Wiring diagrams for HVAC air handlers can be found on an number of online sites, from generic diagrams on Electrical Knowhow, to specific models on the Lennox and Carrier manufacturer sites. Most of the diagrams available are in a downloadable format either in separate diagram packets or in installation manuals.
Electrical Knowhow contains general wiring diagrams only, showing how wiring is installed in most air handler systems. While the diagrams aren't highly detailed, they can provide basic wiring directions for those already familiar with electronics.
The Lennox support site is limited to Lennox brand air handlers only. Each model can be looked up by product category and model number. The wiring documentation is specific to each separate model and is available in Spanish and French as well as English.
The Carrier site is searchable using the product name of the HVAC air handler or an abbreviated model number consisting of the first three or four numbers of the model being searched for. Just as with the Lennox site, once the product is found, the documentation containing the wiring diagram for that specific model can be downloaded. A full listing of all available wiring diagrams is available by doing a generalized search without choosing model or product ranges.