I need to get my 90K miles service performed, and I started to think about changing the spark plug wires and the distributor cap. I recognize that Volvo doesn't specify changing the wires, and I can't remember a manufacturer that does. Yet mechanics almost routinely replace the wires, sometimes with pricey versions. What could actually fail with the wires? The insulation could break down which would cause parallel paths to ground, reducing the current to the plug, possibly preventing the spark from reaching it. However, if I was engineering the elastomer in the wire, I think I would make some bounding assumptions about the temperature under the hood over time, and use an ageing thumb rule based on known degredation as a function of temperature, and then I could specify when to change the wires. Yet this is not done, which leads me to the hypothesis that the wires really don't need to be changed except under extreme conditions or if they are damaged in some way. Anybody have any thoughts on this issue? Thanks.

I gotta ask about this one....when my '94 855T was being revived by the dealer from my self-inflicted unpleasantness last month, the mech made a comment that my plug wires had "very high resistance" and I should consider replacing them. I haven't seen fit to do that yet, and am still puzzled about what would be considered correct resistance and what constitutes high resistance. In fact, I remember something about resistor spark plugs that force the voltage to build up to give a real hot spark instead of letting the spark bleed off too early. Anyone with knowledge on this one??

If you have a volt meter you turn the thing on to read Ohms and like any piece of copper wire, the resistance should be "0" when you touch one end of the wire and the other with the meter. If you have high resistance, that would mean that you either have crud on the ends or that the current is escaping into the insulation or you have crimped wire causing the resistance to increase. The resistor plugs from what I understood were just to keep the electronic noise down so you didn't have that buzz on the radio.......

OK....checked a spare set of wires and found that they varied from 2500 ohms to 4200 ohms depending on the length. Short ones were lower, longer ones higher in resistance, but nothing like "zero" ohms like you'd get with a piece of copper wire.

Then I took a look at the website for Bougicord (should have done this first) and found in their FAQ list the following:
"Since 1957, the law has required that all automobile ignition wire sets be fitted with interference suppressors. Standards place acceptable resistance at 5,600 ohms per metre of cable with a tolerance of ± 20% (the range is therefore between 4,480 and 6,720 ohm/m). At that time, interference suppression was made compulsory to avoid disturbing radios or televisions in areas where the vehicle was running. The phenomenon was troublesome but not dangerous. Today, interference may accidentally set off an airbag, or perturb an ABS braking system. Hence the need for using ignition wire sets that correspond to interference suppression standards perfectly."

Hmmmmm.....so at 5600 ohms per meter, the number one cylinder wire is almost exactly on spec coming in at 4200 ohms since it's about .75 meter in length and the number 5 cylinder wire at only about 8 inches length is also right on with 2400 ohms.

Next step--pull apart the set on my engine to check it.....

Hi,
would like to second KlausC, wires do last long. I'm running two 850s-94 with original wires but second set of distributor and. No problems yet ever with wires.
br Tapsa

You are correct, the wires do last a long time. And if there is a problem, you will start to get misfire codes.

Check the date of manufacture on the plug wires. The good ones have a date on each wire :)
If you change plugs,wires,cap and rotor, FCP Groton is good. I got OEM plugs, Bougicord wires, and cap/rotor. 50% less than dealer.

Memorize the #1 plug wire position on the cap, the Haynes manual is wrong.
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My name is Klaus and I am a Volvoholic.

from all the posting on here, i would make sure to replace with volvo bogicord vs. bosch...or anything else. everyone seems to think they are better. Distributor cap, rotor, perhaps new flash shield in the distributor cap...it's an easy job.

I had a set of wires fail on my old 760. The insulation failed on one of the plugs and the spark was jumping right out the plug boot and into the head. The engine worked terribly until I replaced the wires.

Mind you, for all I knew those wires could have been 20 years old - this happened on a 1986 760, in 2006/2007.
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1998 V70 AWD Turbo 180k+