Volvo check engine light 1231 fmi 9 need help

So recently I have been seeing a few volvos that come in to the shop and have a son 1231 fmi 9 for a missing signal. When I ohm out the grounds on the acm ground wires it shows high resistance. But if I disconnect the positive cable the resistance goes away down to .1 or .2 on some of them. Also with the positive wire connected if I turn the key on the resistance goes up even more. Has any one had this problem or a fix for it.