I have the sigma 70-300mmD i just bought for my nikon D40 and i am having trouble with the auto focus. it will not autofocus can anyone tell me what i am doing wrong and or where i can download the manuel from thanks

Good morning. You have likely already gotten an answer to this, but yes, it will.

Unlike less expensive models such as the D40, D60, D3000 and D5000, the D90 has a built in autofocus motor, which means that all Nikon F-mount autofocus-lenses (except two for the rare Nikon F3AF) can be used in autofocus mode for all newer generation autofocus lenses like the Tamron 200-800. However, you may want to look around a bit, as Tamron has come out with a new 150-600 lens that is simply terrific (I know, I own one) and the Big Sig from Sigma is equally great.

FYI

Good luck.

That would depend on what you mean by "ok." Assuming you get the lens with a Nikon mount, it will fit onto the D40 and meter properly. However, the D40 lacks an autofocus motor and thus this lens will not autofocus. You can still focus manually.

Sigma makes some lenses for Nikon cameras-- though the Nikon D40 requires a lens with its own internal focusing mechanism, since it does not have an autofocus motor. This entirely depends on what Sigma lens you have, i.e. if it were a Sigma lens made for a Canon camera, yes there would be an adapter for that-- but the likelihood of success is less with the limitation of the D40 needing an internal focus motor on the lens.

Yes, it will fit. However, lacking a focus motor, it will not autofocus on the D40.

The Sigma 70-300mm f/4-5.6 DG will not autofocus with the D40 since the lens does not feature an internal focusing motor (HSM). The D40 does not have an AF motor in the body and hence it can autofocus only with lenses featuring an integrated motor (eg. Nikon AF-S lenses, Sigma HSM lenses, Tokina DX II series etc). Unfortunately you can use the said lens only in MF mode.

1 Possiable dirty lens contact terminals
2 Auto focus motor in lens is jamed/malfuntion
3 Auto focus engagement pin on camera body is not engaging the lens
4 Correct lens,old nikon lens and some 3rd party lens wont work on the D40

hi ! iam too a D40 holder,u can opt for AF-s versions or HSM sigma,both of them autofocus on d40,can go for 18-200 AFs VR or if u need still longer end go for 70-300 but it is costly than the sigms 70-300APO DG HSM MACRO..,its a much better reviewed lens and dont forget the cost..anyway do some research before considering..good luck

Your 28-90 AF-D lens can't autofocus with the D40, which requires AF-S lenses with a focus motor in the lens. Sigma lenses with "HSM" (HyperSonic Motor) in the name also have the focus motor.

Unfortunately, this lens is not supported by the D40 due to Nikon's unfortunate decision to leave out the autofocus motor from this camera.  This is why I tell folks not to waste their money on the D40 - access to the many, many, many lenses that Nikon has made over the years, many of which are available used for a substantial savings, is lost on this camera.  The only Nikon lenses that work are AF-S lenses, and some third party lenses that have their own internal motors.  I got a list of Sigma lenses that are compatible with the D40 for you:
The list of compatible Sigma lenses for the Nikon D40 as of the current date is as follows:

• 10-20mm F4-5.6 EX DC
• 12-24mm F4.5-5.6
• 17-35mm F2.8-4 DG
• 50-500mm F4-6.3 DG
• APO 50-150mm F2.8 EX DC HSM
• 70-200mm F2.8 DG MACRO
• 80-400mm F4.5-5.6
• 100-300mm F4 DG
• 120-300mm F2.8 DG
• 300-800mm F5.6 DG
• 14mm F2.8 EX
• 30mm F1.4 EX DC HSM
• 150mm F2.8 EX DG
• 180mm F3.5 EX DG
• 300mm EX DG
• 500mm EX DG
• 800mm f5.6 EX DG

from here:  http://www.imaging-resource.com/NEWS/1177682545.html
Sorry for the bad news.  The cheaper thing at this point may be to sell the D40 and get a D80.  I personally have a D70S, but that is no longer made.  

Your camera is only compatable with AFS-type lenses because it has no screw-drive motor.  You can only use AFS Nikkors and Sigma HSM lenses (the lens actually has a motor inside).  You were misinformed.  Bring the lens back and ask for a refund. Take that money and purchase a compatable lens somewhere else.