My coil readings

[TroyGT]

Is my coil bad? I don't seem to get the proper resistance readings that everyone else is getting, but I'm no electrical dude and don't really know what I'm doing either. I have a cheap Sperry meter that has the following ohm settings... rx1, rx10, rx100. I set it to rx1 and touched my leads to the two outer contacts (15 & 1) where the electrical wires were attached... I get a reading of about 6 or 6.5 ohms on primary. I have to jack the setting up to rx100 to get a reading on the secondary (center post to 1 of the elect side posts) and that shows me a reading of about 75. I have an old black Bosch coil with orange top.

Still working on my stranded 911

Thanks.

-Troy

[TroyGT]

Wow, was expecting a ton of replies within minutes. No one?

[Patton]

I'm not sure your reading the ohms correctly. If you were on the rx10 (resistance times 10, and you read 6 - 6.5 on the primaries (pins 1 and 15), that would be around .6 ohms and that would be good.

I'm not sure about the Sperry meter but if you're reading around 6 ohms on rx1 (resistance times 1), that's not a good primary resistance.

Your reading of 75 with the meter set at rx100 (resistance times 100 would put your secondary (center to pins 1 or 15) at 7,500 ohms which would be good for a secondary.

[HarryD]

Check this link for how to test your coil: Bad Coil?

Early_S_Man is a very good resource for stuff like this.

[T77911S]

Originally posted by Patton
I'm not sure your reading the ohms correctly. If you were on the rx10 (resistance times 10, and you read 6 - 6.5 on the primaries (pins 1 and 15), that would be around .6 ohms and that would be good.

how did you come with .6 ohms with Rx10? my electronics experience says 60 - 65 ohms

[TroyGT]

I tested this several times before posting anything, I'm pretty sure that 6 - 6.5 ohms on primary and 7500 on the secondary are correct. If these readings are correct, is this coil bad?

Thanks

-Troy

[Patton]

how did you come with .6 ohms with Rx10? my electronics experience says 60 - 65 ohms

Read what said, again. I was trying to say that .6 is what he should be reading if the meter was on rx10.


Bottom line "is" 6 ohms is not a good primary resistance, and if actually so, it would clearly represent a bad coil.

[TroyGT]

Definitely read 6 on rx1 not rx10... so I must have a bad coil then. Awesome... that's something I can work with. My car has been down for awhile. My 'Stranded 911' post has almost turned into a soap opera.

Thanks.

-Troy

[Patton]

I know how frustrating it is believe me. I just went through pretty much the same thing. My car would start but ran like crap. It turned out to be multiple problems. Bad regulator, fried several components including the DME. I have a battery monitor/alarm now that tells me if I'm undercharged or overcharging.

After I got all that worked out, I had a couple of serious vacuum leaks. She's running great now.

[Patton]

Troy,
Make sure you have a good battery in you meter. Also, put the meter on rx10 and measure your primary again. If in fact your primary resistance is 6 ohms, on rx10 you should 60. If you get .6 I'd question your meter. Double checking can't hurt.

[rick-l]

Originally posted by TroyGT
Definitely read 6 on rx1 not rx10... -Troy

If you just hold the two leads together on the RX1 scale what do you get? You need to subtract that from your reading.

It is really hard to measure < 1 ohm with a cheap meter.

[T77911S]

Originally posted by Patton
Read what said, again. I was trying to say that .6 is what he should be reading if the meter was on rx10.

.6 on a scale of Rx10 = 6 ohms. he needs to be on the Rx1 to measure a resistance less than 1 ohm.

if his meter has a "1" at full scale deflection, then when the needle points to ".6" that = 6 ohms on a scale of Rx10.
it will = .6 ohms on a scale of Rx1.
you said "If you were on the rx10 (resistance times 10, and you read 6 - 6.5 on the primaries (pins 1 and 15), that would be around .6 ohms and that would be good." .6 ohms is not correct, that is 6 ohms.
on a setting of Rx10, full scale deflection = 10 ohms. if the needle is pointing to "6" ( actually .6) then it would have a resistance of 6 ohms.

do as rick said, hold the leads together, there should be an adjustment to "zero" the leads.

[Patton]

Hey, I said it right the first time. My reply to you was in error, pardon me.

[T77911S]

ok

[rick-l]

Originally posted by Patton
If he read 6 ohms on a rx10 setting then his actual primary resistance would be .6 ohms and that would be good.

Usually the switch will indicate the multiplier. I don't have an analog meter anymore but doesn't the meter scale go from 0 - 9 and you multiply the reading off the scale by the number on the range dial? So 6 on the X1 scale would be 6 ohms, 6 on the X10 scale would be 60 etc.

For example, If my tach says 1/min X 1000 and the needle points at 6 what RPM is the engine turning?

Bad semantics? maybe.

[Patton]

Rick-1 you are right. My analogy on rx10 is wrong. T77911S , my sincere apology.

TroyGT says he measures 6 ohms on rx1.

I think this is odd that he has a primary resistance of 6 ohms when a good coil should read around .6 ohms (factor of 10). Maybe it's coincidence.

[Early_S_Man]

Troy,

The numbers you got sound like a conventional Battery-coil ignition coil to me. Coil terminal designations of 1, 15, and 4 indicate a very old coil, perhaps '69 - '71 vintage. My '73 CDI coil is labeled 1, A, and 4, where 'A' is the hot side of the proimary to the CDI, and a newer '76 CDI coil [0.221.121.006, with less than 1" spacing between the primary terminals] is labeled A, B, and 4, where 'B' is the ground common terminal.

What engine is this coil for? Is it a CDI system?

Part number on the bottom [very small numerals] of the coil should be 0.221.121.001 or 0.221.121.006 for Bosch CDI coils, and the coils are smaller than other Bosch coils. If so, the numbers should be 650 - 790 Ohms for the secondary and 0.4 - 0.6 Ohms for the primary!

[Patton]

Warren,
From TroyGT's post "Stranded 911" he as a 1988 911.

[Early_S_Man]

OK, late Carrera coil ... Bentley's missing table:

Primary resistance:
Test between terminals 1 and 15
Value: 0.4 to 0.7 Ohms

Secondary resistance:
Test between terminals 4 and 15
Value: 5.0 to 8.7 k Ohms

Sounds like secondary is OK, and still some confusion about the primary ... 6 Ohm reading would seem like a bad solder joint internally, or corroded terminal externally.

I suggest retest using a different meter, or change the battery in the Sperry. Sperry is not a 'cheap' brand [cheap is $8 -$12 meter at AutoZone or Habor Freight] ... though it may be one of their lower-end models that is only $49 - $99.

[T77911S]

the top red scale on this meter is for resistance. on this particular meter, the resistance multiplier has the following, x1, x10, x100, x10k, x100k. what ever the selector is set to is the max resistance it can measure. for instance, if it is set to x1, then 1000 ohms is the highest resistance you can measure. if it is set to the x10 scale, then 10,000 is the highest. if the needle was on the 2, that would = 200 ohms.
so, on the x1 scale, 10=10 ohms, 1=1 ohm and the sixth little line from the right would = .6 ohms. with it set to the x10 scale, 1 would = 10 ohms so .6 would be half way between 0 and the first tiny line.
all meters are not the same, instead of 1k on the left, it could be 1, or 10, or maybe 2k or anything else.
my other meter even has a x.1 scale. so then, the 1k on the left would be 100 ohms max.
full scale deflection to the right on x1 indicates a short, like a fuse, all the way to the left with a high multiplier setting indicates an open, like a blown fuse.
if the needle goes to zero, try a lower setting, if it does not move try a higher setting.
i hope this helps. this is hard to explain in writing.