Is there any other reason why a diesel mk1 would crank slow on start up other than a bad battery? My battery shows 12 volts when nothing is on. When I turn the key on to check volts at the starter and stop solenoid it's at 10v. I'm pretty sure it's a bad battery but I won't be home for a couple days to switch 1 out. Anything else that might cause it to crank slow?

 

I thought you have a Jetta now you have a Golf?

12 volt is low for the terminal voltage of a battery. You need to do a volt drop test on the starter system. A slow cranking speed generally means a bad connection or poor engine\transmission earth or a duff battery can you get jumper leads on it?

Starter Circuit Voltage Drop