I missed the title with IAP; I gave you controller-based instructions. To turn on 40mhz channels would be the Wide Channel Band Option under Rf on IAP.
It is probably okay to have 40mhz channels in the 2.4ghz space if you only have a couple of access points. If you have more than that the RF performance plunges: http://www.smallnetbuilder.com/wireless/wireless-features/31743-bye-bye-40-mhz-mode-in-24-ghz-part-1 The interference caused would be to your own access points.
Well, I use only one IAP-135 wihtout controller. As mentioned, "Wide Channel Band" was set to "All" (for this option, other values "None", "2.4GHz", and "5GHz" are available).
Just some more information. Here is what happens after IAP-135 reboot:
========================
# show ap arm history
Interface :wifi0
ARM History
-----------
Time of Change Old Channel New Channel Old Power New Power Reason
-------------- ----------- ----------- --------- --------- ------
2013-03-24 23:48:34 136- 136- Max 21 P-
2013-03-24 23:47:32 44+ 136- Max Max INV
Interface :wifi1
ARM History
-----------
Time of Change Old Channel New Channel Old Power New Power Reason
-------------- ----------- ----------- --------- --------- ------
2013-03-24 23:48:51 5 5 21 21 40INT
2013-03-24 23:47:35 5- 5- Max 21 P-
2013-03-24 23:47:34 6 5- Max Max I
I: Interference, R: Radar detection, N: Noise exceeded, Q: Bad Channel Quality E: Error threshold exceeded, INV: Invalid Channel, G: Rogue AP Containment, M: Empty Channel, P+: Increase Power, P-: Decrease Power, 40INT: 40MHZ intol detected on 2.4G, NO40INT: 40MHz intol cleared on 2.4G, OFF: Turn off Radio, ON: Turn on Radio
====================
So, 40MHz is initially activated for 2.4GHz radio, but then IAP-135 detects 40MHz intolerant bit and changes mode to 20MHz. I was trying to find how to disable this option ("Honor 40MHz intolerant bit"), but did not find where this can be done. Any thoughts?