Spark Plug Wire Ohms Question

MoeJr · 10-28-06, 09:11 PM

How many ohms resistance should a good plug wire show? I have one wire on my Vette that I believe is no good as it had come of the plug a little and was giving me a tip in hesitation and stumble. When I took it off it was all white inside. I cleaned it and put some dielectric grease in it and it seemed to clear up for a few days but now it is back and worse soI think the wire may have gotten damaged from having to jump the spark to the plug. I had been chasing this miss for 2 months. Only at low rpm say idle to 1200 or so. It was fine higher up which I found odd. I ohmed it today and it was 5.5. Any help is greatly appreciated.

compyelc4 · 10-28-06, 09:32 PM

If it really is 5.0 ohms that's pretty darn good conductivity. Heck, a total ground is 0.0 ohms! I believe reasonably good stock plug wires on GM vehicles are maybe 600-900 ohm resistance, although very high quality "resistance" wires, such as on Vettes, can go as low as you measured.

pns0ccc · 10-28-06, 11:34 PM

5 ohms seem very low. On a set of wires that I've just removed, the shortest wire indicated 700 ohms. Are you pulling any codes? Can you remove one end of the wire and notice any difference on engine operation?

compyelc4 · 10-28-06, 11:54 PM

Yep. For fear I was talking out my _ss I consulted my '95 service manual (VIN P or J). It reads, "If the meter reads over 30,000 ohms, replace wire(s)." If you are getting 5 ohms something may be amiss. "Normally", as a wire goes south the resistance (ohms) goes up.

GerryLP · 10-29-06, 12:19 AM

Some high performance wires maintain a 50 ohm per foot of wire. So if the wire is approximately 2 feet long, then one would expect about 100 ohms (at best). Once the wire decays, the resistance will increase, so something is amiss on the 5 ohm reading, IMHO.

GerryLP

MoeJr · 10-29-06, 12:30 AM

No code. Car runs fine going down the highway or cruising and even getting on it. Only time I notice it is when I give it light throttle going away from a stop light or if I am say in city at 35 and rpm is like 1200-1400. If I put the car in gear with my foot on the brake and bring the rpm up to like 1200 you can feel it but it is not like it has a dead cylinder as the shake is a pulsing kind of shake not the constant shake you get if the wire is off. Taking the wire off makes a difference. Also the car does not miss if in park and idling. I did not try raising the rpm to 1200 in park and seeing if it did it. Right now I have the cap and rotor and wires off as I was inspecting them. Cap had that white ashen film on the contacts inside. Rotor had some arc marks on it but nothing I think is too serious. I have that tune up kit coming from mid america and I am also gonna order the 2 injectors and tps. Car has 95k on it so I am sure the original injectors are getting tired. I did however change all the tune up parts within the last 3 years. I tried the 20kand 200k settings on my ohm meter. It is a digital brand new so it should be accurate. Only thing I can think of is do you have to have the neg and pos in a certain end of the wire?Thanks for all the help.

compyelc4 · 10-29-06, 01:11 AM

Nope. Ohms is ohms, either polarity. Are you certain you know how to use an ohm meter? No dis intended. You never know.

How are you measuring that wire? The wire is disconnected from one or both ends, right?

MoeJr · 10-29-06, 01:34 AM

No dis taken(now where is my gun?)

I put the negative probe into one boot and touched the terminal and put the positive in the other boot and did the same. I believe that is the correct way? I did it at the 20k scale and 200k scale and both times got 5.5 and I made sure I was on the terminals good.

compyelc4 · 10-29-06, 01:08 AM

Correct way.

I use an auto-ranging DVOM so setting a scale is no longer a requirement, but as I remember I think you would not be able to read a 5.5 ohm reading on a 200,000 (200K). Even if you are reading 5,500 ohms and not 5.5 ohms, on a 20,000 (20K) ohm scale 5,500 (5.5) ohms would be about 1/4 of the full scale. On a 200,000 ohm scale even 5,500 ohms would be less than 3% of the scale. A difficult measurement to read indeed, unless one is an educated flea.

I don't know, maybe my old skills fail me. It's been a long time.

Can someone else help out here?

Robertwav1 · 10-29-06, 08:15 AM

Okay guys....here's my opinion. I wouldn't be too concerned about the ohms on a spark plug wire. When the meter is attached very little voltage is applied to get an ohm reading. These wires are high voltage as we all know and that's when they break down under a heavy load along with heat from the engine. Windings of a motor may read okay through an ohm meter but with a megger, a piece of equipment that applies enough voltage through the windings, only then can you tell if the windings are bad. The old way of checking a plug wire would be on a dark night preferably damp and to check for arching or shorting while the car is running. Personally....I would just replace them and get them out of the equation.

MoeJr · 10-29-06, 10:04 AM

Replacement AC delcos are on their way. I am just gonna replace the cap rotor and wires along with the map,tps and injectors. The car has 95k on it so the injectors gotta be geting tired. I'll snuff it out. Thanks and I'll keep you all posted. I took pics of the cap and rotor. Not the best but you can take a peek at them.

KOPBET · 10-29-06, 12:26 PM

5.5 on a 20k or 200k scale is 5.5k ohms.

Measure your AC Delco wires when they arrive. My guess is they will be in the 500ohms/foot range (e.g. a 32-34 inch wire will read around 1.5 on the same scales).

As mentioned, DC resistance measured with an ohmmeter is not indicative of performance of the wire, as 50k volts will behave much differently, including skin effect phenomenon of the wire, than the few volts output by an ohmmeter.

MoeJr · 10-29-06, 02:29 PM

So 5.5k would be 55000 ohms? That is not good if I am not mistaken. I learn something new everyday Thanks.

KOPBET · 10-29-06, 02:40 PM

Quote:

Originally Posted by MoeJr

So 5.5k would be 55000 ohms? That is not good if I am not mistaken. I learn something new everyday Thanks.

5.5K ohms is 5500 ohms.

mylotcat · 10-29-06, 04:19 PM

The "k" is scientific notation shorthand for 1000, and means to move the decimal point 3 places to the right, and insert zero's as needed. So, as KOPBET correctly indicates, 5.5k is the same as 5500. The other common symbols you might run into are "m", or 1 million, which means move the decimal point six places to the right, and "g", or 1 billion. You see this stuff a lot in computer lingo, e.g. a typical compuer might contain 1 million bytes of memory, abbreviated 1Mb: = 1 mega byte, or 1 million bytes.

Is this getting a bit "dry?"

Dave