I have a 91 nissan sentra with a electrical issue. Won't start,battery,starter,ignition module tested good.

The first things that I would check are your battery terminals, positive battery cable, the ground (negative) battery cable, and the main power line that goes to the starter. -Loose or corroded battery terminals do not conduct electricity well, which could explain why your battery tests do not show anything wrong. -Similarly, heavy corrosion within the positive or negative battery cable(s) would also result in a severly reduced ability to conduct electricity, which would be most easily noticeable when using high-draw parts such as the starter (that could explain why the starter just clicks - there's enough juice flowing through the cable to power the solenoid - a relatively low-draw item - but not enough for the actual starter motor itself, which can draw upwards of 100 amps). I usually check for cable corrosion by cutting the insulation with a razor blade for about three to four inches from the end of the cable, then pulling it apart to check for green dust in between all the fibers that would indicate that the cable is corroded to the point that it should be replaced. -In addition to checking the negative (ground) battery cable for signs of corrosion, you should check to make sure that there is no rust, grease, dirt, or anything else where the negative cable bolts to the frame (generally within twelve to eighteen inches from the negative battery terminal) and where it bolts to the engine/transaxle assembly (just keep following the negative cable from where it bolts to the frame, it should be pretty easy to see where it connects to the engine/transaxle). In my experience, a wire brush (especially if attached to a drill) can easily handle the job of cleaning up the points of contact, and a little bit of dielectric grease once you're done will help ensure that the contact points remain clean and corrosion-free. -Last, but not least, check the heavy gauge wire that connects to the starter itself, or more specifically, the ring terminal at the end that helps the relatively small machine-thread screw (which may or may not have a hex head on it) keep it attached securely to the starter. If this ring terminal is corroded, damaged, or if the small bolt that holds it in place has worked itself loose at all (try wiggling the wire - if the bolt is tight, you shouldn't be able to move the ring terminal) then it may not be capable of transmitting the total amount of power required by the starter motor - which would again explain why you would hear a clicking noise from the solenoid without getting the motor to turn over as well. Hope that helps!