Jan 24, 2009 · TDI. 2001 ALH Jetta, 2005 BEW Wagon. Sep 28, 2011. #2. I've seen as low as 9V at the glow plugs with a fully-charged 12.6V battery so it can be worse. Here's the deal: that 50 amps of current has to travel from the battery, thru some long wires to the relay panel, thru the relay socket, relay contacts, back thru the firewall to the 50A fuse ...