Yet another glow plug question

[grw800]

I have literally spent hours reading previous threads on glow plugs, and I'm still puzzled about something. The document in antennaclimber's GP Module Theory of Operation sticky says this:

Glow Plug Protection.
If the voltage on the glow plug side of the relay exceeds approximately 12.5 volts, the GPM will cycle the glow plug relay on and off to protect the glow plugs. Note, if the glow plugs receive power thru the resistor and one or more glow plugs go high in resistance then the remaining glow plugs will see a higher than normal voltage applied to them. Higher than normal voltage will damage the remaining good glow plugs resulting in a shorter than expected life.​

If the GP module opens the GP relay when the voltage exceeds 12.5 volts, how is it possible for the glow plugs to see higher than normal voltage?

 

[antennaclimber]

The GP module samples the voltage on the GP side of the relay, if the voltage is above approximately 12.5 volts the GP module will open the relay. The sample voltage drops to zero and the GP module starts the sample process again. This is why the GP relay cycles the power to the plugs on and off. It does not open the relay and keep it open.

At the same time all of the above is happening, the GP module is processing the amount of time that it needs to keep the wait light on along with the amount of time and repetition rate that the GP relay is closed. This is done with RC timing and the internal logic of the components.

Now with all this being said, the GP's can have a greater than 12.5 volt source applied by a few ways.
1. The dropping resistors are shorted, hard to do but possible.
2. The charging system has malfunctioned and is applying more voltage than designed.
3. One or more of the GP's have gone to a high resistance or open state, this will change the characteristics of the voltage divider that the resistors and GP's use to maintain a nominal 12 volt source to the glow plugs.

Applying a higher voltage to the glow plugs will cause them to become hotter faster. The glow plug module keeps the plugs from seeing this higher voltage to a minimum thus keeping them from over heating.

I hope this helps you understand how it works.
Karl

 

[grw800]

OK, I understand how the source voltage can be higher than 12.5 volts, but if the GP module is functioning correctly, would it not prevent the glow plugs from receiving the higher-than-normal voltage?

Just today I replaced all 8 glow plugs which had gone to an open state. The truck was not starting, and the GP module was cycling quickly. I installed new 60G glow plugs and bypassed the resistor, running 12V to the GP relay. It's starting better than ever now; I just like to fully understand this stuff.

 

[thedeliverator]

OK, I understand how the source voltage can be higher than 12.5 volts, but if the GP module is functioning correctly, would it not prevent the glow plugs from receiving the higher-than-normal voltage?

No; the voltage to the plugs is determined by the resistor bank, plug resistance, and battery voltage. The GP module has no way to 'know' what the voltage will be until it closes the relay and completes the circuit.

 

[grw800]

No; the voltage to the plugs is determined by the resistor bank, plug resistance, and battery voltage. The GP module has no way to 'know' what the voltage will be until it closes the relay and completes the circuit.

Right. But once the circuit is complete, the GP module will sense the voltage going to the glow plugs, and will cycle off if the voltage exceeds 12.5. So I can see that the glow plugs might see a higher than normal voltage momentarily. Is that enough to damage the glow plugs? Or perhaps it's the rapid cycling that causes damage?

 

[antennaclimber]

You would have to ask the glow plug mfg what the plugs were designed for.

The monitor part is to prevent the glow plugs from being destroyed all at once by a higher than designed voltage applied to them. Allowing them to cycle gives them time to cool between voltage applications.

A simple analogy: It's like drinking a very hot beverage, taking smaller sips with some time between sips to allow your mouth to cool is much better than taking one large gulp where your entire mouth will get burned.
Karl

 

[thedeliverator]

I suppose it would be possible (but not easy) to have a high enough voltage such that the plugs would fail in the short time they cycle on, mostly depending on the type and condition of the glow plugs. The idea is that the GP module cycling when the voltage is high will prevent damage to the plugs by giving them some time to cool between cycles. I can't think of how the module cycling would cause damage, any more than turning a light bulb on and off damages it.

As a bonus, the noise of the relay switching on and off lets you know something has gone wrong. You don't get that with a manual button.