Glow Plug Switch Wiring

[buffumjr]

I have an L175. Its glow plug switch is starting to fail. Instead of giving up Benjamins to get a new one, I'm going to use two old school, waterproof starter switches. Cost, under $10. i just need one detail. Is the indicator coil in series or in parallel with the glow plug circuit? Wiring. What fun. BTW, how do you get the old switch off?

 

[85Hokie]

here is the diagram from the switch to the controller and plugs

 

[buffumjr]

Thank you. I have that, already, in the service manual. The service manual doesn't say what the circuitry is INSIDE the switch. I'm going to be replacing the one switch with two momentary on switches. Of course, I COULD test the present circuit, but the switch has become unreliable. Age.

How do you remove this switch? I can see no nut, there's no collar and screws. ??????

 

[buffumjr]

Easy test. It's 105° here, right now, so I'll wait until tonight. I'll hook up the battery and turn the switch to heat. I'll take all the wires off connector 17. If connector 17 is not "hot", I'll check the black and white wire. If the black and white wire is hot, it's series, as the current would go through the indicator coil, first. If #17 is hot, it's parallel, and both would get current from the switch.

How DO you dismount that switch?

 

[Roadworthy]

The coil serves as a voltage dropping resistor as well as an indicator. It is wired in series with the plugs and the plugs themselves are wired in parallel with each other. The idea is to drop the twelve volt supply to the 10.5 volts the flow plugs want to see. On my Mitsubishi D2350 I used one fifty amp momentary contact toggle switch to power the glow plugs. As I recall you want to figure ten amps per glow plug. I did have some problem with corrosion on the coil. That will prevent any current flow and of course the resultant open circuit prevents the glow plugs from heating.

 

[BruceP]

I know I have seen a "logic diagram" for that keyswitch somewhere. (it may be on the schematic somewhere)

As I recall, there are two 'modes' of operation.

1. The 'glow indicator' is in series with the glowplugs
   • +12 is connected to #19 on diagram above
   • #30 jumpers to #19
2. The glowplugs receive full battery power.
   • +12 is connected to #17 on diagram above
   • #30 jumpers to #17

Ask yourself this question:
"Would #17 even exist if there was no need for the switch to send full +12 to the GPs?"


I work on all manor of equipment several times a week and often have to make my own schematic diagram using ohmmeter.

I have been known to REMOVE keyswitches and use ohmmeter to make my own logic-diagram for them so I can understand operation.

Perhaps, you might consider doing the same. There are only 4 pins on the keyswitch... how complicated can it be?

 

[Dieseldonato]

I know I have seen a "logic diagram" for that keyswitch somewhere. (it may be on the schematic somewhere)

As I recall, there are two 'modes' of operation.

1. The 'glow indicator' is in series with the glowplugs
   • +12 is connected to #19 on diagram above
   • #30 jumpers to #19
2. The glowplugs receive full battery power.
   • +12 is connected to #17 on diagram above
   • #30 jumpers to #17

Ask yourself this question:
"Would #17 even exist if there was no need for the switch to send full +12 to the GPs?"


I work on all manor of equipment several times a week and often have to make my own schematic diagram using ohmmeter.

I have been known to REMOVE keyswitches and use ohmmeter to make my own logic-diagram for them so I can understand operation.

Perhaps, you might consider doing the same. There are only 4 pins on the keyswitch... how complicated can it be?

Kubota uses a glow relay in the newer equipment with a simple light as the glow indicator. The glow plugs are rated for 12volt (nominal) operation. The above amperage should be pretty close to correct, plus factoring in a safty margin at least a 40amp continuous duty relay should be used. There is also a wire that comes off (most) starters to supply the glow plugs with voltage when cranking the engine. A diod is also inline in this circuit to prevent feedback to the starter when the key is in the glow plug only position. I would reccomend the glow coil be replaced with an indicator lamp, and a heavy duty relay be installed, instead of trying to pass that much current through a push button. It's a simple circuit to wire up.

 

[BruceP]

I would reccomend the glow coil be replaced with an indicator lamp, and a heavy duty relay be installed,

in these older Kubota, it is NOT a good idea to feed full 12v to the GPs for very long. That indicator (see ckt above) is INTENDED to drop the voltage to the GPs as it heats up and begins to glow. (more heat = higher resistance)

The voltage to the GPs may start around 11v. Then, the nichrome wire in the indicator begins to heat up and glow...this drops the voltage to the GPs down below 8v.

 

[Edke6bnl]

in these older Kubota, it is NOT a good idea to feed full 12v to the GPs for very long. That indicator (see ckt above) is INTENDED to drop the voltage to the GPs as it heats up and begins to glow. (more heat = higher resistance)

The voltage to the GPs may start around 11v. Then, the nichrome wire in the indicator begins to heat up and glow...this drops the voltage to the GPs down below 8v.

If the glow plug light fails you eill np longer have power to gp?

 

[Dieseldonato]

in these older Kubota, it is NOT a good idea to feed full 12v to the GPs for very long. That indicator (see ckt above) is INTENDED to drop the voltage to the GPs as it heats up and begins to glow. (more heat = higher resistance)

The voltage to the GPs may start around 11v. Then, the nichrome wire in the indicator begins to heat up and glow...this drops the voltage to the GPs down below 8v.

Interesting, I just looked up several ngk glow plugs for voltage reference and they (for my father's l245dt, same set up as above glow plug listed as good for the l175 as well) list it as a 10.5 volt glow plug, the glow plug for my B series is listed as a 11 volt glow plug. (Idiot light and relay as factory) I don't understand why they would need to drop the voltage to 8 volts, on something rated higher.
Now I do know in some instances (7.3l ih) they used a 6 volt glow plug and the glow plug controller supplied 9 volts to facilitate fast start up.
Not really trying to argue, just trying to understand why it would be wired like this, when there seems to be no real need for it.

 

[BruceP]

If the glow plug light fails you eill np longer have power to gp?

YUP, that is how it works.

But be aware that is is not a "light" it is a loop of nichrome wire which has special electrical properties.

 

[BruceP]

Not really trying to argue, just trying to understand why it would be wired like this, when there seems to be no real need for it.

No problem... perhaps I did not explain well enough.

Firstly: You need to understand the physical principal that resistance goes UP with heat.

The GP indicator is a loop of nichrome wire (same metal used as filament in light-bulbs).


With that said, when you initially hit the power to GPs thru that nichrome wire indicator, the GPs see FULL battery voltage for a moment.

Then, as the GP indicator (nichrome wire) begins to heat, its resistance slowly climbs. As this resistance goes up, the voltage at the GPs goes DOWN.

This is a self-regulating circuit which allows the GPs to heat quickly... but at the same time protect the GPs from burning up. The longer you hold the GPs "on".... the lower their voltage gets until the nichrome wire equalizes its resistance.

Keep in mind this is how things were done LONG before there were computer-controlled components. Using basic, physicial properties of nichrome wire, a self-regulating, robust circuit is used.

 

[Dieseldonato]

No problem... perhaps I did not explain well enough.

Firstly: You need to understand the physical principal that resistance goes UP with heat.

The GP indicator is a loop of nichrome wire (same metal used as filament in light-bulbs).


With that said, when you initially hit the power to GPs thru that nichrome wire indicator, the GPs see FULL battery voltage for a moment.

Then, as the GP indicator (nichrome wire) begins to heat, its resistance slowly climbs. As this resistance goes up, the voltage at the GPs goes DOWN.

This is a self-regulating circuit which allows the GPs to heat quickly... but at the same time protect the GPs from burning up. The longer you hold the GPs "on".... the lower their voltage gets until the nichrome wire equalizes its resistance.

Keep in mind this is how things were done LONG before there were computer-controlled components. Using basic, physicial properties of nichrome wire, a self-regulating, robust circuit is used.

Interesting, I wonder if back then they had issues with the quality of glow (not nessisarily quality, but not as good of materials) plugs that nessesitated this.

 

[Russell King]

I have an L175. Its glow plug switch is starting to fail. Instead of giving up Benjamins to get a new one, I'm going to use two old school, waterproof starter switches. Cost, under $10. i just need one detail. Is the indicator coil in series or in parallel with the glow plug circuit? Wiring. What fun. BTW, how do you get the old switch off?

The glow plug switch is also the starter switch so consider that

There is a small screw underneath the bottom of the switch to remove the handle then there is a nut to remove

 

[buffumjr]

There is a small screw underneath the bottom of the switch to remove the handle then there is a nut to remove

OK, the handle is off, but I see nothing resembling a nut. There is a collar. Do I use a channel lock on that?

 

[Russell King]

Yes you can use channel lock pliers on the collar (nut).

Try to protect the collar with leather or similar in case you want to reuse the switch

 

[buffumjr]

Yes you can use channel lock pliers on the collar (nut).

Try to protect the collar with leather or similar in case you want to reuse the switch

Nah. The switch is no longer starting the starter each time. Time to replace. Many thanx for the post. HUGE help.

 

[ve9aa]

Even on my old VW diesels, from many moons ago, the Bosch(sp?) glow plugs were rated at something much less than resting battery voltage. I want to say maybe 10.5V ?

You definitely don't want to bypass that little chunk of nichrome ("glow plug light" <not really a light per se> or you'll find out how many cycles your glow plugs will last at the full 12.8VDC battery voltage (or whatever your battery is at rest (ie: not charging)

 

[buffumjr]

You definitely don't want to bypass that little chunk of nichrome ("glow plug light" <not really a light per se> or you'll find out how many cycles your glow plugs will last at the full 12.8VDC battery voltage (or whatever your battery is at rest (ie: not charging)

Don't intend to. Will wire in series. One 6-32 bolt and two washers, some tape, and good to go.

 

[ve9aa]

Understood. We are waiting with baited breath buffum,jr !