*EDIT* sorry Barry, I saw you mention the test light and didn't even notice you also mentioned the bypass wire, sorry for the repaet information. *EDIT*

A parallel electrical circuit is superior in (almost ?) every way to a series circuit. Now, im not saying this alone would merit the time/money for a conversion, but it is just a bit of truth.

The main issue with a series circuit is that the entire circuit has one power source and one ground. If the circuit is broken anywhere along the circuit, the entire circuit goes dead. If you have a faulty glow plug that is open internally, all four glow plugs stop working. In a parallel circuit, only the faulty glow plug would stop working, the other three would work normally.

This is because in a parallel circuit each consumer( in this case each glow plug) has its own power source, and its own ground. I have series glow plugs in my 123, I don't plan on converting to a parallel system, unless somehow the parts end up in my lap for cheap.

I agree with the idea of keeping a test light in the vehicle for diagnosis reasons. Another little piece i keep in the trunk is a bypass wire to allow me to get home, after i have located an open glow plug.

I went and got 2 alligator clips rated at 70 amps at 12 volts from radio shack, connected them together with a strand of 6 guage wire about a foot long. If an open glow plug is located, for ease of description, lets assume the glow plug for cylinder 3 is faulty. if you place one clip between glow plug 3 and 4. and one clip between glow plug 2 and 3, the alligator clip/wire will bypass thew faulty glowplug and allow the other three to heat up the motor.

this is in no way a permanent fix, and should only be used in emergencies to make it back to your destination. Doing this greatly increases the electrical load on the other three glow plugs and may result in premature failure of the other three if done continually. Hope i have managed to help a couple people out with some peace of mind. for 10 bucks, I can't imagine not carrying one in my trunk.

Branden