98 12 valve auto cruise control problem
 

I have a problem with my cruise control. The cruise light comes on, I can raise speed with the accel button. I can't get the set button to set speed. But if I push the set button a second or third time the truck will accelerate for a moment and then slows back down. I checked the vacuum system I have good vacuum and have repaired a known vacuum leak and a couple that were questionable. Is there a way to check the diaphragm in the cruise control servo?

One of the steps in the diagnosis section of the FSM says to check operation of the one-way check valve. It's in the line between the speed control servo and vacuum pump. If you connect a hand operated vacuum pump to the reservoir end, vacuum should not bleed off. If it does the valve is bad. Connect the pump to the other side of the valve. Vacuum should flow through the valve, if not replace. Then seal the other end with your finger and apply vacuum, if vacuum doesn't hold the diaphragm is ruptured and the valve needs to be replaced.

Also paraphrasing from the FSM, the speed control has an adaptive strategy to compensate for different length cable length variations from vehicle to vehicle. This might cause what you're describing.

To "unlearn" the overshoot/undershoot condition, the vehicle operator has to press and release the set button while maintaining the desired set speed with the accelerator pedal (not decelerating or accelerating), then turn the cruise control switch to the OFF position (or press the cancel button if equipped) after waiting 10 seconds. This procedure must be performed approximately 10-15 times to completely unlearn the overshoot/undershoot conditions.

Hope this helps. :tu:

Also :hi:

One way valve is good.
 

Yes sorry I did check the one way check valve it is good. I'm aware of the adaptive strategy this is not what I'm experiencing. When I press set if allowed to the truck would roll to a stop. If I press set again the truck will start to accelerate and then if allowed would roll to a stop again. I can press accel and the truck will accelerate as long as my finger is on the button. Thanks for the help though.

:humm: I'll look around and see if I can help out.

Yes Sir
 

Yes Sir, That is basically it. When I hit the set button nothing happens. But if I hit it a second time and third time I get a short burst of acceleration and then nothing. I'm kinda thinking the diaphragm in the cruise itself is leaking.

Ok, so I'm not sure how to test the diaprhagm in the servo. Maybe apply vacuum with a hand pump to the servo and see if it holds? FSM doesn't really go into that. There is a section about how to test the electronic side of the servo. If you want, I can post that up for you.

OK so....
 

If I take my hand vacuum pump and put a vacuum on the servo it should stay at whatever vacuum I leave on it?

That's what I'm thinking, not 100% sure though. Unfortunately, the FSM doesn't really say anything about it.

Please Post
 

Dave O would you please post the electronic portion troubleshooting also? Thanks

Sure thing.

1. Start engine
2. Disconnect 4-way electrical connector at servo.
3. Turn speed control switch to "ON" position
4. Check for battery voltage at pin-3 of wiring harness 4-way connector. This is the 12 volt feed from the stoplamp switch. When the brake pedal is depressed, voltage should not be present at pin 3. If voltage is not present with brake pedal NOT depressed, check for continuity between servo and stop lamp switch. Also check stop lamp switch adjustment.
5. Connect a small jumper wire between the disconnected servo harness 4-way connector pin 3, and pin 3 on the servo. Check for battery voltage at pins 1,2, and 4 of the servo. If battery voltage is not at these pins, replace the servo.
6. Turn ignition switch to OFF position. Check for continuity between disconnected servo harness 4-way connector pin 4 and a good ground. There should be continuity. If not OK, repair open circuit to ground as required.

Hopefully this well help.

Tonight
 

Thanks Dave O with any luck I'll try this tonight and post the results.

Sounds good. :c:

No Power
 

Well I'm checking this now and another clue maybe that I have to push the cruise on button four times before the cruise light will stay lit. Then when I check pin 3 (I checked both middle terminals thinking one of them was pin 3) There is no power there. Seems weird that all the buttons I press for cruise need to be pressed repeatedly until they try and work.

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The wire continuity check from the brake stop switch to the cruise control servo is good. Going to get a brake stop switch and see if that takes care of it.

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Thank You Dave O without your help I'm not sure I would have figured this out! The brake light or brake stop switch under the dash was the culprit. One thing that lead me astray was I saw a post which said if the brake stop switch is the culprit you can lift up on the brake pedal and the cruise will work. Well in this case it didn't work for me.

Thanks again DaveO :c:

glad you let us know what fixed your problem:c:

Glad I could help. And yes, thanks for posting what it was too. :c: