Can I use the newer W126 Cruise Control Amp w/ Relay on Older W126?
 

Can I use the newer W126 Cruise Control Amp w/ Relay on Older W126?

I acquired a 1988 Cruise Control Amp (with the black relay on the plug side) and I would like to try it on my 1985 which originally did not have the separate relay.

Will it work, or do I need some additional modification.

Thanks

The black plug is not a relay, it's a programming module that sets the main module's characteristics based upon the power, gearing, and weight of the car. Inside, there are some resistors that set the characteristics of the op-amps inside the cruise control unit. I know this, because I reverse-engineered a programming plug for my 560SL out of one that was designed for a 190E. Of course, no one in their right mind would do this...but I did this by examining the test values given in the 107 shop manual.

MB went to a standardized cruise control module and separate programming plug instead of individually built (and catalogued, and stocked) cruise control modules in 86 (?).

If you have a post-85 module AND THE CORRECT PROGRAMMING PLUG for that car, it should drop in to your older car. If it's not right, the cruise control won't operate properly and might even surge, lag, and be very dangerous to drive. You should ask the dealer which programming module is correct. I can't believe that it's very expensive - it's 4 or 6 1/8 watt resistors in a plastic connector, fer goodness sakes.

Thanks - that is pretty much everything I needed to know. It also explains why it hadn't worked. It came from an 88' 560SEL and I tried it in a 85' 300SD. I'll try resoldiering my old one.

No rush on all of this project though, I just spent a couple of hours over the last few days with my head under the dash extracting and replacing the expansion valve and flushing the evaporator. I need to finish the A/C project first. Access to the cruise control module seemed rather easy comparatively once I was down there and during a moment of frustration I felt like I needed to accomplish something and trying out the other amp was it.