had my 90 scanned and a code of 42 came up which is the electronic spark timing which the computer controls. the computer also showed no values for the knock sensor which the manual says is the only "part" in this system. if the knock sensor is not working is it possible it is generating that code instead of 43 which is the knock sensor code? thanks for any help.

EST advances or retards the timing, depending on sensor data from more than just the knock sensor. It also uses TPS, MAP/MAF, coolant temp, etc. The knock sensor data will only show "Yes" or "No" on a scanner. If the car is idling, it will show "No". The only time the knock sensor will change from "No" to "Yes" is on heavy acceleration, or part throttle cruise conditions, unless there is a distributor timing issue, such as it being out of time, or, if the engine is overheated.
The "test" for checking knock sensor and wiring is to take a small hammer, and "tap" the engine block around the sensor while watching the data on a scanner. It should change from no to yes when tapped.

An EST code can be set for any number of reasons, and a bad ECM(computer) is one, and, unfortunately, the most common.
If the wiring to/from the ECM to the dist. is good, and the ign/ module is good, there are other things that can cause the code to set. The knock sensor only informs the ECM that it needs to retard the timing.
If you have the shop manual, follow the troubleshooting tree for the code, and you'll find the culprit.
In some instances, if the ECM sees heavy knock for a long enough period of time, and it is not able to retard the timing enough to stop it, it can set the EST code. This could happen from using too low grade of fuel, or simply having too much timing. I'd suggest checking the timing first, to see that it is in spec, and go from there.

The knock sensor isn't very hard to change, and I see quite a few that have had the wiring, and the connector damaged. Check to see that it is plugged all the way in, and that the wiring isn't oil soaked/damaged.

thanks joel. found the test procedure for it and will have to get into it. car runs pretty good anyway, it is just that the check engine lite would flicker sometimes stay on and sometimes wouldn,t come on at all. when we were checking it, my friend happenned to point out that it appears my altenator might be too small as it might have been changed and one just put on.things like that aggravate me as if you are going to replace something you should put the right part in especiaaly if it is a common part.