HOW-TO: Cruise Control Amp Repair 14-pin - PeachParts Mercedes-Benz Forum

JamesDean · #1 03-02-2012, 10:41 PM

Hello everyone,

I decided I'd try to fix the cruise control on my 190E and 420SEL. I figured I'd write up a how-to for all of you out there. Special thanks goes out to forum member sidoog for helpful info and tips.

APPLICATION: 1987+ All Cars that uses the 14-pin VDO unit with Coding Plug.

1. TESTING THE ACTUATOR--PART A
First we must test the actuator and make sure it is within normal operating limits:

-Locate and disconnect the actuator connect inside the engine bay
-Perform the following on the actuator side of the connection

-Get an ammeter and a 2-amp battery charger.
-Connect the ammeter in series with the 12V supply
-Connect the positive probe to PIN 4
-Connect the negative probe to PIN 5

Observe the current draw. It should be within 120mA and 160mA.
The Actuator should make an audible spinning sound.

My 190E was around 130mA with my good meter. My 420SEL/560SEL measured 200mA with a cheapo meter. I blew out the fuses in the good meter before I could test the 420SEL/560SEL.

If the actuator is within this range, you can go to the next step. If not, it has the possibility of damaging the amplifier.

2. TESTING THE ACTUATOR--PART B

Next you need to test the solenoid in the actuator. This is fairly simple. Take your battery charger, and briefly tap the positive to pin 7 while maintaining ground on pin 6.

If you hear a click, then you've passed this test.

3. REPAIRING THE AMPLIFIER UNIT

If your cruise control amp looks clean and healthy (i.e no black spots indication a servo failed and hurt something) the following has a fairly high chance of success.

You'll need to open up the unit and replace a few electrolytic capacitors.

I ordered mine from Mouser.com. As long as you get a capacitor of the same capacitance and the same or higher working voltage you'll be fine. I ordered enough for 3 boards.
Here's my parts list:

The first step in replacing these is to clean off the coating VDO has on the back of the unit.

I used this varnish remover, it worked out pretty well. After it was done I took the board to the garage and used the air compressor to blow of the remains.

Here's the old and new capacitors:

All replaced!

Next Step is to re-flow the solder points. This step take quite a bit of time. Its not difficult just time consuming.

You basically want to tap the soldering iron and solder to each and every solder point on the board. Don't stay more than a few seconds. Especially on the large IC chips. They're sensitive to heat. Once you've done that you should be good to go!

On my first board I used my de-soldering iron and removed all the old solder, then re-applied new. This is a bit much as all I did on the second board was reflow the joints and it worked fine.

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Repairing 14-Pin Non Coding Plug Version ~1980-1985:

Earlier Mercedes were outfitted with these 14-pin engine/chassis specific units. They do not have a coding plug on them and as far as I can tell cannot be used on other Mercedes of the era. They do have the same pin-out as the Coding Plug variants. So testing is exactly the same.

The procedure is pretty much the same as above. Strip, Replace, Resolder.

I've not had the ability to test this unit yet as both of my 300SD's are missing their servos and mounting brackets. I have a spare servo but no good way of mounting it for testing. So tomorrow I'm going to go to the junkyard and see about finding one so I can test.

INFORMATION ON CODING PLUGS:

According to Auto Haus AZ. The coding plugs often fail before the amplifier itself. I've not experienced this yet the 420SEL, 560SEL and 190E's plugs were all intact.

Here's some pictures in case yours is bad and you need to replace the resistors.

If you have any questions or comments let me know.