What is the problem of full dark black colour?

what kind of printer are you using. Is your colored ink nearly out. if so, replace it.

Whenever you ask about any problem please explain in detail.There are couple of reasons behind the above mentioned problem.Its not clear that copies are all black (full black) or just dark in print density? or its giving a dark shade? As per asked question, the solution is Check the followings1-Scanner mirrors are dirty2-Drum is damaged or faulty3-Charging corona 4-LD unit5-Developer

try this link.

The easiest way to get to the heating element would probably be to take off the front panel. Be sure to disconnect the power first as you may have to disconnect the door switch to give yourself more room. Even by doing that you don't have a full view of the element but taking out and putting in the barrel/barrel belt is a much bigger pain so you may want to try installing the heating element without pulling the entire thing apart.
As far as the wiring goes There should be two wires, one light coloured, and one dark coloured(probably white and black). The dark one usually goes to a small disk-like switch(called the 'limit switch) near the element before connecting to the actual element. The light coloured wire should go to the connector opposite the dark coloured wire.
It really doesn't matter which side of the element you connect it to as long as you don't connect them together.

My black printer cartridge does not print dark enough. How can I print darker?

Most Colour machines have three settings.

Auto Colour, Machine will determine if the original is Colour or Black and White from its scan. Then print in the format that best suits the original.

Full Colour, Machine will copy in full Colour regardless of Original type.

Black and White, Machine will copy or scan in B & W regardless of original type.

Hope this is of some help.
Ed

Dear friend, your Black Ink Cartridge is Empty thats why your printer is using the Colour Cartridge to produce the Black Colour. This is the reason you are having Dark Reddish Brown colour instead of Black.

It's never going to be perfect (unless the background is quite light in colour), because the copier basically "sees" the coloured background as "gray", and the darker the background the more "gray" it looks. Try setting the original type to "photo" mode, and then adjusting the exposure a little lighter/darker....that's all you can basically do.

with all brother printers all the ink cartridges must be full or the printer will not print brother tech

He's not saying that the black cartridge won't print (which would warrant cleaning the heads, or messing with the cartridges) -- I have the same problem that MarkDan is having as well... no matter what you select as the output from any "Print" dialog window, the HP 21xx and 12xx series printers _always_ print in full color. You can choose Black and White, Greyscale, Color -- doesn't matter what you choose in the dialog boxes, it utterly ignores you, and will always print in color.
I have no idea why it does this, but it's eating a _ton_ of color ink for no good reason. I have searched all over the Web and on HP's site, but solution has been found.