Hello,
I have had great luck asking questions in this forum, so here I go again: the cruise control in my 1988 F-150 (L6, 5-spd) suddenly stopped working with no warning of any kind. I figured it was the servo, and since it seemed cheapest and easiest to change, I bought a remanufactured one and put it in. The cruise still does not work. The buttons in the steering wheel are getting power and they all pass on different voltages (purposely I presume) so I do not know what to check next. I noticed one other threat about a cruise control, but it did not get to a solution. If anyone has any ideas, maybe even an order to check things, I would greatly appreciate your assistance. Thank you for your time.
William
1988 F-150 4.9L L6

 

The Haynes manual shows the wiring for the cruise system, so check it out.

There is a small Vehicle Speed Sensor stabbed into the t-case where the speedo cable connects, and it sends pulses to the cruise computer. Older ones were prone to locking up or breaking, but '87-92 VSSs were pretty good. Pull yours (it's easy & intuitive) and put the speedo gear into a drill and see if it's producing a tiny A/C voltage (should change with drill speed). It should spin VERY freely.

A more likely cause would be a loss of vacuum supply to the servo (check the hoses for leaks) or a leak in the line that goes thru the firewall to the brake pedal valve. If that valve is leaking or its line is, the servo will always think you're trying to stop, and it won't engage.

 

to check is the cutout switches on the brake/clutch pedals.