Glow plugs not so glowing

[Selkirk_D]

So my L2850 glow plugs don’t seem to be glowing . When I measured voltage on cyl #3 glow plug , I got 6.0 volts. Seems pretty low. #1 glow plug measures 0.0v. Looking at the wiring diagram someone posted on these forums, it appears the power comes straight off the key switch? No relay? Seems strange. Any ideas?

 

[Russell King]

I don’t see how the voltage would drop from one glow plug to another unless they are wired improperly.

From looking at the illustrated parts list there should be a glow plug indicator (resistor) in line to the glow plugs, between the key switch and the glow plugs. It should reduce the voltage but only down to 11.5 volts or so. It should also glow dimly so you may need to be in fairly dark place to see it.

You should also see voltage going to the glow plugs in the crank position. This path bypasses the glow plug indicator since the starter will normally be dropping the voltage (and the glow plugs should already be glowing from pre-heat). So assuming you have to push the clutch pedal down to get the tractor to crank, you can leave the clutch pedal up so the engine doesn’t crank and turn the key to start and you should be getting battery voltage to the glow plug connection at the top of each glow plug.

The wiring on my older L185 is through a starter switch that is after the key switch so there are some pretty large gauge wires from the battery/starter to the key switch and then onto the glow plug indicator and glow plugs. I only have two cylinders and two glow plugs but they are connected together with a heavy (uninsulated) wire at the top studs and grounded to the block where they screw into the block. The wire to the glow plugs is heavy gauge wire about the same gauge as comes off some aftermarket battery cables (to hook up positive side electrical accessories).

 

[Selkirk_D]

I’m thinking that key switch or a harness connector may have bad contacts in the glow position, which would account for the drop. I’m using a pretty nice digital Fluke meter so I’m pretty confident that it is accurate

 

[85Hokie]

So my L2850 glow plugs don’t seem to be glowing . When I measured voltage on cyl #3 glow plug , I got 6.0 volts. Seems pretty low. #1 glow plug measures 0.0v. Looking at the wiring diagram someone posted on these forums, it appears the power comes straight off the key switch? No relay? Seems strange. Any ideas?

Have you done an Ohm test on each? looking at about ONE ohm each

 

[Selkirk_D]

Have you done an Ohm test on each? looking at about ONE ohm each

Not to that point yet since I’d have to pull the intake manifold to remove the glow plugs.

 

[85Hokie]

Not to that point yet since I’d have to pull the intake manifold to remove the glow plugs.

You do not need to pull intake to take reading.

Take top lead off each ..... test top of plug to ground, should be about one ohm or damn close.

 

[Selkirk_D]

Well testing the plugs means that you have to take them out. And I know from recent experience that you can’t get them out with the intake on. besides there’s an obvious voltage drop before the juice ever gets to the plugs.

So after taking dash apart, I’ve found that voltages are following:

input wire to ignition switch 12.5v
Output of glow indicator: 9.25v
Glow plug connection in engine bay 9.25v (disconnected from plugs)
Input wire of glow ind disconnected 11.5v
resistance through ignition switch .25 ohm
Resistance thru glow indicator .05 ohm.

so I’m losing voltage at the switch, although .25 ohms doesn’t seem like much of a loss, the meter is pushing like 9volts and Milliamps to get that measurement. That value doesn’t mean much when a 10A of current is trying to get through that switch.

I’m thinking switch is bad. Thoughts?

 

[North Idaho Wolfman]

Well testing the plugs means that you have to take them out.

You DO NOT need to take the glow plugs out to do an ohm test.
Disconnect the buss bar and read each individual glow plugs ohms.
I think your getting false readings on OHM's and volts at the switch and wire.

Disconnect the wire at the glow plugs and put your meter to that wire and ground and crank the tractor.
What voltage are you getting?

 

[Selkirk_D]

You’re right , I don’t need to remove plugs to test.

however, starting at firewall, I disconnected plugs from circuit, then the glow Indicator from circuit to take these measurements. So I don’t think the measurements are faulty. I’m losing voltage at switch and at glow indicator in dash. And that’s a fact

once I get the circuit resistance issues worked out, I’ll focus on the glow plugs themselves

 

[Lil Foot]

If your machine is like the B7100, the "glow plug indicator" is actually a current resistor, thus the voltage drop.

 

[Selkirk_D]

If your machine is like the B7100, the "glow plug indicator" is actually a current resistor, thus the voltage drop.

Ya maybe 1 volt, not 3 volts. The current drop in the circuit is causing a 3 volt drop before it gets to the glow plugs.

 

[North Idaho Wolfman]

Ya maybe 1 volt, not 3 volts. The current drop in the circuit is causing a 3 volt drop before it gets to the glow plugs.

I told you how to check for it but for some reason you chose to think I'm full of ....
So best of luck to you.

 

[Hugo Habicht]

Measuring voltage here is not telling you anything. And measuring such low resistances accurately is not easy.

Measure the current through each glow plug individually.

 

[GreensvilleJay]

FWIW ...
3 glow plugs in parallel will draw 30 amps. They're about 1 ohm each, nominal 10 volts from GP ind.

If the switch really is .25 ohms, you'll only get 5 volts to the indicator and glow plugs as 7 volts ( .25 *30 ) is 'lost' at the switch,providing all glow plugs are good.

Disconnect the busbars connecting the glowplugs and measure each plug's resistance. Should be 1 ohm. Be sure to compensate for lead resistance. You can also put a 15 amp ammeter in series, should get 10 amps+-.

Most DVM today can read low ohms, down to 0.1 ohms, IF you subtract lead resistance first.