Working with the Case IH 1660
The Case IH 1660 is one of those combines that shows up when you need it most and refuses to cooperate the moment harvest starts. It was built in the late 1980s and early 1990s, and the service manual for it is not a single neatly bound booklet anymore. What exists is a collection of parts catalogs, wiring diagrams, adjustments, and troubleshooting sheets that were originally shipped with the machine or printed later by IH dealerships. You will find them under part numbers like P-6700, P-6701, and the more recent reprint sets from Case IH itself. The most practical route is to grab a digital copy from a dealer-parts site or an agricultural literature reseller. I usually check the official Case IH parts portal first, then fall back to sites like TractorParts.com or manual-reprint vendors who scan from actual print runs. The originals from the 1990s are fine for reference, but if you are doing real repairs, a clean reprint without faded text matters. A PDF scanner that turns the shop manual into a muddy mess is worse than having no manual at all. I also recommend checking whether the copy includes the supplement sheets. Case IH issued field revisions for the 1660 throughout its production run. The ones covering the 1990-1992 production years have known issues with the rotor drive belt tensioner and the chaff spreader adjustment. If your manual does not include those supplements, the troubleshooting sections will point you toward incorrect specs.
What the manual actually covers
The service manual splits into sections roughly like this: engine specifications and adjustments, clutch and drive train, rotor assembly, cleaning shoe, grain handling, hydraulics, electrical, and finally a large diagnostics section with troubleshooting tables. The troubleshooting tables are the main reason people buy the manual instead of guessing. They map observable symptoms to probable causes and list the expected measurement values next to them. The most useful part is the wiring diagram section. The 1660 uses a fairly standard 24-volt system with multiple ground points. When an electrical gremlin shows up at 2 AM during harvest, having the diagram in front of you is worth more than the rest of the book combined. Without it, you end up pulling harnesses apart pointlessly and burning through spare fuses.
How I actually use this manual in the field
I do not read the manual cover to cover. I go straight to the symptom. Say the feeder chain keeps slipping under high moisture. The troubleshooting table for the rotor and feeder chain area lists belt tension, chain slack, and feeder speed settings. It also gives the proper deflection values. I measure, adjust, and move on. That is the intended workflow. For something trickier, like intermittent loss of power to the hydraulic fan clutch, I follow the electrical diagnostic flow. Check voltage at the solenoid, then trace back through the relay and fuse block. The manual's wiring diagram shows which pins correspond to which colors. The factory wire color codes change over the production years, so verify against your serial number before swapping wires based on color alone. That caught me once when I assumed a brown wire was ground and it turned out to be a switched feed on a 1991 model.
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A specific problem I ran into and how the manual helped
Last fall I had a 1660 that would not crank steadily. It would turn over fast for a second, then hesitate, then start. The manual's starting system troubleshooting section pointed me toward the starter solenoid circuit and the battery isolator. I tested the isolator voltage drop under load and found about 0.8 volts across it, which is above spec. The manual lists acceptable drop as under 0.5 volts during cranking. I replaced the isolator and the crank stutter disappeared. I also double-checked the ground strap from the frame to the engine block because the manual calls that out as a secondary failure point, and sure enough, the bolt was coated in grease from a prior adjustment and barely making contact. Cleaned and torqued it down, problem solved. What was interesting is that the wiring diagram did not show a dedicated ground strap. It assumes the engine mounts provide sufficient grounding unless the manual's fault tree directs you to check the frame-to-engine bond. That is one of those small omissions that costs people hours if they are not following the diagnostic sequence step by step.
What the manual does not do well
The 1660 manual was written for a dealer technician with factory tools. Some of the torque specs assume a clean, dry bolt. If your bolts are gunked up or corroded, the values can be off by 10 to 15 percent unless you back them out, clean them, and reapply the spec. The manual does not explicitly call that out in every section, but the fastener torque tables are based on clean threads. Another limitation is the parts catalog. The diagrams are detailed, but some sub-assemblies were updated mid-year. If you order a part based on the main diagram and it does not fit, check the supplement pages for that serial number range. I have seen this happen with the rotor bearing kits and the separator concave clips. The main diagram shows the older part, and the supplement shows the revised version. Ordering from the wrong page gets you a part that looks right but does not seat properly.
Practical tips that actually matter
Keep a highlighter and a pen in the manual. The wear items section will get marked up quickly. Circle the intervals that match your crop conditions. The standard intervals assume average grain moisture and field speed. If you are running high-moisture corn, the cleaning shoe and sieve adjustments need more frequent attention, and the manual's suggested intervals do not account for that. Write your own intervals in the margin so you remember next season. Scan or photocopy the troubleshooting tables and keep a duplicate set in the cab. Paper manuals crack at the spine within a year of real use. The cab environment is hot, dusty, and full of vibration. A laminated copy of the diagnostic pages stays legible. I use the original for detailed measurements and reference work, but the cab copy is what I flip through during actual repairs. If you are replacing the rotor drive belts, do not mix old and new belts on the same sheave set. The manual gives the correct deflection for a fresh set, but the deflection drops as the old belts stretch. You will get uneven wear and premature failure if you install new belts alongside worn ones. The manual does not spell this out in glowing letters, so pay attention to the belt replacement section and replace all belts in a circuit together.
The hydraulic filter change interval in the manual is conservative. For routine maintenance under normal conditions, I stretch it to about 500 hours instead of the 250-hour recommendation, but only after checking the hydraulic fluid condition first. If the fluid looks dark or has a milky tint, change it immediately regardless of the hour meter. The manual gives the fluid specification, so verify you are using the right grade before assuming the interval alone tells the whole story.
Bottom line
The Case Ih 1660 Combine Service Manual is a solid reference if you know how to use it. It is not a quick read. It is a working document. The troubleshooting sections, wiring diagrams, and torque specs are accurate when you follow them in order and verify your serial number range. The parts diagrams can trip you up if you ignore supplements. The electrical section saves you time when things go wrong at night. Treat it like a field tool, not a book to passively read, and it will serve you through multiple harvests.
