1 · Get into Configuration mode

From Gross/Net (G/N) mode, press and hold HOLD/MENU until it beeps three times (about 3 seconds).
Key in the password 640 on the number pad.
Press PRINT/SELECT to open CONFIG.

2 · Standard calibration — the CONFIG number

For grain carts the CONFIG number is a 5-digit code: 98 [increment] [printer] 0

Digit	Meaning
98	Standard grain-cart setting
3 / 4 / 5	Increment: 3 = 10 lb, 4 = 20 lb, 5 = 50 lb
9 / 0	4th digit: 9 if using a printer, else 0
0	5th digit

Common codes: 98300 · 98390 · 98400 · 98490 · 98500 · 98590. Factory default is 18033.

Which weigh bars? The manual's own advice: it's impossible to know exactly which bars are on a system, so try a recommended CONFIG number and keep trying until the scale weighs properly — then fine-tune with the CUSTOM number below.

3 · Custom calibration — dial it to a known weight

Enter a Configuration Code in the 97XXX–99XXX range, then access the CUSTOM number.
Start from the value for your system: 3-bar w/ resistor 37500 · 3-bar w/o resistor 27775 · 5-bar 45000.
Do you have a resistor? Look at the junction box: external 5-pin plugs = has a resistor (use 37500). If the bars are hardwired into the box, there's no resistor (use 27775).
Put a known weight on the scale and read what it shows. Correct with:
NEW CUSTOM = ACTUAL WEIGHT ÷ INDICATED WEIGHT × CURRENT CUSTOM
Enter the new CUSTOM number and re-check against the known weight.

Worked example: scale shows 59,240 lb, true weight is 58,654 lb, current CUSTOM is 37500 → 58654 ÷ 59240 × 37500 = 37129. Enter 37129.

Check it hitched, not on a jack stand. Verify the calibration with the cart hitched to the tractor — its true working position. A cart checked on a jack stand loads the bars differently and won't read the same in the field.

Source: CCS Quick Calibration page & 640 Series Indicators manual (ccscales.com).

Tap the keys — no scale required