Glow Plug Voltage

[gyroman123]

I have a 70ish kobota L285 and it takes awhile to start, i checked the glow plug voltage and it reads 6volts.
Is that normal or do i have a problem? Thanks for any reply

 

[JWR]

The answer is maybe -- which needs explanation. The glow plugs draw a lot of current so when you turn them on there is considerable drop in voltage. I suggest that you disconnect all the glow plugs from the buss that feeds them. Then try the switch and turn on the voltage for glow plug operation. That should give you the full battery voltage = 12v. at the glow plug bus, open circuit. I assume the L285 is a 12v system. Then ohm out each glow plug -- they should be very low resistance, on the order of an ohm or a fraction of an ohm probably. If any are "open" (e.g. showing very high resistance) then of course that is a bad glow plug.

It is quite possible that when you turn on the glow plugs the large current going through the wiring to them causes a voltage drop of 6v across the plugs and 6v across the rest of the wiring. I don't know, but you should run the test I described and go from there.

 

[Tinhack]

Six volts at the glow plugs sounds about right. The "Glow Plug Controller" on the dash is a HUGE resistor. In the dark, you can usually see it glow when you turn the key to the left. If your glow plugs aren't getting warm, check the connections at the controller. Those typically corrode and the resistance goes up.

Also check your battery voltage. If it reads 12.0v or less, it's discharged.

 

[Roadworthy]

The brand of your glow plugs can be instrumental in learning the exact voltage. Most NGK plugs take 10.5 volts. The older tractors used a dropping resistor in a kind of indicator on the dash. The terminals would corrode further dropping the voltage. If you have one of these you can try cleaning the terminals or replacing it. Normally the chrome cover unscrews and the unit drops through the dash.

 

[CobyRupert]

Six volts at the glow plugs sounds about right. The "Glow Plug Controller" on the dash is a HUGE resistor. In the dark, you can usually see it glow when you turn the key to the left. If your glow plugs aren't getting warm, check the connections at the controller. Those typically corrode and the resistance goes up.

Also check your battery voltage. If it reads 12.0v or less, it's discharged.

Why wouldn’t they just deliver 12volts to the glow plugs? Why would they have a “glow plug controller” that really is a resistor (heater) that heats the air at the dash?

6 volts at the glow plug seems really low even if significant current is causing voltage drop in the wiring before it gets to the glow plug. Are two plugs connected in series?

Glow plug is just a resistor. Their fixed resistance will determine the current draw in accordance with Ohm’s Law and what voltage is supplied. This current across this resistance will determine the power (wattage/heat) they give off. They’ll take any voltage, unless the voltage is so high it breaks down insulation, or creates a current so high the heat generated burns things up.

With many resistors, their labeled wattage is based on an assumed voltage. This is true for incandescent light bulbs, heater elements, etc.. too. They’re not always operated at this voltage and will have a different wattage output.

 

[Tinhack]

Why wouldn’t they just deliver 12volts to the glow plugs? Why would they have a “glow plug controller” that really is a resistor (heater) that heats the air at the dash?

12 volts would probably burn the glow plugs out. Applying 12v to a 6v bulb will probably burn those out too. My Kenbota has the same design. I've never checked the voltage applied to the glow plugs because they've always worked. My hour meter makes a tick sound about every three seconds. For cold starts, I hold the key on preheat for 5-6 of those ticks. If it's really COLD, I hold it for 8-10 ticks. It's always started. Why fix it if it ain't broke.

 

[Tinhack]

Forgot to mention:
There's many reasons why a design is done one way and not another. It could be anything from the cost of the filaments to breach of other patents. The design works. As Roadworthy mentions, clean all the contacts--From the ignition switch to the glow plugs.

 

[JWR]

Six volts at the glow plugs sounds about right. The "Glow Plug Controller" on the dash is a HUGE resistor. In the dark, you can usually see it glow when you turn the key to the left. If your glow plugs aren't getting warm, check the connections at the controller. Those typically corrode and the resistance goes up.

Also check your battery voltage. If it reads 12.0v or less, it's discharged.

I will nitpick Tinhack's (good) post about only one thing -- A lead storage battery for 12v systems is never going to read higher than 12v and is NOT discharged just because it reads 12v. The only time the voltage reads over 12v at the battery terminal is during charging when the alternator shoves a higher voltage over there allowing the battery to charge. Standing alone it will rarely read less than 12v except when one cell or more is totally dead. A "bad" battery (or for that matter one severely discharged) typically reads 12v until you put load on it. That's what a load test is about at your friendly shop or battery store.

 

[number two]

A fully charged 12 volt battery will read 12.6 volts.
6 cells @ 2.1 volts per cell.
A 6 volt battery will read 6.3 volts(3 cells @ 2.1 volts each).
Charging voltage will read 14 volts +/- a bit.

Not sure if this helps the glow plug question.
Good Luck!

 

[Fallon]

I will nitpick Tinhack's (good) post about only one thing -- A lead storage battery for 12v systems is never going to read higher than 12v and is NOT discharged just because it reads 12v. The only time the voltage reads over 12v at the battery terminal is during charging when the alternator shoves a higher voltage over there allowing the battery to charge. Standing alone it will rarely read less than 12v except when one cell or more is totally dead. A "bad" battery (or for that matter one severely discharged) typically reads 12v until you put load on it. That's what a load test is about at your friendly shop or battery store.

A fully charged "12v" lead acid battery produces 12.6v, possibly a bit more if it was charged recently. If it's discharged below 12v it degrades battery life. Charge voltage is about 13.2v.

AGM batteries are a bit higher, lithium batteries are around 14v.