Hi all - I don't know much about cars, so please bear with me. I drive a 1991 740 with about 184k miles. A few days ago the car started shaking on acceleration, so I brought it into the mechanic and they said the transmission mount was broken. That seemed to fix the shaking, but when we picked it up we noticed that the speedometer is now jumping all over the place, generally anytime the car exceeds 20 mph.

The mechanic says that it's the speedometer sensor and that it's not their fault and has nothing to do with moving the transmission around. Are they being honest? I was under the impression that the speedometer cable and sensor would have been moved along with the transmission. I have a hard time believing that the speedometer, which worked perfectly before, would suddenly go bad on its own when they were coincidentally messing around with a related system.

They're saying that it's not only not their fault, but that they won't fix it ("can't get the part") and we should take it to the dealership. Help would be appreciated!

Thanks.