172 Cup - No TB, ECU on OBD after winter storage

[SweCup]

Hi!

I have had a recurring issue after taking the car out from winter storage, the black relays under the bonnet have had a bit of corrosion on the pins.
Usually just cleaned them off and it would have been fine.

The problem is indicated by;
no relay click apart from big yellow one
no squeal from TB
no ECU OBD (OBD reader shows there is voltage however)

Taking the car out this year the usual fix has not worked.

Tried the following as of yet:
All relays have been swapped
Relay board pins cleaned with rotating wire brush
White connector pins cleaned
Measured OK voltage (12,5V) on fuses and relays under the bonnet (all red cables basically)
Replaced negative battery to chassis cable and cleaned bolt plus attaching area of body work
Replaced chassis to gearbox braided cable for direct ground to battery

And for the weird part...
Swapped battery from my V50 into the Clio, turned the key and stuff came alive for a few seconds as in TB started whining and the lamp check for shift light came on.
However, the shift light faded slowly out and TB went quiet. Repeated with the Clio battery and that worked as well for a few seconds and then just went back to being dead.

Not sure where to go from here, can't seem to find any similar issue while searching.

The car is a UK import. Fun fact all 10 Cups sold in Sweden were ready built Group N cars..

 

[colesy]

Check the earth on top of the gearbox, it’s pretty much directly under the throttle body area. I’ve had that break before with similar symptoms.

 

[SweCup]

Ah right, thanks for the tip! Was looking for it but forgot where it was since changing the gearbox a few years back and could for the life of me not spot it.