One more cruise control amp repair success story - PeachParts Mercedes-Benz Forum

rhodes2010 · #1 04-10-2006, 01:05 AM

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.

PatricdeBoer · #2 04-10-2006, 02:37 AM

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?

pawoSD · #3 04-10-2006, 02:45 AM

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.......

rhodes2010 · #4 04-10-2006, 03:25 AM

Quote:

Originally Posted by PatricdeBoer

How many different connections did you have to re-solder?

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.

Geodad · #5 04-10-2006, 02:01 PM

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...

NewBenzOwner · #6 04-10-2006, 07:53 PM

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

Jeremy5848 · #7 04-10-2006, 09:48 PM

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