A soon as you enable flash on your S90, Func Set Button, Flash On, the minimum shutter speed will be limited to 1/60 second whereas before with your Ixus camera and no strobe you may have been shooting at speeds as slow as several seconds, thus the blurry photos. At 1/60 second and greater 9S90 syncs at 1/60 to 1/500 second with flash on and tends to favor the slower end.
For the engine room scenario which is a horrible place to take photos in "pitch" black", I would probably set ISO to 100 (and experiment from there upward to 400) and set camera to Av mode, set camera flash to Auto under the Menu button and then set the Inon strobe to sTTL high position, camera f stop to f2.8 and shoot. Then adjust from there.
On the deck, I would shoot ISO100, f stop probably around f 4.0, camera in Av mode, camera flash in Auto under the Menu button, flash set to On (Forced) under Func Set button on camera, Inon strobe in sTTL mode normal.
Not being mean, trying to help, but, I think one of your issues is you need to read your camera manual, need to read a few books on basic photography because your questions IMO are too basic to answer, since I cannot be in that engine room, really, anything I said above is a WAG.
You have two other options, rather than Av mode, shoot in P (Program) mode or Manual.
1. In Manual mode on your S90 the sTTL will not function with the S2000. You will have to go to Manual control on the strobe also. You will need to install the magnet switch. Now you match f stop on camera to strobe and start at ISO100 and work up to 400 or more in that engine room. Remember that the strobe power level and the camera f stop control near/subject exposure and that shutter speed choice tends to set exposure for the background (along with f stop). So I would choose as low an f stop as practical in the engine room and a shutter speed around 1/60 to 1/125 and on the deck I would choose again f4.0 to f5.6 and a shutter speed of 1/125 to 1/250 and ISO100.
2. You can also use Program mode on the camera, set your S2000 to sTTL high or normal as needed. Turn the knob to plus or minus to increase or decrease strobe exposure.
The strobe always assumes ISO100 by the way. There is no adjustment for that so when you set other than ISO100 you are playing tricks with the camera and strobe.
The magnet switch installed tells the S2000 when in Manual mode (assuming you also have the camera set to manual) not to expect a pre-flash.
The magnet, in or out, does not matter when in sTTL, sTTL selection overrides the magnet switch and assumes a pre-flash and thus you must set your camera to either Program, Auto, Av(camera flash to Auto under Func Set) or Tv (camera flash to Auto under Func Set).
Canon S90 pre-flash operation:
Program--always a pre-flash
Auto--always a pre-flash
Av--you can select flash Auto or flash Manual. There is a pre-flash in Auto and no pre-flash in Manual.
Tv--same as Av
Manual--never a pre-flash
Inon S2000 strobe pre-flash expectations:
sTTL-must have a preflash from the camera to operate
or
Manual-can be set to expect a pre-flash (magnet not installed) or not to expect a pre-flash (magnet installed)
BTW, in that engine room, you need two or three or more strobes if ytou really are pitch black and intend to photo anything much.
N
