The link on down this post offers a few solutions to id the sender,gauge or anti slosh module as the cause of your issue,but it doesn't list a specific testing procedure for the module.Once you open the link,scroll to post 2 & begin reading/trouble shooting as stated in the post.
Some members have had success,if the issue is related to the module,by removing the module from the cluster,cleaning the module & cluster contacts with alcohol then reinstalling the module.If that doesn't help, some had stated that the replacement of the large capacitor at the top of the module solved the issue.If you want to try this method,The following info is for the capacitor on the 87-89 module,so check the electrical specs on the capacitor in your 90-93 module to verify if the specs are identical electrically between the capacitors before using the info directly below to buy/order a new capacitor.If different,use the info on your cap instead.BTW- the catalog number listed is for Radio Shack (now closed) so its likely not valid when you're buying a new cap from a different source.
***** 100µF 35V 20% Axial-Lead Electrolytic Capacitor/ Model: 272-1016/Catalog # 272-1016 *****
Members have also mentioned bypassing the module as another option,but if you do that,the gauge needle will bounce around everytime you hang a curve,slam on the brakes,go up or down a hill,etc,but it wont affect the reading once the vehicle is sitting stationary.Another option (although it'll be hit or miss on both finding a car & module or a correctly functioning module) would be to go to your local Pull-A-Part & try to locate a 90-93 Mustang.Im unsure if the modules are identical between the 5.0 & 2.3 models or not because the ads usually just say 87-89 & 90-93,but dont specify if the module is for a 5.0/2.3 or a 5.0 or a 2.3?? However some members have had success at finding working modules at PAP,so its worth a shot,just like cleaning the contacts & replacing the capacitor is.
Here is the trouble shooting thread I mentioned.Good Luck https://www.stangnet.com/mustang-for...luster.871618/