It gets the info from the glowplugs themselves. Have you heard of the "Z bar" on the controller? That is a very very low calibrated resistance on the controller block. All the current flowing through the glowplugs flows through this z bar. The very low resistance creates a very small voltage. Yes, it is actually stealing this voltage from the glowplugs, but it is very very small. The more current flows through the z bar, the higher the voltage. Lower current means lower voltage from the z bar. The circuitry in the glowplug controller monitors this very small voltage. That is how it determines the glow time.
When the glowplugs are cold, they draw a lot of current so the z bar voltage is high and the controller holds the plugs on for a long time. On a warm engine the glow plugs draw less current, so the z bar voltage is less, so the controller glows the plugs less.
The above is all well and good until things happen. If you have one glowplug go out, that lessens the current flow through the z bar so much that the controller will not glow the plugs long enough to start the engine. The wiring. If the connectors or the wiring has some corrosion and is limiting the current flow to the plugs, then of course that limits the flow through the z bar and the controller will short cycle again. If you do not have the exact correct glowplugs in the engine, they may draw more or less current. This of course changes the glow time.
An EXCELLENT explanation and thank you!
All OE Beru GP's and the only 'tests' I can and have performed over the decades of owning my truck since NEW is the 6-7 steps for the GP System in my Haynes 7.3 Diesel book. And over the decades of performing those tests all steps showed either everything in the system was good or/and I know when I had a bad GP (very short cycling/clicking) or the Controller was bad, as it was acting-up late last summer and I replaced it with a Ford part.
The one test in the book is for 'ambient temperature vs GP burn time' with the key-on using a test light and the "wait to start light". And unfortunately, like what it's currently doing as winter has set-in it's not performing as the book states and the book says to... "disconnect the battery and replace the GP controller."
I'll just say, the original controller from 1988 and controller #2 I replaced were beefy units. Controller #3 (this one) from Ford looked exactly like the cheaper controller from Napa but, the more costly Ford controller has a two year warranty and that's the one I purchased.
So, maybe the controller is faulty or, the resistance in the GP's are off ... they were all replaced just a few years ago.
Thanks again for your explanation.
