Unusual electrical problem: measure the amps

I bought a Jubilee that was converted to 12v and the wiring was
a mess.
I rewired it so the ammeter would work.
In the process I installed a 30a circuit breaker to protect the
10g wire connected to the alternator.
I used 14g wire for the lights and installed a 15 amp fuse.
I used 14g wire for the ignition coil and gas solenoid and
installed a 15 amp fuse. All worked just fine for about 15 years
until yesterday.
The 15 amp fuse going to the ignition coil and gas solenoid
blew.
I installed a new 15 amp fuse. The tractor ran for about 5
seconds and the fuse blew again..

The gas solenoid always clicks when power is applied.
The gas solenoid wasn't shorted to ground.
The gas solenoid was working fine.
I measured the solenoid amps which should have been close to 2
amps.
Something happened to the coil causing the gas solenoid to use
18 amps.

Go figure, something can work and still be bad..
An ohmmeter is the least accurate meter and only tells me there
is a path for current.
Measuring the current told me the rest of the story..

This is the second time this year a coil of wire has gone bad.
The coil on my central AC compressor contactor had the same
problem.
It was using 7 amps instead of 1 amp blowing a 3 amp fuse.

I'm posting this to let people know an ohmmeter isn't the answer
when troubleshooting a coil of wire. Measure the amps..

I ordered a new gas solenoid and plan to install a separate 5
amp fuse going to the solenoid to make the troubleshooting
easier.

Yep, ohm meters will lie!

They are so sensitive, real world conditions often confuse them. Load testing is the only true test.

It's difficult to accurately measure resistance less than a couple of ohms with a typical multimeter, due to lead and contact resistance. And for anything less than an ohm, you need a lab-type meter capable of making four-wire resistance measurements. A four-wire lab meter can measure down to around a milliohm.

The difference in resistance in Ohms to go from a 2A draw to an 18A draw on 12V is miniscule.

6 Ohms of resistance will give you 2A draw.
2/3 Ohm will give you an 18A draw.

In college we used A Wheatstone bridge to accurately measure resistance.
I used to calibrate electronic equipment at Crane Naval
(USNAD). The ohmmeter is only a great way to measure conductivity. It doesn't measure ohms accurately.
When it comes to testing coils, measure the current.

I do not know how many times I have had this argument, the fuel solenoid on the Kubota tractors do that and hard to convince folks it is bad. They say I checked it and it is not shorted.

> In college we used A Wheatstone bridge to accurately measure resistance.

I think the Wheatstone bridge circuit is implemented in many multimeters, particularly analog meters. But by itself, the Wheatstone bridge doesn't solve the problem of lead resistance. To do that, you need to eliminate current through the test leads. Which is what a four-wire measurement does. You have separate 'source' and 'sense' leads; all the test current goes through the source leads while the sense leads measure voltage. Since the input impedance of a DMM is typically 10 megohms, the voltage drop through the sense leads is insignificant.

Jerry.
Use a 12v battery and an ammeter.
Show them what current a good solenoid uses and what their bad solenoid uses.

This past summer a 1 year old central AC had a bad contactor.
I used a 24v transformer and an amprobe. I measured 7 amps and the furnace transformer had a 3 amp fuse.
A good contactor uses 1 amp.

Mark
No ohmmeter is accurate.
Look at the owner's manual and see the accuracy of the meter..
The old simpson 260 you could zero the meter to compensate for the lead's resistance. They were at best + or - 5% (maybe more?) of the full scale.

Can you zero digital ohmmeters?

If a person wants to test their ohmmeters accuracy, just buy some precision resistors.

Good post Geo.

First, don't put all your eggs in one basket. By no means do I rely on just a ohm reading. Relying on OEM trouble charts early on This was one of my pet peeves. Ohm this ohm that ohm till ell freezes over. If all fails replace the ECM/part. If they would have just told us what the voltage should be life would have been good. That info was out there it just was not available to a independent in OEM form.

I did not throw my ohm meter away : ) It still comes in handy for a quick check. OK time to kill wire the solenoid up let it get warm and check the ohms. For the poor boys and giggles.

Lets use that useless ohm meter to check you new solenoid, For the poor boys and giggles.

With so much high tech equipment on hand I can tell ya a simple ohm check on a new/rebuilt part would have saved me countless lost of billable hours.

I feel I may have coined this phrase as I do get some strange looks from highly accredited instructors when I take an on going training class.

A ohm meter can tell you its bad but it can't tell ya if its good. Let that sink in and don't throw away that ohm meter.

My shop ice maker quit years ago a simple ohm check of the water valve is all I needed to confirm the solenoid was bad/good.

I put a rebuilt engine in a 94 Toyota truck as always I put a set of rebuilt injectors on it. It was a dud no power it ran great just low on power. Long story short If I had only ohm the new injectors while I had them in my hand.

Don't throw away they ohm meter yet : )

This post was edited by Hobo,NC on 11/16/2022 at 07:07 am.

Coils in electric motors can do that as well. AMProbe is very handy. Knowing when to suspect an internally shorted magnetic coil is the hard part because they have (or should have) no moving parts. Jim

Mornin Good Neighbor

DITTO X2 I agree

I'm posting this to let people know an ohmmeter isn't the answer when troubleshooting a coil of wire. Measure the amps.

For over 50 years of use and including labs back at Purdue University us sparkies all know typical run of the mill low to mid grade Ohm Meters are not greatly accurate ESPECIALY at low readings WELL DUH LOL

Ive used Simpson 260 Analogs up to Fluke digitals and found this to be the case grrrrrrrrrrr

Thanks for informing the non sparkies, you have a good day now, keep warm Im slowly working my way South but its still cold brrrrrrrrrrrr

John T

(quoted from post at 08:29:09 11/16/22) Yep, ohm meters will lie!

They are so sensitive, real world conditions often confuse them. Load testing is the only true test.

Click to expand...

y position is not that Ohmmeters lie, but rather, the operators lack of ability to correctly assess the measurements. & interpret the meaning. H3ll I venture to guess that 75%+ of users think "continuity" means zero Ohms. That is about as wrong as it gets.

I am curious -what is the ohm reading across the old, failed coil, versus the new coil? As far as I know, all coils have to have some resistance, but they can't be zero ohms, more like 2-3 ohms. I bet the old coil has less resistance than the new one. Mark.

(quoted from post at 12:28:59 11/16/22) I am curious -what is the ohm reading across the old, failed coil, versus the new coil? As far as I know, all coils have to have some resistance, but they can't be zero ohms, more like 2-3 ohms. I bet the old coil has less resistance than the new one. Mark.

Click to expand...

indings can short to each other as well as to be fine, one to another, but still have winding short to housing.

Does the solenoid have a pull coil and a hold coil? Some do, and the pull coil is momentarily and high amps, the hold coil is lower amps.
The plunger of the solenoid hits a switch and shuts off the pull coil.

> Can you zero digital ohmmeters?

The ability to zero an ohmmeter has no effect on its accuracy or precision. The real question is, 'can your meter distinguish between a resistance of, say 0.1 ohms and 0.05 ohms? Most can't.

Back when I was a tech rep at Bell Helicopter Amarillo working on AH-1S Cobras, there was an EMC issue with the Helmet Sight System interfering with some other avionics. The EMC guys determined the HSS wasn't grounded well enough, and that its control box needed to be bonded to airframe with less than something like 10 milliohms resistance. Obviously, the electricians' Simpson 260s weren't going to make that measurement. My colleague took a DMM out of the metrology lab, set it up for four-wire ohms, and was easily able to measure down to one milliohm.

> I venture to guess that 75%+ of users think continuity means zero Ohms. That is about as wrong as it gets.

There is, of course, no such thing as 'zero ohms'. It's a matter of how much precision and accuracy you have when making a resistance measurement.

Another thing most DMM users don't know is that the continuity or 'diode check' range on their meter does not measure resistance. It measures VOLTAGE.