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Cruise control

Started by beagle2022, 25 May 2011, 10:53 PM

beagle2022

Hi folks,
1979 280sel with demon-possessed cruise control. The lever will happily accelerate the car but won't set a cruise speed.  I have tried it at speeds from 80-110 kmh with no success.

I have no idea how this system works, so would appreciate dummy-level advice, including informed suggestions to walk quietly away from the problem if it will require a second mortgage or a degree in engineering.

Thanks for any insight available.

Peter
Sydney, Australia

jbrasile

Hi Peter,

Typical problem... most likely your cruise control amplifier is bad. It is a little  module that based on a signal from the speedometer controls the position of a vacuum throttle actuator to maintain your set speed.

This silver box is located under the dash on the driver's side right above the brake pedal in LHD cars so I assume it would be the same for RHD.

It is possible sometimes to have the unit repaired by re-soldering its board connections but I usually advise clients to buy a rebuilt unit on exchange, they are available in the US.

Let us know if you wish to pursue a solution and will give you some options and prices.

Tks,

Joe

WGB

Do a search on Cruise control amplifier - there will be lots of detail including pictures.

Basically the condensors fail and soldered joints dry out.


Bill

Hemersam

Amplifier, most likely, possibly the actuator (or both). On my '78 280SE, I had a rebuilt amp ($150) and used actuator from another 116 installed, and the cruise works like it should (more or less). When it gets warm outside, it will lose some speed over a number of miles, but when the weather is cool (or cold), it stays right on the mark. But, since I don't use it anymore, it has become irrelevant.
Hemersam

Casey

Mine usually works, but sometimes it doesn't and I have to hit the Accel again.  Is this probably the amplifier as well?

Also, once I set a speed, it slowly loses speed over time - if I set it to 80, in 5-10 minutes I'll be down to 70, in another 5-10 minutes I'll be down to 60, while the cruise is still obviously engaged though.  I was guessing this was a vacuum leak?  It was worse when I first tried using the cruise control and has gotten a lot better over time, mysteriously.

Also, sometimes the cruise will suddenly disengage and I'll hear the revs drop and start losing speed rapidly.  Generally, I can just hit Accel and reengage the cruise okay, but sometimes it ends up in a state where the cruise just fails to respond altogether for a while.  Try again in 10 minutes and it works fine.

Once in a while, I'll hit Accel, and when I release it, the car just keeps accellerating like mad, with no end in sight until I manually brake the car.  That seems real safe...  Fortunately that doesn't happen often.

I quite like having a cruise control, but it takes so much fiddlery now that it's frustrating.  I'd like to get it in perfect working order at some point...

1980sdga

I have a "part time" cruise as well  ::)  Seems like the accel function keeps working but it has lost the ability to lock in a speed.

I think the main components are the amp, the actuator and the speed sensor on the instrument cluster. Is the speed sensor the 2 prong locking connector on the speedo?  I could be completely wrong on this one...

If that is the speed sensor then it could be another failure point.  Maybe wishful thinking because it's easy to get to  ;D

jbrasile

Casey, I'd say the amp is at fault as in most cases.... my old 79 300SD did almost exactly the same thing, rebuilt amp in, problem gone...

Tks,

Joe

jbrasile

Casey, yes that's your speed sensor, but again, accelerating but not holding is typical of an amp failure.

Tks,

Joe

1980sdga

I've had my cruise amp out to gain access to vacuum lines.  It's not hard to remove if you happen to be a double-jointed dwarf with 3 arms.

Casey

Joe,

Can you get a price on the amp?

jjb-w116-hu

can cruise conrtol be added ? i'd love it in my old girl

jbrasile

It can but you would need to get most of the components from a donor car and perhaps just buy a rebuilt amp to be on the safe side.

Also, if your car doesn't have a speedometer transducer, you will have to get one tha does.

All of the harnesses, etc must be removed from the donor car as well since it is probably NLA from MB

A good friend has added a factory cc system to a 300E-24 stick shift and it looks totally stock! So it is not an impossible project.

Tks,

Joe

jjb-w116-hu

hehe solid! thanks.... this is worth some consideration indeed! thanks Joe.

talking of harnesses, do you mean the electrics?

i was wondering as i played with fuses and bulbs this afternoon if its possible to get all the wiring new? i hear a funny clicking through my speakers when i play with the light switch

jbrasile

jjb,  some of the harnesses (electrical cables) are still available but to be honest with you I think it's  overkill to replace any of them.

Do you have a factory radio in your car? I ask because if you have an aftermarket stereo, the installer who put it in may have taken power from a wire in the lighting system hence the clicking sound when you turn the lights on/off.

Tks,

Joe


beagle2022

Wow.  Thanks folks.  I will attempt to find the triple jointed dwarf and have a look at the amplifier gadget. 

Peter
Sydney, Australia