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Discussion Starter · #1 ·
When my dad bought his 1688 4 years ago, it came with a 1680 manual, and although it is uselful for most of the combine, there are a few things that we would still like to have the proper information for. One of the biggest things is the factory engine speed. The guy that dad bought the combine from had it sped up to almost 2800 rpm and from what I can find after hours of google searches is that the 1688's were turned down to 2200 rpm. We did turn it down to 2300 but seems like we loose too much rotor and fan speed (rotor maxes out at 940 and fan at 950 when running 2300engine rpm). So we need to find out if something else has been changed (eg. slow down kit). If anyone has a manual they would like to sell or possibly give some elaboration on how these machines were set up as apposed to a 1680 would be tons of help. Thank you in advance!
 

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This sounds a lot like a 1680 not a 1688. I wonder if the engine and the top end separator drive aren't from a 1680. Did someone put in a cross flow fan? The 1680 didn't come with that. Does it have a gearbox on the unloading auger vertical auger or is it helical open gears? Does the valve spool box next to cab have a hinge or is it bolt down? Are the hydraulic valves inside the box mounted vertically or horizontally? If stacked one on top of the other it is a 1680. 1688 has them mounted horizontally. What does the serial number on the LH side of the combine tell you? I would suspect if you have a 1680 manual and the machine has been setup to run the Cummins engine at greater than 2300 - it is a 1680 and not a 1688. The only thing I can think of is that it maybe one of the first few models of 1688 which may not have had the changes but I really doubt that they would have placed a new model out with old materials. The radiator is and the air cleaner is way bigger on the 1688. The 1680 had the same air cleaner as a 1460 - way too small for that much HP!
 

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Discussion Starter · #3 ·
Its definitely a 1994 1688, I think what happened was the people dad bought it from were trying to spin the rotor as fast as possible, when we got it home we realized that the rotor transmission was cranked all the way to the left, thinking that they would pull the belt tighter and spin faster, the concaves were way out of adjustment (1/2 inch on pipe side and rub bars would hit somewhere in the center of the concave) And then the engine speed was cranked up way to much. Its been so frustrating trying to get a clean sample out of this machine without either filling the hopper with white caps or throwing out massive amounts. The old worn out 1480 would be almost clean enough to put right into the air drill without cleaning and would never throw over.
 

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fan speed on a squirrel cage fan on our 1688 tops out at near 1200 rpm which is where we run it for everything (peas and durum). Can you get it to this value at 2300 eng rpm? You should be able to achieve at least 1150 without making sure your sheaves pull up to their absolute maximum. I did have to clean ours up to get 1200! I am not exactly sure about rotor rpm but we typically hit 950 rpm in durum which is about what you are saying you can get to.
 

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I two somehow have a 1680 book with my 1688 I do know that whe setting the roter belt the measurements are different for the 88 than the 80 I too can just get 1200 for my fan .there are a few ther specs that are different from set up ....
 

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Engine rpm should be 2500 I believe, 21-2388's are anyway. Also how did you lower the rpm? Are you getting spring tension on the cable when you put the lever in the max rpm slot? Mmake sure you use the high idle stop bolt on the pump and that the last 1/4" of travel on the throttle lever is spring tension.
 

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Discussion Starter · #7 ·
It was last fall that we changed the engine rpm, so how we did it escapes me at the moment, I'm sure it was just tightening/loosening the retaining nuts on the cable at the throttle on the pump, and it didn't make the lever in the cab any tighter when at full throttle. RunningREDharD, is your rotor maxed out at 950? Or will it spin faster? We may need a new fan belt to get the rpm up to 1200, but we rebuilt the entire rotor drive last summer and put a new belt on but still can't get over 1000 rotor rpm with the engine at 2200. But then again if the sheive measurements and settings are different between 1680 and 1688 then that could be part of the issue. Which would lead me back to needing the proper manual or I may as well just have a john deere 9600 manual to go by. Thanks a lot for the help guys, I really appreciate it, its a darn good machine if we could just get it to run properly.
 
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