Cruise Amp Repair
Inoperative or malfunctioning cruise control is often caused by a bad
amplifier. The amp can often be repaired simply by resoldering the
connections on the circuit board. I used this procedure to fix the amp
on my 1980 240D as well as my current 300D.
The amplifier is a small rectangular box measuring roughly 4" x 6" x 1".
It's usually located underneath the driver's side dash.....above your shins.
Remove the long connector attached to one end and then unscrew the
whole box and bring it to your shop. You'll need to remove the aluminum
part of the amp. Simply pry back the metal tabs that hold it on.
Slide the cover off and you'll expose the circuit board. Look carefully at
each component to make sure there aren't any burned parts. If so you'll
have to replace them or the whole amp. You probably won't find anything.
Get a good solder iron and heat it up. Make sure you have good lighting
and a comfortable chair. A magnifying glass will help. Your objective is
to quickly reheat each solder joint. A hot iron is essential. You need to
make sure that you re-melt the whole joint. You may notice a sort of
shockwave ripple across the joint as it melts. This is how I determined
that the entire joint was indeed melted and re-set. Sometimes I had to
push hard and actually pierce through part of the hard solder before it
would melt. It helps if you put a small amound of solder or flux on the tip
of the iron before touching it to the joint. The longer you hold the hot iron
on the joint, the more likely that you will damage a component, so heat it
quickly and move on. Make sure that you don't inadvertantly bridge
joints that weren't meant to be connected. Use very little solder, if any,
and look at what each joint connects to or touches before you solder it.
There are lots of joints and it will be hard to keep track of which you have
already done. It helps if you break things down into sections. I did all the
large connector joints near the plug recepticle and then did the rest of the
board by separating it into four sections.
I didn't touch any of the microchip connections. I was afraid the heat
might damage them and I figured that I could always go back and solder
them if it didn't work. Luckily, resoldering all of the other components
and the connectors fixed my problem.
My amp had a small relay type box at one end. I took this apart and
resoldered it as well. I also roughed-up all the connection pins with
some very fine wet/dry sandpaper.
When you've finished, inspect all your joints with the magnifying glass
just to make sure none are bridged. Reassemble the amp and reinstall
it into the car. It's okay to let it just hang there while you test things out.
If it doesn't work then you can try soldering the rest of the components
or begin testing individual components. Sometimes you'll find a resistor
that reads very high resistance or much higher than resistors that have
the same color bars. This would indicate a faulty resistor which is
easily replaced.
Hope it works!