318is Cruise Retrofit Failure... ???

[Schnitzer318is]

Well, followed the directions on m42club's DIY and installed cruise today. Of course I go to try it out and it does not function.

My question is where should I start troubleshooting? I don't know if everything worked prior to install as it came out of a yard car. I'm guessing the wiring is good as it looked to be in good shape.

Is there a way to test the actuator or computer without just replacing with a known good one? I can do that but would rather not tear apart my convertible's working cruise to start swapping parts.

Thanks for any suggestions.

[DaveSmed]

I would start with disconnecting then grounding the wire at the clutch switch that goes to the cruise control module, and checking to see if you are getting a speed signal.

[Schnitzer318is]

^I was wondering that as well... what sort of signal should I be looking for? A voltage change or a resistance change?

[Schnitzer318is]

Getting signal for speed on the appropriate wire... I am leaning towards the computer or the actuator being shot, hell I guess it could be the stalk itself. I also verified the switched 12v+ and the ground wires were good at the computer so I am pretty sure the wiring is good.

I work at a BMW indie shop (been going there since I could drive) and one of the techs said that the cruise computer goes kaput fairly regularly in the e30 and that the stalks and regulators almost never go bad so I guess I'll start there.
Guess I'll have to start swapping components to find the culprit... weaksauce.

[DaveSmed]

What kind of signal is it? Pulse to ground, float to battery voltage?

Anyway, the stalk should be easy enough to check, but you definitely verified you were seeing a ground on the wire going to the module from the clutch switch with the brakes and clutch released?

[Schnitzer318is]

Ummm... not sure honestly, I just verified that it was reading the same on both cars at 0. Guess that's not guaranteeing anything, but lol it sounded logical at the time. I also swapped the suspect cruise computer into the car with working cruise, and it worked as well. Strike the computer... leaning towards the actuator honestly.

I did verify that the computer was getting ground with both the clutch and brake pedals released.

Edit: Wish I had a signal generator... lol would make testing a lot easier.

[Schnitzer318is]

Ok... tested the computer, actuator, and stalk in the other car and everything is good. I wonder if there is a break in the harness somewhere? Going to pull the harnesses and check for continuity from one end to the other.

I doubt the speed sensor plug on the back of the cluster could be the problem, but with my luck that's what it will turn out to be.

[Schnitzer318is]

For those that are interested in a puzzle (dave)... I figured it out. And I can't believe I didn't figure it out sooner!

I had everything hooked up correctly from the get go and was wondering why I was getting small voltages on the red/green wire on the brake switch without the switch being depressed. It's supposed to be grounded with no depressed pedal.

Soooooo... I let that go and tested every component of the cruise in the convertible one by one. They all tested good. So then I tested continuity between all pins/harness points of the cruise circuit. All 13 tested good at all their designated connections.

Hmmmmm... so I check out the ETM for the cruise circuit and verify all my connections are correct. They are, GOD DAMNIT!!! So then I get to reading it more... "The green/red wire is used to sense ground when the pedal is not pressed. Ground is removed electronically at the brake switch and mechanically at the clutch switch."

EUREKA!!! Remember that very small voltage I was getting on the green/red wire of the brake circuit? Well disconnected the cruise harness from it and grounded the green/red wire on the cruise harness (chassis ground). HEY! Working cruise control... pressing the brake or clutch won't turn it off, but the stalk still will.

So now I am wondering about the root cause of the problem... and it hits me like a hammer. You idiot!!! You're using resistors to emulate bulbs on your brake circuit because of your LED tail setup. Apparently I have the voltage low enough to not freak out the signal relay and cause hyperblinking but not enough to give me a full ground signal on green/red.

Problem solved! Now to install a kill switch on my cruise control so no one else can use it but me. Wouldn't want anyone assuming it would turn off with a brake press or a clutch pedal press now would I.

Thanks for the help and only posted this for the intensely curious... not like anyone else out there is running a 91 318is and LED tails and retrofitting cruise. :facepalm:

[DaveSmed]

Ha! Wouldn't have guessed that one, does that stuff still run through your sho-me module?

[Schnitzer318is]

Ha! Wouldn't have guessed that one, does that stuff still run through your sho-me module?

Ya I wouldn't have guessed it either which is why it took 6 days to install what is a really simple system. Yes, the tail lights setup still runs through the sho-me. I need to pick up another one actually so that I can run the front signal lights through one. Right now my front lights don't blink with my rear signals because of this... they just run off a relay on the front light signal. Not really a big deal since you can't see them both at the same time... but it irks me.