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JamesDean 03-02-2012 11:41 PM

HOW-TO: Cruise Control Amp Repair 14-pin
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.

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.


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.


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 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.


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.


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.

big dog 2 03-03-2012 11:26 AM

You have a lot of patience! Congrats!

JamesDean 03-03-2012 01:43 PM

Until we got the 420SEL, I've never driven with cruise control on a Mercedes.

I think Monday I'll venture out to the junkyard and try to cobble together cruise systems for my 300SDs.

If anyone is interested, I'd be happy to repair your cruise control amp. I enjoy soldering. lol.

d.delano 03-03-2012 06:59 PM


Originally Posted by JamesDean (Post 2895336)
If anyone is interested, I'd be happy to repair your cruise control amp. I enjoy soldering. lol.

I have a sneaking suspicion that with this, you have crossed the threshold into starting a new business. You might not be aware of this yet.

You have now:
1) fixed the LCD temp readout, a perenially malfunctioning item in almost all the older MBs still on the road
2) fixed the cruise amp, another common problem

Not only have you successfully repaired these items you have shown the rest of us how. It doesn't seem like it's particle physics either! Brilliant!

There used to be a guy on the left coast who did this stuff, General Development Labs. I don't know if he does it anymore, who knows. At any rate, this kind of repair is in demand.

Next, you should try to figure out the CCU pushbutton unit! There's one for you. Although a common pattern I'm seeing is that it's those caps giving up the ghost every time.

You should throw a simple web page up and advertise repair services for this stuff. I'm sure there are a lot of units out there needing some similar repair across all makes of European autos. The electronics in these cars are ridiculously primitive, for a modern electrical engineer a cakewalk. Most people would sooner buy a rebuilt unit than replace individual components on the PCB.

I'd take you up on your offer to fix, but I've been lucky that my cruise amps have remained operational. I will however use your information to repair my LCD temperature readout. I like to solder too.

mbzjag8090 03-03-2012 09:29 PM

HVAC push button repair on Gen. I W126 would be awesome.

I bought a new one 5-6 years ago and it broke just recently. If I press the middle or second from the right button, the fan doesn't kick on. I've got to press the fourth from the left button to get the fan to kick on and then click EC or the middle button if I desire those settings.

Based on what I've read on the forums this is a common problem with the units. I got a few from the yard to try out and they all exhibited the same problem!

I've tried soldering numerous times on two different units with no success.

JamesDean 03-04-2012 04:52 AM

I'd love to try fixing a first gen 126 climate control unit. Sadly I've got no spares to work on. The ones in my 300SD's work fine.

One forum member on here has already contacted me about fixing a unit for his 124.

I think I might head to the junkyard on Monday and get a few cruise control units and see what I can do. I dont think there are any first gen 126s at the yard though.

I probably wouldn't want nearly as much as the other places to fix these. I've seen prices up to $200+.

JamesDean 03-04-2012 05:03 PM

I've been looking into the possibility of using the newer "with coding plug" style units on the older cars.

Here's what I've come up with:

The coding part sets the impulse per kilometer range for the unit (As evidenced by: VDO Catalog Tempostat Coding Plugs) This range differs from transmission to transmission it would appear. The VDO numbers on the two plugs I have don't match up to that image at all. I've cross referenced MB stuff a bit and came up with the following:

My 300SD uses a 722.303/722.416 transmission.

These amplifiers work in it 002 545 16 32, 002 545 82 32, 003 545 86 32.

These units also work in a variety of other chassis, namely the 201.122,
. The 190D 2.2 and the 200D/E200 Diesel respectively, both use the 722.403 transmission.

In the 190D 2.2, the amplifier was replaced by the newer 14-pin coding plug type with plug 005 545 11 32

So there is a chance that this 005-545-11-32 plug coupled with the 14-pin newer style cruise unit could be used in all 722.303/722.416 enabled cars.

I don't have this plug on hand, and its $40 from the dealer! So if anyone has this on their car. I'd love to get a peek inside at the resistors so I can order up some resistors and test this theory out!

compu_85 03-04-2012 05:25 PM

I a picture of mine:

The bottom "resistor" is just a straight through wire.

I don't recall what number is on my plug :(



JamesDean 03-04-2012 05:38 PM


Originally Posted by compu_85 (Post 2895913)
I a picture of mine:

The bottom "resistor" is just a straight through wire.

I don't recall what number is on my plug :(

Is that from your 350SDL ?

compu_85 03-04-2012 06:29 PM

I'm 99% sure it is. It is possible it's from the 420SEL's cruise amp I got at one point, but I really don't think so.


JamesDean 03-04-2012 06:48 PM

Yep. Those resistors dont match up to the V8 plug I have in the first post. Must be for the 350SD/SDL's 722.361 transmission.

compu_85 03-04-2012 06:51 PM

Here's the FSM info on these cruise controls:

Hit Man X 03-06-2012 04:16 PM

Kris, you best purchase your bits from! My woman has been there nearly a decade... so going to Digikey is like a stab in the back to me! :D

I have a 1st gen unit I will mail you soon. It is a late 90s reman from Programma. The lack of low speed fan is irritating.

JamesDean 03-21-2012 06:55 PM

I found a large reference table and it looks like I can redo the coding plugs for any specific model that I need.

So far I have a 100% success rate repairing these units. I just repaired a unit for a 85 300D for 79Mercy. I've got enough spare parts to repair probably 5-6 more units if anyone is interested. Send me a PM.

79Mercy 03-22-2012 12:27 AM

I got my amp back today from JamesDean my cruise control works flawlessly, used it to and from work tonight, about 20 miles each way and it held perfect constant speed the entire time. All other functions work properly, all thanks to JamesDean!

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