Join Date: May 2014
Location: Espoo, Espoo, Finlnd
Indeed, the problem was the relay and it's welded contact.
This relay actually has a bad design flaw IMO (as an engineer) which most likely leads to this particular failure instead of just not allowing current to flow - what an insulating spike normally would/could cause. The relay has two identical parallel contacts for the motor power current, operated simultaneuosly with a single coil. The other contact had a spike hindering the pair of contacts from releasing. Not so hard to imagine that the current will keep on flowing through the other contact.
Instead of this kind of construction both contacts should have their own coils or there should be just one single larger contact. In the latter case the current would just stop flowing when a spike appears but the brakes would still work. As it is now when the relay fails on the road you may get a broken ABS motor as well.
The other contact was not badly contaminated, the other one was clean. I cleaned the relay and now it's ok, for some time at least.
Last edited by robopoulos; Jun 8th, 2014 at 8:10 am.