Cruise Control Ups and Downs

Hi everyone,
I was commenting on another thread about my progress with fixing the cruise control. Originally the cruise did not work at all... so after doing some tests I thought it was the amplifier.
Well after replacing the amplifier with a rebuilt one, the car began to surge once engaged. Usually it doesn't slow down.... it speeds up past the set speed, coasts for a little while, and then speeds up again.
I've talked to a number of local techs and my responses have been that it could be a vacuum leak, a bad shut-off valve on the injection pump, or a bad actuator.
What do you guys think?? Do you think the speed sensor could be an issue? Any ideas of how to narrow down the culprit? The actuator is like $356, the shutoff valve is like $76, and I don't know about the speedsensor.
Some info. on the vehicle and system are the following:

10 pin amplifier with old Vacuum Motor Actuator
616.912 engine
123.123 chassis
81 240D

The guy at the shop said he could test it... 1 hour min. or $100 min. (I wish they would just give you a price instead of minimums...)

Everyone's input is very much welcome... I could really use your help, experience, and expertise.

SB

PS: I am going to post this in the diesel forum as well since I think it applies in both arenas

['79 Super Turtle]

Maybe this will help.

cruze control/not

Good luck! My cruise had a bad switch.

Patsy

[Richard Wooldridge]

I don't think your sensor is at fault. I think your vacuum control valve is defective. What's happening is that when the vehicle reaches the desired speed, the valve isn't shutting off the vacuum quickly enough and is causing the cruise system to overshoot the mark. Then the system coasts back down to just below the speed and gives the vacuum system another shot of too much vacuum, reproducing the cycle.... You might get some useful information by checking this site: http://www.fix.net/~gdl/mbz01.html
They rebuild cruise controls for a living, and seem to know what's what. I haven't had any dealings with them, but did use some info on their site to troubleshoot my cruise control...
Richard Wooldridge

Thanks for the post...

What is the proper procedure for checking resistance. I have a voltmeter and when I plug it in to the the item I am suppose to be checking resistance it pings past the 0 mark. I have no idea how to reverse it? Please advise.


-Kofi

Thanks a lot Richard... what you say makes sense and that is exactly what happens... so it is probably the shutoff valve that is connected to the injection pump?

I'll check it out... thanks again everyone.


-SB

Well, I checked the vacuum pressure at the acuator. It was 21 in at idle and 7 in when engaged. This seems to be within the 300 mbar and 100 mbar range the service manual talks about. This means the actuator itself is functioning properly right?

So it is either the speedsensor or shut-off valve...

I was looking at the GDL website and it says that my original amplifier was for a manual transmission whereas my current one isn't. But at the same time it says that my current rebuilt amp is "less surge prone".

Question: with the shutoff valve being the broken link... would this make the speed tend to want to surge to a faster speed? This seems to be the trend with my cruise... I am going to get on a very flat road though to properly test it.

Thanks again everyone.

-SB

I went on my long flat drive and it seemed like after I drove a good ways... the cars cruise performed pretty much perfect... it held the speed...let off the throttle for down hill... picked it up for the uphill climb. I am going to take it out again today and see if it just needed to get used to my system or is the shutoff valve working sometimes (on its way out)... and at other times not...

Any comments would be appreciated.

SB