Cruise control only works intermittently

It sounds like you've eliminated the clockspring, the normal culprit, since the dash light is on.

I've seen the switches available on eBay. If there's a market for those, there may be a reason and I'd suspect that first. They may be easy enough to troubleshoot with a good ohmmeter and wiring diagram--only three wires and some internal resistors as I recall. You need to pull the airbag to get at it--use caution and get good instruction if needed.

You may be able to check that at the ECU, but I suspect taking the wheel apart might be easier. But checking at the ECU will also check the wiring harness and clockspring, the next logical step.

If that's not it, you may need to diagnose the ECU. Do you have a Mini VCI? If not, it might be worth it for this.

 

Some easy testing. Turn the steering wheel ( in different positions) to see if the cruise ON light will go on. If it does it indicated the ribbon cable is bad.

 

If you want to DIY, it is time for you to obtain the factory repair manual at techinfo.toyota.com so that you can follow the voltage measurement and the comm line connections between the engine ECU, hybrid vehicle ECU, skid control ECU, and traction battery ECU. The first two are behind the glove compartment. The third is located near the steering column while the last ECU is within the traction battery case.

 

My experience with Cruise Control relates to "other than Prius," but basic principles should be somewhat the same.

First, most cars develop cruise control problems as they age -- duh. Most of us figure we can live without cruise control and don't bother to fix it.

But...In terms of your cruise control, the first thing I always checked was the fuse....yours is an intermittent problem and the fuse is probably not involved....but I would still check it. That would be the cheapest thing to fix.

I don't know the details on the Prius system, but if the fuse is fine, then the switch at the brake pedal could be worn out, or out of adjustment. A bad switch or a sticky switch could cause troubles.

Personally, I would not bother...

 

The cruise control works via using ohm values to send to a decoder/multipexer/module to tell it what switches are pressed.
The states are:

on/off
Resume/set
Cancel

These values are transmitted via two wires. When these values are sent to the ECU the ECU does the command which the value dictates.
So if you have a bad ribbon cable and the command value is not transmitted. Also the value is not recognized as its transmitted but the value does not fall in the values of the program data list. Either no data was transmitted or value not within the parameters.as well.
Repairs or diagnosis should always start with the mickey mouse parts first and most certainly the most physical to wear. Of course I will add in cheapest if this is to be a shotgun repair lol! Thats why its prudent to checkout the basics first before ripping into the ECU or any other wiring.

 

There are other threads here on the clockspring, also called spiral cable, replacement. And some vids on Youtube. Pay heed to a recent one about being wary of the cheap knock-offs available for $30 on eBay, etc. Use your VIN to get the right part number--various configurations are available for the same model year. Pay attention to transferring the steering angle sensor if you have VSC. Don't do it if you're not comfortable working safely with the airbag. Otherwise it's a straight-forward DIY repair.

 

Okay well I usually get my parts on eBAY and you will have to decide on price point and/or go genueine (OEM) or after market.
I went the cheap way and bought but not installed one yet. Initially my cruise ctrl was flaky so I bought a used one and put it in box under breakfast table as we type/speak here. Then I got this brainy storm to get a leather wrapped steering wheel...done,..now after market ribbon cable...LOL! I see it's time to slam dunk them in..For install check out YouTube for videos. Even If I am a seasoned mechanic 39 years experience its always good to go thru someone elses run thru so as not to screw up! Oh yeah..even a pro messes up if this is the first time! sometimes. But check those videos out and as well as search for website still pics on this subject. It is not difficult but as always never get too cocky. Some fear is always good to keep you sharp. When I joined here there was a post here on someone who did install a ribbon cable and they had codes now which he didn't when he started. Ended up he did not pay attention and forgot to plug in one cable!
ALWAYS DC (double check) your work as sometimes your own repair can result in more problems than it should have.

In closing the only thing to remember is that once you release the airbag and remove make sure the steering wheel is centered at all times.
Make sure the ribbon cable install centered orientation is pointing at itself when installing. As I said just cruise thru the videos..the more you see them the plan becomes a blue print in your mind.

 

I thought there was one on the Prius clock spring but looking..NOPE MY BAD!

Well then this is the procedure. UNBOLT the negative terminal of the 12 Volt battery so your safe. Wait a couple of minutes to be sure. Look for procedure to remove airbag on Youtube or search under the PriusChat Search box. Straighten steering wheel so its at 12 o'clock.
You can unbolt the steering retainer nut. unattach allt he wiring to the cable. Then supposedly you can pull the steering wheel off...normally you need a puller but this is a first for me insofar as pulling it off bare hands but there it is. Then you see the clockspring cable device.
Let me see If I can get some pics..

This thread has some info of use:

New Prius Clockspring Part# 84306-0E010 | PriusChat

 

Yeah I drive the 04 while wifey drives the 09. Mines looks to be the Package 3 but Iwith CD and seperate Toy CD in the bottom bay below radio head. Check this thread and PDF dload of packages: About three msgs down:
2009 Option Packages | PriusChat
Also I scored a new trim ring on ebay for $20 last week. He had four listed genuine and NEW! in the orig bag!

 

Let me bump this thread for future readers.

For more than a year, I have been trying to diagnose why my cruise control has not been working. I went to the local dealer and spent $150 on diagnosis and waited for hours on end. They told me the cruise control switch needs to be replaced. I came back home and I put in a brand new OEM cruise control switch. But it still didn't work.

I took it back to the dealer and they worked on it a few more hours. Or at least they pretended to. I am almost certain they did not work on the car since the battery was never disconnected when I got back into the car (the trip meters, clock, seat belt and reverse beeps all get reset when the battery is disconnected but none of them did). At the end of the day, they said they need to open up the entire dashboard and get to the wires and will need at least 2 more hours and I will have to pay for that. I walked away.

After reading what Dion Kraft mentioned about trying to switch on the cruise control at different positions of the steering wheel, I was shocked to find that my old supposedly non-working cruise control switch is indeed working! By the way, I had just taken out the new switch to return to get my money back given that the cruise control switch was never the problem to begin with. So I have my original switch back in my car.

For those of you in the future diagnosing this in the future, please try this first -- rotate the steering wheel at 5 degree steps and keep pressing the cruise control switch. It may indeed work. And if it does, then the clockspring is likely the issue.

 

That was a lot of reading to get to the result......clockspring is the issue

 

The internet, smartphones, social media and multi-tasking has dramatically reduced our patience to read, unfortunately.