It's not hard to test the servo to see if it will function electrically.
The servo contains three large electrical coils inside the servo. Two are used to open the vacuum and vent ports and the third is used to tell the BCM the movement and position of the throttle cable. Two of the three coils are shown in the photo below. The third (not shown) is on the other side of the servo.
NOTE:
- The coil with green wrap is the pull vacuum coil measured via pins C & E.
(Smaller inlet fitting.) - The coil with the red wrap is the vacuum release coil measured via pins A and C.
(C is common ground for both vacuum coils) (Larger outlet fitting.) - A taller red coil, hidden behind the pins in the second photo, is the position sensor.
Its resistance is measured via pins B & D.
If you remove the cruise servo from the car for testing, be very careful when disconnecting the vacuum hoses. You can easily break the plastic fittings on the servo. The large one is the most prone to breaking. Carefully split the hose down the side with a sharp knife and/or use a small screw driver to carefully pry the hose off the fitting.
To verify that your servo is good electrically, use your Digital Volt Meter [DVM] set to the lowest ohms range.
Using the DVM measure the resistance between the center pin, C, and the two outer pins, A and E. You should measure around 40 to 42 ohms between prongs (A & C) and (C & E) .
Typically if one of the coils is bad it won't return any reading (open). If the reading is close but not quite within range, touch the two meter probes together and look at the reading. Most inexpensive DVMs don't allow the meter to be zeroed out, and will have a reading of a couple of ohms or so when this is done. This small error will be reflected in the reading of the coils and can be taken into account to see if the reading would then be within range.
If one of the readings is below 40 ohms, and a meter error is not suspected, I would caution against using that servo as the circuits inside the BCM that control these solenoids are running close to maximum current, and if the resistance falls below 40 ohms, the extra current can damage the output circuit requiring the replacement or repair of the BCM.
Next, take a reading between pins B and D. This should return a reading around 20 ohms. If not the BCM won't be able to track the operation of the servo in and out. If this reading is not correct the servo should be replaced.
If the servo tests good electrically, you can test the vacuum bellows inside the servo by sucking in on a piece of vacuum hose connected to the large vacuum fitting. With the throttle cable disconnected, the bellows should move the in, and stay in, as long as you are sucking on the fitting. It doesn't take much to suck the bellows in if it is good. If you have to suck pretty hard then there probably is a hole in the bellows and the servo should be replaced. - Thanks Jim Finn for this information!
Thanks to Mc_Reatta, member of the Reatta forums, for providing this information and the photos!
