I have a 1990 D250 Vin5 with a 5.9 I have a automatic transmission problem. When it is cold it works fine but after 5 miles at 65MPH or so on the freeway it starts shifting back to 2nd gear. I'm not sure but I may have put in a quart of ATF instead of ATF+4 a while back but it had been working fine for some time. This problem started when I was moving from southern California to Northern California about 50 miles into the trip.I would be very greatfull for any ideas of what could be causing this.

 

what transmission is in it? if you have a 727(3speed) that shouldn't have hurt it. that is what is recommended in a 727 or any older transmission. usually the atf+4 is only needed in trucks with overdrive. does the transmission fluid smell burnt?

 

It is an A518 (46RH) which does have overdrive and the fluid does not appear or smell burnt.

 

When was the filter changed last? You might want to do a pressure test and see what you have.

https://www.trucktransdiag.com/downl...nsmissions.pdf

 

I really don't know when or if the filter has been changed. I think I will try changing it since that may be the most cost effective place to start.