OK... if I could address a couple of mistaken assumptions in this discussion and try to set things streight. To address the original posters question, it is necessary to review both the 6.9 and 7.3 glow plug controller operation in factory trim. Remember that both systems use 6 Volt glow plugs.
The 6.9 - The controller has no real intelligence. This may be an oversimplification, but it relies on a couple of bimetalic strips which form a contact arrangment similar to a turn signal flasher. When you turn the key on, the strip has returned to the cool position, and it energizes the fender relay. As the contact strip heats, it breaks contact, turning the glow plugs off. This cycle repeats briefly as a second contact strip heats, and eventually remains open, preventing any further energizing of the glow plugs. Typical glow times will depend on the ambient temperature. The colder it is, the longer it's going to glow, as that bimetalic strip only bends out of the way at the same temp. The lower the temp - the longer it has to be on to achieve whatever that higher temperature is. So, you may see a glow time in the range of 15 to 18 seconds in severely cold weather. Battery voltage and current also dips in these fridgid temps, extending the glow time further.
The 7.3 - This controller has current sensing technology and makes the following fact it's operating principle - That a hot glow plug draws much less current than a cold glow plug. In fact, it pretty much knows how to keep a glow plug at it's maximum acceptable temperature without burning it out. On a cold start, the controller, using an analog electronic circuit, energizes the glow plug relay. Current is fed to the glow plugs, and they start to heat. While this is going on, the controller is measuring the voltage drop across the Z strip. It's tempting to call this a resistor, and it is, but only to the extent that any piece of wire is actually a resistor. All conductors have an electrical loss component through their lenght. The Z strip is just a handy way to have an easily manufactured piece of wire with a known electrical loss characteristic that is consistent and easily reproduced. The controller will watch the voltage drop across the Z strip, and as the glow plugs warm, the voltages (whatever they may be depending on battery status - the genious part of this) will get closer and closer to being the same. From a technicians point of observance, you may watch this phenomena by measuring the voltage present on the #2 glow plug. At initial cold glow, this voltage will start down around 5 volts, and as the plugs heat, it will slowly climb towards battery voltage minus one or two volts. Once the controller sees this voltage differential at it's optimium, it will begin to cycle the glow plug relay off and on, to maintain that voltage difference. Once it has started to do this, a timer circuit starts that counts down X number of seconds, and when that countdown is complete, the controller will keep the relay off until the next key off/on cycle.
The primary problem with the 6.9 controller, is that a bi-metalic strip may stick, locking the glow plug relay in the on position. Also, a cycling of the ignition key will cause a full glow cycle on potentially already hot glow plugs. Either way, you can eat your plugs in short order. However, the 6.9 controller does not care if you have 8 working glow plugs or only 1. It will cycle them all the same.
The primary problem with the 7.3 controller, is that the predetermined values for voltage differential comparison are corrupted by faulty connections and bad or wrong glow plugs. As resistance in the circuit increases (as in open circuit conditions caused by disconnected or burned out glow plugs), the controller is fooled into believeing that the glow plugs are hotter than they are. This results in the remaining good plugs being starved of "on" time which prevents them from heating properly. Also, the total system resistance is based on BERU brand glow plugs. ANY OTHER BRAND may cause eventual glow plug failure.
Bypassing the 7.3 controller in favor of a push button is an extremely bad idea in my opinion. The 7.3 controller when properly serviced, is an exceptional design that will actually save your glow plugs from failing (assuming you use Beru) and allow them to stay hotter, longer, than any other method.
Bypassing the 6.9 controller with a push button is also risky, but given the known issues with the 6.9 controller, it is something I have done myself. However, I should point out, that I have gone through 3 sets of glow plugs on the Moosestang (which is manual push button at this time) but the Moose Truck has NEVER burned out a set of plugs and is running on the ORIGINAL now 28 year old 6.9 GPC. Likewise, the Scarlet Moose is running the factory 6.9 controller and is outside all the time. Granted we have only had it for a year, so that may not be saying much. I did have to put a set of glow plugs in it when we first got it, but I attribute that more to previous owner neglegence than GPC failure.
My goal is to replace all my glow plug control systems with the 7.3 style controller. In my analysis, this is by far the best design and optimum goal attaintment for trying to get your truck started.
Obviously, glow plugs are a bit like light bulbs, in that they will eventually, even if correctly used and activitated, fail. The 7.3 system is extremely vulnerable to anything less than perfect glow plugs and perfect wiring, with perfect connections. However, all of these things are easy to maintain. What leads to owner frustration, is when multiple points of failure exist, and extensive troubleshooting and guesswork introduces more problems. The relay itself (not the controller) is a wear item as well as the glow plugs, and should be replaced on a preventative schedule, but ONLY with a Motorcraft HIGH QUALITY relay with a disc contactor and not a bar contactor. A common fault among owners is to swap in "any ol" relay. Bar contact type relays will not last more than a few cycles before becoming unreliable in this application. They are designed for starter motors and not glow plug systems. Consider that in 3 cold starts of the engine, this relay may have activated over a months worth of times in comparable starter motor service. Neither the 6.9 or 7.3 system is a place to try and save money. It PAYS to spend for the good stuff.
mines manual the controllers just sitting there. I should try using it without the resistor/heat dissipate'r spring thing
