this might help, on my 87 190d-the tach pickup is on the front side of bell housing,drivers side, one wire goes up to a connection on firewall, behind battery.(then i'm not sure, where else) but it's connected to the klima and over voltage relay(somehow). my tach was intermittent-I took the ovr apart & found a broken solder connection-had to use a magnifing glass- but when resoldered it works all the time........jerry