The series glow plug system is actually very simple to work on, and very reliable. (Though it doesn't work as well as the later parallel plug systems.) The main feature to recognize is that each glow plug has two non-ground terminals and is a 1-volt plug. String N of them in series, and then use rather hefty dropping resistors to consume the remainder of the voltage (at 60A or so). The oldest systems don't have a controller, just a pull knob, and use an additional glow-plug-like element in the dash that glows red hot when ready. (This can be considered the N+1 glow plug.)
Each plug, when working, should have approximately 1V measured across its two leads. If not, something somewhere is wrong. That's where it can get tricky if you're not very savvy electrically. The usual diagnosis technique seems to have a very involved table of electrical voltages (measured to ground) at each point, every one different. I find that using the differential plug voltage is much simpler and more consistent. A higher voltage across a plug points directly to that plug being a problem. 12V is an open plug, less than this indicates a poor connection. A lower voltage _usually_ indicates that there is insufficient current flow due to a poor connection elsewhere. Keep looking for a too-high voltage, and correct that first. Burned-out series plugs are relatively rare (when compared to the parallel plugs), what plagues them is bad connections, it's a hostile environment what with all the heat and the many connections for the 'toaster rack' series dropping resistors. Heavy soot buildup can also short the element to ground inside the head, though if you have this problem something else is wrong (perhaps just your driving style/regimen). I remain convinced, in the absence of evidence, that if the toaster rack heat could have been captured in the intake manifold the series system would be just as effective at cold-starting the cars as the later parallel system, though not as quick. (Parallel systems work to lower temperatures, and more quickly.) -- Jim
