Grid Heater??
 

wheres the timer located on the grid heater change the solenoids still not working want to find the timer and try changing that next any one know?

Its in the computer.

ya the computer runs the grid heaters and somethin else and thats all on these trucks. pretty basic

what would i have to do to fix it

On a Non-intercooled '91 the "timer" is on the firewall, near the column.
On a '91.5 (Intercooled) it's behind the battery in the fender.

There are several reasons the grid heaters won't activate and a bad timer is only 1.
You could have a bad air temp sensor or bad wiring connections from the timer to the solenoids.

Troubleshoot it like you would anything else, verify that each peripheral component works, such as the solenoids and the grid heating elements.

Mark.

solenoids are good where is the air temp sensor located

top of the manifold(2 pin). their is an entire book about grid heater diagnosis and operation in the sticky that shows its location( i believe)

If it's a non-intercooled '91, the air temp sensor is located in the head, directly below the grid heater elements and passes through the large throttle bracket assembly.

Mark.