How do i change to the smaller black ink tank on the canon pixma MP630 when the large one has run out? Thanks, Lynne

It has been reported that some manufacturers have issued an update which can cause the printer not to run generic cartridges, could also be a faulty cartridge, change this first.

The small black tank is called "Photo Black Ink Cartridge" and is usually only sed when printing color pages or photos. There is no way to bypass that. Use the ********* ink since it is a cheaper cartridge judging by volume. Sets can be purchased on ebay for very low price. Link below:
7 BLACK Printer Ink with chip fits Canon PGI 5 MP600 MP800 MP830 MX850 eBay

If you need further help, I’m available over the phone at https://www.6ya.com/expert/alex_22751bbefa74c3d0

This "photo" printer has two separate BLACK inks -- one using vegetable dyes, and one using suspended carbon micro-dots, in order to get the "best-possible" BLACK colour.

Replace the empty cartridge.

No!

The large cartridge is used for text only printing.
The smaller black cartridge is used for black in images such as pictures.

You have a "C (cyan) M (magenta) Y (yellow) and B (black) color system. This actually saves the color inks for you. If the Black was not there, you would need to make a composite black from the color inks.

Don

The big black tank is pigment based ink used mainly for printing text documents and copies and the smaller black is dye based ink used along with the color inks during photo printing.

Error 6502 is the ink tank position sensor error. Basically means the printer can't see the inks. Try reseating them. If this fails you need to send it back.

The large cartridge is for text and black pictures and the small one is for toning colours so you need to replace it. I found an absolute bargain on e-bay (2 packs of complete cartridges for £12-$18) when a set of origionals are £24 each set.

It won't default to the small one...I emailed Canon support; this is the response I got.

The reason for the two different black ink tanks is for the different types of printing your unit is capable of. They are not intended as backups to each other. The larger black tank is pigment based, and is primarily used for standard black text printing or output. The smaller black ink tank is dye based like the color inks, and is used for photo and graphics output. They are designed to complement each other, allowing the printer to produce the best possible black text output or photo quality as appropriate.

This machine is not meant to switch between the two black cartridges. The large one is used when you are using black only, and the small one is used when you are printing in color.

Are you using Canon brand ink?

Could be dirty contacts in the carrange behind the cartridge. Clean with the eraser on a pencil.