I will need to pulse a solenoid-controlled air valve on/off at a variable rate to modulate air pressure (for turbo boost control on my car). The valve itself is specifically designed for this purpose.
#1 Is a SSR connected to a PWM output what I need to accomplish this?
#2 Can I use a SSR that was originally used to control 120vac to control 12vdc? The reason I ask this is because I have many SSRs that I salvaged from old copiers (excellent source of parts and wire :D). On the copiers they are used to control the fusing lamps which are 120vac.
#1 Is a SSR connected to a PWM output what I need to accomplish this?
Yes
#2 Can I use a SSR that was originally used to control 120vac to control 12vdc?
No
You need an SSR that will handle DC, this is very different inside from one that handles AC. You can't pulse the AC one because once it is on it only turns off at the end of the current AC cycle.
There are also speed limitations on how fast you can pulse SSR units, typically 10s of milliseconds to turn on or off - see the spec sheets for the model you're interested in. This may well relate to the period of the mains frequency that the SSR is controlling (20mS @ 50Hz) You'd probably be better going for a simple FET driver to control your solenoid.
And many of the DC ones operate in the microsecond speed range so no problems with PWM switching frequency either. Not quite as cheap as a simple FET but the way to go if you are unsure about the circuitry. Just make sure you get the 5 volt logic driven ones (TTL in old money ?)