A new issue surfaced: I had a sudden acceleration from cruise control, so I decided to finally investigate the servo.
Based on this Youtube video slowing down like I had before is usually a case of a dirty release valve. I suspected that acceleration could be caused by dirt in a vacuum valve.
xc_hide_links_from_guests_guests_error_hide_media
I documented the process for anyone troubleshooting their system.
Servo removal is straightforward, 3x 7mm screw on back hold the servo to the bracket on 350 TBI engine.
Remove the 2x Torx to reveal valve body. Red coil with metal plunger is a release valve, the green one with plastic top is a vacuum valve. The middle one is a position sensor.
You must be registered for see images attach
At this point plunger and a rod from release valve can be removed and I advise to do so as it’s easy to loose. To disassemble solenoids you need to remove three further torx screws, however you can keep the top one in palce if you want to remove valve assembly as a whole. If you do remove it then watch the coil wires as they are fragile, there are also couple of small parts in between that will fall off.
You must be registered for see images attach
Thee valve body is friction fit via o-ring to the back of diaphragm. With two (or three) screws removed you can pry it off.
Here are the solenoids removed and disassembled from the top, and a description of parts:
Release valve (normally open)
Base -> spring -> sealing cup -> metal plate -> piston (?) -> wavy spring washer -> coil -> thin rod -> plunger
Vacuum valve (normally closed)
(From top) Base with a rod that sticks out -> o-ring – metal plate – o-ring – coil.
(from the bottom) White orifice tube -> spring -> rod with a rubber seal -> base.
Once disassembled you can see the dreaded foam filter. Mine looked intact, however it turned into dust upon touching. After cleaning the remains I replaced it with a strip cut form dish sponge as it is resistant to high temperatures.
You must be registered for see images attach
The bigger issue was the vacuum valve. I tried to remove the white orifice, but it cracked into pieces and a spring from valve popped through the now-enlarged hole. The plastic was like a very hard chalk. I had to remove it with a tiny milling bit and bunch of sharp tools to scrape the remains.
You must be registered for see images attach
I found that a brass stovetop gas nozzle is a good replacement for that white orifice. They come in various nozzle sizes and the standard M6 x 1 mm thread can be filed down a bit to make it press fit into opening (tip: mount it into drill to file it down)
You must be registered for see images attach
I tested the cleaned, reassembled valve by sucking on a piece of hose and suppling 12V to the coil to actuate it. I still got intermittent leak upon closing that I couldn’t get rid of with any further cleaning. I replaced the spring with the stronger one I had (the bigger one above) adjusted by shortening to actuate at about 10 V applied to the coil. This solved a valve leakage. Another source of leak I found was from two flattened o-rings, sandwiched on both sides of metal plate in a valve assembly. I didn’t have replacement so I cleaned them as much as I could and applied some Vaseline with hopes of softening the rubber, it did the trick.
The above was enough to fix my cruise control, it holds the speed nicely, roughly +/- 1 mph of setpoint, acceleration via the "ACCEL/Resume" slider works correctly and I also no longer can feel kind of cycling that I occasionally felt before, where every second or two it slightly accelerated and slowed down. Vacuum valve is also holding vacuum correctly, I can change HVAC controls hours after shutting off the engine.
FYI: Connector has 5 spades, from left to right:
Vacuum valve + | sensor coil | common ground | sensor coil | release valve +
Applying 12 V at any polarity actuates valves, they actually start to operate at about 6 to 7 V. Solenoid coils draw about 0.25 A at 12V, so they are roughly 50 ohm each. As they share common ground and work at any polarity you can conveniently actuate both by applying 24 V (or less) to Vacuum + and release +.
I hope it helps anyone struggling with a malfunctioning cruise control servo.
