Should glow plugs receive any voltage while engine is running?

[KYKub]

I have a glow plug dash light malfunctioning. Flickers, then goes off while key is in the "heat" position. I used a multimeter to verify that glow plugs do receive 12v while the key is in the heat position, regardless of whether or not the dash light is on. However, I did notice that when I started the engine there was still a steady 2.8V going to the glow plugs. After engine stop, voltage to glow plugs slowly returns to zero (about 3 minutes).

I would have thought glow plugs would be full 12V while heating or 0V when not heating. Is there any reason 2.8V would be applied while running? Or, is 2.8V so low that it's incidental and not enough to cause the plugs to heat?

Thanks for any assistance. Jeff

 

[MrGiggles]

Not sure about your specific engine, but its common for them to operate for a short while after a cold start.

 

[Qapla]

Do you have to hold your key in the "heat" position or is the heat cycle automatic, one that comes on when you turn the key to on and then wait for the light to go off before starting?

 

[KYKub]

Thanks, MrGiggles and Qapla. I didn't run the engine long while measuring voltage. I'll give that a try and see if it stays steady at 2.8V. I probably ran it for only a couple of minutes.

Qapla, the "heat" position is between "off" and "start." You turn to heat, hold, then turn farther right to start. (Kubota)

 

[oldnslo]

Could 2.8 Volts be stray voltage? Seems like an odd voltage on a 12 VDC system

 

[flyerdan]

It's probably thermoelectric effect. That would explain the voltage from the glow plugs when being heated by the engine. It will be low amperage and not go past the relay, so it shouldn't create any problems. The flickering indicator light is probably a bad connection somewhere, I'd check and make sure that the circuit for it originates on the battery side of the relay, and get it working correctly to warn you if for some reason the relay continues providing high amperage power to the glow plugs when it shouldn't.

 

[Texasmark]

I have both kinds, aka turn the key from ON to Start and half way there the dash lights (oil and gen) go out signaling the thermostart is getting voltage where you stop and count out your warm up time, and then on over to START where it's still getting voltage. Releasing the key to spring loaded On position cuts the voltage to the device......no longer needed.

The Branson has a start inhibit relay (the relay that applies power to the glow plugs) in that when you go to ON and are below a certain temp, the glow plug light comes on and stays on till something happens...timer, plugs get to some temp, sensor gets to right temp, I don't know but when the light goes out you can start the engine.

Once the engine starts, there is no reason for any voltage to be applied to the plugs as their purpose is to heat a cold chamber and if the engine is running the chamber sure-nuf tain't cold.

On thermoelectric effect, I don't know what that is. If you are talking about a bimetallic thermocouple, that's one thing. But a tungsten (assumption) wire that is cooling down and generating a voltage across it while it does, doesn't make a lot of sense to me.....where's the mechanism? However, if you were on the ohms scale and are talking about resistance buildup, I can buy that since the heat of combustion could heat the plug wire and increase it's resistance slightly....not near what it would be when glowing, but noticeable. Just brainstorming this; no fact, just assumptions on the part of the thermoelectric effect thing.

 

[Coyote machine]

I have a glow plug dash light malfunctioning. Flickers, then goes off while key is in the "heat" position. I used a multimeter to verify that glow plugs do receive 12v while the key is in the heat position, regardless of whether or not the dash light is on. However, I did notice that when I started the engine there was still a steady 2.8V going to the glow plugs. After engine stop, voltage to glow plugs slowly returns to zero (about 3 minutes).

I would have thought glow plugs would be full 12V while heating or 0V when not heating. Is there any reason 2.8V would be applied while running? Or, is 2.8V so low that it's incidental and not enough to cause the plugs to heat?

Thanks for any assistance. Jeff

Seems possibly 2 seperate issues, maybe connected. Check what your exact switch positions are: mine are: off, accessories, glow, crank to start. In glow one can leave the key there and wait for the light to go out, OR can go back to glow, and be held there, with pressure against the spring mechanism to reglow the plugs when the tractor is very cold.
(Note: section in blue above is rewritten to make a correction to what it said earlier- see my next post to understand why...)

Your's sounds like the relay circuit is not working correctly, causing the light to flicker, then go out, when it should be staying on until the timer releases the relay and the plugs shut down. The 2.8 Volt anomaly is likely part of the relay/timer glow circuit malfunction.
Find out how exactly it's supposed to work on your tractor and tell us that, so we can help further troubleshoot it.

Model, # hours on the clock, battery and cables, to/from condition, etc., please. HST or shuttle/stick?

 

[PILOON]

I would say that no voltage should be available to the glow plugs when the engine is running.
At least with any tractor whereby the operator has to manually select a heat position before he can engage the starter.
Think of it, you have to hold a spring loaded switch B4 you can even engage the starter.

 

[Coyote machine]

I would say that no voltage should be available to the glow plugs when the engine is running.
At least with any tractor whereby the operator has to manually select a heat position before he can engage the starter.
Think of it, you have to hold a spring loaded switch B4 you can even engage the starter.

You may be right about the no voltage after engine having started; I may have mis-spoken about the actual sequence in my earlier post, and now I'm rethinking it. I will go back and edit it to reflect more accurately what happens.

 

[KYKub]

Thanks for the replies. The machine is a Kubota G1900, 3 cylinder D722 engine. I posted in this area because I figured glow plug voltage would be pretty much standard across tractors. The correct key switch order is: 1) "lightning icon" which illuminates dash lights, 2) glow plug heat, 3) start/run.

Did some additional testing today. I have two G1900 machines so I checked voltage on glow plug operation on the second machine. 0 volts> turn key to glow plug heat position = 10V> start tractor = back to 0V and stayed there.

So, it seems that voltage should drop to zero volts during operation. It appears unlikely that thermoelectric effect would be present in one tractor but not its twin.

I let the engine run enough to get good and warm (on the original tractor in question) and monitored voltage to glow plugs. It dropped to 2.3V and seemed to stay there.

Would 2.3V indicate enough current to be concerned about?

Thanks again for the good replies.

 

[skyhook]

It all depends, if its been like that for a while, it doesn't seem to be hurting anything, but "I" would be concerned. As you found out, there should be 0 volts when running, these glow plugs take allot of juice and my concern would be battery drain. What made you decide to check their voltage?

 

[PILOON]

Low voltage vs the high suggests leakage of some sort. Could be as simple as a wet wire harness or corrosion.
While you can read a low voltage I bet there is minimal amperage.

To demonstrate an example, wet your fingers and grab the voltmeter probes in a low resistance setting and you will get a reading.

And yes battery drain could result over time.

 

[zzvyb6]

"There is a timing module with 2 temperature inputs: ambient and engine. If the temps add up to a need for glow plugs, an indicator light comes on for up to 8 seconds. Glow plugs are lit and remain fully powered for up to 30 seconds to ensure the engine will remain running. When the engine reaches a preset temperature, the glow plugs are turned off."

Sounds like a few possibilities for you. A bad ambient temp sensor, a bad engine sensor, a bad timing module, wrong timing module (there have been 3 used since early models, or a retest in warm weather with the engine warmed up.