Just bought a 1991 F250 with a 7.3 liter diesel and my delay wipers don't work. Turning the wiper switch to low position doesn't work either. They only work in Hi with a bit of a delay between cycles. Replaced the switch and bought a used wiper delay module from a guy in Pa. It didn't seem to help. Any ideas, suggestions, or wiring diagrams would be greatly appreciated....

 

I had the same problem when I bought my 1987 F250 Diesel. There is a black box behind the wiper switch. I don't know if that is the delay module or not but that fixed my problem. The auto parts stores don't have a listing for it. I ended up getting mine from the local junk yard.

 

When the wiper switch is in Hi, 12 volts goes direct to the wiper motor.
When Lo speed or delay is selected, the power for the wiper motor comes
from the wiper delay module which is know to have cold or cracked solder joints
on the circuit board.

Here's the schematic for a 88 with delay wipers.
The wiper motor and delay module is the same for 87-91 so I would think the schematic
is the same.