How to use a multimeter tester

[Ron Pett]

I am testing a 1948 Ford tractor coil. Specs call for resistance between 1.06-1.17 ohms. Using Southwire multitester that is unadjustable. When touching leads of tester together, tester reads .4 ohms. Testing the coil shows 1.1 ohms. Is the reading of the coil really 1.1 ohms or net of the testers .4 ohms, meaning the coil is reading .7 ohms. Thanks

 

[Petej999]

I am testing a 1948 Ford tractor coil. Specs call for resistance between 1.06-1.17 ohms. Using Southwire multitester that is unadjustable. When touching leads of tester together, tester reads .4 ohms. Testing the coil shows 1.1 ohms. Is the reading of the coil really 1.1 ohms or net of the testers .4 ohms, meaning the coil is reading .7 ohms. Thanks

You need to take the res of the leads into consideration

 

[Ron Pett]

You need to take the res of the leads into consideration

Thank you!

 

[PEG]

Hi,without knowing the model of tester,it is not possible to know if the leads require nulling or their read resistance being taken in to consideration.
Good quality multimeters,even if fitted with fixed leads,normally have some means of ensuring accuracy from zero,to their resolution.

What coil are you testing?

 

[Ron Pett]

Hi,without knowing the model of tester,it is not possible to know if the leads require nulling or their read resistance being taken in to consideration.
Good quality multimeters,even if fitted with fixed leads,normally have some means of ensuring accuracy from zero,to their resolution.

What coil are you testing?

The tester is a Southwire Model 10031S.

i am trying to determine ignition issues on a 1948 Ford tractor with a square mount coil that sits on top of the distributor, before the side mounts were introduced. It burns points to fast and I have changed condensor and rebuilt distributor to address any wear in the bushings, even though there was none visible. So I checked coil resistance that should be 1.06-1.17 from top input to bottom output to points and it reads 1.1, but that is with a tester that read .4 testing itself. So I dont know if I have 1.1 ohms or .7 ohms. If the .7, then this could indicate a problem..Thanks!!

 

[PEG]

Hi,a 0.7 ohm reading could potentially indicate shorting in the windings,but don't dismiss a faulty condenser,even if new....i always test them straight from the packet.

Does that model run with a ballast resistor? Incorrect leads,plugs and voltage at the coil,can all have a similar effect on points wear.

 

[Ron Pett]

Hi,a 0.7 ohm reading could potentially indicate shorting in the windings,but don't dismiss a faulty condenser,even if new....i always test them straight from the packet.

Does that model run with a ballast resistor? Incorrect leads,plugs and voltage at the coil,can all have a similar effect on points wear.

PEG..Thanks so much for your response. Yes, this model has a ballast resistor on the "dash" board behind the voltage gauge. I doubt it is the leads as I have not removed any wiring since I got it years ago except for the one wire at the top of the square coil. Other forums gave me tips on checking different items, so after rebuilding distributor, I did alk the "\shorting" test to make sure there was nothing inside the distributor. I checked voltage going into the coil, 6.1 volts (original 6 volt system). Once I did all the tips, I bought a service manual that provides the specs for each component and checked the ohms, thus the reason for the first question. No, I did not check the condenser since I did put in a new one, but I will do that since so much stuff is bad from the beginning.

Now for a "electrical question for dummies". If the coil resistance is almost cut in half, does that indicate more current flowing out of the coil? That seems to be what is happening since I replace points, it runs for a few hours, then they are burned and no spark.

 

[PEG]

Hi,i take it you are using the 6V batteries and equipment,so it may be worth checking you are getting the full 6V during starting,and then a reduced voltage,via the ballast resistor,at ignition "run" position.

If the ballast resistor is damaged,bypassed,or not in the run circuit,the coil may be receiving full voltage,and after a prolonged period,could cause overheating of the windings,and other issues.

I have had similar problems on old machinery,and trikes,etc,where the clue is a burn on touching the coil casing.

Some ballast resistors are part of the loom,and the direct full voltage is derived from either a feed from the starter motor,or the ignition switch direct,but if the coil is rated for it's 100% duty cycle at reduced voltage,it won't like full voltage.

 

[johntheo]

Hi,i take it you are using the 6V batteries and equipment,so it may be worth checking you are getting the full 6V during starting,and then a reduced voltage,via the ballast resistor,at ignition "run" position.

If the ballast resistor is damaged,bypassed,or not in the run circuit,the coil may be receiving full voltage,and after a prolonged period,could cause overheating of the windings,and other issues.

I have had similar problems on old machinery,and trikes,etc,where the clue is a burn on touching the coil casing.

Some ballast resistors are part of the loom,and the direct full voltage is derived from either a feed from the starter motor,or the ignition switch direct,but if the coil is rated for it's 100% duty cycle at reduced voltage,it won't like full voltage.

I think that on 12V systems anyhow the coil was actually 9V and the resistor/resistance wire was by passed while operating the starter motor but the resistor reduced the voltage while engine was running.

 

[Ron Pett]

Hi,i take it you are using the 6V batteries and equipment,so it may be worth checking you are getting the full 6V during starting,and then a reduced voltage,via the ballast resistor,at ignition "run" position.

If the ballast resistor is damaged,bypassed,or not in the run circuit,the coil may be receiving full voltage,and after a prolonged period,could cause overheating of the windings,and other issues.

I have had similar problems on old machinery,and trikes,etc,where the clue is a burn on touching the coil casing.

Some ballast resistors are part of the loom,and the direct full voltage is derived from either a feed from the starter motor,or the ignition switch direct,but if the coil is rated for it's 100% duty cycle at reduced voltage,it won't like full voltage.

Thanks, I will replace resister when I replace coil. Only a few dollars to becsafe. Battery was fully charged at the auto parts store. 6 volt, notb12 voot conversion

 

[johntheo]

Just always remember that whether a 6V or 12v system that the resistor/resistance wire is always in service after starting the engine.