W126 Cruise Control acting all funny - PeachParts Mercedes-Benz Forum

JustinMorrison · #1 01-09-2007, 04:13 AM

I tried to search this one, but to no avail.

Overall my crusie control works as it should. The four positions on the cruise control stalk all do what they're supposed to (set/accell, decel, resume, cancel.) The only problem is that when engaged, the cruise control acts as though it is tapping on the gas pedal the entire time. If I place my foot on the gas pedal I can feel it going in and out.

Does this sound like a problem with the cruise control amplifier? If so, is it advisable to attempt to repair it?

Oh...the car is a 1987 560SEL, 242,000 miles, very loved

dpetryk · #2 01-09-2007, 07:12 AM

Amplifier problem. Needs to be re-soldered. How are your electronic repair skills?

JustinMorrison · #3 01-09-2007, 03:26 PM

Good enough to give it a shot, I successfully re-soldered the buzzer/light relay thing last year.
Is it okay to remove the amplifier and still drive the car around? Any hints I need before getting into this?

Rick76 · #4 01-09-2007, 05:02 PM

The circuit board is dipped in a conformal coating to protect it from moisture. Don't attempt to re-solder without stripping the coating off (solder side only). Use a gel type stripper from the local hardware store. Brush on and let it wrinkle - don't leave on any longer than you need to. Wash off with a blast of water from the garden hose and immediately dry with compressed air. The bad solder connections are usually around the rectangular capacitors.

There is no problem driving the car with the module out.

JustinMorrison · #5 01-09-2007, 09:39 PM

Okay, thanks for the heads up. I'll pull it out tonight and hopefully have good news to report in the next few days...

dpetryk · #6 01-09-2007, 10:06 PM

Just solder right thru the coating. You dont want to add any harsh chemicals to the conductors. It might cause electrical leakage.

Rick76 · #7 01-10-2007, 06:09 AM

You can solder though the coating but I find that the contamination of the lacquer coating makes the job much more difficult. You may tend to keep the heat on the joint too long to get a good solder connection. The electronic supply companies sell conformal coating strippers, but they appear to be chemically the same as the hardware store type but for more money.
I strip the board, wash, dry, re-solder and then wash with isopropyl alcohol and an anti-static brush (cheap china bristle will do) to remove the solder flux and any other contamination then blow off with compressed air. Then re-spray with conformal coating (probably optional since the module is mounted inside the vehicle).
You can leave the flux on the board but in that case you should, still do the alcohol wash to remove any traces of the stripper before soldering.

JustinMorrison · #8 01-10-2007, 08:29 PM

Well, I pulled the amplifier out today and looked it over and I can see no obvious problems. I also checked the little relay that plugs in next to the wire harness, and it looks fine as well. What would be the best method to test the amplifier, or is it possible that the problem lies elsewhere (wire harness, actuator, or whatever...)?

Rick76 · #9 01-10-2007, 08:56 PM

You say that the gas pedal is going in and out when the cruise is engaged. The gas pedal will move in and out as the actuator adjusts to maintain the setpoint. Is it maintaining speed or is there a wide variation?

Solder connections around the leads of the capacitors crack and are very hard to see. Re-solder them all before you put the unit back together.

JustinMorrison · #10 01-10-2007, 09:07 PM

The gas pedal constantly moves in and out very slightly, as though someone were tapping the pedal. The speed fluctuation is minor, but very noticeable. Imagine if someone were gently tapping the brake 10 times per second, you notice it but the speed remains constant on the speedometer.

Is there a good method to test the solder points with a multimeter? I'd rather know it's fixed than have to reassemble everything and go for a drive just to learn that I didn't actually fix anything.

dpetryk · #11 01-10-2007, 10:15 PM

You cant see the problems with the solder joints with the naked eye. You need a microscope. Trust me - I know what I am talking about. You will see micro cracks on the leads of the larger components. Their mass and vibrations from the car weaken the solder joints and funny things begin to happen. Get a scope and check it out. Look carefully at the leads on the large components as compared to the smaller ones like the resistors. You will see what I mean if you have enough magnification. Just re-solder all the joints and it will work. I have done it dozens of times. One of my cars did exactly the same thing as yours is doing now. I have soldered numerous amps for forum members and it worked every time.

Look at this thread;

Cruise control amplifier on 124 (1990 300E).

JustinMorrison · #12 01-17-2007, 02:14 AM

Well, I finally got around to working on this and sure enough everything now works as it should.

The hardest part was getting the bracket that holds the cruise control amplifier and the door buzzer relay back in place. I realized as I was trying to get it back in that I could have taken the amplifier out without removing the bracket to begin with. Oh well, live and learn!

Thanks to everyone for all the help!

hank3rd · #13 11-02-2007, 09:20 PM

I do not mean to resurrect this thread, but my problem is similar. I have a w202 and my cruise was working intermittently at best. I took out the amplifier and soldered the unit and it stopped working altogether. I guess my soldering skills were not as good as I thought!

I ordered a used amplifier, and while I was waiting for the unit, my CEL light came on and the code stated that there is a problem with the speed sensor. I installed the new amplifier and the cruise did not work. Probably because of the speed sensor? Is there a way for me to find out which sensor it is?