Cruise control will not come on

[rf900rkw]

The CC module is looking for a valid ground on pin 18.

The module biases the line high

The clutch pedal switch is normally, NC, and breaks the circuit as soon as the pedal moves.

The brakelight bulbs act as a half ohm ballast to the ground sink. Low enough resistance that it appears to be nearly a ground to low current levels. But applying the brakes floods the circuit with 12V

So, clutch depressed, line floats high.
Clutch engaged, bulbs pull line low
Brake lights flood the circuit to high again.

LED bulbs are very high resistance. They will not sink the signal current. Many a person has fried the transmission in their '90's Ford because of the same circuit design being used to ransmission control.

You could rewire the clutch switch so that it goes to ground via the NC contacts of a small relay. Drive the relay from the brake light signal such that the contacts open when the brake lights are on.

[Vinci]

Excellent info. That's an interesting method for shutting off the cruise control.

I may well rig up a relay. That would still work if the incandescent bulbs were swapped back in. Win win.

[Dsbhn]

Thanks for confirming the two functions are separate switches. No LEDs on my car.
I'm hoping the replacement cc cutout switch fixes it. I have a couple of long drives I want to do. It would be nice to have the cc working for the just getting there part.

I have a similar problem, probably the same exact problem. Can you post what your final result was after replacing the switch?

[Muzz258]

If you are going to change the Cruise control clutch switch please note that the early Z3's had a 2 wire switch and the later Z3's had a 3 wire Hall Effect switch. Order the right one for your car. You either have a look to see which one you need or go to www.RealOEM.com and put your VIN number in to search for parts for your particular car.

The most common cause of the Cruise Control not working is a faulty brake switch. There will be a "BLS/BTS plausibility" code logged in the DME.

[amancuso]

If you are going to change the Cruise control clutch switch please note that the early Z3's had a 2 wire switch and the later Z3's had a 3 wire Hall Effect switch. Order the right one for your car. You either have a look to see which one you need or go to www.RealOEM.com and put your VIN number in to search for parts for your particular car.

The most common cause of the Cruise Control not working is a faulty brake switch. There will be a "BLS/BTS plausibility" code logged in the DME.

I wonder if the different switches have any effect on LED brake lights?

[BimmerBreaker]

I wonder if the different switches have any effect on LED brake lights?

Cant imagine so. LEDs trip the cruise control because of a physical issue - the bulbs themselves dont have the proper resistance. This, to the car, means the brake light circuit is broken, and the cruise wont work