Cruise control question... - PeachParts Mercedes-Benz Forum

jonnyo · #1 10-31-2008, 12:20 PM

Hi, guys,
I've been without a diesel since I gave my Mom my '85 300dt for her Birthday last year so I've not been around snooping for tidbits of wisdom, here much.

But now I'm the proud owner of a '81 300SD!

Question for you 'smert' guys: What causes my cruise control to surge 5 miles past where I flip the switch? Meaning, I'll flip it on at 65 mph and the car accelerates rapidly to 70 and settles down at 70 not 65.
Like 'Kentucky windage', what I do now is just turn it on at 60 to have it stay at 65.
But what gives?
Any ideas?
It does this rapid acceleration to 5 mph past where I turn it on at all speeds.

Thanks in advance!

Graplr · #2 10-31-2008, 12:40 PM

Wow, nice car. I have an 84SD in the same color except yours is a bit shinier with better paint!

My cruise control sticks where it is set at, actually if anything I have to set it about 1-2mph high as it drops a bit when it settles.

My two thoughts are- (and they are thoughts as (knock on wood) the cruise is one of the few things I haven't had to mess with)
1-Something in the amp
2-Linkage from acuator to the engine

I would start with #2. Inspect the linkage from the actuator.

bgkast · #3 10-31-2008, 12:42 PM

Just be glad yours works!

hangit · #4 10-31-2008, 12:47 PM

Nice Car! Looks like my 83 300SD. You might call Peter at General Development lab and ask him. He has a trouble shooting page on his website. I have had him rebuilt both of my cars' cruise controls and they work flawlessly. www.gdl-online.com/begin.html.

John

hangit · #5 10-31-2008, 12:48 PM

Also, put the year and model of your car somewhere in your signature so members know what they are dealing with.

John

Jeremy5848 · #6 10-31-2008, 01:05 PM

Most probably in the mechanical drive component under the hood. The electronic box under the dash is usually an all or nothing type of failure. The drive can be opened up, cleaned, regreased, etc., or simply readjusted to not pull as much on the IP linkage.

lutzTD · #7 10-31-2008, 01:06 PM

i had one that did that too, when I pulled the case off of it there were a few toasted components on the pcb.

2.5Turbo · #8 10-31-2008, 01:26 PM

I got a remanufactured cruise amp for mine a while back. It did this the minute I put it in. Thinking that a new reman unit couldn't possibly be bad, I picked up a couple used actuators for cheap and tried them, same result. I didn't want to open up my gauge cluster and check the speed sensor so I sent the reman amp back to the manufacturer to see if it was bad. Sure enough they sent me a new one which works perfectly.

I love the cruise in these cars, it keeps the speed constant better than any other car I've driven. Starting up an abrupt hill, my Subaru will lose up to 5mph before giving it enough gas to keep going. Then it will give it tons of gas and drop a gear or two to get back up to the set speed, which it will overshoot by a few mph. When coming up to a long hill in it, I just stop the electronic cruise and use the organic cruise

jonnyo · #9 10-31-2008, 02:11 PM

But they're not cheap. ( Though luckily I work for a shop and can get a Programma (rebuilt) for $145.

)

Cr from Texas · #10 10-31-2008, 02:37 PM

It works. I'd live with the problem till it quits.

rcounts · #11 10-31-2008, 03:02 PM

Quote:

Originally Posted by Cr from Texas

It works. I'd live with the problem till it quits.

Ditto, but I wonder if whatever is wrong with mine is similar to what's wrong with yours.

Mine accelerates when you set it too - but it doesn't stop at 5 mph over, it just keeps right on accelerating. A gain of 15-20 mph up to around 75-80 is as far as I've ever let it go, but it doesn't seem like it would stop there. If I just let it go it seems like it would keep right on accelerating until it topped out.

I've been told it is the amp...

turbobenz · #12 10-31-2008, 03:39 PM

Quote:

Originally Posted by jonnyo

Hi, guys,
I've been without a diesel since I gave my Mom my '85 300dt for her Birthday last year so I've not been around snooping for tidbits of wisdom, here much.

But now I'm the proud owner of a '81 300SD!

Question for you 'smert' guys: What causes my cruise control to surge 5 miles past where I flip the switch? Meaning, I'll flip it on at 65 mph and the car accelerates rapidly to 70 and settles down at 70 not 65.
Like 'Kentucky windage', what I do now is just turn it on at 60 to have it stay at 65.
But what gives?
Any ideas?
It does this rapid acceleration to 5 mph past where I turn it on at all speeds.

Thanks in advance!

the CC amp. Mine does the same exact thing

jonnyo · #13 10-31-2008, 03:44 PM

I'll let you guys know how it goes as soon as I can.

Thanks

lutzTD · #14 10-31-2008, 04:19 PM

you can get CC amps at the PNP for $10. if it doesnt work you take it back and they refund you. couple trys you should be able to get a working one. I dont even pull them any more.

jonnyo · #15 10-31-2008, 05:56 PM

Quote:

Originally Posted by lutzTD

you can get CC amps at the PNP for $10. if it doesnt work you take it back and they refund you. couple trys you should be able to get a working one. I dont even pull them any more.

I don't know about you but I value my TIME as well as cash. I'm too old to go crawling in and out of cars 'til I get 'one that works'. An hour to drive to one of these PNPs, another hour to see if one even works? And then for HOW LONG will it work? Not a chance. Me no play that game.

ANYWAY, the unit I just put in fixed the problem and comes with a three year warranty. Took me less than an hour to order it, have it delivered and installed.
SWEET!
Thanks for all the input!