Reinstalled the cruise control servo and the cruise control is not working?

TWEBB101

MEMBER
1984 CHEVROLET TRUCK
7.4L
V8
2WD
AUTOMATIC
170,518 MILES

I just reinstalled the cruise control servo into the pickup (had to repair the rubber diaphragm). Everything is plugged in, but the cruise isn't working. I have manifold vacuum connected to the smaller port on the unit, and what I think is the break-controlled vacuum connected to the larger port. Wires are connected as is the throttle linkage. Can you help me diagnose the issue? I think it's electrical, possibly the turn-signal switch, but that's expensive so I don't want to go there first. Is there a fuse for it, or some other way/item to diagnose?

Wednesday, July 5th, 2023 AT 3:40 PM

21 Replies

KEN L

MASTER CERTIFIED MECHANIC
54,144 POSTS

I would start by checking the brake light switch adjustment because this will cause the cruise control not to work. Here are the system wiring diagrams so you can see how the system works and where to check for power and ground.

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

Check out the images (below). Please let us know how it goes

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Thursday, July 6th, 2023 AT 7:04 PM

TWEBB101

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Thanks Ken. I want to update you. I was looking around for the brake switch you mentioned, and noticed that the wire for the cruise, from the turn signal switch, was cut and not even plugged in. (Face palm). So, I replaced that, but still nothing. So, back to square one. I found the brake light switch, it has 2 plugs in it, and seems to be working. The brake lights come on :) haha on the same bracket is a black switch as well; is that for cruise or something else? How do I check them for proper adjustment/functionality?

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Saturday, July 15th, 2023 AT 6:18 PM

KEN L

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Yep, one is for the brake lights and the other is for the cruise control. If the brake lights are coming on the switch is working. But to test the cruise side use a voltmeter on continuity and when you press the brake pedal it should connect.

This guide can help:

https://www.2carpros.com/articles/how-to-use-a-voltmeter

Please go over this guide and get back to us.

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Sunday, July 16th, 2023 AT 10:02 AM

TWEBB101

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I tested it this morning, and here’s what I’ve found while pressing the brake pedal:

- No readings on the gray plug at all. The gray wires feed into the black hose that’s connected to the black brake switch.
- With red lead on orange and black lead on white: 3.89 ohms.
- With red lead on white and black lead on orange: -2.46 ohms.

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Sunday, July 16th, 2023 AT 11:04 AM

KEN L

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You need to test the switch with the wiring connectors disconnected. Please let me know, test while pressing the brake pedal and not pressing the brake pedal as well.

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Monday, July 17th, 2023 AT 10:06 AM

TWEBB101

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Okay, tested the switch with both plugs disconnected. Bottom plug (orange/white), I got 1 with no brake pedal, and 0.00 with brake pedal, on both combinations of red and black leads. On the top plug (gray), I got the opposite, 0.00 with no brake, 1 with brake.

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Monday, July 17th, 2023 AT 1:02 PM

KEN L

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You tested through the switch, correct? Let's go over the vacuum lines to make sure they are correct. Check out the images (below). Please let us know what you find.

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Tuesday, July 18th, 2023 AT 10:18 AM

TWEBB101

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Yes, I disconnected the plugs from the white switch and touched the multimeter leads to the switch prongs directly.

Attached are pics of my vacuum setup currently. Manifold vacuum > T fitting > one end to the brake booster, one end to check valve > one end to vacuum accumulator ball, one end to another T fitting > one end to climate control, one end to cruise control servo. Other outlet on servo goes into a black hose that goes through the firewall to the brake pedal area (I believe.)

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Tuesday, July 18th, 2023 AT 4:28 PM

KEN L

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Yep, the vacuum lines look good, you might have a bad/defective servo. But I do remember that the switch stock (blinker handle) would break the wires just outside of the switch. I think I found the wiring diagrams for the cruise control, but it is a little hard to read.

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Thursday, July 20th, 2023 AT 10:07 AM

TWEBB101

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Thanks Ken. Is there a way to test the servo? Or the power going to the servo?

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Thursday, July 20th, 2023 AT 10:42 AM

KEN L

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I am sure there is, by looking at the wiring diagrams I would test the signal from the cruise switch and the power and ground to the system. Did you replace the turn signal stock? They had a high rate of failure.

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Saturday, July 22nd, 2023 AT 8:43 AM

TWEBB101

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Hi Ken, I wanted to follow up with this thread. A long story, but I found that the optical speed sensor that mounts to the rear of the speedometer was disconnected. I re-mounted that and the cruise started working! Hooray! However, the truck will “jerk” when cruise is engaged, as if trying to hold speed, it gets too much acceleration, and then too little acceleration. Any thoughts on that?

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Tuesday, December 26th, 2023 AT 9:23 PM

KEN L

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Good job getting it fixed. I am not sure what you mean by "jerk"? Do you mean the engine will misfire?

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Wednesday, December 27th, 2023 AT 12:05 PM

TWEBB101

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No, the engine fires fine; it's like if someone is pushing the accelerator pedal then releasing, then pushing again, and releasing again. The truck will surge back and forth with the acceleration changes. It will hold the set cruise speed, but it's not smooth at all.

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Wednesday, December 27th, 2023 AT 12:21 PM

KEN L

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Does it fell like the cruise control is "hunting"? If so this is caused by the cruise control unit, can I ask if the speedometer is working smoothly?

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Thursday, December 28th, 2023 AT 7:48 AM

TWEBB101

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Yes, the speedometer is working great; a few MPH slower than actual speed, but it operates smoothly.

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Thursday, December 28th, 2023 AT 7:49 AM

KEN L

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It sounds like it is the transducer itself. Do you have another one you can swap out to see if it makes a difference?

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Friday, December 29th, 2023 AT 9:33 AM

TWEBB101

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Are you referring to the Servo? That attaches to the carb and vacuum lines. Or something different? I'm not sure what the transducer is.

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Friday, December 29th, 2023 AT 9:42 AM

KEN L

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Yes the servo, the cruise control head. That is the only thing that can make the cruise control hunt.

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Saturday, December 30th, 2023 AT 10:43 AM

TWEBB101

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Confirmed: Servo is all over the place. I stuck a GoPro under the hood and got this footage. I have a used one I can try swapping out, but it’s a pain to get those screws back in! :)

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