S Helper F7 ABA unit and cannot get it to run

One suggestion is the the unit has a Dallee Locomatic card in it that will only respond to signals supplied by a Locomatic controller. S-Helper supplied many of their early F3 and some F7 units with the Locomatic option until DCC became more popular. You can open up the lead A unit to check to see if such a card is in it. If so, you have the option of removing the card and losing the sound (or installing a DCC card at some point) or getting a Locomatic controller from Dallee Electronics and controlling the motion, sound, and classification lights on an S-Helper F7 unit.

If the lead A unit does have Locomatic control and the other A unit and B unit do not, you will have to connect the B and trailing A units to the lead with the tiny MU electrical wires that S-Helper provided.

The non-DCC version F7 could be run conventionally with AC or DC, or "slave" to a 10-button Locomatic control box depending on how specific dip switches inside the A unit rear door were set.  Check those.  DCC versions must be used with a DCC system and came with supplemental documentation in addition to the aforementioned version Owner's Manual.  

Click here for the the Conventional/Locomatic F7 Owner's Manual.  Including MU cable use and a whole lot more.  Here are a few excerpts:

Thank you for purchasing our F-7 diesel cab unit. Your new diesel comes ready-to-run on American Flyer® com- patible S gauge track with either AC or DC power.

3-6 AC/DC LocoMaticTM Sound Units
Your new SHS F-7 locomotives come complete with Loco- MaticTM Control. LocoMaticTM Control features total train control with the push of a button. To activate your Locomatic Sound Units, you will need to move dip switch- es #1 and #2 to the “on” position (these are located inside the “A” unit rear door. 

3-8 DCC (Digital Command Control) Sound
One option for our F-7 is factory installed DCC sound de- coders made for us by Soundtraxx. Supplemental in- struction booklets are included with this option.

If you'd like to hear directly from Don Thompson, former owner of S-Helper (who along with his wife Robin were and remain two of the nicest people and greatest ambassadors of the S-trains hobby), join the showcaseline Yahoo Group.   Don frequently replies to questions posted there with the same courtesy and nearly limitless detailed S-Helper product knowledge as ever.