I am pretty sure that the glow plug controller is a 12 volt relay.

I understand why you want the grid heater tied into the glow plug relay but it seems needlessly complicated.
Why not just use the grid heater as a pre heater for those really cold mornings.
Turn the grid heater on for X number of secons, then off.
Attempt to start the engine, then turn it back on when necessary.

Eventually you will figure out the best ways to operate it and can modify the circuit if necessary.

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U.S. Army Retired

Yes, I'm sure I can figure out the best way to operate it, but this is my wife's daily driver. I want to make this as simple as possible. But......maybe you are correct. Maybe this is needlessly complicated and operating the glow plugs and grid heater simultaneously make not be a problem.

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2006 CRD Sport
GDE FT ECO tune, GDE TCM ECO tune, ARP studs, HDS-001 203F T-Stat, 3.7L nylon fan & Hayden 2905 clutch, Carter P76148M in-tank pump, Racor 245R122 filter head & 2 micron R25S fuel filter, Provent 200, Samcos, Fumoto F-102, Litens 920834A de-coupler, PML rear diff cover, OEM trans pan with welded in bung, JBA UCAs, full skids.

Not sure exactly what's inside our controller but given the level of diagnostics we have available it's almost certainly not a simple on/off relay like the old days. Modern controllers are quite sophisticated and use a PWM signal from the ECM to optimise the glow plug voltage allowing rapid initial ramp up and accurate steady state control. They also provide phased switching of the individual plugs so that no more than one of them switch on or off at the same time thus spreading the load and minimising electrical noise. Finally the module manages monitoring and failure detection passing the results back to the ECM as a serial data stream.

The signal between the ECM and GP module is 5V and will not be able to drive an external relay without additional buffering and signal conditioning to convert the PWM to a steady level.

Here's an example of the type of device that Bosch is likely integrating into the controller module http://www.st.com/web/catalog/sense_pow ... 7/PF151746

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After a year of double and triple glow to start, I replaced glow plugs at 212,000K with 7v etecno drop-ins. To my great surprise all 4 came out with very little effort, fully intact. Thanks to fuel additive for 3 months prior they were very clean. This morning 1 glow and start. This is the way it used to work;)