McDonald

On its own motion this court transferred this case to itself for hearing and decision after the Appellate Department of the Orange County Superior Court reversed Bruce Robert McDonald’s convictions for driving under the influence of alcohol and driving with a blood-alcohol content of .10 percent or more. McDonald contends the trial court erred in its instructions to the jury on the use of the blood-to-breath partition ratio in determining the accuracy of a breath test. He also argues he was denied the effective assistance of counsel. We reverse McDonald’s convictions.

I

McDonald was tried by jury for the misdemeanor offenses of driving under the influence of alcohol (Veh. Code, § 23152, subd. (a)) and driving with a blood-alcohol content of .10 percent or more (Veh. Code, § 23152, subd. (b)). He was also charged with having suffered two similar prior convictions, although we do not know whether these allegations were later proven. At trial, California Highway Patrol Officer Pat Dillon testified he detained McDonald at about 1:30 a.m. on October 11, 1986, after observing him driving erratically. McDonald smelled strongly of alcohol, his eyes were red, watery, and…