I have a 1990 f150 2 wheel drive automatic transmission with only around 107,000 miles on it and I just bought it and the first time I drove it for longer then 15 20 mins my trans started warming up and it would act like it would drop out of gear and go into neutral then after a couple seconds of reving it would go back into gear and act fine for a couple mins and keep doing it but if I stay going around 50 55 mph it wont have a problem it only does it as I'm accelerating up to speed. That said theres a few issues I know of the neutral safety switch is bad and needs replaced on it and I'm pretty sure the rear main seal is bad and needs fixed its leaking out of the inspection plug/dirt plug on the bell housing its only a smaller slow drip after it runs for a few mins its not pouring out of the plug but it needs fixed so does anyone know what is wrong with the trans? Will it need a rebuild or just a part for it or maybe just a fluid change or could it be related to the neutral safety switch I dout it cause I never get lucky enough for it to be a easy fix but I wanted to ask around before taking it somewere and paying a arm and leg or selling it for dirt cheap cause the trans slips . if you need any more info let me know ill get back with it asap.

if anyone has any answers I would greatly appreciate some help and thank you for it

 

welcome to FTE.
first thing to do is check the trans fluid level and see if it is low or not.
the neutral safety switch has nothing to do with it, that only prevents the engine from starting if the trans is in gear.

 

The fluid level is good it dosent show that its low

 

Actually,this problem is caused by the range sensor,which also has the neutral safety switch inside.Upon failure of the range sensor (formerly known as the MLPS) a disengagement feeling,then a random and often harsh re-engagement occur.Replace this sensor and your "slip into neutral" problem,along with the other issue (I'd assume a no start in park,but a start while in N) will also be resolved.
This sensor is most often found under the neutral safety switch name in parts stores,however I strongly recommend a Motorcraft sensor.This sensor is found on the drivers side of your E4OD transmission (door jam sticker code: E / big "METRIC" lettering on bottom of trans pan). You should find good instructions on how to replace it via a search,so I'll not retype that unless you have issues.Usually the first and most important step is left out though and that is to first set the e-brake and chock the wheels before shifting to N and getting under your truck.

 

A 1990 F150 can have an AOD, mechanical 4 speed overdrive automatic transmission, or an electronic controlled E4OD. Check the driver side door tag for the Trans code.

Example tag here:


Trans code T = AOD
Trans code E = E4OD

If the truck has the E4OD then I agree the most common cause of "neutraling" is the MLPS. If you have an AOD you should check the TV cable adjustment.

The transmission fluid leak is most likely caused by the torque converter seal. You have to drop the transmission to replace it.

 

Here' a pic of the sticker which of the types of neutral safety switchs will i need to buy cause there I think 4 different ones online that are for different transmissions