Cruise control mind of its own

Hello again all

How are we all this fine easter Sunday?

Another gremlin with the 2011 RRS TDV6 auto.

Set the cruise control to a set speed but regardless of the set speed it continues to accelerate beyond and continues until you hit the brake pedal or cancel the cruise using the button. They work fine.

I am.about to attempt a drive from West of Scotland to Norfolk as my brother has passed away and I need to take care if his last officer etc and ideally hoping there is a quick driveway fix for this one of you have unearthed having encountered this issue?

The resume button works the set button works but the speed is not remaining at set speed. No warning lights or anything.

Wheel speed sensor? Steering angle sensor?

Just replaced the brake light switch and that hasn't helped.

Cheers dudes and dudettes.

Diagnostics may be the only way forward. A dangerous fault!

Datatek said:

Diagnostics may be the only way forward.

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Other than the codes (now dissapeared) relating to my 3 amigos and loss of power thread, there are no codes when I scan it.

Datsun770 said:

Other than the codes (now dissapeared) relating to my 3 amigos and loss of power thread, there are no codes when I scan it.

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Sorry, in that case I can't help. From what you say, I'm not sure I would ever trust it.

I have never trusted cruise control since I experienced a 'surge' when travelling up the M1 in my old Jag, scared the bejesus out of me because it did n't respond to braking or adjusting from control unit, turned out to be a faulty relay, but put me off them for life and probably scared the pants off the guy driving the AUDI in the fast lane in front of me that I suddenly approached at 'AUDI' driver speed!
As Data siad , full diagnosis is the best option, you don't want to get caught out with a 3 ton beast at full chaff!

The cruise control is a relatively basic function, there's a resistive ladder in the switch pack that sends a specific voltage to the ECM - which has the CC supervisory function on HS-CAN.
The first step is to look at live values from the switches - particularly the '+' switch. My guess is that someone has spilt something sticky into the switch pack and it isn't fully releasing, causing the set speed to keep incrementing.
Of course, none of this will set a DTC because the switch pack and the ECM are just responding correctly to driver input....

Check all the cruise related inputs & relays with both diagnostics & multimeter. It must be getting some speed data, or should not normally engage.

Graculus said:

The cruise control is a relatively basic function, there's a resistive ladder in the switch pack that sends a specific voltage to the ECM - which has the CC supervisory function on HS-CAN.
The first step is to look at live values from the switches - particularly the '+' switch. My guess is that someone has spilt something sticky into the switch pack and it isn't fully releasing, causing the set speed to keep incrementing.
Of course, none of this will set a DTC because the switch pack and the ECM are just responding correctly to driver input....

Click to expand...

Doesn't seem sticky but I noticed the '-' button doesn't slow it down. It just races away past the speed limit. I figured if it was sticking it woukd set itself also. But doesn't. I'll get a look at the live values. What's the best and most accurate way of doing this?

Thanks all for the response so far.

pwood999 said:

Check all the cruise related inputs & relays with both diagnostics & multimeter. It must be getting some speed data, or should not normally engage.

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Know of a how to for this at all dude?

Datsun770 said:

Know of a how to for this at all dude?

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Sorry no. Don't have access to manuals for RR after 2006. The earlier L322 manuals say it's done by the Engine Management ECU, so maybe there's info there ?

Couldn't see anything about troubleshooting thus in the manual. Thanks anyway man.

There are no relays on a L319 CC system, the inputs are direct voltage from the switchpack, brake pedal & composite speed signal from the ABS, broadcast on HS-CAN and transmission temperature (as an inhibit), again, on HS-CAN. It's a pretty simple system.

Easy when you know how. That's another language to me any help on ways of diagnosing for dummys appreciated dude.

Check the live data for speed & the set & resume switches.

Graculus said:

The cruise control is a relatively basic function, there's a resistive ladder in the switch pack that sends a specific voltage to the ECM - which has the CC supervisory function on HS-CAN.
The first step is to look at live values from the switches - particularly the '+' switch. My guess is that someone has spilt something sticky into the switch pack and it isn't fully releasing, causing the set speed to keep incrementing.
Of course, none of this will set a DTC because the switch pack and the ECM are just responding correctly to driver input....

Click to expand...

A resistive ladder? How crude is that

Yet it works - mostly.

Datatek said:

A resistive ladder? How crude is that

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Easy way for the ECU to figure out what switch is being pressed & save wiring, but I agree it's a bit crude & prone to error.

Seemy silly to me to save wires & connector cost, considering all the excessive ECU's in there !!

Graculus said:

Yet it works - mostly.

Click to expand...

Expensive when all that is needed is a pulse count from the wheel sensors and a comparator to the set speed pulse count, dead easy with all the computing power there

pwood999 said:

Easy way for the ECU to figure out what switch is being pressed & save wiring, but I agree it's a bit crude & prone to error.

Seemy silly to me to save wires & connector cost, considering all the excessive ECU's in there !!

Click to expand...

Sorry, it doesn't even save on the wiring.

Two wires through the clockspring to the ECM, the voltage indicates the function requested, the composite speed etc. is already on HS-CAN so other than a 1 or 0 in the CCF, it's not a 'bad' way of doing it, just different.