resistance spec for LB1914 glow plugs?

[dbotos]

Didn't see it in the service or owner's manual. Service manual had a line drawing of how to check it, but didn't give a value. P/N is E5205-65512 (replaced E5205-65511).

 

[Patches]

I've been told so long as you measure any resistance in the plugs, they are good. If you get a "O" ohm reading, they are open and not functional. Also, remove/disconnect the bridge between the plugs, so as to check each plug individually

 

[ritcheyvs]

A good glow plug is less than one ohm when cold. That's lower than you can accurately read on an ordinary ohm meter. But the resistance goes up substantially as the plug heats up to about 800 degrees. A healthy glow plug will stabilize at about 8-10 amps a few seconds after it's connected to 12 volts. About all an ohm meter can tell you, for sure, is if it'a an open circuit or high resistance (almost open circuit). So there is no useful ohmage spec.

 

[dbotos]

Thanks. Perhaps that's why they don't give a value. I do have a clamp-style DC ammeter, so I could check the current as you described.

 

[ritcheyvs]

Thanks. Perhaps that's why they don't give a value. I do have a clamp-style DC ammeter, so I could check the current as you described.

That is the ideal way to test the plug. Older/cheaper clamp-on meters only measured AC but new hall-effect probes do DC too. I wish I had one. They are particularly useful for cars. tractors, boats, etc. with high-current DC systems.

 

[dbotos]

Figured I'd check the cold resistance just for grins while I had the wiring off the glow plugs. All three measured 1.0 ohm from the threads on the glow plug to a threaded hole in the head just forward of the front glow plug. Used a Klein MM100 multimeter on the 200-ohm setting (0.1 ohm resolution - lowest of the resistance ranges on it). Manual says accuracy of the resistance measurements is +/- (0.8% + 4 digit)...

+/- (1.0 x 0.008 + 0.4) ohms
+/- (0.008 +0.4) ohms
+/- 0.408 ohms

R = 1.0 +/- 0.408 ohms
R = 0.592 --> 1.408 ohms

So as ritcheyvs said, not the most accurate measurement (~41% accuracy).

Planning to measure current this afternoon. The Extech 38389 has a +/- (2.5% + 0.6 A) accuracy when measuring 0-70 A DC, so for 10 A, that would be +/- 0.85 A or 8.5 %.

 

[ritcheyvs]

I can't comment on your specific meter, but normally those percentage specs (e.g. 0.8%) are percentage of full scale, not percent of the reading.

When you first slap 12v on a cold plug, the current will spike to about 20 amp and then decrease exponentially (over a few seconds) to a steady-state value, probably around 8 amp.

 

[dbotos]

Didn't have an FS or full scale callout, so I was assuming percent was relative to reading. 0.8% of 200 ohms is 1.6 ohms, which would be pretty horrendous on a 1.0 ohm measurement, especially when you tack on the 0.4 for a total of +/- 2 ohm.

 

[k0ua]

I've been told so long as you measure any resistance in the plugs, they are good. If you get a "O" ohm reading, they are open and not functional. Also, remove/disconnect the bridge between the plugs, so as to check each plug individually

You have that just backwards. They should be near 0 Ohm's. Slightly less than 1 ohm . open circuit is infinity. Not 0. Anything less than 1 ohm is difficult to measure on most meters. A piece of copper wire is near 0 Ohm's as well. Some call that a dead short. A piece of wire across a battery will draw a lot of current. Make a lot of heat and light, and maybe some smoke. Don't ask how I know these things.

 

[dbotos]

Here are the currents I measured yesterday afternoon:

front: 15 A initial, 8 A settled
middle: 14.6 A initial, 7.6 A settled
rear: 14.9 A initial, 7.8 A settled

FYI - the little hex nuts that hold the wiring on are M4 x 0.7 (in case you happen to drop one and then can't find it). I did come up with a technique to help avoid that:



Essentially something to hold the nut until it catches the threads when installing or to catch the nut when it comes off the end of the stud when removing.

 

[k0ua]

Here are the currents I measured yesterday afternoon:

front: 15 A initial, 8 A settled
middle: 14.6 A initial, 7.6 A settled
rear: 14.9 A initial, 7.8 A settled

FYI - the little hex nuts that hold the wiring on are M4 x 0.7 (in case you happen to drop one and then can't find it). I did come up with a technique to help avoid that:

View attachment 457767

Essentially something to hold the nut until it catches the threads when installing or to catch the nut when it comes off the end of the stud when removing.

Thanks for the report back. I think measuring the current draw is the best way. Others have reported holding a IR thermometer with the spot on the glow plug as a quick and dirty way to see if each is working as a fair method if you cannot measure large DC currents.

 

[ritcheyvs]

I would call those good numbers and healthy plugs. You have the right meter for the job; the initial current (15A) would blow the fuse in the typical consumer multimeter (10A max). Good tip. Dropping any small part into dirt, grass, or gravel is extremely frustrating (and that ain't speculation).

By the way, if you ever remove glow plugs, be extremely careful not to break them. Go slow; use lots of patience and penetrating oil. If you break a plug you generally have to pull the head and take it to a machine shop to get the broken part out. Also, if you ever remove glow plugs, clean out the carbon out of the hole with a wire brush. If you shoot, bronze gun-cleaning brushes work well.

 

[k0ua]

I would call those good numbers and healthy plugs. You have the right meter for the job; the initial current (15A) would blow the fuse in the typical consumer multimeter (10A max). Good tip. Dropping any small part into dirt, grass, or gravel is extremely frustrating (and that ain't speculation).

By the way, if you ever remove glow plugs, be extremely careful not to break them. Go slow; use lots of patience and penetrating oil. If you break a plug you generally have to pull the head and take it to a machine shop to get the broken part out. Also, if you ever remove glow plugs, clean out the carbon out of the hole with a wire brush. If you shoot, bronze gun-cleaning brushes work well.

Good tip, I will try to remember the bore brush idea.:thumbsup:

 

[dbotos]

Good tip, I will try to remember the bore brush idea.:thumbsup:

Just don't turn the handle counterclockwise. I learned that sweeping the chimney one time when I wasn't thinking.

If anyone needs a more reasonably-priced option for measuring glow plug current, I bought a Techman 20 A DC analog panel ammeter on eBay for $12 shipped (I use it on my electrolytic rust removal tank). It's internally shunted and has screw terminals on the back. Here's a Dorman 20 A pushbutton that would work in the test circuit:

Dorman Products - 85935