Cruise control inop

[Stan]

what are the most common causes of cruise control failure?

[Stanley Rockafella]

You certain it's broken?

On my first e30 I also thought it was broken. After speaking with the PO he said it worked fine for him.
Turned out, the light pressure of my foot touching the clutch pedal was enough to prevent the cruise from engaging.

[AndrewBird]

Bad brake and/or clutch pedal switch is pretty common and easy to fix.

[Stan]

I am a noob. Is there a part number or pictures?
how can I fix?

[7pilot]

Did the C/C work previously and just quit? Or was it cutting out beforehand?
Buy a Bentley Repair manual if you intend to DIY. (aka, welcome to your new mistress)
Have a butcher's at the cruise control cable. Is it still routed correctly? Are any of the plastic anchors for the cable broken? Does the linkage at the throttle body work smoothly by hand with no binding, stiffness?
Check the related fuses. Do you have an illuminated brake lite warning on your instrument panel? Even tho the brake lites work?...then replace the brake switch that abuts the flange of the brake pedal .
you'll need to remove the knee bar and the lower cardboard barrier.
I you are of good size, the drivers seat will have to come out. I recommend that you have a repair manual handy.
Easy job but very fiddly.

[Gregs///M]

If your speedometer is not working, the CC will not work either. There is a single yellow/blue wire in the cluster harness that splits off and plugs into the green plug on the rear of the speedometer. Without that connection, CC will not work.

[Stan]

wow, a lot to look at! I just bought the car and it was ions at the time. But the batteries in the instrument cluster is 30 years old, the econometer is not working, & the temp gauge is slow to respond. Maybe it is all related to that.
Thanks guys!

[Stan]

If your speedometer is not working, the CC will not work either. There is a single yellow/blue wire in the cluster harness that splits off and plugs into the green plug on the rear of the speedometer. Without that connection, CC will not work.

Will a dead instrument cluster battery affect this?

[Gregs///M]

Will a dead instrument cluster battery affect this?

No, the SI board has nothing to do with the speedometer.