Cruise control - my test procedures and still completely dead. - Mercedes-Benz Forum

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Old 10-06-2008, 11:26 PM   #1 (permalink)
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Date registered: Sep 2008
Vehicle: 1983 Mercedes 300DT 301500 miles
Location: Salt Lake City
Posts: 179
Lightbulb Cruise control - my test procedures and finally got it fixed!!

I am trouble shooting my completely dead cruise control on the 83 300DT
It has the newer electric actuator.
I am following the test procedures here:
Test Procedures - Table of Contents

Here is my way of doing it (so far unsuccesful ):
First thing I did is test the actuator in the engine bay:
pin signal
1 ground
2 potentiometer
3 potentiometer
4 motor
5 motor
6 ground
7 engage solenoid

So I disconnected the connector of the actuator and started testing:
I apply +12v and ground on pin 4 and 5 and the motor spins, i reverse the voltage and it spins in the other way so it works.
I apply +12v on pin7 and ground on pin 6 and actuator clicks.
I apply +12V on pin7 and pin5 and apply ground on pin 6 and 4 and solenoid engages and motor starts to pull the throttle linkage. (so it works)
I disconnect the linkage and measure resistance between pin 2 and 3 (this is a potentiometer which resistance changes according to the position of the shaft that pulls the linkages. Now this pot was not working correctly so I took apart the actuator cleaned the pot and after that at Idle it shows about 1-2 Ohm as I push the throttle the resistance increases to a few Kohms.
So conclusion is that the actuator is completely working now.

So then I test the Steering column switch and other signals at the amplifier:
(Whit the amplifier disconnected and ignition on)
At the amplifier (cruise control unit) which is next to the brake pedal:
pin signal connected to actuator
1 Power: 12Vdc
2 Decel/Set (from switch)
3 On/Off (from switch)
4 Accel/Set (from switch)
5 Actuator Engage Solenoid (output) 7 Engage Solenoid
6 Resume (from switch)
7 Motor Control 4 Motor Brush
8 Brake Disengage (brake light bulbs)
9 Actuator Positional Feedback 3 Potentiometer End
10 Motor Control (output) 5 Motor Brush
11 Speed Input (from transducer)
12 Ground 1, 6 Grounds
13 Actuator Positional Feedback 2 Potentiometer Wiper
14 Ground
pin 1 does have the 12v and pin 14 had the ground.

Switch Test (meter common to ground)
Pin 1 BV (BV = battery voltage: somewhere between 12.5 and 13.8 Vdc)
Pin 2 Decel/Set 0 Vdc initially - goes to BV for Decel/Set
Pin 3 On/Off BV initially - goes to 0 V for Off
Pin 4 Accel/Set 0 Vdc initially - goes to BV for Accel/Set
Pin 6 Resume 0 Vdc initially - goes to BV for Resume
Pin 8 Brake Disengage MUST BE 0 Vdc for Cruise System to work - goes to BV when you step on the brake.

The system gets a critical ground through the brake light bulb filaments. If both bulbs are burned out or there is not proper ground here the system will not work. This is the mechanism for cancelling the Cruise when you step on the brake: this pin goes from ground to plus voltage and loses a ground which is necessary for the sytem to stay engaged.
Pin 12 Ground
Pin 14 Ground (some cars have no ground here; the system will work without it)

So tested all that and got the right results and yes Pin 8 is 0V and when I press the brakes it goes to +12v.
So this proves that the brakes switch works + control switch on steering wheel works.
I also did this test with actuator connected and amplifier disconnected and measuring on the amplifier connector:
From Pin To Pin Actuator Amplifier Resistance
Number Number Plugged in? Plugged in? in ohms
7 10 yes no 3 to 30 ohms (not 0 ohms!)
7 gnd (12) yes no infinity
10 gnd (12) yes no infinity
5 gnd (12) yes no infinity 002 545 86 32 (or) ~40 ohms 001 545 75 32
9 gnd (12) yes no 3.4 K ohms ±0.6 K ohms
9 13 yes no 0 ohms approx.

on pin 7 to 10 - got 4 ohm, the other 2 got infinity, on pin 5 to 12 got 40 ohm
on pin 9 to 12 got about 3k ohm, on pin 9 to 13 got about 2-3 ohm and when i press the throttle it increases.
So the conclusion:
Steering column switch works,
Actuator in the engine bay works, (I did have to clean the pot)
The connecting cables are intact and work,
break disengage works correctly.
Went for a test drive - nothing (completely dead)
And NO - it is not the fuse, first I have checked it and second there will not be +12 v at pin 1 of on the amp connector with ignition on
So the amplifier you will say... ok I took it apart, resoldered it and put it back in - NOTHING. So then went to the junkyard and got another one out of a W126 560SEL and guess what NOTHING.

So the only thing that I have yet to test is this:
B) Speed Sensor Test (meter common to ground)
This test must be performed with the rear wheels turning. Please have two qualified people present at all times.

Pin 11 Speed Sensor loaded (with the amplifier plugged in) must be at least 2.0 Vrms at 25 mph.

The amplifier actually measures the frequency of this signal. At 60 mph the frequency should be 131 Hz.

I can not measure the frequency because my multimeter is a cheap one, but I guess I could check to see if there is 2.0 V at 25mph. Or I might get a better multimeter from a friend of mine that can measure frequency.

So the speed sensor is my last hope. Any other ideas will be very appreciated.
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1994 Mercedes C220 converted to run on Natural gas.
Bought at 138 200 miles, current odometer: 267 300 mi

Last edited by Tedd300DT; 10-07-2008 at 09:54 PM. Reason: fixed
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Old 10-07-2008, 10:01 PM   #2 (permalink)
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Date registered: Sep 2008
Vehicle: 1983 Mercedes 300DT 301500 miles
Location: Salt Lake City
Posts: 179
UPDATE:
I finally fixed the damn thing. I did not resolder everything on my cruise control unit the first time so I took it out and resoldered completely and I also resoldered some of the solders that looked bad on the other control unit that I got out of a W126 560SEL so now they both are working !! There is some surging from time to time but at highway speed is less. The minimum speed that the cruise control works is about 30MPH. I was thinking maybe the surging is because of my jumpy speedometer? I might have to lube the speedometer cable to see if it helps. I think the CCU that I got out of the junkyard surges a little less then my original one. I will also try to see If I can find the electrolytic capacitors and replace them. By the way the CCU out of the W126 is like almost an inch longer then mine and seems to have more components. Maybe it is an updated version. I think the W126 was an 1986 or something like that.
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Old 10-08-2008, 07:34 AM   #3 (permalink)
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Good info! My CC does not work either.
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Old 04-18-2009, 11:30 AM   #4 (permalink)
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Date registered: Apr 2009
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Brake lights permanently on Cruise control not working

With these two faults on my W163, I changed the brake light switch
(a simple job instructions detailed elsewhere on this forum - thanks)

And both faults are fixed
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Old 11-06-2009, 11:13 PM   #5 (permalink)
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sorry for hopping on an old post, but here goes. i tested my actuator today and got the same responses for it as you did yours, Tedd. when i checked +12V on pins 7 and 5 and applied ground on pins 6 and 4, my actuator chattered (like a ratcheting noise). eventually it did start to move the linkage, but this did not sound "right" to me. i was expecting a smooth, noiseless pull on the linkage. i went ahead an took the actuator off and popped the cover and all the gears looked pristine and unworn (and i really studied it for some time). i reassembled the actuator and reinstalled it and had the same chatter noise when performing the above test. so, is the chatter normal or is the solenoid inside the actuator bad? if it is bad, can this be rebuilt?
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Old 05-25-2010, 09:19 PM   #6 (permalink)
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Quote:
Originally Posted by jmcqueen View Post
sorry for hopping on an old post, but here goes. i tested my actuator today and got the same responses for it as you did yours, Tedd. when i checked +12V on pins 7 and 5 and applied ground on pins 6 and 4, my actuator chattered (like a ratcheting noise). eventually it did start to move the linkage, but this did not sound "right" to me. i was expecting a smooth, noiseless pull on the linkage. i went ahead an took the actuator off and popped the cover and all the gears looked pristine and unworn (and i really studied it for some time). i reassembled the actuator and reinstalled it and had the same chatter noise when performing the above test. so, is the chatter normal or is the solenoid inside the actuator bad? if it is bad, can this be rebuilt?
Anyone have an answer for this? And besides this, after resoldering my amp, as soon as the ignition is turned to on (without the car running), the actuator continues to go nuts with the "ratcheting noise" non-stop until I either hit the brake or the accel or decel lever. Off doesn't do anything, and then the noise stops, the motor continues to spin. I tried two different actuators, both with the same issue. Anyone encounter this?!
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Old 05-26-2010, 04:33 AM   #7 (permalink)
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Quote:
Originally Posted by Tedd300DT View Post
UPDATE:
I finally fixed the damn thing. I did not resolder everything on my cruise control unit the first time so I took it out and resoldered completely and I also resoldered some of the solders that looked bad on the other control unit that I got out of a W126 560SEL so now they both are working !! There is some surging from time to time but at highway speed is less. The minimum speed that the cruise control works is about 30MPH. I was thinking maybe the surging is because of my jumpy speedometer? I might have to lube the speedometer cable to see if it helps. I think the CCU that I got out of the junkyard surges a little less then my original one. I will also try to see If I can find the electrolytic capacitors and replace them. By the way the CCU out of the W126 is like almost an inch longer then mine and seems to have more components. Maybe it is an updated version. I think the W126 was an 1986 or something like that.
Keep us posted if you get the speed sensor frequency measured.
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Old 05-26-2010, 04:44 PM   #8 (permalink)
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Unhappy PLEASE HELP?!

Quote:
Originally Posted by TurboDoug View Post
Anyone have an answer for this? And besides this, after resoldering my amp, as soon as the ignition is turned to on (without the car running), the actuator continues to go nuts with the "ratcheting noise" non-stop until I either hit the brake or the accel or decel lever. Off doesn't do anything, and then the noise stops, the motor continues to spin. I tried two different actuators, both with the same issue. Anyone encounter this?!
So I picked up another amp from a decent 1985/86 300dt that I ransacked at the junkyard... Plugged it in on the way home to see what it would do and it yielded the same thing as my "resoldered" original amp-- a constant ratcheting of the CC actuator before I even turn the engine over. I have tried 2 actuators and it also yields the same results. What else is there to try? I find it odd that both amps and both actuators would be bad in the same exact way to bring the same exact results. I must have something else screwy going on... PLEASE HELP!?
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Old 05-27-2010, 12:09 PM   #9 (permalink)
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This is very, very strange. If you have your ignition on, push the CC lever switch to "OFF," turn the ignition off, and then turn the ignition back on, do you still hear the ratcheting noise?

Let us know and depending on the results, I'll whip out the voltmeter to test what the behavior should be with the ignition on.

There are only 4 components to the CC system...

-switch
-sensor
-amp
-actuator

..and you've covered the only two that should lead to the CC turning on and making that noise.
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Old 05-27-2010, 12:51 PM   #10 (permalink)
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OK I will try that within the next few days... Just to make everything clearer,

My brake switch and bulbs are new and all work correctly. The speed sensor is within spec. I tried the system with both my original lever on the steering wheel and one from a junkyard--no difference. Oddly, the one actuator's servo constantly spins when it is not making the ratcheting noise. Maybe both actuators are bad?

Anyway, I will try what you suggested and report back as soon as I get a chance.

Thanks!
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