Twice today on uphill portions of freeways, I noticed the cruise control engaged itself without any input from me (which was rather scary). I noticed as the car was repeatedly downshifting to 4th, even as I was letting off the gas.

The "cruise main" light was on as I never turn that off, but the "cruise control" light below (which indicates that the cruise is set) was *NOT* lit. However, cruise control definitely seems to be the culprit because it released the throttle immediately when I pressed "cancel".

Any ideas why this might happen? This happened twice over the course of a 2+ hour drive.

 

I was thinking, maybe the buttons were sticking, but if that were the case, the cruise light should've come on to indicate it being set.

So, I think it's likely a failure in the ECM/ECU.

If I were in your shoes, I'd take it in for a dealer (or trusted mechanic) to diagnose.

Possible resolutions (if the fault is the ECM):

reprogram ECM (not likely)
replace ECM (kinda pricey)
replace Fit...

 

"Turn off cruise main and never turn it on again" was (hopefully) the worst-case outcome, in my mind.

 

Have you done any electrical fiddling around lately or been in an accident? Wires touching somewhere might be the "gremlin" engaging the cruise control.

 

Well, a faulty Cruise Control system is IMO just too dangerous for me to want to offer any remote distance DIY advice.
My only advice then is if you feel this is happening, get it professionally checked out ASAP. The symptoms you are describing ARE scary...and dangerous.

 

Well, considering that cruise control engaged without the "set" light being on, the question is... does turning of the main control prevent it from ever engaging?

As others have said, having it come on is pretty dangerous. It won't be completely out of control, so long as you're somewhat attentive (short of total catastrophic failure, you'll have a few countermeasure options)... but it's definitely something I don't what happening to me.

Going back to the question, can it engage without the main... this is not the kind of question I want even being asked.

 

I did disconnect/reconnect the battery last week to adjust the mounting on my 51R, and I have a bunch of random electrical mods (extra horn, aux headlights, aux reverse lights, etc). Nothing that should have affected an engine control circuit, but I'll double check the wiring to make sure there's no shorts.

I drove ~10 miles of partially washboarded gravel road immediately before, which could have jarred something out of place.

As for taking the car to a dealer/mechanic - I'll definitely do that if the issue persists. I drove around quite a bit yesterday and wasn't able to reproduce it, so I have hope that it was a one-off fluke.