coil resistance measurement

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Topic author
letank
Posts: 4009
Joined: Wed Oct 03, 2012 9:16 pm
Location: SF bay area

coil resistance measurement

Post by letank »

Funny how we learn things.... the GF's sport car did not finished the ride as planned... power dwindled, engine labored on inclines... so like a jeep I pulled over, lifted the hood and checked a few things... of course no tools... the engine came back to life after a 5min cooling period, but no overheating in 70 degrees weather, we had power for the next 2 miles... then again... pulled over... recheck... thinking now about the coil/electrical system... coil was hot, uncomfortable to the touch after 1sec... especially that it is not on top of the engine... 10 min wait... and repeat 4 times.

So to the point, how do we measure the coil resistance to compare it with its oem specs, the 450 manual tell you to replace with a known good one... but no specs listed, so a bit of a search let me find this from user jyuma, from https://www.benzworld.org/forums/r-c107 ... -coil.html

At the risk of boring the crap out of people... a transformer works by passing a current through the primary which results in the creation of a magnetic field. As the magnetic field builds (or collapses) the magnetic flux (lines of magnetism) cross the windings in the secondary which will create a voltage in the secondary. The amount of voltage in the secondary is a function of the ratio of turns in the primary to turns in the secondary. It is very much proportional to the degree that double the turns in the secondary will result in double the voltage in the secondary (but half the current). This effect of a current (voltage in motion) in the primary producing a magnetic field that then creates a voltage in the secondary is totally dependent upon the integrity of the coils involved. This is why simply DC resistance checks are all but useless, because the fact that the DC resistance between two different coils is the same doesn't tell you anything about the condition of the windings in the two coils.

Clear as mud?

and for the measurement, How to Measure Inductance of a Coil:

https://sciencing.com/measure-inductanc ... 46890.html

Inductors are sometimes wound by the user instead of bought. In such cases, the inductance would not be stamped on the side but instead may need to be found empirically. The best way to measure inductance for an inductor like a coil (solenoid) is to use an inductance bridge or meter. If you have neither, a more indirect way is to use an oscilloscope.

Connect a resistor of known resistance and the coil in series with the sine wave oscilloscope.

Turn on the oscilloscope and clamp the two clips of one voltmeter to the circuit on opposite sides of the coil to determine the voltage drop across the coil. Then do the same for the resistor with another voltmeter.

Set the frequency of the oscilloscope such that the voltage drop across the resistor and inductor are the same. Finding said frequency may be a matter of trial and error. At that frequency, the resistance of the resistor and the impedance of the inductor will be equal.

Set the resistor’s resistance and the inductor’s impedance equal to each other and solve for the inductance of the coil. Resistance = 2?fL, where \"f\" is the frequency of the oscilloscope and \"L\" is the coil’s inductance. The resistance of the resistor has not changed from the start; it is independent of the frequency. So \"L\" can be solved for by arithmetic.

Have fun... in the mean time I installed one the jeep spare coil and will be doing a drive test to confirm my hypothesis of a bad coil
Michel
74 wag (349 Kmiles... parked, next step is a rust free body)
85 Gwag (229 Kmiles... the running test lab)
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tgreese
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Re: coil resistance measurement

Post by tgreese »

Mmm. Interesting and simple way to measure the impedance of a coil. However, I think big iron core inductors like our automotive ignition coils are not going to be linear with frequency, or with current. So the L you measure will be dependent on the R you choose for the test, and may not be the L you get when conducting 12 volts.

I kinda think this is academic since most here don't have an oscilloscope and frequency generator. You can buy an instrument to measure inductance for cheap. Again, the L is going to be frequency and current dependent, but you'll get an idea of the inductance. If you shop around, you can buy a spare coil for nearly as cheap too.

https://www.amazon.com/Multimeter-HOLDP ... ref=sr_1_5
https://www.amazon.com/Standard-Motor-P ... ref=sr_1_3
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
Blockchain the vote.

Topic author
letank
Posts: 4009
Joined: Wed Oct 03, 2012 9:16 pm
Location: SF bay area

Re: coil resistance measurement

Post by letank »

tgreese wrote: If you shop around, you can buy a spare coil for nearly as cheap too.
Thank you, I agree with a quick purchase, but I found it an eye opener for our daily beasts, where most TSM indicate a resistance measurement. I remember Tom and Ray's car talk show when they mentioned that coil could not be tested by resistance.

A simple voltage measurement was an insider, the oem coil was at 2 volts, engine purring upon start up and the jeep blaster was specs of 7v.
Michel
74 wag (349 Kmiles... parked, next step is a rust free body)
85 Gwag (229 Kmiles... the running test lab)
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tgreese
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Location: Medford MA USA

Re: coil resistance measurement

Post by tgreese »

It's hard to measure low resistances accurately, both because of the contribution of the test set-up, and the inaccuracy of the cheap-ish multimeters that are available to most of us. With a coil primary, it's just a bunch of wire, and the total resistance of hundreds of feet of wire is still quite low. This may be the root of the Tappet Bros reasoning.

My '82 TSM shows a test resistance of 7700-9300 for the secondary (ok - we can measure that) and 1.13 to 1.23 for the primary at room temperature (Ha! Good Luck!).

A resistance measurement can still help you, even with coils and motors and such. Recently I replaced the starter in my J20 after I determined it was defective using a multimeter. Spec for the hold-down coil is like 4 ohms, which the starter passed when standing on end on the bench. Lay it down in its normal position and it measured ca 2 ohms. Not good... Remarkably, the TSM suggests that you unsolder and replace the hold-down field coil! Must have been available through the parts department in the day. The J20 got a brand-new starter through Amazon. Works great.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
Blockchain the vote.

Topic author
letank
Posts: 4009
Joined: Wed Oct 03, 2012 9:16 pm
Location: SF bay area

Re: coil resistance measurement

Post by letank »

update on low resistance measurements, the inductance meter is back ordered. One of my tech friends told me to do a differential measurement with various resistor added to the circuit... so I used a 36, and a 67 ohm resistors... at 36 ohms I got any number around, even repeated measurements where not precise... but the 67ohms was precise , aka reproducible, mainly 67.2, and added to the coil gave me 67.6 or 67.7 which is in agreement with the merco .33 to .46 ohm specs, the fluke vom was better than the noname brand as expected. So on next outing I will have a debugging kit to see where I stand.
Last edited by letank on Wed Sep 25, 2019 11:47 am, edited 1 time in total.
Michel
74 wag (349 Kmiles... parked, next step is a rust free body)
85 Gwag (229 Kmiles... the running test lab)
User avatar

tgreese
Posts: 7118
Joined: Fri Jun 08, 2012 6:31 am
Location: Medford MA USA

Re: coil resistance measurement

Post by tgreese »

Good tip, thanks for posting!
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
Blockchain the vote.
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