting out on level ground it accelerated then almost stops before surging ahead again, same happens in reverse.
Engine does not surge, it runs great.
Has anyone had this problem before?
Thanks to all for any help.
Keith
 
4getgto said:
Usually you hear the other way around..
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It really does act backwards. I have a steep hill on my woodlot , it climbs up without hesitation and hold a constant speed. when I reach the top and the ground levels out the forward motion slows down to an almost complete stop then surges forward for a bit only to repeat again.
 
Hey Keith your symptoms are somewhat backwards. Is your control lever staying in the same position, most have a broken or weak spring that holds the lever in place that’s at the bottom of your lever. Another thing to check is underneath where the lever linkage is connected to your tranny. That arm that travels back in forth is very sensitive. There is a bushing that rides inside of a plate that’s connected to your lever. If this is worn or out of adjustment your fast, slow, neutral, and reverse ranges can be affected. Do you have a manual. Keep us posted.
 
the tension spring is broken, I have a notch that I put the lever in for forward gear, it holds it securely. I have gone over the linkages, made adjustments and tested function with the rear end up on jack stands. forward and reverse work great as selected and disengage when brake is applied. The only manual I have is for my 960 , I will have to download one. Thanks for the input.
 
Keith R said:
the tension spring is broken, I have a notch that I put the lever in for forward gear, it holds it securely. I have gone over the linkages, made adjustments and tested function with the rear end up on jack stands. forward and reverse work great as selected and disengage when brake is applied. The only manual I have is for my 960 , I will have to download one. Thanks for the input.
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Does that thing have any sort of filter or screen on or in the hydraulic's.
 
I can understand it pulling good uphill and not on level if your fluid level is low. As you start uphill the oil would move to the rear raising the oil level so the pump isn’t starved. It could be that the tube going from the pickup to the pump is cracked or broken off. Just thinking out loud. o_O
 
Another thing if you have just a slight leak allowing air to enter the unit will cause issues. Going through the shifting ranges back and forth including both hi and low will normally bleed itself. You could like Chris mention have a cracked pickup tube. Usually if it’s starving for fluid it will whine a lot.
 
Sawdust said:
Another thing if you have just a slight leak allowing air to enter the unit will cause issues. Going through the shifting ranges back and forth including both hi and low will normally bleed itself. You could like Chris mention have a cracked pickup tube. Usually if it’s starving for fluid it will whine a lot.
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The fluid level is good, it stated whining late last Fall around the same time I noticed the issue, so I may have a cracked or hole in the pickup tube.
when the weather warms up a bit I will check that out.
Thanks
 
Check that the hydro/rear end is tight in the frame. Sounds just like my John Deere 430 did. Frame was all cracked where axle tube's bolted to frame, but where the cracks were you couldn't see them. Then I put a crowbar on Trans near diveshaft and I could rock the whole drive train. On hills the load kept it snug, but on flats it could go either way, like a surge, as it caused the control shaft to move in relation to trans.
 
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olcowhand said:
Check that the hydro/rear end is tight in the frame. Sounds just like my John Deere 430 did. Frame was all cracked where axle tube's bolted to frame, but where the cracks were you couldn't see them. Then I put a crowbar on Trans near diveshaft and I could rock the whole drive train. On hills the load kept it snug, but on flats it could go either way, like a surge, as it caused the control shaft to move in relation to trans.
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Those control shafts can be very sensitive, that sounds like a logical but hard to find reason for a problem Daniel.
 
olcowhand said:
Check that the hydro/rear end is tight in the frame. Sounds just like my John Deere 430 did. Frame was all cracked where axle tube's bolted to frame, but where the cracks were you couldn't see them. Then I put a crowbar on Trans near diveshaft and I could rock the whole drive train. On hills the load kept it snug, but on flats it could go either way, like a surge, as it caused the control shaft to move in relation to trans.
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Thank you for that information, I will also check that out also. I spent the last week putting a new wood stove and chimney in my shop, I now have a nice warm place to work. I will be going over all the hoses, fittings and linkages looking for any leaks or miss adjusted parts.
 
I'm almost certain you have a loose transmission/rear end. When I bought my JD430 it acted identical to yours. On uphills it acted as it should, but on flats to uneven ground the speed would constantly change, from almost a stall to putting me back in the seat. But it will be worse depending on how loose things are. I simply unbolted my axle tubes, then tack welded the broken pieces, then ground welds smooth. Then I slipped in 12" sections of 3/8" thick pieces of 2x3 angle iron and welded securely to the tractor frame, drilled and rebolted in place. There was an upper mount bolt on the transmission that wore the frame hole oblong, so that got a very thick hardened washer welded on as well. It only took a couple hours once I seen what was going on. I bought the 430 for just $700 as I figure the guy thought it had a bad hydro unit which would be very expensive to replace and he actually worked at a JD dealership. It had lots of brand new parts including a new deck shell that at the time cost $700 alone! A couple hours time turned my $700 purchase into at least a $2500 tractor! That was around 15 years ago and I still have it and it's at my daughter's house where they mow over 2 acres.
 
olcowhand said:
I'm almost certain you have a loose transmission/rear end. When I bought my JD430 it acted identical to yours. On uphills it acted as it should, but on flats to uneven ground the speed would constantly change, from almost a stall to putting me back in the seat. But it will be worse depending on how loose things are. I simply unbolted my axle tubes, then tack welded the broken pieces, then ground welds smooth. Then I slipped in 12" sections of 3/8" thick pieces of 2x3 angle iron and welded securely to the tractor frame, drilled and rebolted in place. There was an upper mount bolt on the transmission that wore the frame hole oblong, so that got a very thick hardened washer welded on as well. It only took a couple hours once I seen what was going on. I bought the 430 for just $700 as I figure the guy thought it had a bad hydro unit which would be very expensive to replace and he actually worked at a JD dealership. It had lots of brand new parts including a new deck shell that at the time cost $700 alone! A couple hours time turned my $700 purchase into at least a $2500 tractor! That was around 15 years ago and I still have it and it's at my daughter's house where they mow over 2 acres.
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The symptom you had with your tractor is identical to how mine acts, that may be the first thing i look at.
 
Wow when I read Daniel’s suggestion about the frame I thought a few days ago about that hole in those frames. I can’t remember but only a few years of those had that hole. Mine is a 76 and it does not have that. It is very common for this area to crack. What I’ve seen is to not only weld the crack but plate it also.
 
Sawdust said:
Wow when I read Daniel’s suggestion about the frame I thought a few days ago about that hole in those frames. I can’t remember but only a few years of those had that hole. Mine is a 76 and it does not have that. It is very common for this area to crack. What I’ve seen is to not only weld the crack but plate it also.
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I believe mine is a 1974, i will be welding a steel plate over the hole after I weld the cracks.
 
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