question on wiring GPs from resistor to 12v terminal block on fire wall

[llong66]

Hello all. i have a quick question with, I hope, a simple answer. after a GP card failure caused a cascade failure in my GPs, I bypassed the resistor on the firewall and wired the relay to the black diamond shaped terminal on the firewall. As I understand the original function of the system, the resistor drops the 24v from the rear battery to 12v for the GPs (AC60Gs) This way, if one GP goes bad, or you get a loose wire, you wont get the cascade failure and fry the rest of your GPs.
My question is, how does this work? If the firewall resistor drops the voltage to the proper 12v to the plugs, unless you get a GP failure/loose wire and the remaining plugs get more voltage than designed for, causing the remaining ones to fail, why does bypassing the resistor to a 12v only source keep the remaining plugs from burning out? It seems to me that once you loose a plug, no matter where your pulling the 12v from, you would eventually get a cascade failure.
What am I missing here?

Thanks much!
Greg

 

[goldneagle]

With the resisters in line each time you loose a GP the voltage goes up. The GPs are also resisters in a way. When all of them work and the resisters are in line they pull 12volts from the original 24 volt circuit. Each time you loose a GP the remaining resistance is lower which raises the voltage going to the rest of the GPs. That higher voltage causes the remaining GPs to start failing do to too much heat from the higher voltage. The more GPs that fail the higher the voltage going to the rest of them. Causing them to overheat and fail.....

I think I got that straight

 

[Barrman]

The 12 volt glow plugs won't fail if they have 12 volts going through them. That is the purpose of the resistor bypass. Another plus with the 60G is that they are self regulating. Meaning they will heat up, turn off, heat up, repeat. At least in theory. Watching one sitting on a vice with the power turned on, they just get red and stay red hot. I haven't had the nerve to keep the power on until it fails.

 

[Richard86]

With the resisters in line each time you loose a GP the voltage goes up. The GPs are also resisters in a way. When all of them work and the resisters are in line they pull 12volts from the original 24 volt circuit. Each time you loose a GP the remaining resistance is lower which raises the voltage going to the rest of the GPs. That higher voltage causes the remaining GPs to start failing do to too much heat from the higher voltage. The more GPs that fail the higher the voltage going to the rest of them. Causing them to overheat and fail.....

I think I got that straight

Actually this is not absolutely correct. But the result is the same.

The reason for the rising voltage at the glow plugs has nothing to do with a resistance drop because of less working GPs. Actually the resistance of the System rises as more GPs burn out.

The problem is the nature of a resistor. Voltage drop after a resistor is depending on the actual current flow.

You calculate it with the formula U=R*I

U = Voltage (drop)
R = Resistance
I = Current flow

The Resistance of the Resistor is constant but every single Glow plug less that is working means less Current flow through the resistor. And less current flow means less Voltage drop after the resistor.

That is why you see a Voltage rise at the GPs when one or more burn out.



Hope I could make it more understandable for you guys and sorry about my English.

Richard

 

[cucvrus]

I have never had an issue with any of my CUCV's with the resistors in place and wired up as designed. I had the same truck 20 years and 173K miles and never even changed the relay or the resistors. I did change glow plugs as needed but never had any failures on any of the other units. Strange little things here and there but never a catastrophic failure. When I put a complete set of wellmans in once that was a different story. Big trouble. I use the stock AC Delco 13 G glow plugs. I have put the heat to them in a vise till they popped. Have a nice day and thank you for the fine explanation of the glow plug ballast resistors.

 

[llong66]

CUCVRUS, When my OEM card went, I lost a whole set of GPs, the were 60Gs