Reduced Voltage at Glow Plugs

[wallymortar]

I have a 1995 Kubota L2650 with about 1500 hrs. I replaced the glow plugs and contacts and bridge wires are okay. Compression is within spec and fuel delivery is fine. The tractor turns over well but will not start when cold after using the glow plugs. I get smoke and soot but it will not quite fire on any cylinder. If I tow the tractor, it will start easily and it runs strong, i.e. no apparent power issues. Once it is warmed up, it will restart using the starter. There is current at each glow plug when the key is held in the glow plug position. I isolated each glow plug and checked resistance. Each has resistance. I check current at each glow plug and at the disconnected lead wire and the current measures about 8.5V. I consulted several experts and they believe, as I do, that current at the glow plugs should be close to 12V but at least >10V. I had this issue on a diesel truck and the problem was a defective glow plug relay but my Kubota dealer tells me the L2650 has no glow plug relay. There are two electrical items on the firewall in the glow plug circuit. One is called the "starter relay" and the other is called the "timer relay". I am in the process of checking the glow plug wiring from the starter switch terminal to the lead in the engine compartment, but I could use some advice on troubleshooting the issue.

 

[foobert]

I assume when you measured individual resistances, they were all about the same value?

Otherwise, I think you are on the right track with your diagnostics. Almost certainly a bad connection, nearly broken wire, or bad switch contacts. I'd continue tracing the wire to its source, and measure the voltage there.

 

[BruceP]

current at the glow plugs should be close to 12V

Current is not the same as Voltage... but I believe I understand the jist of what you are saying.

My 1980s era Kubota uses a "glow indicator" which is actually a glowing loop of wire to limit the current to the glowplugs. (More heat = more glow = more resistance = LOWER voltage at the glowplugs)

In other words, the voltage at the GPs starts out at full battery voltage... but after several seconds the voltage starts dropping due to the "glow indicator" resistance increasing. It is designed this way to protect GPs from burning out.

In colder temperatures, I have to glow the engine about 90 seconds before starting. (the manual states this)

i mention all of this to ask you..... Does your tractor have a "glow indicator", light-bulb or some other means to indicate the function of the glowplugs?

If I tow the tractor, it will start easily

This suggests that the starter is not spinning the engine fast enough to start it. A diesel engine NEEDS to spin fast enough to generate the pressure/heat required for combustion.

This is not the same as a gasoline engine which can start by barely turning it over in cold weather.

-------------
A couple things you could try.

1. Use jumper wire to put FULL battery voltage on the GPs for perhaps 30 seconds... then try to crank engine to start it.
2. Remove and clean the 3 main electrical connections required for starting. (Battery posts, Big Ground wire on engine, Big wire on starter) Make certain there is SHINY metal before snugging down connection-bolts.

If #1 makes engine start.... suspect the GP circuit
If #2 makes it start... the connections were the problem (and now are fixed)

Also, I have been known to remove starter, take it to bench, take it apart and grease all moving parts. This goes a LONG way to making the engine spin faster. (Old grease adds friction which slows the starter)

 

[wallymortar]

I assume when you measured individual resistances, they were all about the same value?

Otherwise, I think you are on the right track with your diagnostics. Almost certainly a bad connection, nearly broken wire, or bad switch contacts. I'd continue tracing the wire to its source, and measure the voltage there.

Thanks. Yes, resistance measured was nearly the same on each but I didn't write down the readings. Zero resistance would have indicated a burned out glow plug and I was just interested to see if there was resistance and there was.

 

[wallymortar]

Current is not the same as Voltage... but I believe I understand the jist of what you are saying.

My 1980s era Kubota uses a "glow indicator" which is actually a glowing loop of wire to limit the current to the glowplugs. (More heat = more glow = more resistance = LOWER voltage at the glowplugs)

In other words, the voltage at the GPs starts out at full battery voltage... but after several seconds the voltage starts dropping due to the "glow indicator" resistance increasing. It is designed this way to protect GPs from burning out.

In colder temperatures, I have to glow the engine about 90 seconds before starting. (the manual states this)

i mention all of this to ask you..... Does your tractor have a "glow indicator", light-bulb or some other means to indicate the function of the glowplugs?



This suggests that the starter is not spinning the engine fast enough to start it. A diesel engine NEEDS to spin fast enough to generate the pressure/heat required for combustion.

This is not the same as a gasoline engine which can start by barely turning it over in cold weather.

-------------
A couple things you could try.

1. Use jumper wire to put FULL battery voltage on the GPs for perhaps 30 seconds... then try to crank engine to start it.
2. Remove and clean the 3 main electrical connections required for starting. (Battery posts, Big Ground wire on engine, Big wire on starter) Make certain there is SHINY metal before snugging down connection-bolts.

If #1 makes engine start.... suspect the GP circuit
If #2 makes it start... the connections were the problem (and now are fixed)

Also, I have been known to remove starter, take it to bench, take it apart and grease all moving parts. This goes a LONG way to making the engine spin faster. (Old grease adds friction which slows the starter)

Bruce, Thanks. I was trying to condense my diagnosis write-up and glossed over some of the steps I performed. I understand that current and voltage are different. I first checked for current just to be sure something was getting to the glow plugs when I turned the key. This is one of the older setups where you turn the key backwards (opposite the start position) and hold it against a spring. There is no light or feedback showing that the glow plugs are being actuated. Then I measured voltage at each glow plug and that is when I found only about 8.5 volts at each, which I think is insufficient to heat the glow plugs correctly. The battery isn't new but it was fully charged and load tests okay. However, I have a commercial charger with a high start voltage and used that when cranking. I have 5 other Kubota tractors ranging from 1979-2022 plus multiple other diesel tractors and construction equipment, so I'm sure the tractor was cranking sufficiently fast to start had there not been another issue. This tractor doesn't have a glow indicator like you mentioned. My 1979 and 1981 Kubota's have an open coil visible via a hole in the dash. Two newer Kubota's have indicator lights as you noted and the 2022 is all digital and automatic. One thing I don't understand the function of is the device on the firewall called a relay timer. My Kubota dealer parts person couldn't explain it either. This happens to be the only Kubota that I can't find a good wiring schematic for and the parts and service manuals don't have a good diagram either. Since my wiring diagram seems to indicate that relay timer is in the glow plug circuit before juice goes to the glow plugs, I'm thinking that timer is a safety to limit the time current goes to the glow plugs to keep from burning them out and that timer may be failing. Thanks again for your suggestions. I'll keep plugging away at it (no pun intended).

 

[Dave_eng]

Not wise to apply 12 volts to the GP's as they can explode leaving sharp pieces of ceramic inside the cylinder.

Resistance must be very close to 1.5 ohms.

Here is another serious caution regarding testing GP's outside the engine.

The relay timer has to do with stopping the engine. Some engine stop solenoids are powered to make the engine stop. The timer relay makes that happen for long enough for the engine to stop and then cuts the power otherwise the battery would die. Other engines apply power to the stop solenoid to make the engine run.

Dave

 

[whitetiger]

I have a 1995 Kubota L2650 with about 1500 hrs. I replaced the glow plugs and contacts and bridge wires are okay. Compression is within spec and fuel delivery is fine. The tractor turns over well but will not start when cold after using the glow plugs. I get smoke and soot but it will not quite fire on any cylinder. If I tow the tractor, it will start easily and it runs strong, i.e. no apparent power issues. Once it is warmed up, it will restart using the starter. There is current at each glow plug when the key is held in the glow plug position. I isolated each glow plug and checked resistance. Each has resistance. I check current at each glow plug and at the disconnected lead wire and the current measures about 8.5V. I consulted several experts and they believe, as I do, that current at the glow plugs should be close to 12V but at least >10V. I had this issue on a diesel truck and the problem was a defective glow plug relay but my Kubota dealer tells me the L2650 has no glow plug relay. There are two electrical items on the firewall in the glow plug circuit. One is called the "starter relay" and the other is called the "timer relay". I am in the process of checking the glow plug wiring from the starter switch terminal to the lead in the engine compartment, but I could use some advice on troubleshooting the issue.

Here is the switch test from the service manual, you should have battery voltage at the lead.
I included a wiring schematic for the l2650.

 

[GreensvilleJay]

hmm, I thought maybe one plug was bad but according to the diagram WT supplied they're in series, not parallel.
If each plug does measure 1.5ohms, then they should be good. You could unplug the 'extension' wire and test..they should read 4.5 ohms ( 1.5 x 3). If they do ,then that 'extension cable or the connections could be 'bad'. The 3 Gps only draw abot 3 amps ( 12/4.5) so really not a 'load' on a battery. As you only see 8 volts, the other 4 must be lost in bad wiring or perhaps the ignition switch.
simple test... bypass the ign sw. carefully supply battery +12 to the GPs using the wire at the ign sw.and read the voltage. if 12, then it has to be 'bad' switch.