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  #1  
Old 01-11-2010, 07:09 PM
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Location: North Carolina
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Question Car bucking with cruise on



I found that when cruise is on in the mountains it does fine however in the hills of SC it is like the car is bucking gas/brake/gas/brake, etc.......

Sometimes in both the mountains and the flat lands the cruise will just turn itself off will not resume and somewhere down the road engage again (usually when I am exiting a highway).

Any ideas folks??? How would I test out those ideas?


Dave H...
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  #2  
Old 01-11-2010, 07:17 PM
79Mercy's Avatar
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We would love to know what kind of car you have.
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1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg
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  #3  
Old 01-11-2010, 07:21 PM
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I know you haven't stated what your vehicle is, but when you do have you done anything to the throttle linkages under the hood?
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Going back to the original post: "Can you get the vac to blow instead?" No. Vacuums are low pressure so they by nature "suck" and nature abhors them.
1984 380SL
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  #4  
Old 01-11-2010, 07:52 PM
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Look at your linkage from the cruise servo to the throttle linkage,any excessive loose play can cause that as the servo tries to control the speed and due to lost motion in the linkage ends up over correcting in one direction which causes more of a change in speed than was called for so then over corrects in the other direction and repeats, when my car was doing that it was always worse on level ground than going up hill.Another possibility might be a touchy brake light switch,as you may know 12volts applied to the brake lights is what shuts off the cruise when you step on the brake,but i dont think it would come back on if that were the case it would just be off until you switched it on again. Don
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  #5  
Old 01-11-2010, 08:07 PM
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OK I guess I get the "Sunglasses" award for brightness.

This particular car is a 1984 300Dt

I'll check for play in the linkage but no I have never adjusted any of the linkages before.

Thanks folks.


Dave H...
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  #6  
Old 01-11-2010, 08:10 PM
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Quote:
Originally Posted by a52daveh View Post
OK I guess I get the "Sunglasses" award for brightness.

This particular car is a 1984 300Dt

I'll check for play in the linkage but no I have never adjusted any of the linkages before.

Thanks folks.


Dave H...
You might want to go ahead and lubricate the all of the throttle joints.

When I went and did mine I popped off every little end of the ball joints and shot some white lithium grease into it. When I did some driving with cruise on the highway I was getting that funny push, break, push, break bucking you are describing.

In my case the problem worked itself out. Maybe for you, you may have to check the make sure everything is tight with little to no play.
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Quote:
Originally Posted by Yak View Post
Going back to the original post: "Can you get the vac to blow instead?" No. Vacuums are low pressure so they by nature "suck" and nature abhors them.
1984 380SL
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  #7  
Old 01-11-2010, 08:12 PM
79Mercy's Avatar
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Use ATF to lube the linkages, it is listed in the FSM.

White lithium grease can turn thick and cause them to stick after a while.
__________________
1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg
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  #8  
Old 01-11-2010, 08:13 PM
okyoureabeast's Avatar
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Quote:
Originally Posted by 79Mercy View Post
Use ATF to lube the linkages, it is listed in the FSM.

White lithium grease can turn thick and cause them to stick after a while.
Ouch I'll buy some when I do my oil change next week. Last time I follow my haynes manual on fluids!
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Quote:
Originally Posted by Yak View Post
Going back to the original post: "Can you get the vac to blow instead?" No. Vacuums are low pressure so they by nature "suck" and nature abhors them.
1984 380SL
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  #9  
Old 01-12-2010, 08:04 AM
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Thanks so much folks I'll check the linkage and use ATF to lubricate.


Dave H...
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  #10  
Old 01-12-2010, 09:33 AM
92 300D 2.5L OBK #59
 
Join Date: Feb 2007
Location: Central FL
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Mine surges on declines. Works well on flats and inclines. It's expensive to fix the stock system. Might want to look at an aftermarket setup. Check out audiovox CCS The search function works great. http://www.peachparts.com/shopforum/search.php?searchid=4687205
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  #11  
Old 01-12-2010, 10:40 AM
Craig
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I would also lube the linkages. At one point the system in my 300D was a little "underdamped." When it started acting up I replaced the amp and the actuator, I discovered that some idiot had installed the incorrect amp (from a 240D) so the damping was incorrect. It works perfectly now.
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  #12  
Old 01-12-2010, 09:17 PM
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Could it be corroded solder joints on the Cruise Amp?
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  #13  
Old 01-12-2010, 10:13 PM
'86 300SDL 205K
 
Join Date: Jan 2010
Location: Summerville, SC
Posts: 19
Most likely in absolutely no way related, as the linkage between the cruise and the throttle sounds like it, but this happened to me about a year ago -
86 300SDL
The door light on the underside of the rear passenger side door popped loose and my son shut the door while the car was still running...crunch...half the accessory power in the car went off...the seat belt warning light blinked constantly...a few other weirdities...
But the strangest was the cruise control was fine...unless I hit the left turn blinker...then the cruise kicked off...
It was about 19 degrees outside and I didn't want to mess with it but I took the light's remains apart and taped off the contacts, replaced the blown fuse and all was fine...
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