Glow plug resistors

[Kellyboy]

While trying to diagnose a hard start problem I decided to bypass the glow plug resistors. Before doing so I took a voltage reading on it. I found that it only had 12V feeding it from the first battery in series. I believe it should be fed with 24v (correct me if I'm wrong). The wierd thing is it still has 12v on the down stream side. Pulled it off and checked resistance from end to end and got perfect continuity. It doesn't seem to have been modified in any way (see pic). If the unit was burned up shouldn't there be a dead short or loss of continuity. So confused. Either way the relay is now jumped to the 12v fusible link above it. Should I run a separate line to the front battery or is it fine feeding from the fused terminal?

 

[MarcusOReallyus]

While trying to diagnose a hard start problem I decided to bypass the glow plug resistors. Before doing so I took a voltage reading on it. I found that it only had 12V feeding it from the first battery in series. I believe it should be fed with 24v (correct me if I'm wrong).

You are correct.

The wierd thing is it still has 12v on the down stream side.

Unless the glow plugs are activated, there is no current flowing. If there is no current flowing, the resistor will not drop the voltage. You should see the same voltage at both ends when the glow plugs are not activated. You will only see a difference while the glow plugs are drawing current.

Pulled it off and checked resistance from end to end and got perfect continuity.

Meaning zero ohms? Are you using a digital meter or an analog meter? The resistance is very small, so you can easily misread it on an analog meter.

If you are using an audible continuity check, it will probably beep even when the resistors are fine. It's only .28 ohms.

It doesn't seem to have been modified in any way (see pic). If the unit was burned up shouldn't there be a dead short or loss of continuity. So confused.

Usually failure mode is an open, or high resistance. For it to become a dead short would be unusual.

Either way the relay is now jumped to the 12v fusible link above it. Should I run a separate line to the front battery or is it fine feeding from the fused terminal?

Yes.

Meaning, either way works. Plenty of people have just moved the input to the GP relay from the resistor to the diamond shaped terminal block right above it, and have used it like that for years with no problems. Short and sweet.

I took the direct to battery approach. You can read about it in the "Helpful Threads" sticky.

 

[Kellyboy]

You are correct.




Unless the glow plugs are activated, there is no current flowing. If there is no current flowing, the resistor will not drop the voltage. You should see the same voltage at both ends when the glow plugs are not activated. You will only see a difference while the glow plugs are drawing current.




Meaning zero ohms? Are you using a digital meter or an analog meter? The resistance is very small, so you can easily misread it on an analog meter.

If you are using an audible continuity check, it will probably beep even when the resistors are fine. It's only .28 ohms.




Usually failure mode is an open, or high resistance. For it to become a dead short would be unusual.




Yes.

Meaning, either way works. Plenty of people have just moved the input to the GP relay from the resistor to the diamond shaped terminal block right above it, and have used it like that for years with no problems. Short and sweet.

I took the direct to battery approach. You can read about it in the "Helpful Threads" sticky.

10-4 ! I did not know resistors only work under load but it would explain why the voltage would drop to 6v when glow plugs were on. Just kinda figured they pulled that much load to cause a voltage drop. Thanks Marcus. Much appreciated.

 

[Kellyboy]

Oh yeah, and I was using a good fluke digital meter. The resistance was only fluctuating between .01-.02

 

[Rvitko]

I had a similar low resistance issue when I got mine. It cost me a set of glow plugs. Try taking it off the firewall, rinse it in distilled water after using an old paint brush or cleaning brush to get off what you can and then let it dry, mine was caked in mud and after it was clean resistance returned to .26 ohms from .17 Ohms started with.

 

[Kellyboy]

Bypassed the resistor(don't plan on ever using the slave) @30 degrees this morning the glow plugs only came on for a few seconds and it fired right up like it was a warm engine. Problem solved and potential future problem eliminated ahead of time. Happy camper here thought I had a fuel problem.

 

[Tinstar]

I had a similar low resistance issue when I got mine. It cost me a set of glow plugs. Try taking it off the firewall, rinse it in distilled water after using an old paint brush or cleaning brush to get off what you can and then let it dry, mine was caked in mud and after it was clean resistance returned to .26 ohms from .17 Ohms started with.

That's good to know, especially with mud daubers everywhere now.

 

[Tinstar]

Bypassed the resistor(don't plan on ever using the slave) @30 degrees this morning the glow plugs only came on for a few seconds and it fired right up like it was a warm engine. Problem solved and potential future problem eliminated ahead of time. Happy camper here thought I had a fuel problem.

Glad you got it sorted out!

 

[MarcusOReallyus]

10-4 ! I did not know resistors only work under load but it would explain why the voltage would drop to 6v when glow plugs were on. Just kinda figured they pulled that much load to cause a voltage drop. Thanks Marcus. Much appreciated.

Thar ya go!


Think of it like plumbing. Let's say you have a 3" pipe running horizontally. There's a water source on the left end, and a valve on the right end. In the middle is a short section of 1/2" pipe.

Let's say that water source is a tank mounted 30 feet up in the air, so we have some pressure on the system. Measure the pressure in the left side of the pipe, and let's say it's 25 psi. With the valve closed, the pressure in the right side will be the same. No difference.

Now open that valve so the water can flow out (like turning on the GPs). The pressure in the left side will stay pretty close to the original 25 psi, but the pressure in the right side will drop very low.

That 1/2" pipe that divides the 3" pipe is like a resistor - it impedes the flow of the current. When there is no current flowing (the valve is closed), the pressure equalizes on both sides of the 'resistor'. When the valve opens, the pressure (voltage) is no longer equal. If course, it takes a few seconds for the pressure on the right side to adjust. You can see it happening. With electricity, this same "adjustment" takes place at (almost) the speed of light, so you'll never see it.

Does that help?

 

[MarcusOReallyus]

mine was caked in mud and after it was clean resistance returned to .26 ohms from .17 Ohms started with.

You mean, like this?

http://www.steelsoldiers.com/showth...-GOOD-thing!&p=1657647&viewfull=1#post1657647