Thread: Cruise control and ECU (td5)

Hi, it's premature to blame the ECU from the beginning if the cruise controll doesnt work cos there might be many reasons... one quite common which might not be noticeable is the bralke switch, it has a NC contact too which if it's not closed the CC doesnt work...normally if this contact is not closed you should feel the car a bit sluggish but it depends on the fuel map version too

would be good to plug in a tester to check the brake switch input, untill then adjust it: fully depress the pedal and pull the switch's plunger untill it reaches the pedal

also the CC switches can be at fault as well or some bad connection along the circuit,

if it's manual the clutch switch can affect it too

are your horns working? a faulty horn relay would stop the CC as well

the rotary coupler can be at fault too, quite common

many things to check before getting to the ECU IMO

Originally Posted by sierrafery

Hi, it's premature to blame the ECU from the beginning if the cruise controll doesnt work cos there might be many reasons... one quite common which might not be noticeable is the bralke switch, it has a NC contact too which if it's not closed the CC doesnt work...normally if this contact is not closed you should feel the car a bit sluggish but it depends on the fuel map version too

would be good to plug in a tester to check the brake switch input, untill then adjust it: fully depress the pedal and pull the switch's plunger untill it reaches the pedal

also the CC switches can be at fault as well or some bad connection along the circuit,

if it's manual the clutch switch can affect it too

are your horns working? a faulty horn relay would stop the CC as well

the rotary coupler can be at fault too, quite common

many things to check before getting to the ECU IMO

Thanks for the reply,

Not the brake switch as I replaced this earlier - I have a nanocom so could use that to diagnose if possible.

Not the actual switches, replaced them thinking that was the case.

its an auto and the horns are working.

Don't know what the rotary coupler is.

It's been looked at pretty closely and both came to the same conclusion - what exactly happens in the ECU if it were to go? I read on here that taking out the horn relay can cause permanent damage to the CC.

It seems that you missed only the rotary coupler then ... it's in the center of the steering wheel, you can check continuity of the circuit involved in the CC operation if you know how to identify it from the diagram... i dont think that removing the horn relay would ruin the ECU though ... do you get the cruise controll request reading with nanocom from the engine ECU... when you speak about ECU you mean CC ECU dont you?

Connect nanocom to the engine ECU and in ''INPUTS'' menu see if the ECU gets the request if you activate CC from the switch... if it doesnt then the rotary coupler is very suspect

Originally Posted by sierrafery

Connect nanocom to the engine ECU and in ''INPUTS'' menu see if the ECU gets the request if you activate CC from the switch... if it doesnt then the rotary coupler is very suspect

From memory the off registers but the set does not - I will confirm later today

The CC ''Set/Accelerate'' and the ''Resume'' signals are from the switch directly through the rotary coupler to the ECM so if it doesnt get the Set/Accelerate command from the switch to the ECM input and you are certain that the switch is good the fault is almost sure the rotary coupler(provided it's not a wiring issue between the coupler and the ECM but quite unlikely)

see that piece of diagram

Damn. I was hoping that one of them didn't register in the inputs menu but they both do, indicting no issue with the switches.

Then it seems that the first assumption that you might need a CC ECU was right, they are not expensive second hand , search for AMR5700, no worries cos it's cheaper than o rotary coupler