I've read a couple of different statements on how the relay acts under normal operation. My 92 wasn't starting as well as it should. Checked glow plugs and found one bad. Replaced it with a MC/Beru.
Before replacing bad GP control relay would heat plugs and then cycle on and off. After replacing bad GP it would heat plugs and not cycle. Now it's not starting as well and control relay is cycling again. Makes me think another GP has gone bad. I'm now about 600 miles from home on a hunting trip.

 

93 year info.......other than some fuses mentioned the 7.3 system 87-94 is basically the same.

GLOW PLUG CONTROL
HOW THE CIRCUIT WORKS
The Glow Plug Control circuit applies power to the Glow Plugs, which heat the combustion chambers so that the diesel engine can be started.

Glow Plug Controller
The solid-state Glow Plug Controller, attached to the top of the engine block, controls the Glow Plug pre-glow and after-glow time. It also controls the circuit’s operation by sensing engine temperature, Glow Plug voltage and after-glow voltage
from the start/run circuit.

When the Ignition Switch is turned to START or RUN, voltage from Maxi-fuse K is applied through the Glow Plug Controller to the Wait-To-Start Indicator Lamp.
The Glow Plugs heat up in zero to fifteen seconds, depending on engine coolant temperature. After the Glow Plugs heat up, the controller cycling switch opens and turns the Wait-To-Start Indicator Lamp off. The Glow Plugs are now warm enough for the engine to be started.

At the same time the Ignition Switch is turned to RUN, voltage from Maxi-fuse K is applied to the after-glow timer (located inside the Glow Plug Controller). The after-glow timer cycles the Glow Plugs for up to two minutes, depending on engine temperature. The after-glow timer then opens. The Wait-To-Start Indicator Lamp will not light during the after-glow period.
If the Ignition Switch is turned OFF, it can be turned to ON immediately, and the Glow Plug heating cycle will start again.

Diesel Start/Run
The diesel engine uses two batteries to provide extra power for starting and Glow Plug heating. Power is applied from the batteries, through heavy gauge wires, to the Starter Solenoid (located in the Starter Motor assembly). When the Wait-To-Start Indicator goes out, the Ignition Switch can be turned to START.

With the Ignition Switch in START or RUN, voltage is supplied to the Fuel Heater, Fuel Shutoff Solenoid, and the Engine Temperature Switch through Maxi-fuses K and U.
The Fuel Heater is in the Fuel Filter/Separator. It heats the diesel fuel, melting any wax that might clog the filter. The heater has an internal thermostat to turn it on as needed at 32*F.

The Fuel Shutoff Solenoid controls the flow of fuel into the injection pump. With the Ignition Switch in START or RUN, the Solenoid is energized, and fuel is allowed to flow into the injection pump. When the Ignition Switch is turned off, the solenoid is deenergized, fuel flow stops, and the engine stops running.

The Engine Temperature Switch provides voltage to the Cold Timing Advance Solenoid and the Cold Idle Solenoid. When the engine temperature is below 112°F (44°C), the Engine Temperature Switch is closed. When the Ignition Switch is turned to START or RUN, the solenoids are energized, advancing injection pump timing and engine idle, allowing the engine to run more smoothly when cold. When the engine temperature reaches 112°F (44°C), the Engine Temperature Switch opens. This deenergizes the solenoids, returning the timing and idle to normal.

Can you plug in the block heater for an hour before starting.......

Check the fuses in the engine compartment box. Check for a very good ground on the controller.........

 

Thanks for the excellent run through. I'll have to print that out when I get home.

 

what tells the glow plug relay what the engine temp is?

 

83 thru 86 trucks have the old style system.
The glow plugs are controlled by a timer and a coolant temp switch.
Old style controller


87 thru 94 trucks have the new style system, it uses resistance feedback from the glow plugs to time the glow plug cycle.
New style controller


This is why glow plug questions need a model year listed with the question.
The systems are completely different in the way they operate and the components and locations are different.

Old style system, spade connectors on the glow plugs.
New style system, bullet pin connectors on the glow plugs.

 

Mine used to cycle on and off when there was 2 GPs out. But when there was 4 burned out last year, it wouldn't cycle, but neither would it turn the GPs on long enough to heat the combustion chamber.

 

Just finished checking for bad GP's. One bad. Found a couple at the I-H dealer.
Sure glad I found this place and for the people who provide their knowledge so willingly!

 

Back home now.
Changed out bad GP in Montana. After changing out the bad one the relay wasn't cycling on and off.