Cruise Control question - PeachParts Mercedes-Benz Forum

Tanksowner · #1 09-16-2011, 02:41 PM

Okay so before I shell out money to fix the cruise control, I'm looking for a little insight...

I've tried using it, and it does attempt to hold the speed steady. So it's getting power, I guess, but for some reason, it doesn't stay steady for long. As an example, if I set it at 65, it'll stay there, then after a few seconds it'll slow down to 60, then speed back up to 65, then slow down, then speed up.

I'm no expert on this, but it seems to me that it could be the amplifier, due to the constant fluctuation of speed. I'm guessing that it's just not getting enough power through the unit to keep the speed set. What do you guys think?

LandYaghtLover · #2 09-16-2011, 03:03 PM

In my experience when the amp is bad cruise cuts out 100%. I never had mine fluctuate or re-engage.

Cal Learner · #3 09-17-2011, 08:16 AM

Isn't there a hall effect sensor in that circuit somewhere? Is that a possibility?

Yak · #4 09-17-2011, 11:58 AM

I think there were anecdotal comments about not holding speed and jumpy speedometers.

If you don't want to wade through the pages and pages of troubleshooting in the manual. There's a concise troubleshooting description here:

http://userweb.windwireless.net/~jimc/mamerepairs.html#acctest

<<>>

The last tests are done while driving the car. Pin 11 is the signal from the speedometer, an AC voltmeter between it and ground (pin 12) should show a voltage that rises with speed. (And if your meter also measures frequency, like mine does, it certainly should show a rise in frequency with speed; frequency, in fact, is what the circuit is actually reacting to.)

An inadequate supply of vacuum can cause strange problems with sinking set speeds, surging, etc. If this is your problem tee a vacuum gauge (a common vacuum/fuel pressure gauge or a MityVac will do nicely) into the black/yellow line that feeds the actuator, as close to the actuator as you can make it, and place the gauge where you can see it safely while driving. (A long hose can be used to snake the gauge into the passenger compartment, or you could try strapping the gauge to the wipers.) Try to use the cruise control normally. If the vacuum supply drops below 5" (Hg) during operation the cruise control cannot work properly: the value should normally be twice this number in operation. A low vacuum supply can be caused by leaks or occlusions in the piping feeding the actuator, and must be corrected before suspecting electrical problems. Of note is that the system consumes the most vacuum while holding a set speed, not while accelerating, so the ability of the system to accelerate well does not prove that the vacuum supply is good.

tangofox007 · #5 09-17-2011, 02:15 PM

Quote:

Originally Posted by Yak

An inadequate supply of vacuum can cause strange problems with sinking set speeds, surging, etc.

Do you see any vacuum lines associated with the cruise control actuator on a 1985 300D?

charmalu · #6 09-17-2011, 08:47 PM

YAK you may be thinking of the older style cruise control like my 80 240D. It is vacuum operated with the black/yellow hose and a black one.

The op has a 85 with the newer style.

Charlie

Yak · #7 09-17-2011, 08:55 PM

Quote:

Originally Posted by tangofox007

Do you see any vacuum lines associated with the cruise control actuator on a 1985 300D?

So you go to the link, move six inches to the right and read about troubleshooting a servomotor unit style instead of the vacuum controlled style. Is that so difficult?

<<<>>>>>>>>>>>>>>

The connector that plugs into the servomotor unit amplifier looks something like this (facing the little sockets):

o 2 O 1 o o 4 3 o o 6 5 o o 8 7 o o 10 9 o o 12 11 o o 14 13 o 2 Decel 1 Power 4 Accel 3 Cancel 6 Resume 5 Clutch 8 Brake 7 Motor + 10 Motor – 9 Feedback Pot 12 Ground 11 Speedometer 14 Ground 13 Feedback Wiper
The first series of tests is done with the key off. Pin 12 and 14 should be grounded, a resistance test of them to the car's chassis should show little/no resistance. The servomotor's clutch unfortunately has a series diode, so its resistance is difficult to measure. You can either connect an ammeter between Pins 1 and 5 and see approximately 300 mA, or you can defer testing it to a later step. (If it's broken the feedback resistance test will fail.) If you have a diode test range on your meter you can test it as a diode, though you won't see the actual resistance of the clutch coil. The resistance between Pins 7 and 10 (the motor) should be approximately 5 ohms, and there should be no continuity to power or ground. The resistance between Pins 9 and 14 should be approximately 3000 ohms, as should be the value between Pins 13 and 14, and likewise there should be no continuity to power or ground. If any value is substantially more than specified the actuator under the hood is broken, it's unplugged, its connector is extremely dirty, or the wiring to it is broken. Find out what's wrong and correct it.
The next series of tests is done with the key on (though the engine need not be running) and the ground lead of the meter in Pin 12. Pin 1 is power, it should register +12 volts. Pin 8 is the brake lights, it should register 0 V normally, and +12 volts when the brakes are applied. Pin 3 should register +12 V unless the stalk switch is moved to Cancel. Pin 6 should register +12 V when the stalk switch is moved to Resume. Pin 4 should register +12 V when the stalk switch is moved to Accel. Pin 2 should register +12 V when the stalk switch is moved to Decel.
The next test is done with the key on and the engine definitely not running. Connect one jumper between pins 5 and 3, another between pins 8 and 10, and another between pins 7 and 4. The ohmmeter should be connected between pins 13 and 14. When the Accel switch is activated the measured resistance should decrease smoothly to zero. Moving the switch to Cancel or pressing the brakes should return the resistance to the approximately 3000 ohms value. Repeat this test several times, especially trying to 'tease' it slowly. If the resistance jumps to a large value at any time the actuator itself has suffered internal damage to its feedback potentiometer, usually due to wear. I have heard of this being repaired by painting conductive substances on the damaged resistance track, but I have never done so. Procuring another actuator (used?) may be the best option at this point.
Next start the engine, leaving the preceding three jumpers in place. When the Accel switch is activated the engine should rev up, moving the switch to Cancel or pressing the brakes should return the engine to idle. Be particularly careful on gasoline cars as there is no governor to prevent the engine from over-revving. If this doesn't work check the actuator and its mechanical linkage to the throttle. The last test is done while driving the car. Pin 11 is the signal from the speedometer, an AC voltmeter between it and ground (pin 12) should show a voltage that rises with speed. (And if your meter also measures frequency, like mine does, it certainly should show a rise in frequency with speed; frequency, in fact, is what the circuit is actually reacting to.)

If your car passes all these tests and the cruise control still doesn't work right, then the amplifier is definitely at fault. Send it in! (If you wish to read about some of my personal trials with these systems, refer to here and the 240D's log starting here for about two weeks for the vacuum system; refer to here and the 380 SL's log starting here for the servomotor system.)

shertex · #8 09-24-2011, 04:33 PM

Sounds like a bad amp. But keep in mind that a bad amp can be cause by a bad actuator....get them both evaluated before spending big money. Or, if you want to take a chance, get a known good amp and see what happens.

sjh · #9 09-24-2011, 05:18 PM

When I bought my used '90 300D 2 years ago it had the exact symptoms you describe.

I set the speed and then it would oscillate over a 5 MPH range or so.

I bought a new amp for about $150 and its been fine since.

jbach36 · #10 09-24-2011, 06:02 PM

I was just talking to a mechanic about my 1991, the cruise doesn't work, and he said it's probably the amplifier.

Is there any way to fix them (I saw a resistor for sale on ebay, maybe that's the part it needs?), or just replace?

Also, what model and years will fit? There are some people parting out cars near me, maybe I could grab one of those.

1991 300d, 168k.

shertex · #11 09-24-2011, 06:27 PM

Quote:

Originally Posted by jbach36

I was just talking to a mechanic about my 1991, the cruise doesn't work, and he said it's probably the amplifier.

Is there any way to fix them (I saw a resistor for sale on ebay, maybe that's the part it needs?), or just replace?

Also, what model and years will fit? There are some people parting out cars near me, maybe I could grab one of those.

1991 300d, 168k.

For your car:

Amplifier Part # 005 545 0532

Actuator Part # 002 545 8632

Go to Germanstar dot net to see which models/years use that part.

The resistor (or coding plug) makes that amp work with your particular car. I doubt that's the problem.

If you want it done right, send both your amp and your actuator to GDL for evaluation then take it from there....or get dealer to evaluate for an hour's labor.

Some people re-solder all the joints....sometimes that works....BUT it would then become ineligible for a GDL rebuild.

On my 91, I did actuator tune-up and amp rebuild with GDL.

On my 92, I took a chance and got an actuator and amp off of Ebay for pretty cheap (original a & a were toast). Works perfectly....for now.

sjh · #12 09-24-2011, 08:10 PM

GDL (who is great) wanted $150 to fix my amp. I got a new one from MB for $150 instead.

shertex · #13 09-24-2011, 08:33 PM

Quote:

Originally Posted by sjh

GDL (who is great) wanted $150 to fix my amp. I got a new one from MB for $150 instead.

Really? A brand new VDO from the Mercedes dealer for $150? That's a great deal! Hard to believe....the rebuilt Programas online go for about that. Why'd they give you such a price break?

GDL's rebuild prices are actually a bit higher....$250 give or take.

Price List: Mercedes-Benz Cruise Control AmplifiersTemplate

sjh · #14 09-24-2011, 08:55 PM

Then my friend must have given me a great buy. This happened last year.

I called Peter at GDL and he said send the part and a check for the repair. I told my friend who has a used-car lot specializing in MB and he came back a few days later and installed a new one for me.

I had worked with Peter at GDL before and he had given me a price break on a rebuild job for my 123 so he and I already knew each other. He's a great guy.

I was having major surgery at the time. Apparently my friend gave me a better deal than I realized.

Thanks for pointing that out.

shertex · #15 09-24-2011, 09:03 PM

Quote:

Originally Posted by sjh

Then my friend must have given me a great buy. This happened last year.

I called Peter at GDL and he said send the part and a check for the repair. I told my friend who has a used-car lot specializing in MB and he came back a few days later and installed a new one for me.

I had worked with Peter at GDL before and he had given me a price break on a rebuild job for my 123 so he and I already knew each other. He's a great guy.

I was having major surgery at the time. Apparently my friend gave me a better deal than I realized.

Thanks for pointing that out.

Yeah, I would expect a dealer to charge somewhere in the 350-450 range for a new amp.