intermittent disengagement of cruise control
 

Hi all. I have an annoying intermittent problem with my R75 cruise control system where it either fails to engage or disengages whilst driving. The unit has been opened and checked out and all is clean, no corrosion, and no fault codes are registered on the ECU,via a TestBook diagnosis. Normally when failing to engage at all, if you pull over and switch off engine for a minute, then set off again, it all works fine for many miles. Any ideas ?? Really starting to p me off now. Its a 75 Saloon, Conno SE Petrol 2.5 Auto, Dec 2004.

I’d say a pedal switch is the culprit, try clicking it further towards the pedal

Hi David,
I would second Trikeys advice about checking the pedal switches.
If you ask whoever did your t4 diagnosis they should be able to check the operating point of the switches using the live data on the t4.

Or if you want to give me a shout I can check them for you.
Regards Chris

The other thing to check is if there are any abs faults - if the abs ring on the front right gives misleading speeds, then the cruise control could give up trying to maintain speed

Did you ever get this sorted?

I have posted a few years ago the exact same problem with my 2.5 auto:

https://www.the75andztclub.co.uk/for...d.php?t=303544

as did this forum user:

https://www.the75andztclub.co.uk/for...d.php?t=282970

Since then I have:
(1) changed the centre console switch
(2) changed the cruise relay module

(3) changed the cruise interface
(4) fitted a new engine

OK the final one wasn't only in a futile attempt to get the cruise working again. But even with a new engine, same problem as before. Sometimes cruise works, sometimes it doesn't engage. When it engages I hear the clicking behind the glovebox, not when it doesn't work. If it stops and I turn the engine off and restart, often it will start working again (for a while).

Occasionally on a long motorway drive with cruise engaged it just disengages and won't come back on, randomly and by itself.

Also, I've tended to find it's most likely NOT to work on the first drive of the day. So wait until engine's warmed up, try to engage cruise - nothing happens. Stop, restart engine - works fine.

Car's olf to the garage for some work soon - can anyone suggest anything the garage should be looking out for, given the symptoms and previous work done? What's the "vacuum system" mentioned on other threads? And is there a specific cruise ECU, or would I be looking at changing the entire engine ECU if checking for an ECU fault? That sounds like it could be an expensive and complex pain.

i believe the cruise vacuum module lives somewhere down below the offside bulkhead below the firewall. it does have a vacuum pump together with its gubbins. i cant remenber if it has a plug to the main under bonnet engine harness or where thats located.but you could try to remove and clean the contacts and reseat it a few times. are the bellows on the throttle body punctured or not seated well ? also check for fowling on the piston connection alongside the body. check the pipe connections for cracks and seating along its length. most of this is for the cable throttle versions but i dont know about the fly by wire versions that dont have a throttle cable. other members will know im sure . .. ;)

i think a run over to chris is your best way forward on this, after checking out what you can it will be your best option before you go to a garage and unzipping your wallet big time ... :D

This might be a red herring, but…

About 10 years ago, I had a similar problem with a Ford Mondeo. Cruise either wouldn’t engage, or it would suddenly disengage for no apparent reason.

Main dealer looked at it on several occasions, but couldn’t find a problem. Eventually, they decided to order a switch pack for the steering wheel. I left the car with them for the day and when I went to pick it up, they said they had fitted the new switch pack, but it didn’t work, so they refitted the original ones.

After that, I never had another problem with it, so it looks like it was a bad connection with the switches.

quite possible indeed , as is any intermittent bad connection. hence a visit to chris might reveal some answers.