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To add to your good reply, Bill
In my mind the symptoms are indeed consistent with CPS/rpm sensor failure. An eventual complete no start would seem a likely next stage.
Talk about which CPS sensor brands are best intrigues me at the moment. In the past I've always tried to stick with Genuine Volvo or Bougicord. But I'm now setting my sights wider. Art B. has experimented with cheap CPS sensors from China and found them surprisingly satisfactory, at least initially, but longevity remains an unknown
CPS, aka rpm sensors don't wear out in the true sense, rather they fail. The sensors actually aren't all that complicated to manufacture, a simple pickup wire coil around a metal rod in a plastic sensor head and a small gauge wire pair to deliver a millivolt pulse to the ignition controller, which also passes that signal on to the ECU. A foil shield is used in the cable to minimize/eliminate signal interference along the way, grounded at one end only for best shielding.
What the controllers need to see is enough of a voltage pulse to detect a revolution. What they don't want to see is significant signal interference to make it think there's another revolution happening. If the shielding should suddenly fail and if the engine should then die (rather just a stumble) then you might expect the engine wouldn't easily restart until something changes, which is one symptom we often see. Another common symptom is not being able to do a cold start, but being able to do a warm start. Another variation is having the engine die when rounding a corner at speed and flexing the CPS cable. Then there seems to be everything in between.
In my mind it doesn't add up that a bad CPS is mostly a damaged shield allowing interference. Damaged cable insulation is of course associated with and the common cause of a damaged shield and a less than perfect signal getting from the sensor to the controllers, but just how easy is it for interference to ruin a pulse or create a false pulse?
What I'm wondering, actually theorizing, is that in addition to an intermittent/ broken wire, that a CPS failure might be due to moisture getting in through damaged insulation and even behind the foil shield, eventually working its way down to the sensor. It may then be that it's corrosion of the sensor coil and pickup rod that can be more a problem than interference. It would take a few careful autopsies to confirm my suspicions.
Damaged insulation, often starting out as a brittle area from age and heat, will occur first at the flex points, quite often where the cable goes into the sensor. It's often difficult to spot a small split down there unless you remove the cable and flex it for inspection. Broken or missing plastic standoff clips on the water pump return line and dipstick allow the cable to flex there. If the standoffs are still in place then the main flex point is at the upper standoff. Damage there can often be spotted visually by unclipping the cable.
With all that in mind and as well the wire strands breaking from being able to flex too much at a damaged area, I believe the key to quality and longevity is the cable construction, most important being the thickness, quality and longevity of the cable insulation (casing).
So having theorized this, I bought a Facet CPS made in Italy with cable construction that looks slightly better than the Volvo/Bougicord originals. Facet is OEM for a number of automakers. It was cheaper than the Bougicord I wasn't able to find in stock along with reasonable shipping. It also includes two new standoff clips that you don't get with Bougicord, only with the Genuine Volvo (the clips themselves are now NLA from Volvo). I've now put this in one of my 940s.
At the other end of the scale, I also bought a cheap one from China at almost 1/10th the Volvo price. Of the dozens available on Aliexpress, I chose one that appeared it might have better construction than the very cheapest ones. I actually purchased it as a spare to pack, but my current plan is to try using it in my other 940 to see how long it lasts. I'll likely not be able to fully report back on this for few more years.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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