W124 intermittent cruise control fault

[BlueQuinn]

My cruise control refused to turn on yesterday during a motorway run at 70mph.
Slightly annoying but I pressed on, thinking it might be the switch.
Then an hour or so later I tried it on a 60 limit and it worked fine.

Any ideas?
The car is a 1990 S124 300TE-24 with no ASD/ASR and the 4 speed auto.

 

[renault12ts]

Your rear tyres may need changing. If the rears are badly worn, the difference in diameter between front and rear will render the CC inoperative above about 65mph.

 

[BlueQuinn]

Tyres are practically new Contis.

 

[WDB124066]

Brake light switch...??

 

[BlueQuinn]

Brake light switches are cheap. That's good if it's that.
If it was a wheel speed sensor issue would I get the ABS light on?
Can I diagnose this with a blinky light code reader?

 

[WDB124066]

Yes.
Normally.
Yes.

 

[300D-91]

Could be a sign that the cruise unit is going bad. Do some googling for advice on how to check it. And if your good with a soldering iron, tips for fixing it.

 

[Olivier]

I look at the speed sensors ring on the hub? Might be corroded and need a clean.
On the E300TD the rear are easy to look at with the wheels o n but for the front you need to take out the brake disc .