[Green 912]

You have given 2 good clues that in some ways contradict each other. In one way you have described the classic hot start problem but in another you have described a charging problem. My bet from what you describe is a possible charging problem based on the installation of the second battery, successful restart and the reading of .34 volts (if correct) on the running system.

If it won’t crank, check the battery voltage both with just the lights on and when attempting to crank the motor with the lights on. If same when cranking as not, suspect the starter connections and leads, if it drops considerably when you attempt to crank, recharge or replace the battery, start the car and test the charging voltage with the car running. It should be about 14.5 at 3000RPM or so. If you are starting out with a charged battery, leave the volt meter on the battery and see if the voltage is (1) not going up to 14.5V with the car running at more than idle and (2) dropping continuously as the car is running. Both would indicate a problem in the charging system. If you have nether symptom then move on the hot start testing. If you have charging problems then it will require more testing on your part in order to give direction on where to look.

If the car will not crank when hot regardless of battery charge, start with all the connections from the battery to the starter and all on the starter as well (ground strap too) clean and tighten as you go. If you still have the problem with hot cranking then check the starter and its bendix.

Not on the top of the list but another thing to check is the ignition switch. I have seen Ig. switches get hot and fail as they get old and on a 912 it will be old. If you have the problem, just reach under and see if it is hot and if so push on the back and wiggle it some and see if it will work. The backing plate on the Ig. switch has a bundle of connections on it and it can loosen and make intermittent and weak connections.

Amp output from the generator will vary widely depending on the load placed against it but the voltage will vary little. In order to charge a 12-volt battery you must overcome the batteries internal resistance. To do so in an automotive battery you need to put more than 13.4 Volts against it in order to induce current flow into the battery. A generator that is responding to the regulator correctly and capable of charging the battery will produce greater than 13.5 Volts and normally 14.5 Volts regardless of load.

Don’t start replacing things without knowing what is wrong first. That is the best way to introduce a second problem, and having 2 related problems will make each very hard to find.



Kurt