1991 Ford Thunder Transmission Not Shifting When Cold

Change the fluid and filter. Take note of the condition of the old oil and filter ie. does it smell burnt? Are there any metal filings/fragments? Use only mercon ATF, no additives. If this doesn't fix it, it will need a pressure/road test. Only other thing is the vacuum modulator valve. To check it, look on the pass side of the trans towards the rear for a small vacuum actuator. You'll recognize it because it has a vacuum hose going to it and it has one bolt holding it on. Remove it with an oil drain pan handy. There is a small push rod in the hole, be careful not to lose it. Now if you somewhat clean it, you can put a short piece of vacuum hose on it, and **** on it to see if it holds vacuum. There is a spring loaded diaphram inside that you should be able to move if you **** hard enough. If it's nfg, then air will be able to flow throuh the diaphram and it won't work. Hope this helps.