bhensarl

Okay, in all actuality the cruise hasn't worked since I've gained ownership of the car, but I've decided to get tough and make a concerted effort to fix it. Here's what I've done so far (In accordance with Adrian's book):
-Checked fuse #19
-Replaced brake pedal microswitch (Part # 96461330100)
-Replaced clutch microswitch (Part # 96461341500)

That brings me to :
Internal failure of steering column control switch.
Before I tear apart the column, any ideas on how I can know whether or not this has failed? Is there any test I can do?

Brian

springer3

Brian:

Mine works, so I don't have direct 964 troubleshooting experience. I have owned three cruise-controlled Mercedes. The VDO system looks the same, and I have lots of experince with this trouble-prone system. Fortunately, most problems are minor.

First of all, you are shooting in the dark without the wiring diagram. You can waste a lot of time and money replacing parts, when the only problem is a loose connector somewhere. Always start at the cruise control "black box". In my 964 it is under the passengers seat, and I think is is the same on the 89 model. Pull the connector, and you have access to all the various switch contacts and control signals. The wiring diagram has the pinout for the connector. I make a sketch to document all the readings. This is from memory, but I think this is the list of things to verify. Any missing signals can be tracked down by checking each connector and wire all the way to the part that is not sending the correct signal. If all the signals check out, it is either the controller or the throttle actuator. They are expensive, but in my experience don't fail.

1) Power supply. Always check power first. I once had a number of electrical problems all traceable to a bad alternator. DC supply should be about 14 volts +/- 1/2 Volt with the engine running. Switch the meter to AC and check to see if your alternator is putting out a lot of AC ripple. Caveat: cheap meters read high AC voltage even on a pure DC voltage. Check again with the engine off if you read more than a Volt on the AC scale. If you still see AC, you have a junk meter. Check with an oscilloscope if this is potentially a problem.

2) Brake switch contact (seldom the problem, but easy to check). I know you replaced it, but what about the wires and connectors?

3) Clutch switch contact (same comment)

4) Resume, set/accel, and coast contacts (column-mounted switch). These also seldom fail.

5) Throttle position signal (internal to the throttle actuator, you check the risistance as the throttle is moved). The sensor does wear. This is a problem I had once. The sensor can be fixed without replacing the throttle actuator, at least on the older VDO systems.

6) Speed signal. This one is the other problem I had once. The signal comes from the speedometer. If the spoiler control works properly, the speed pulse is probably present. It does require an oscilloscope to look at this one.

As you can see, it is not simple, but the problems are usually easy to fix. The cruise control is the best way I know to keep from getting speeding tickets on long trips, so I keep it in perfect order.

bhensarl

Thanks again for all your help, Springer. I just printed out the wiring diagram so I'll check those connections. I'm definitely a novice when it comes to using a multimeter, but I think I can figure it out with your directions.

Brian

jrb964

My cruise control did break. No real warning...just quit. At first did not think anything of it...I'll fix it later. On the same trip I put my foot in it and the engine went automatically to redline...needless to say it sacred the #$%#$ out of me. I managed to get the engine stopped and the car stopped. The accerlator pedal was stuck on the floor board. Giggled it a little and it poped right back. Started up and drove off. Everytime I put accerlator pedal to the floor the car redlined.
These two problems were directly related. It turns out the connector (highlighted in the pictures) which is plastic, broke. Causing both problems. The factory replacement is metal and preset lengthwise so it just snapped on and corrected the problem. Less than $20.

bhensarl

Okay, first things first. Do I check the DC output between the battery terminals? Then leave the probes there on the terminals and switch my meter to AC to test the alternator?
Also, how do I test the black box (it's actually a silver box, right? )? I understand what you mean about being able to tell if something that is supposed to be sending a signal to the black box is not, but I don't know where to put the probes of my meter. Thanks for all your help, and the elementary lesson in multimeter usage!
Brian

Michael Delaney

Brian, My cruise control failure was caused by a faulty speedometer. The cruise function does not really get speed input from the speedometer, but it does function as a kind of terminal board. The only way I know to troubleshoot this is to swap speedometers. On my car a different speedometer allowed both my cruise and rear spoiler to operate when they hadn't before. This is certainly not the only thing that could cause this, but it worked for me. Many things to look at. Good luck.

bhensarl

Roy,
Thanks for your input, but would the spoiler still function normally if the speedo was bad? Are there two different connections for the spoiler and cruise? It seems the signals being sent to my spoiler are good because even if the spoiler doesn't physically raise, I get the warning lights. Could my speedo still be bad?
Brian

Michael Delaney

Brian, I really can't answer that for sure. Both my problems were fixed simultaneously so I really didn't look at this any further. If I had to guess I'd say they were wired indendently of each other. Most of the wiring on these cars doesn't seem to integrat things. All I'm saying is that if you can swap a speedo with someone it might eliminate a part of your troubleshooting.

bhensarl

Okay, good idea. At least I would know whether or not my speedo was the problem. Then my next question would be what year speedos are compatible?
Brian

bhensarl

I'm looking at the wiring diagram of the cruise control unit and I'm having a hard time deciphering it. Can anybody shed some light on this for me? Specific questions:
-I know what the "Control Unit Cruise Control" is and where to find it, but what is "Drive Cruise Control"? Is that the column mounted switch?
-What are "M+", "M-", POTI A,S, and E, and "Stop Light Switch"?
TIA
Brian

springer3

Brian:

You are starting to "shot gun" again. Get systematic, or you will spend a lot of money replacing things that already work fine. You also risk causing another problem the more things you pull apart. Leave the speedo alone until you diagnose what the problem is.

It is much faster, easier, and cheaper to pinpoint the exact problem first. Then make just one surgical strike, and the problem is resolved. Here is what I would do if I had your problem. I have been successful with this approach on two Mercedes cruise systems that are very similar to the 964 system.

From your wiring diagram, make a big sketch of the plug that attaches to the cruise control module (yes, it is silver, "black" means you don't know what is inside). If you look closely at the plug, the terminals have numbers next to them that correspond to numbers you will see for the connector in the wiring diagram. Identify the function or draw the actual wiring circuit for each pin on your paper sketch. Once you have a good sketch, it should be clear how to check for that connection (resistance to ground, voltage, or pulse string). Record all your observations and readings on the hand sketch. Keep your wiring diagram clean for later needs. Some of the signals are difficult to trace on the schematic, and some may come from a different sheet than the sheet with the connector. Colored highlighters are a big help for tracing down a particular wire all the way to its origin. This step can be time consuming, but you can do this indoors in comfort, rather than doing the same thing in the middle of a bunch of wires and loose parts.

There is at least one pin that is +12 VDC power. There may be one constant power supply and one that is switched through the ignition switch. I have not looked at the 964 cruise, but the spoiler control does have two power terminals. Verify battery voltage is present at each terminal shown as power supply.

There is at least one pin that is ground (check resistance to nearest chassis ground, resistance should be zero to 1 or 2 Ohms).

Draw each switch contact and how it is connected to the module. Some go to ground, and some switch between two pins on the connector. Use your VOM on "Ohms x 1" or Ohms x 10 at the cruise control terminal to see if the switches are doing what they are supposed to. You should read millions of Ohms when the switch is open, and zero to 2 Ohms when the switch is closed.

Draw the line to the speed signal. I believe on the 964 the same signal goes to the speedo, DME, spoiler controller, and cruise contol, but I am not sure on this point. VDO cruise controllers in a Mercedes use a special speed signal picked up off the speedo cable. The 964 speedo is electronic, so it will be different. You will need to figure out where and how the ground speed is sensed. I'll bet it is a pulse signal, and that is difficult to troubleshoot with a VOM. Try rolling the car a few feet with the VOM connected between the speed signal and ground. The ignition will need to be on. You can do this by driving the car very slowly. You should see the voltage change from zero to 12 volts or zero to 5 volts, depending on how VDO did this. Loss of the ground speed signal is a common cause for cruise control failure.

Draw lines from the plug pins to the cruise actuator. The cruise actuator is located next to the throttle plate in the engine compartment. It sends a throttle position signal to the cruise control module. Set your VOM to Ohms x 100 or Ohms x 1000, and measure the resistance of the throttle position signal as you move the gas pedal from the idle to full throttle. In 1988 this was still a resistance signal, but the 964 generation could be digital (pulse). This point you will need to verify from the schematic.

If you find any faults, you have located the problem, and can decide which component has failed and needs replacement. This is the end of the easy stuff. If all the signals are present, the problem is either the cruise module or the throttle actuator. I have not seen either fail, and I therefore am not experienced with troubleshooting past this point. You can reconnect the plug to cruise module, but get a clip lead to measure the signals with the plug attached. There is an output signal to the throttle actuator. If that signal responds to commands to engage cruise, the problem is in the actuator. If the output signal does not respond, and all the input signals are present, that means there is an internal failure in the cruise module.

A 964 cruise actuator sold on eBay recently for $40, but I imagine they are at least 10 X that new. The reason it sold so cheap is that I suspect they do not fail. You can find rebuilt cruise control modules, but be sure to get the same part #. They are interchangeable (my Mercedes module plugs into the 964 no problem), but the control map is different for each car. You will have hunting and surging if you use the wrong box.

The important point is that you should troubleshoot first, before replacing anything. This will take a long time the first time, but it elimates the frustration of trying things that do not fix the problem. The systematic approach is what the factory-trained mechanics learn. It takes less time in most cases.

bhensarl

Thanks so much for your detailed explanation, Springer. I really appreciate all your help.

Brian

bhensarl

Okay, after some careful study of the wiring diagram, a few more questions:
-How do I figure out what sort of connection it is, and how do I check the different connections?(resistance to ground, voltage, or pulse string)
-Verify battery voltage is present at each terminal shown as power supply. (How do I know which terminals are power supplies?)
I'm going to attach a portion of the diagram and see if that helps.
Brian
(Sorry for the size, I shrunk it as much as I could)

springer3

Brian:

Excellent work marking up the diagram. Your "What is this?" is the actuator in the engine compartment. Unfortunately, I can't read the numbers, but the pin numbers are just outside the outline of the "black box". The numbers inside the box appear to describe the function. Any pin that goes to a ground bar should read near zero Ohm to ground. The brake light switch should read "open" until you press the brake pedal, and then should read near zero.

Pin 1 looks like switched 12 VDC power (called "15" on the schematic). Measure it with the black voltmeter lead clipped to ground.

Pin 5 engages the clutch in the actuator. That should change voltage when the cruise is engaged, but that you will need to test while everything is connected.

All the column-mounted switches are shown, and can be checked using the Ohms x 10 scale. Good luck.

Herbie

I repaired my CCUnit, it was a bit difficult, had to simulate speed signal with a square wave oscillator.

The clutch terminal in Tiptronics is not connected.