Yeah, as an automotive and aerospace engineer I find the bulb theory "interesting". I plan to check my owners manual to see what type bulbs are listed and pull a couple to check. I spend a lot of time on the Web trying to dispel various myths. Most of my time is spent with the vintage Corvette community.

One thing I NEVER DO is kill the cruise with a tap on the brake pedal. I always use the cancel function on the stalk. Tapping the brake flashes a bright red signal to any bandits at your six, and I'm not interested in advertising my control actions.

The cruise control is an example of a damped feedback control function. The speed you set is controlled by the speed sensor on the back of the speedo housing, and any variation in the sensed speed from the selected speed is going to cause the amp to correct, so a sticky cable that causes the needle to wiggle is going to cause hunting/surge, and you are right, this could get the amp into a sweat. Whenever the cruise starts surging, I kill it with the cancel control. I recall reading somewhere that the Merc cruise control system speed can be set in 1/2 MPH increments, and my experience is that it's very accurate at maintaining selected speed, which is not the case for some other OEM systems.

Since my car is going into summer storage in a couple of weeks (I use the Merc as my daily driver in the winter and a '91 MR2 in the summer.). I'll probably buy the cable and swap it next fall when the Merc comes out of storage for the next winter season.

Duke

Okay, call me an annally retentive engineer, which I am. But, it's impossible for me to understand how light bulbs that are not lit most of the time while the cruise control is in operation, have ANYTHING to do with cruise control operation.

I bought in on this suggestion without thinking about it several years back before I thought about how ridiculous it is.

So, please explain how a resistance that is not in any circuit has an effect.

Have a great day,

Hi Larry,

I am an electrical-electronics engineer.

I may shed some light (hopefully making sense) on how the CCA can be influenced by the Brake Light Bulbs as to make it disfunctional.

If I remember correctly, Pin# 8 of the CCA reads a ground thru the brake lights (filaments or base, I do not know).
When the brake is applied, the circuit is opened and the resistance becomes infinity, hence disengaging the CC.

IF the resistance (of the bulbs or the casing?) is greater than whatever resistance it is supposed to read (brakes not applied), it will see it as an open circuit, hence making the CCA inoperable. It will make it think the brake is constantly applied, hence keeping it permanently disengaged.

Keep in mind, ground does not necessarily means 0 Ohms. All circuits have thresholds.

I hope this explanation makes sense.

In my humble opinion, I believe the theory of the bulbs affecting the CC is valid.

__________________
1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
609 Certified

But the problem is not with an "inoperable" CC. It's with a unit that works and then surges.

This also happens on my '85 107.

__________________
Rick

'85 380 SL (sold)
'85 Carrera Flatnose
'71 280 SL Signal Red/Cognac

I was addressing LarryBible's request for an explanation, not necessarily the surging problem.
Although I am sure there is an explanation fo tha one too

__________________
1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
609 Certified

An intermittant surging problem would seem not to be logically caused by a brake light bulb, because hitting the brake would either disengage the CC or not disengage it. As I understand the device, the amp cannot reingage after one hits the brakes.

It's probably more likely to be a bad circuit in some other portion of the amp.

__________________
Semibodacious Transmogrifications a Specialty

1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)

"Time flies like and arrow, yet fruit flies like a banana"
---Marx (Groucho)