missing 3rd brake light effect cruise? - PeachParts Mercedes-Benz Forum

ngarover · #1 11-20-2010, 12:24 AM

Been looking over the threads on cruise control for my 87 300D and before I start tearing into it, would missing the 3rd brake light (the one in the back window) cause the cruise not to work?

mines totally gone. wires are just hanging in the trunk for it. At first I thought I would be smart and just jumper them but of course blew the fuse...

So I figured I better ask to see if there would be another solution for testing.

cirrusman · #2 11-20-2010, 01:02 AM

I don't have a w124, but I really don't think that Daimler-Benz would design a CC unit that depends on the third light as it would be a huge safety issue...

Jeremy5848 · #3 11-20-2010, 01:17 AM

The only thing a missing or burned-out third brake light will do is to cause the light-out alarm in the instrument cluster to illuminate. The cruise control is canceled by the brake light switch, not by the lights. All of the stop lamps could be out and the cruise control would still work and cancel normally.

Jeremy

compu_85 · #4 11-20-2010, 01:25 AM

I don't think ALL the lamps can be out, the cruse unit checks for ground on the brake light line, when that ground goes away it assumes the brake pedal is pressed and the lights are on. Having all the lamps burnt out causes the same thing.

-Jason

ngarover · #5 11-20-2010, 08:36 AM

Good, then at least I know I can move forward with some of the other fixes and feel comfortable that when I do find and fix the issue it should work even without that 3rd brake light.

shertex · #6 11-20-2010, 01:22 PM

Helpful info from GDL site:

Brake Light Bulb Controversy

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.

Jeremy5848 · #7 11-20-2010, 01:50 PM

Quote:

Originally Posted by compu_85

I don't think ALL the lamps can be out, the cruse unit checks for ground on the brake light line, when that ground goes away it assumes the brake pedal is pressed and the lights are on. Having all the lamps burnt out causes the same thing.

-Jason

According to the schematic, pin 8 of N4, the "Cruise Control Amplifier" is connected to S9, the "Stop Lamp Switch," through connector X20. When the brake pedal is depressed, pin 8 goes high and that shuts off the cruise control.

Additionally, the "clutch release solenoid" in M16, the "Cruise Control Actuator" (the thing on the engine) appears to be grounded through the brake lights. If that is so, then at least one brake lamp must be good or the cruise control will refuse to engage. Note the slight semantic difference between "turn off" and "refuse to turn on." But at least one brake light bulb must still be good for the cruise to work.

So you are right and I sit corrected.

Incidentally, some cars have a little green "CRUISE" light in the dash to tell you that the system is working. I wonder why Mercedes didn't do that? Looks like you could wire an LED to pin 5, the "Clutch Control Output," of N4, the "Cruise Control Amplifier." I'll have to try that.

Jeremy

compu_85 · #8 11-20-2010, 02:58 PM

On the W124 I think there's even an unused green segment in the cluster face! My car has a red one, I might do that at some point. Or have it be an alarm armed light or something (the alarm module grounds a pin when it is armed)

-J

Jeremy5848 · #9 11-20-2010, 07:32 PM

Quote:

Originally Posted by compu_85

On the W124 I think there's even an unused green segment in the cluster face! My car has a red one, I might do that at some point. Or have it be an alarm armed light or something (the alarm module grounds a pin when it is armed)

-J

My green segment is already in use as an "AFTER/GLOW" light but that's certainly an option. Good suggestion.

greenrickoshea · #10 11-27-2010, 03:34 PM

Quote:

Originally Posted by shertex

Helpful info from GDL site:

Brake Light Bulb Controversy

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.

Hi everyone my 88 w124 cruise has just stopped working after pressing my brake pedal in cruise mode. Both brake bulbs seem OK could it be the transister in the amplifier as mentioned or is there a reset I could do? Thanks very much. GreenO.

ppal · #11 10-08-2017, 11:36 PM

Quote:

Originally Posted by Jeremy5848

The only thing a missing or burned-out third brake light will do is to cause the light-out alarm in the instrument cluster to illuminate. The cruise control is canceled by the brake light switch, not by the lights. All of the stop lamps could be out and the cruise control would still work and cancel normally.

Jeremy

In some cars burned out brake light disables cruise control - first hand knowledge - Hyundai Santa Fe 2005 - fixed cruise control by changing burnt out lightbulb (central light)