1991 Ford F-150 transmission only engages sometimes

Hi there,

If the transmission has not been serviced in some time, flush the oil and change the filter, it would be a good idea to have a line pressure test done, as this will give a better indication as to the internal condition of the transmission including the valve body, as I feel that, that is where you problem is, a sticky or leaking shift valve.

Mark (mhpautos)