One more cruise control amp repair success story
 

1980 300D.
Cruise accelerates car, does not hold speed at all when setting.
Tested the actuator under hood (vacuum can type), replaced
speed sensor on speedo with donor. Replaced amp with donor
(but did not know for sure if it worked), it still did same thing.

Opened metal can of cruise amp like the link/sticky/diy says to do.
Convinced myself nothing was wrong with the cruise amp.
I have seen bad solder joints on hardware and usually can pick them out.

Was sure that re-soldering as instructed would do no good.
Reluctantly started the job with my 180 watt trigger soldering iron
that I used for blower motor brushes, radio leads, and rear window defroster.
I found Mercedes solder to need fairly high temp.

Way too much heat, after creating burn marks on the first 10 or so connections
I switched to a 35 watt pencil type soldering iron.

Touched each joint, melted it and let it cool.
If the connection looked like the solder was being sucked out I added solder.
Tried not to overdo it, sometimes I did.
Lost track, sort of, of which ones I had done and which needed doing.
Sometimes you can tell by the color.

With even more reluctance I switched in the resoldered unit, convinced I had ruined it with my work.

The surprise was when I went to set it at about 40mph and it held.
I could not believe it.

So the lesson is, resolder the board, even if it looks perfect.

I'm thinking I might as well give that a shot. My cruise control would accelerate my car and then totally let off the gas, then accelerate and let off again. Then one day, it did nothing. How many different connections did you have to re-solder?

I tried the resoldering, I spent over an hour on it and soldered a ton of them, using a 25watt pencil style iron, worked pretty good.

Results: worked ok at 70+ mph for a few days. Then it stopped entirely for a bit.....now it only rapidly accelerates (same problem it had before soldering) or it does a 5mph "hunt" where it accelerates 5mph as fast as it can, then drops off completely, looses 5mph, and then goes at it again, over and over. When its cold out it doesn't turn on at all....which makes me wonder about those solder connections again......I am leery to spend $200 on having it rebuilt when I can just get a whole aftermarket cruise system for less than $120.......

I can't say how many connections are there. Maybe 50.
I kinda went down one side, then the other.
The whole process did not take more than one hour.

It took me 2 tries
 

First re-soldering attempt just re-heated the joints (all of them). This worked ok for about a month or two then started going flaky on me.

Second attempt I added solder to any joint that looked like it needed it. This has held for the last 6 months or so and is still working well.

Both times I went over all the joints. A faulty joint can still look good...

I tried the re-solder route and had a little luck for a couple of months. I have noticed that when I hit a bump sometimes the cruse kicks in and will work for the rest of the trip, and sometimes it quits as soon as it starts. Could it be a low voltage to the card? I charged my battery overnight, (not much, just a couple of amps) and the cruse worked the next day. Any good suggestions would help.
Thanks

Bump in the night
 

I doubt that battery voltage is related to your cruise problem, NewBenzOwner. It might be due to a poor connection. Try tracing all of the cables and wiggling or unplugging/replugging all of the various connectors in the cruise control circuit.

Jeremy

Well if my actuator was working properly in the SDL, I could then check to see which of these amps I have work. :o

Mine does the +/- 5mph hunt... time to bust out the soldering iron, I guess! I wonder if new motor mounts will help in the longevity of solder joints, as I'm sure vibrations have a hand in their failure in the long run.

It works!
 

I am happy -- overjoyed -- to report that the soldering trick worked. Thanks to all who kept urging me to "resolder the board even if it looks good." (Would that I had the time to solder one pad at a time and then test -- I don't know which one fixed it.) Actually, it probably doesn't matter, just that it now works. We'll see for how long this fix is good.

Note for thos comparing their problem to mine: this is a 1985 300D Turbo (123 chassis). My cc would (sometimes) engage and operate for a minute or two, then stop. At other times, it would fail to work entirely. Once, it accelerated with no end in sight, until I shut it off (at least that part worked!).

Some comments:
1. I found it easier to remove the board from the aluminum can than remove the can from under the dash. The screws, especially the rear one, are just too hard to get out (how did M-B get them in???). I carefully unbent the can where it was bent in against the board's mounting thingy, unplugged the wiring harness, and slid the board down and out. Carefully.
2. The board was made by VDO and wasn't as well designed as it could have been. The holes are not plated through -- this makes soldering harder and less reliable, as there's less surface for the solder to attach to.
3. The board was dipped in some kind of shellac or varnish, probably as a moisture barrier. While that may have been a good idea, it makes resoldering all that much harder, as you've got to burn through the coating, which also interferes with the soldering process. I added solder to almost every joint, more for the rosin (flux) that it contained than for the extra metal.
4. The pin structure for the connector is composed of multiple riveted brass components. On my unit, the brass was rather corroded; I'm surprised that there wasn't a poor contact there somewhere. I decided not to try soldering those joints because I don't know how soft the plastic composing the mounting base is.

One nice thing about this kind of problem is that if it recurrs, the same treatment will work as well as it did the first time. I hope I won't ever have to do that job again but at least I know how, now.

Jeremy in Santa Rosa, California

They can also fail if a component on the board actually fails, which I do believe happens quite a bit of the time, so re-soldering won't always cure the problem...I may have another go at resoldering at some point next month, I want to see one last time if I can get it to work right....taking that dash panel covering in and out is the biggest pain in the world though. :o

just took mine out and back in yesterday.
 

pulled the board. ya know the two bronze doodads on one side? well there was a resistor next to the one on the left that must have gotten hot because the paper cover had come off. on the meter it read 151 milliamps. chad(the electrical guy) resoldered it than went to the pin plug and did them as they looked dark grey not shiney like the rest of the board.

had no idea where the actuator was on the 300(gdl-online) so we just pluged back in and went for a ride. nothing.

2 question here.....
what is the proper reading for that resistor?
where is the actuator on a 1981 300td-t?

Still have some suspicions on what is going on here exactly. People describe a very funny alloy mix in the original solder for starters. Plus it is slightly concievable that the specific type of electroylitic capacitors might be temporarily rejuvinated by the resoldering heat travelling up the legs. The capacitor type used on this board is such a high failure item that the german site suggests changing them out. I think this is possible for the guy with minumim electronic knowledge or less to acomplish as well. Another thing that comes to mind is to remove the manufactures solder with a solder sucker or prefferable a solder wick. Available at any electronic parts outlet usually. Then replace the solder with a normal electronic rosin core solder. If there has been an induced failure at that point a simple reheat may or may not be succesful as the corrosion that may be present is not really properly overcome. There is a slight possibility that some form of dissimular metal is having a reaction at the junction and going open. Possibly an incombattability exists from new. Why they did not use normal solder originally kind of leaves me cold or their alloy mix is aging strangely. To make sure you get all the joints just mark each one with an ink marker as you do the board to eliminate missing any. I ran across strange solders out of europe a long time ago on grundig and telefunken equipment on occasion and even then did not know why they were not using standard values of metal alloy solder mixes other than to save money perhaps.

where is the actuator on a 123 either a 300td-t or a 240?

bump
 

c'mon now. someone knows. share now.