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I have a 1989 GMC Vandura 2500 conversion van with a 350 engine.
The speed control fails intermittantly. When I first start out engine cold it will hold the speed, however a little weakly, as a hill will cause the van to lose speed. When the engine warms up after about 30 minutes, the speed will slowly drop, although the control is still engaged. I can bring the speed back up to the preset speed with the throttle and it will hold, sort of, then slowly drop. I can press the accelerate control on the speed control knob on the stick on the column and it will "hold" my speed, not accelerate. I have changed the cruise control brain box under the dash and the actuator on the engine. Any ideas or tests I can do? I have a GMC service manual for this vehicle.
Answer -
Hi,Dave
I was leaning towards the Cruise servo located at the engine but it looks like you replaced it,sounds like you are loosing vacuum to the cruise servo,make sure the vacuum lines are ok,not broken or leaking,replace the vacuum lines if they are,next you need to locate the vacuum reserve canister,it is loacted on the hood and looks like a black plastic ball and has a vacuum hose attached,make sure that the hose is ok and that the tank itself is not leaking,also run the engine and see if there is vacuum to it,next check the brake vacuum release valve,this is located under the left of the dash at the brake pedal,it the switch with a large vacuum line attached,to adjust,depress the brake pedal,push the switch all the way inward release the brake pedal giving a little force upward slowly untill it stops,you will hear the switch ratchet,recheck the cruise control,if it is still doing the same then i would suspect the brake vacuum release switch.if you need further help contact me.Thanks
ED
I snooped around today and found that when I sucked or blew on the vacumn line that is attached to the brake switch I heard air rushing in the servo. When I removed the big vacumn line from the servo and plugged it I could not blow any air. I removed the servoand checked it and it appears to have an internal air leak. I ordered a new (again) servo.
Answer -
Hi,Dave
Hmm,ok,but check the GM service manual that you have and if the cruise control diagnosis is listed in the manual,usually the servo when not active,it will bleed vacuum,upon activation the exhaust valve in the servo will close,in the manual there is a test for this,if you cannot locate it,let me know and ill attempt to do the diagnosis on my end,or install the new servo and recheck cruise operation,let me know what happens,Thanks
ED
Dear Ed,
Success!!!! A three year old problem is gone!!. I replaced the servo with the new one I picked up today, and as you said it "leaked" air just like the old one. I should have known it was not leaking when I took off the vacumn lines and it still had vacumn after engine sat all night. But, AS YOU SUGGESTED the brake switch caught my eye. I removed it, disassembled it, greased the O ring with vasoline, cleaned the electrical contacts with WD-40, Reassembled and then got it adjusted so there was no vacumn leak when the brake was not depressed. VIOLA!! I am crusin' in comfort. Thanks thanks & thanks!! Have a wonderful resurection Sunday.
your Pal, Dave
AnswerAlright! Dave {clapping hands} great job! im glad you solved the problem,and that cruise control is being put to work...a little team work and we can make it happen.
If you need more help or advise just contact me and ill give you a hand.Thanks
ED
