Hello Gentlemen!

My '85 300D Turbo's cruise contlol suddenly quit working and I tried the "re-soldering trick" but still haven't been able to revive and make it work again. The turn signals, instrument cluster, tachometer, and speedometer all works and properly functioning which rules out the fuse. Upon inspection of the inner guts of the amplifier/module, there were no visible and obvious signs of burned or molten component or any floating and cracked conductor circuits on the board.

Now I have some few questions for the experts:

1) What are the chances out of 10 that:
a) cruise control stalk/switch is bad?
b) cruise actuator/motor is bad?
c) speedometer sensor is bad?
d) the amplifier/module is bad?

2) How about MB inter-model compatibility and inter-changability? Are W123 amp/module interchangable with that of W201, W124, and W126's? I am aware that they have electronic speedometer sender as compared to the mechanical of W123, but since they use the same cruise motor/actuator, I'm just wondering if they use the same amplifier/module even though their part numbers vary?

Thanks in advance!

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'85 300D Turbo - CA Version

bump........got pushed out to the third page already

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'85 300D Turbo - CA Version

Have you been to GDL's website and performed the tests? Lots of info. over there and on here.

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AJ

1985 300D (SOLD)

Some of us have heard for years about brake light bulbs and their effects on the cruise control system. I suppose many beliefs are firmly grounded somewhere in folk lore because of some caveman's personal experience. Don't forget... the world was originally flat.

Here are the facts:

I'll begin with some moderately technical information and then boil it down to very simple lay conclusions.

In all VDO Cruise Control Systems the filaments of both brake light bulbs are connected to chassis ground at one end and are switched to approximately battery voltage at the other. When the brake lights are off the switched end sees a wire (the filament) to ground; when on, that ground disappears and that end of the filament goes to somewhere in the vicinity of 13.8 Vdc, current flows and the filament glows. The key is that at the "hot" (in electrical terms) end of the bulb the ground is lost when the bulb lights.

The Cruise Control System uses the above condition in the following manner. Amongst the couple hundred electrical connections inside the Cruise Control Amplifier, there is one, that's ONE! connection to the switched end of the brake light bulb. In the systems from 1981 to 1993 this connection provides a ground path the to emitter of an npn switching transistor which controls the digital flip-flop circuit which holds the cruise system on. In earlier systems the configuration is slightly different. In either case it is a totally passive connection. In other words, it is just a wire which provides a current path to chassis ground for that circuit. When the ground is lost the brake lights turn on, the cruise system switches off. It's a simple as that.

Thus, while there may be several ohms difference between brake light bulbs of different brands, there is no way they can affect operation of or damage the Cruise Control Amplifier. Use the brand of brake light bulb of your choice!

Since the bulb provides a critical system ground, it is true that at least one, preferably both, brake light bulbs must be working and seated properly for the Cruise Control System to work. If your brake light bulbs are burned out the cruise control system will not work.

The notion that it matters what brand of brake light bulb you use is a myth which has no merit based in technical reality.


http://gdl-online.com/bulb.html

http://gdl-online.com/testproc.html

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96 E300d

My brake lights, left and right, are both working but the cruise control suddenly stops working last week, but I will still clean off the bulb contacts and terminals just to make sure that they getting good ground contacts.

I just started checking what the gdl website tips and test procedures and I may get into that to make sure that my switch, sensor and amplifier are in good working order. Thanks for the reply

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'85 300D Turbo - CA Version

I meant the actuator and not the amplifier that I'm going to test and make sure that is in good working order

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'85 300D Turbo - CA Version

More info here:

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

Snippet:

The first series of tests is done with the key off. The resistance between Pins 7 and 3 should be approximately 12 ohms and there should be no continuity to power or ground. If it's substantially more than this 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. Pin 12 should be grounded, a resistance test of it to the car's chassis should show little/no resistance. (Leave the ground lead of the meter in this socket for the next series of tests.)