Cruise control

[Chadwell]

The cruise control on my '90 will not operate, the light in the instrument panel does not come on. I'm getting a b671 code which is a cruise position sensor. Can anyone tell me where this sensor is located, and is it easy/hard to check or change?

[Guest Mc_Reatta]

The position sensor is the center coil of the three coils inside the servo module. It can be checked using an ohm meter (DVM) and measuring the resistance of the coil.

Here's how to check out the coils inside the servo:

The servo contains three large electrical coils inside. 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.

To verify that your servo is good electrically, use your DVM set to the lowest ohms range.

There are five prongs at the electrical connector. Using the DVM measure the resistance between the second and fourth prong. 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.

While your at it, measure between the center prong and the two outer prongs. (1, and 3 and 3 and 5). Each of these should measure around 40 to 42 ohms apiece. Typically if one 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 any of the coils don't return the proper resistance you should replace the servo. Same servo was used on many of the larger sized GM cars of the era, or you can get a rebuilt one from most FLAPS.

[Ronnie]

Mc, Thanks for posting the testing instructions. With your permission, I would like to post your instructions for testing the cruise control servos on ReattaOwner.com. Do you possibly have a photo of the connector that I could use to show which prong is #1-5?

[DAVES89]

I can help you with a good used servo. $45.00 delivered. PM me if interested.

Price includes the three mounting screws and cable clip. [You will drop/lose at least one].

Edited April 17, 2012 by DAVES89 (see edit history)

[Guest Mc_Reatta]

Ronnie here's my original post on testing the cruise control servo:

It's not hard to test the servo to see if it will function electrically.

The servo contains three large electrical coils inside. 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.

To verify that your servo is good electrically, use your DVM set to the lowest ohms range.

There are five prongs at the electrical connection. Using the DVM measure the resistance between the center prong and the two outer prongs. (1, and 3 and 3 and 5). Each of these should measure around 40 to 42 ohms apiece. Typically if one 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.

Then take a reading between the second and fourth prong. 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.

Additional great information can be found in these thread links:

http://forums.aaca.org/f116/91-cruise-control-vacuum-fittings-317541.html

http://forums.aaca.org/f116/cruise-issue-1988-a-319911.html

Notes for the photos:

FSM identifies pins as A thru E not 1 to 5. Pin A is at the bottom of the attached photos, E at the top.

The taller center coil partially hidden behind the pins in the middle photo is the position sensor. Its resistance is measured via pins B & D.

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.)

The coil with green wrap is the pull vacuum coil measured via pins C & E. (Smaller inlet fitting.)

[Ronnie]

Thanks Mc. I've got this on my short list of things to do.

[Chadwell]

Great information. Thanks for the response. I plan on working on it this weekend when my son, who's much more adept at things electronically than I, will be here to lend a hand.

Again, thanks.

[Ronnie]

The cruise control testing tutorial is now on ReattaOwner.com. Please let me know if additions or corrections are needed.

Cruise Control Servo Electrical Testing*-*ReattaOwner.com

Thanks to Mc_Reatta for the information and photos used in the tutorial.

[SeanR]

Are these interchangeable with other GM models of the era? Say 88 Fiero? How often do they go bad?

[Ronnie]

A lot of other GMs will fit. Not sure about the Fiero.

[jonlabree]

I have had very good luck cleaning the contacts on the servo and returning it to service.

[DAVES89]

Here you go Jon, complete with input from you...