- This topic has 21 replies, 11 voices, and was last updated 12 years, 6 months ago by
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Well, that depends on the meter used – some meters will show zero ohms when the connection is just really low resistance, others will show zero ohms when the circuit is open. Best to use a meter that is definitive enough to tell the difference between 0.1 ohms and 1 ohm or 100M ohms and infinite (open circuit).
Resistive heating elements such as glow plugs probably have a few ohms resistance when good and infinite resistance when burned out (open). A heap meter may not be sensitive enough to tell a low resistance. Pull one of them out of the head and hook it to a battery and see if it heats up. Hard to go wrong that way.
The 40 amp relay mod is easy. Get a single pole 12 volt relay at the auto parts store and wire it so that the actuator (coil) is hooked to the key switch glow plug wires in place of the glow plugs themselves. Run a #8 or #10 wire from the battery positive terminal on the starter to one of the relay's switch contacts and from the other switch contact to the glow plug buss. Now when you turn the key switch to the glow plug position it will activate the coil in the relay to close the heavy duty contacts that will supply power directly from the battery to the glow plugs through wire that is heavy enough to carry the current load without having excess voltage drop.
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a infinite reading will not pop a fuse but it will not heat either if you have infinite readings on the glow plugs and are still poping fuses you have more than on issue.if the golw plugs are reading close to 0 ohms they are internally shorted and that will pop a fuse.I seem to think that good glow plugs are less that 20 ohms so there is not a big window.
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Allow my to stick my nose in here.
One day in the shop one of my employees was testing three glow plugs. He was using meters for resistance tests etc. Anticipating his answers I asked how many ohms resistance should we have. He did not know. So I asked what the point was to the test. He did not know other than to see if they worked. As the employer I knew I had about $100 in labor in just his testing and fooling around with the glow plugs for 1/2 a day.
Here's what I posted on the shop wall:
Proper test of glow plugs.
Use a known charged 12V battery.
Connect a wire of sufficient gauge to carry 15 amps to the positive post and a wire to the negative post.
Touch the positive wire to the connector on the glow plug. At the same time touch the negative wire to the body of the glow plug. Continue to hold the wire there for 10 to 15 seconds.
Touch the glow plug “the part that is supposed to get hot”
Possible outcomes:
1. Damn I turned lose of that in a hurry before it burned the crap out of me. Nice! The glow plug is good!
2. The glow plug did not heat or only heated a little. The glow plug is bad.
In case 1 if the glow plugs are not working via the normal circuity on the tractor then you should look elsewhere other than the glow plugs.
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For inquiring minds:
My 354 pulls over 30 amps when the glow plugs are on cold (they pull less hot).
Power (in watts)= volts * current ( in amps)= 12v * 30+A= 360+ watts
So each glow plug ( I have four) is 90+ watts cold, somewhat less hot.
Also:
Power( in watts)= (volts squared)/ resistance (in ohms)
So 90=144/R, R= 144/90= 1.6 Ohms. Unless you have an expensive ohmmeter it's not going to be able to differentiate between 1.6 Ohm and a dead short. The simple field test is that a dead short will blow your fuse.
If the resistance of a glow plug is close to zero, it's getting voltage, and it's not blowing the fuse, then it's working. Note that you don't have to remove them from the tractor to measure resistance. You do, however, have to separate them from each other. If one is open and the others are good the change in resistance is probably not measurable.
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Thank you all for replies.
I checked all three plugs with a ohmmeter,
All read zero ohms. I then measured the resistance
of a 60 watt light bulb to see if the ohmmeter was working.
The lightbulb measured 19.2 ohms.
I then took a new glow plug from a different engine
and again measured zero ohms.
Then I connected each of the old glow plugs directly to a 12v battery
for about 10 seconds. Each one glowed red.
Then connected new glow plug. It also glowed red.
So can I correctly conclude that the plugs are still usable?
I wish that I had a meter that could measure the current while
an old plug is glowing and compare to a new plug.
But my meter only goes to 10 amps.
I know that the 20 amp fuse in the tractor would blow.
And that replacing with a 30 amp fuse would start to melt the fuse block.
So I am thinking that the 40 amp relay circuit posted by RichWaugh
is the best solution.
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Well actually I just have a flat copper strip
that bolts down to the top of each plug
connecting them all together.
Then one wire that connects to the copper strip
so that they are all energized at the same time.
It's pretty roomy around the plugs and I keep the area
clean with the power washer.
I've had each plug out and could see no visible damage.
But I suppose maybe something could short out inside
that I can't see.
Thank Tinbender.
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in a pinch take a amp meter like you would have in the dash and place #10 stranded wire with some gator clips on it most of them are 30 amp .I keep one loose to check for load on dc stuff. something makes me think that the glow plugs are ok,your meter just can not see the resistance in them. I would concentrate on the wiring.alot of it on theses tractors are undersized to the things that pull some amps. I had a issue to the starter added a relay and my starting issues were gone.that might be your best choice even if a temp. one to see what it does,I have seen that it is better on the keyswitch life not running the amps thru it also.
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I most definitely agree.
Have already had to change key switch a few years back.
I too think that my meter will not read the low ohms
and that the plugs might still be ok to use.
Yes I have isolated the glow plug circuit to see if there was a short somewhere else.
But starter and lighting circuits works great.
No other shorts or fuse problems.
I would have to agree that the wire gauge does
look too small for the glow plug circuit.
And that the plastic body design of the key switch
does not seem adequate for higher amperage.
I will try to do the 40 amp relay circuit. But want to find a 12V push button type switch. My key switch from a local Farm Supply doesn't have an isolated/designated terminal for the glow plugs.
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that sounds good. though you might want to consider the relay will not take much amps to activate use the lead that goes to the glow plugs to go to the hot side of the button switch, then go to the relay.that way when the key switch is off, the power is off to the glow plug switch .I prefer to keep to a minium stuff that is hot all the time. just a thought
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Go buy you a good toggle switch and run you a wire from hotpost on battery to one contact on toggle switch, other contact on toggle run to the bus bar that connects all glowplugs, hold toggle switch to on position for about 15 to 20 seconds, start tractor.
Ben
Circle G Tractors
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