I have a 89 F250 XLT Lariat 4 wheel drive with a 460 and twice now on my way home from work it’s shifted from second to first to second to first then shifted normally after that. What would cause that?

 

which automatic trans do you have, have you checked for codes even if CEL isnt on, could have stored codes.

 

I don’t know which transmission I have and I haven’t checked for codes. I’ll have to get a reader

 

Three speed? Four speed with overdrive cancel button? What is the transmission code on the driver’s “B” pillar?

 

it’s a three speed no overdrive

 

That's a C6. A code reader isn't going to do you any good. First, check your fluid level. Secondly, pull the vacuum line off the vacuum modulator on the passenger side, and see if it's got ATF dripping out of it.

 

should I do this cold, hot or does it matter? Either way if the diaphragm is leaking it’ll leak regardless correct

 

To get a good fluid reading? It needs to be warmed up. But start it up, and check it cold, to make sure that it's not low cold. A C6 will take alot of abuse, including low fluid, before it starts breaking or wearing things out, but it's still not a good idea. And yes, if the diaphragm is leaking, it's going to do it hot, or cold.

 

This will sounds strange but it might be your lights... I read before about turn signal lights causing the tranny to down shift due to mismatched bulbs sending signals to the computer... ya strange but I personally know this to be true, just not sure it pertains to this year truck... the one I helped with was a mid 90s. Did you recently change a directional bulb to LED?

 

The OP's truck has a mechanical C6 transmission. There is no computer control therefore using LED bulbs has zero effect on the transmission operation.