Welcome to VxON!
You should scan it for ABS error codes...with an appropiate scanner. Be aware that GM cars are really picky when using multibrand scanners especially when ABS, ESP, airbag etc implied. Post codes here in their genuine spelling.

 

Thanks for the welcome Bibu16v - yeah the codes say 'out of range signal errors' on front left and right ABS sensors. But (and obviously i should have included this information before, sorry!) Ive replaced the front right one and getting the same errors.

Im presuming (but trying to avoid) that the computer in the ABS module is just for some reason unable to process the front sensor feedback at higher speeds.. but that doesnt sound quite right.

The other idea I thought up last night was that at higher speeds, something in the shape of the hardware involved (wheel, sensor 'clamp' etc) are bending slightly at higher speeds and causing intermittent / inaccurate readings .. but that seems equally unlikely.

I guess i'll just have to get a new ABS module and fit that and hope that fixes it :/

 

If front sensors are Ok (new as you lately say) and their wiring too, probably the ABS module is the culprit...
Did you clean its conector pins? Maybe it'll help to read some voltages on the pins if possible...

 

yeah maybe i'll try cleaning the pins at the module end - havent really been there yet, i think if i AM gonna go that far, i'll probably go for a ABS module replacement - its the only real logical conclusion (maybe except for a wiring harness issue, but i don't wanna go there as really that'll spell the end of the van for me at that point )

 

damn my diag tool (after software update) has wiped my history.. i'll plug it up to the combo again at lunch time and get the exact codes and wording

 

Make sure the front ABS rings are in good condition. Any excess rust or minor cracks can affect the signal.

 

hmmmmm yeah that could make sense, i'll look at that this weekend.... thanks!
p.s. didnt notice this post before i put in the "story" mode....

 

A little story to demonstrate. (numbers in graphs are in now in MPH - click on pictures to see larger):

Step 1) cleared all the ABS errors with diag tool (didnt think to take picture of clear dashboard, but it was clear).

Step 2) started the van up on the driveway and sat there:

Step 3) reversed out of the driveway, then headed to end of road and waited for clear traffic (still no ABS trigger yet):

Step 4) Set off down main road around 20mph - still no ABS light yet but green and blue 'wobble' once each:

Step 5) incease to 30mph and whilst the readings for blue + green (front sensors) is going metal, still no ABS light, UNTIL, about 3/4 of the way to the right of the graph (02:08+) and at that point the ABS light comes on.

 

Step 6) pulled over to collate data (ABS light still on, but readings seem 'normal' as the vehicle slows down):

Step 7: faults:

Thanks for any feedback / thoughts

 

Post #10 shows what I found on mine.

https://www.vauxhallownersnetwork.co.uk/threads/oddly-behaving-brake-pedal.831767/?post_id=3975091#post-3975091

 

thanks Koicar - i really hope its a simple as that - thanks for the info

 

Happened to me too with Astra F: one (front) ring cracked with no consequences and over time(I'd speculate years) got deformed and ABS got activated at slightest braking on hard and dry surfaces. An annoying grinding noise was present, but no ABS error... probably normal for an old ABS generation...